340 CYLINDER HEADS

[LJ67barracuda340]

why run zero deck? is it a modern gas issue? I have KB 243s in my 340 and I think they are stock above deck type. So far no issues on pump gas.

so I could get a tight quench .038" with closed chambers for the now gas.

 

[Dartswinger70]

so I could get a tight quench .038" with closed chambers for the now gas.

Got it. What heads are you using with closed chambers? aftermarket ones?

 

[LJ67barracuda340]

Got it. What heads are you using with closed chambers? aftermarket ones?

Edelbrock 63's

 

[Kendog 170]

why run zero deck? is it a modern gas issue? I have KB 243s in my 340 and I think they are stock above deck type. So far no issues on pump gas.

My 340 KB 243-20 / eddy heads 60179 65 cc/ .039 Head gasket/ stock cam / around 10.5-1 160 cold cranking comp./ 93 Octane haven't try less runs good but may have a failing lobe.

 

[AJ/FormS]

Was that cam custom ground?

no that was a Hughes HE2430AL FTH, 270/276/110
The key to this cam is the short 6* split that allows plenty of power extraction. In at 110 the numbers are;
270/115/112/276/53 overlap/ Ica of 65
intake,compression/power/exhaust. Effective overlap is 50*
At 180 psi, she was very fuel-thrifty in point to point cruising, but she liked a lot of ignition advance, as they usually do.
It's more cam than you need at your lightweight and application.
But it had the Ica that I was looking for, for my design and elevation. I loved it.

If you even think that you might someday want a bigger cam, you need to push the pressure envelope today, so there is enough pressure left over later at the later closing intake angle, else if it dips down too far, she will be a dog at low rpm. Two cam sizes, could be a minimum 7 degrees of Ica and a predicted drop of around 13 psi. Your only way to get the pressure back is more Scr. But to get the low-rpm performance back, at the pressure deficit, would require a higher stall, or bigger rear gears, or some combination.

Of course at your stated useage, you could easily get away with iron heads, closed chambers and tight-Q preferred but at your weight, could be open;
There is such a thing as too much bottom end,lol.

Here is an exercise;
Lets say we went down one size from mine, say a 262/268/112
and chose iron heads; in at 111 your numbers would be
262/118/113/268/41overlap/ Ica of 62*
With closed chambers and tight-Q, you can push 160 psi for say 89 gas at WOT, so 9.9Scr at 800 ft will get you there. With open chamber heads and NO-Q, you will need to step it up to 91 gas or drop the pressure maybe 5 psi, so say 9.7 Scr@800ft.
The thing is that the VP falls several point in the dropping of the pressure, which is already soft at 160psi! This may or may not want a higher stall depending on what you started with.

going back to;
262/118/113/268/41overlap/ Ica of 62*
the numbers in bold, predict that this combo could be very fuel-thrifty.
113* of power extraction is quite a lot, we could give some of that up, to get some top end power. Or we could increase the compression degrees to get the VP back. Or we could give up some absolute power to broaden the torque curve.
Or we could switch to a solid lifter cam, and make more absolute power with similar Part Throttle / low rpm characteristics.

 

[coyote42]

why run zero deck? is it a modern gas issue? I have KB 243s in my 340 and I think they are stock above deck type. So far no issues on pump gas.

It was what I chose for forged pistons at the time. I was looking at quench and saw with a closed chamber I would need a flat top piston if I were to run a closed chamber without crazy compression. I have open X heads but also looked at the Magnum heads I have too. Now I am looking into Aluminum heads with the short block I have.

 

[AJ/FormS]

Just to throw a monkey wrench at you, and IMO, and for your combo only;
Ima thinking some small-port closed chamber heads and a small-port intake and a short period cam would be dynomite in your lightweight combo, at your stated application. To whit; a 5.2Magnum top end, and a 5.2Magnum type cam.
This would make, IMO, a fantastic low-rpm and midrange machine. While not making big horsepower, with 3.23s you will hardly notice it........ because 65 mph in second gear with 26.7" tires comes to about 4200rpm@10% slip. Sooooooo, what good is power at 5200 if you never go there.
I built a hi-compression 340 like this while I was still a young man, about 1975. To this day I remember it fondly. It was a tire-fryer.


