Help on 408 build

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Indeed, the best advice here is to plan everything carefully and double and triple check during the building stage. Take your time and don’t crowd the project, make sure you educate yourself and know exactly what the final outcome will be before you start.
 
If the piston is a true zero deck flat top you are easily 13 to 1 compression. You really have 2 problems. You initially said it lacked power. I would be that your heads are choking that thing really bad unless they are pro ported to death with really big valves. Even then it would still hold it back. I totally agree with PRH, you should really do your homework before spends money on a band aid fix. The 230 psi cranking compression confirms the 13 to 1 compression. Changing the camshaft will help with low speed part throttle ping but will actually hurt more at high RPM full throttle. If I were you I would swap the heads to a Edelbrock Performer RPM. The big chamber version. It will probably lower the compression by half a point (do some research on this) and couple it with a little thicker head gasket and you will be near 12 to 1. The aluminum will help soak up the heat energy and it would be equivalent to 11 to 1 with the iron head. The chamber is really good on the edelbrock and will help with quench and hot spots further helping with detonation. You could go further with one of the CNC options from indy or Hughes and have a bigger