IIRC the small-port Magnums come in around 60cc, so your total chamber volume might be guestimated at
60+8.8+5=73.8 for a Q of .039(gasket thickness) and so the Scr is predicted to be
706.74 +73.8/73.8=10.576, The only way this would work is with alloy heads; (181psi) then I can get the Ica down to ~58degrees@800ft.
But get this, the VP is up to 150, something in the neighborhood of what the 440Magnum made back in the day. I can tell you that a VP of 150 is a ridiculous tire-fryer.
To get an idea of what that Ica of 58* might look like; here are some
256/122/115/262/112/35overlap/ Ica of 58*.......... or
262/122/113/268/110/45overlap/ Ica of 58*.......... or
268/122/110/276/108/56overlap/ Ica of 58*..........
So you got plenty of choices.....
Do you need a VP of 150?
No, but that's where your combo is gonna come in at, with a 58* ICA, so why fight it,lol.
The 256* cam above will make ridiculous fuel economy.
The 268* will make a lotta power, at the expense of fuel economy.
the 262* goes up the middle.
But all make the Same strong bottom-end because they all have the same 58* Ica. So if you get the stall-speed right the first time, it will be right every time.
So the money you don't need to spend on a TC and gears, can go into buying alloy heads. And then you can sell the X-heads to recoup most if not all of the rest of the costs associated with the alloy heads.
And, furthermore, because the ICA stays at 58*, you can run any of these cams without having to re-adjust the Scr. This is the beauty of running Alloy heads at the bottom of their pressure capability, versus running an iron close to the top of it's pressure ceiling..
Now,
the 256 cam can use small-port heads.
the 268 should really have big-ports
the 262 is sortof in the middle, for your application I wouldn't be afraid to run the smaller ports.
Whatever ports you run, I would run the same sized intake ports.
For this application, because the VP is so high, I would, me personally, would try to find a small-port, small-plenum singleplain, which will give you a lil something on the top, albeit, at an rpm you might never legally drive it at, lol, something like a Weand Excellerator.. I mean with 3.23s, 5000 is ~85 mph..... in second gear, lol. But that power bulge will be right where you need it.
You should probably pick out your intake first, because the Magnum heads will not accept every intake out there at the standard bolt angle. It's probably best to select the LA bolt angle, if available on the heads you choose.Then you can run any LA intake.

Thinking it over, because of the very high VP, You could probably run any sized ports. I myself, because of your stated application, would run the smallest available, because having done it once already, I sorta remember what it was like, namely; outrageous.
Happy HotRodding.

read about VP here; V/P Index Calculation

 

[Dartswinger70]

You don't see the guys in the 318 threads talking about chamber ccs "Cross thread points!" anyway that was the thing, throw a set of 360 head on a 318 and have 7:1 compression LOL! we were kids what did we know... however it seems smal block mopar the LA specifically used the piston deck height to vary compression ratio, and that is convenient with full floating rods. you can change things out easily at home. I would think closed chamber you'd need the flat tops exclusively.

 

[coyote42]

no that was a Hughes HE2430AL FTH, 270/276/110
The key to this cam is the short 6* split that allows plenty of power extraction. In at 110 the numbers are;
270/115/112/276/53 overlap/ Ica of 65
intake,compression/power/exhaust. Effective overlap is 50*
At 180 psi, she was very fuel-thrifty in point to point cruising, but she liked a lot of ignition advance, as they usually do.
It's more cam than you need at your lightweight and application.
But it had the Ica that I was looking for, for my design and elevation. I loved it.

If you even think that you might someday want a bigger cam, you need to push the pressure envelope today, so there is enough pressure left over later at the later closing intake angle, else if it dips down too far, she will be a dog at low rpm. Two cam sizes, could be a minimum 7 degrees of Ica and a predicted drop of around 13 psi. Your only way to get the pressure back is more Scr. But to get the low-rpm performance back, at the pressure deficit, would require a higher stall, or bigger rear gears, or some combination.

Of course at your stated useage, you could easily get away with iron heads, closed chambers and tight-Q preferred but at your weight, could be open;
There is such a thing as too much bottom end,lol.

Here is an exercise;
Lets say we went down one size from mine, say a 262/268/112
and chose iron heads; in at 111 your numbers would be
262/118/113/268/41overlap/ Ica of 62*
With closed chambers and tight-Q, you can push 160 psi for say 89 gas at WOT, so 9.9Scr at 800 ft will get you there. With open chamber heads and NO-Q, you will need to step it up to 91 gas or drop the pressure maybe 5 psi, so say 9.7 Scr@800ft.
The thing is that the VP falls several point in the dropping of the pressure, which is already soft at 160psi! This may or may not want a higher stall depending on what you started with.

going back to;
262/118/113/268/41overlap/ Ica of 62*
the numbers in bold, predict that this combo could be very fuel-thrifty.
113* of power extraction is quite a lot, we could give some of that up, to get some top end power. Or we could increase the compression degrees to get the VP back. Or we could give up some absolute power to broaden the torque curve.
Or we could switch to a solid lifter cam, and make more absolute power with similar Part Throttle / low rpm characteristics.

Thanks AJ, I am trying yo digest the numbers. I was thinking a 268 type cam myself. Now looking at the aluminum head choices, 63cc looks ok now or the 65cc to lower pressure a little. This leads to what runner size. Do I get 170cc to keep up port velocity for torq on my stock 3.31 stroke or the heads that are 180-190cc to let it breathe better? Is the 180-190cc a good compromise?

 

[AJ/FormS]

Now looking at the aluminum head choices

Because we have so many Alloy head choices now, It's starting to be an adventure. All I can tell you is that some FABO members have stated that they are running 200 psi on pumpgas. I'm assuming 91 or better. Allowing 5 psi per grade gasoline (with a tight-Q), that allows up to guessing 190 on 87E10 with a correct power-timing curve. I have run 185 on 87E10 no problem. For a short time, I was running close to 200
The point is this; I don't think we should be paying any attention at all to the Scr when using alloy heads, because in all likelyhood you will leave some performance on the table.
Do you need to run at the pressure ceiling with a streeter?
Well no you don't, and there is such a thing as way to much VP. IMO, a VP of over 150 is pointless unless yur running Caltracs or something, because they don't make decent 15" tires to withstand any more VP, so I'm always either spinning, or on the verge of spinning.
Yaknow, spinning in a straightline is ok, but when the back lets go around a turn when yur not expecting it, that's a recipe for disaster. I had to fit 295s in the back before I could corner with a 750DP carb,lol.
So really, we should be looking at the Dcr and pressure, and never mind the Scr, let the drag-racers worry about that.

So then, because of the flexibility in chamber sizes in alloy, if you have not yet bought heads, then with zero-deck flat tops, you should probably pick the cam first, so you can know the installed Ica. Then figure out what Dcr and pressure you wanna target, and let that guide your chamber size selection.
_________________

And BTW, the 268/276/114 cam was designed in an age where that's what it took to make the 340 the engine that it was, with iron heads. But, the power was up around 5000rpm.
Your modern streeter running ~185 psi, no longer needs that cam, because;
a power peak around 5000 leads to a shift point with an automatic and alloy heads, of around 5800, which in first gear with 3.23s, is about 53 mph, and the tires will still be smoking; on a good build. And second gear will hit ~90 mph with those same 3.23s.
So, IMO, the 3.23s are totally the wrong gear or
that 268 cam is way too big.
But, going to a the right gear, is gonna make the tires even easier to spin, so WTH do you do?
Well IMO, you gotta at least think about bringing the powerpeak down, or even downsizing the engine. The 340 being a bigbore, is a great sized engine to downsize the cam on.
Yakno, people are always saying the 318 is only a lil smaller than a 340, but often overlooked is that the 5.2M is gonna go back together at stock bore, whereas the the 340 will almost always need to go to 4.07 to clean up. Now yur at 160 thou difference. That doesn't sound like much but convert that to area; the 3.91 bore is 12.007 sq inches, versus 13.020 sq inches for the 340, or 8.35% bigger. That is no small number.
Anyway; the point is this, those alloy heads are gonna be a life-saver, making plenty of power due to the small chambers. So lets say the pressure jumps from 165 on a borderline detonating open iron chamber X head, to 185 on a barely even breathing hard alloy head still burning 87E10. That alone is gonna be worth a lotta cam duration, so already that 268 is "obsolete" in a streeter. IMO, you can easily scub off 7 to 10 degrees intake duration, and throw away that 114 LSA and end up with a much sweeter engine. So what if the power is down; yur still spinning the tires at the top of first gear, and the power at 65 mph is likely to be at least as high as it was with the 268 iron headed 340, with the same gears. You won't notice the power loss until you are fighting wind resistance, say 85mph, well into speeding territory.
So that opens the door to a 262* cam or even less. A 256* cam, for example, will bring the power-peak down about 400 rpm (as compared to the 268); and bring the bottom end down about half that so 200 rpm. Between that and the increased pressure, maybe you can use a regular stall and 3.23s or a lil more stall and a lil less gear. Or some combination.

I can tell you this;
I have had iron-headed hi-compression 340s on and off since my first one in 1970.
When I was in the engine selection stage for my Barracuda, I had 5 340 engines sitting around and one 360.
I chose the 360 and put alloy heads on it from day one. I have never been sorry about that.
I was sorry I ever put a 292/292/108 cam into it, yes.
But I soon put a 270/276/110 cam into it (and later a 276/276/110) and was never sorry about that either. In both cases and the third one,lol, the Scr was readjusted to maintain the 180psi or so pressure.
After I installed the 276 cam, I realized I shouldda gone the other way. Whereas; the 292* cam hated 3.55s, the 270*cam liked 3.23s and the 276* cam was very unhappy about the 3.55s until I jumped from a 2.66 low gear to a 3.09 low gear. ( plus 12.8%). This is the same as from 3.55s to 4.00s. That was with just a 7* intake duration increase! You can't really get a sense of that from perusing the catalog.
Bottom line is this; When it comes to cams, a 7 degree change is very significant.
If you design your pressure around a certain cam, you can always go bigger. But to go smaller often results in failure. Then the cam gets blamed for what is really a pressure problem. Don't be that guy, lol.

BTW; all this talk is particularly important for a streeter with a manual trans...... because the engine is sorta married to the tires. If the Dcr is low, (with street gears), taking off will usually require a lot of rpm to get moving. There is no Hi-stall convertor to help.
and of course;
for a dragracer with an automatic, this is of almost no importance, because his engine is never required to work at the rpms where optimum Dcr or VP matter.

 

[LJ67barracuda340]

Thanks AJ, I am trying yo digest the numbers. I was thinking a 268 type cam myself. Now looking at the aluminum head choices, 63cc looks ok now or the 65cc to lower pressure a little. This leads to what runner size. Do I get 170cc to keep up port velocity for torq on my stock 3.31 stroke or the heads that are 180-190cc to let it breathe better? Is the 180-190cc a good compromise?

You said you wanted a cruiser and not be a street hero. Now your talking flow and I feel your mixing things up. Torque is king in a cruiser. With tons o' torque you dont need RPM. Ive see 6700 but most of the time I dont need to go past 4500/5000 on the street or back roads. The Edel head runners are 177cc I think.

 

[LJ67barracuda340]

Ive been trying to tune a 340 69 Dart GTS 727 for a friend. X heads and a big (funny car sounding) cam. I dont have any idea on the specs. Car used to be (previous owner) a drag car and now friend is trying to cruise with it. Cranking pressure with the iron heads is between 185 and 200. Ive taken all the timing I can out and it wont stop rattling unless its on 94 or higher. It might be loaded with carbon I dont know but I know the PSI is too high for iron heads and todays fuel.

 

[Dartswinger70]

Ive been trying to tune a 340 69 Dart GTS 727 for a friend. X heads and a big (funny car sounding) cam. I dont have any idea on the specs. Car used to be (previous owner) a drag car and now friend is trying to cruise with it. Cranking pressure with the iron heads is between 185 and 200. Ive taken all the timing I can out and it wont stop rattling unless its on 94 or higher. It might be loaded with carbon I dont know but I know the PSI is too high for iron heads and todays fuel.

Do you think it may have 12.5:1 pistons in it or something? I had a similar setup, a Crane ssh 320-nc cam and the old TRW L23223 pistons , I took it apart and replaced the pistons and put a mild cam in it for that reason, I figured it too much for what I wanted to do with the car. I think in the old days with leaded gas it may have been ok.

 

[AJ/FormS]

I know the PSI is too high for iron heads and todays fuel.

I gotta agree; 185/200 is running into the ceiling for alloys already, never mind iron, even with tightQ
I think the only guy on FABO admitting to running iron heads at over 165 on pumpgas, successfully, is Yellowrose, at 170. But I don't recall his elevation.

 

[LJ67barracuda340]

Do you think it may have 12.5:1 pistons in it or something? I had a similar setup, a Crane ssh 320-nc cam and the old TRW L23223 pistons , I took it apart and replaced the pistons and put a mild cam in it for that reason, I figured it too much for what I wanted to do with the car. I think in the old days with leaded gas it may have been ok.

Ya probably. Ive had the oil pan off for a main seal and thats all theres numbers on the bottom of the pistons but i cant come up with any ID.

 

[coyote42]

You said you wanted a cruiser and not be a street hero. Now your talking flow and I feel your mixing things up. Torque is king in a cruiser. With tons o' torque you dont need RPM. Ive see 6700 but most of the time I dont need to go past 4500/5000 on the street or back roads. The Edel head runners are 177cc I think.

Thanks LJ67, not mixing up, just asking to be sure. I was figuring on keeping port flow up but I thought I would ask if there was a different thought about it anyway.
A cruiser with torque when I wanted it.

 

[coyote42]

Because we have so many Alloy head choices now, It's starting to be an adventure. All I can tell you is that some FABO members have stated that they are running 200 psi on pumpgas. I'm assuming 91 or better. Allowing 5 psi per grade gasoline (with a tight-Q), that allows up to guessing 190 on 87E10 with a correct power-timing curve. I have run 185 on 87E10 no problem. For a short time, I was running close to 200
The point is this; I don't think we should be paying any attention at all to the Scr when using alloy heads, because in all likelyhood you will leave some performance on the table.
Do you need to run at the pressure ceiling with a streeter?
Well no you don't, and there is such a thing as way to much VP. IMO, a VP of over 150 is pointless unless yur running Caltracs or something, because they don't make decent 15" tires to withstand any more VP, so I'm always either spinning, or on the verge of spinning.
Yaknow, spinning in a straightline is ok, but when the back lets go around a turn when yur not expecting it, that's a recipe for disaster. I had to fit 295s in the back before I could corner with a 750DP carb,lol.
So really, we should be looking at the Dcr and pressure, and never mind the Scr, let the drag-racers worry about that.

So then, because of the flexibility in chamber sizes in alloy, if you have not yet bought heads, then with zero-deck flat tops, you should probably pick the cam first, so you can know the installed Ica. Then figure out what Dcr and pressure you wanna target, and let that guide your chamber size selection.
_________________

And BTW, the 268/276/114 cam was designed in an age where that's what it took to make the 340 the engine that it was, with iron heads. But, the power was up around 5000rpm.
Your modern streeter running ~185 psi, no longer needs that cam, because;
a power peak around 5000 leads to a shift point with an automatic and alloy heads, of around 5800, which in first gear with 3.23s, is about 53 mph, and the tires will still be smoking; on a good build. And second gear will hit ~90 mph with those same 3.23s.
So, IMO, the 3.23s are totally the wrong gear or
that 268 cam is way too big.
But, going to a the right gear, is gonna make the tires even easier to spin, so WTH do you do?
Well IMO, you gotta at least think about bringing the powerpeak down, or even downsizing the engine. The 340 being a bigbore, is a great sized engine to downsize the cam on.
Yakno, people are always saying the 318 is only a lil smaller than a 340, but often overlooked is that the 5.2M is gonna go back together at stock bore, whereas the the 340 will almost always need to go to 4.07 to clean up. Now yur at 160 thou difference. That doesn't sound like much but convert that to area; the 3.91 bore is 12.007 sq inches, versus 13.020 sq inches for the 340, or 8.35% bigger. That is no small number.
Anyway; the point is this, those alloy heads are gonna be a life-saver, making plenty of power due to the small chambers. So lets say the pressure jumps from 165 on a borderline detonating open iron chamber X head, to 185 on a barely even breathing hard alloy head still burning 87E10. That alone is gonna be worth a lotta cam duration, so already that 268 is "obsolete" in a streeter. IMO, you can easily scub off 7 to 10 degrees intake duration, and throw away that 114 LSA and end up with a much sweeter engine. So what if the power is down; yur still spinning the tires at the top of first gear, and the power at 65 mph is likely to be at least as high as it was with the 268 iron headed 340, with the same gears. You won't notice the power loss until you are fighting wind resistance, say 85mph, well into speeding territory.
So that opens the door to a 262* cam or even less. A 256* cam, for example, will bring the power-peak down about 400 rpm (as compared to the 268); and bring the bottom end down about half that so 200 rpm. Between that and the increased pressure, maybe you can use a regular stall and 3.23s or a lil more stall and a lil less gear. Or some combination.

I can tell you this;
I have had iron-headed hi-compression 340s on and off since my first one in 1970.
When I was in the engine selection stage for my Barracuda, I had 5 340 engines sitting around and one 360.
I chose the 360 and put alloy heads on it from day one. I have never been sorry about that.
I was sorry I ever put a 292/292/108 cam into it, yes.
But I soon put a 270/276/110 cam into it (and later a 276/276/110) and was never sorry about that either. In both cases and the third one,lol, the Scr was readjusted to maintain the 180psi or so pressure.
After I installed the 276 cam, I realized I shouldda gone the other way. Whereas; the 292* cam hated 3.55s, the 270*cam liked 3.23s and the 276* cam was very unhappy about the 3.55s until I jumped from a 2.66 low gear to a 3.09 low gear. ( plus 12.8%). This is the same as from 3.55s to 4.00s. That was with just a 7* intake duration increase! You can't really get a sense of that from perusing the catalog.
Bottom line is this; When it comes to cams, a 7 degree change is very significant.
If you design your pressure around a certain cam, you can always go bigger. But to go smaller often results in failure. Then the cam gets blamed for what is really a pressure problem. Don't be that guy, lol.

BTW; all this talk is particularly important for a streeter with a manual trans...... because the engine is sorta married to the tires. If the Dcr is low, (with street gears), taking off will usually require a lot of rpm to get moving. There is no Hi-stall convertor to help.
and of course;
for a dragracer with an automatic, this is of almost no importance, because his engine is never required to work at the rpms where optimum Dcr or VP matter.

Whew, great info. I have collected 8 3/4 centers with 2.92, 3.23, 3.55 and 3.91 over the years so I have a selection to choose from to see what runs better, lol. So a smaller cam works better with higher cyl pressure than a larger cam. ok. I will review all this as I proceed.

 

[AJ/FormS]

I dont have any idea on the specs.

what I have done in the past,
is to install a timing tape on the balancer, then inject air into the cylinder at low pressure to find the crossover from compression to the end of the intake cycle. As I get nearer to the point I have to turn the pressure down to keep the piston from bouncing.
Once I get that Ica, I can plug it into the Wallace calculator and fudge the numbers with the pressure reading to estimate the compression ratio.
Having said that; if you are getting readings from 185 to 200, there is likely something wrong inside anyway, 15 psi to blame on carbon is IMHO a bit much.
And I don't think you can get to, say 192psi, with a funny-car sounding cam; even with a true 12.5/1 Scr. But I guess that would depend on my interpretation of "funny-car sounding.
The Wallace spits out an Ica of 74* at sealevel/ 72* at 1000 ft, which correlates to about a 292 cam. It's a long way from a292* cam to "funny car sounding".

 

[rumblefish360]

why run zero deck? is it a modern gas issue? I have KB 243s in my 340 and I think they are stock above deck type. So far no issues on pump gas.

There is an answer below by LJ67, he took the words right out of my mouth. Read on to the Q&A by 512... LJ67

I will ask Rumble the same question
What cranking psi have these cams produced?
Sorry again for the hijack
Maybe I will create a new thread

I did t test cylinder pressure. I really never do. Some will say “How come? Why would you NOT do that? What if it is to high in pressure?”

The simple answer to not doing the math and/or an actual cylinder pressure test is that I have been there and done that’s ouch to know without calculating the numbers or doing the test what will work and not. Now before you make “Pfsssst” and huff and puff sounds like I’m sounding off my horn as a know it all arrogant bastard, this knowledge comes from decades of doing it and paying close attention to what others are doing and that is just not any yahoo on the net or stranger at a car meet, it comes from trusted friends only who I have full faith in and know there on the money telling the truth. If my buddy Mark says this is what I did, you can get the house and retirement on it and win.

In the days of “PI” (Pre-internet, LMAO!) everything was a buy it and try it deal. And that I did. This is where I picked up my love for tinkering for tinkering sake in the search of knowledge. Etween spending a lot on engine parts, a dozen plus intakes easily more than a dozen different sets of cylinder heads, over 25 carbs, different sets of headers and cams... OH man! I changed cams a lot and just for the fun of it.
After a while, you learn!

so I could get a tight quench .038" with closed chambers for the now gas.

Flat top or dome?
In my current 340 @ a .030 overbore, I have the KB243’s at zero deck on the pistons flat area and the dome sticks up it’s listed amount, a number I can’t recall at the moment. I’m using Edelbrock 63cc heads and a thin gasket for. 10.25-1. (I sharpied the ratio on the drivers side of the block to remind me.)
There is a ton of room, so says the clay on top of the piston. It runs on pump gas no problem. It takes a great street strip curve advance from the distributor. All in by 34* before 3K. The HFT cam card below.

 

[LJ67barracuda340]

what I have done in the past,
is to install a timing tape on the balancer, then inject air into the cylinder at low pressure to find the crossover from compression to the end of the intake cycle. As I get nearer to the point I have to turn the pressure down to keep the piston from bouncing.
Once I get that Ica, I can plug it into the Wallace calculator and fudge the numbers with the pressure reading to estimate the compression ratio.
Having said that; if you are getting readings from 185 to 200, there is likely something wrong inside anyway, 15 psi to blame on carbon is IMHO a bit much.
And I don't think you can get to, say 192psi, with a funny-car sounding cam; even with a true 12.5/1 Scr. But I guess that would depend on my interpretation of "funny-car sounding.
The Wallace spits out an Ica of 74* at sealevel/ 72* at 1000 ft, which correlates to about a 292 cam. It's a long way from a292* cam to "funny car sounding".

Hahaha .read funny car as "Big". 292 is pretty big for cruisin @ 2000 rpm. The engine has been run hard. I did see H beam rods went the pan was off. It has a Weiand single plane 850 dp and stock iron manifolds. Total mismatch for a cruiser .IMO

 

[AJ/FormS]

So a smaller cam works better with higher cyl pressure than a larger cam. ok. I will review all this as I proceed.

is that what I said? Chapter and verse please, lol.
More pressure is always better, until it's too much, for the available fuel to prevent detonation; irregardless of the size of the cam.
But with alloy heads, there comes a point when all that more pressure does, is smoke the tires more.
If all you can fit on your street car is 255s,then there is no point in striving for high pressure, for acceleration, in a streeter, cuz all it will do is spin the 255s to past the speed limit. Hi-pressure will still help with making her fuel stingy. So if you can control your right foot, it's still the way too go. And hi-pressure will still blast your car down the track after it stops spinning.
What I did for a while was run a small vacuum secondary carb during the week, and bigger DP on the weekend. That worked pretty good. But when I retired her from DD, the 750 stayed on .............. with predictable results on my bank account vis-a-vis tires,lol.

If you are running a 3000 or more TC, maybe even a 2800, then the striving for a high VP is somewhat misguided. That number is only a comparative tool to help understand performance below 3000rpm.
However your stall-rpm is highly dependent on your engine's torque, which the VP reflects. So if you have a low VP, that points to a situation with low pressure, and so your stall will be different than the same engine sporting a higher VP, which points to a higher pressure resulting from higher torque numbers.

I like to think of a hi VP number with a manual trans, as being akin to what a hi-stall TC is to an automatic. They both increase the launch . But unlike the convertor, in which once the rpm is past stall, the effect rapidly declines; whereas the hi VP never stops working for you. Well unless the pressure forces the engine into detonation, that is.
Another thing with an automatic, is that you can swap stall for VP. If you have a high VP you can getaway with less stall. But if your VP is low, she will need more stall.
This is really only important when the stall argues with the cruise
rpm. So if your gears dictate a 2000 rpm cruise, you might not want to run a 3000 TC, and the higher your VP is the less throttle you will need to cruise with at 2000, and so the assumption is that she will also burn less fuel.
But if your gears dictate a 3000 rpm cruise, then who cares about VP.

 

[coyote42]

is that what I said? Chapter and verse please, lol.
More pressure is always better, until it's too much, for the available fuel to prevent detonation; irregardless of the size of the cam.
But with alloy heads, there comes a point when all that more pressure does, is smoke the tires more.
If all you can fit on your street car is 255s,then there is no point in striving for high pressure, for acceleration, in a streeter, cuz all it will do is spin the 255s to past the speed limit. Hi-pressure will still help with making her fuel stingy. So if you can control your right foot, it's still the way too go. And hi-pressure will still blast your car down the track after it stops spinning.
What I did for a while was run a small vacuum secondary carb during the week, and bigger DP on the weekend. That worked pretty good. But when I retired her from DD, the 750 stayed on .............. with predictable results on my bank account vis-a-vis tires,lol.

If you are running a 3000 or more TC, maybe even a 2800, then the striving for a high VP is somewhat misguided. That number is only a comparative tool to help understand performance below 3000rpm.
However your stall-rpm is highly dependent on your engine's torque, which the VP reflects. So if you have a low VP, that points to a situation with low pressure, and so your stall will be different than the same engine sporting a higher VP, which points to a higher pressure resulting from higher torque numbers.

I like to think of a hi VP number with a manual trans, as being akin to what a hi-stall TC is to an automatic. They both increase the launch . But unlike the convertor, in which once the rpm is past stall, the effect rapidly declines; whereas the hi VP never stops working for you. Well unless the pressure forces the engine into detonation, that is.
Another thing with an automatic, is that you can swap stall for VP. If you have a high VP you can getaway with less stall. But if your VP is low, she will need more stall.
This is really only important when the stall argues with the cruise
rpm. So if your gears dictate a 2000 rpm cruise, you might not want to run a 3000 TC, and the higher your VP is the less throttle you will need to cruise with at 2000, and so the assumption is that she will also burn less fuel.
But if your gears dictate a 3000 rpm cruise, then who cares about VP.

Sorry, misunderstood, lol. Again, thank you for all the details. It will help get me into the right combination without making a big mistake. That’s what I was looking for.

 

[LJ67barracuda340]

Flat top or dome?

I use KB 243 flat Hyper and had them cut down to zero deck to work with the closed chambers and .039" gasket.

 

[65 Cuda 340]

For a brief while, I ran a Comp 282S solid lifter cam in my 340. Kept having to adjust the valves which I figured was due to lobe wear from my not having broken it in correctly. Anyway, I said the Hell with it and swapped in a Mopar Performance 761 hydraulic cam, 270/272 duration, .450/.455 lift, 110 LSA. Mopar's specs for duration at .050 are ridiculously wrong; at .050, I'm pretty sure this cam is 224 on the exhaust and 2-4 degrees less on the intake. I have no doubt that there are better cams out there today, but this one has given me no trouble at all for over 20 years, and I don't think replacing it would gain enough to be worth the trouble. Good all around power, and it will rev to six grand.

Anyway, just FYI, with RPM Air Gap, 750 Holley vacuum secondary, Edelbrock 65 cc heads, TRW forged pistons that stick out of the block .018-.025 or so at TDC, FelPro .039 head gaskets, cranking compression is 165.

Also, TTI headers, 2.5 inch mandrel bent X-pipe exhaust, four speed with 3.09 low gear, 3.23 rear gears.

 

[Krooser]

why run zero deck? is it a modern gas issue? I have KB 243s in my 340 and I think they are stock above deck type. So far no issues on pump gas.

Free horsepower...more squeeze more HP. Your 340 already has an advantage with the heads sticking out of the hole...

Regarding Speedmasters... they do not flow very well OOTB. Don't remember the numbers but a few members here may have some.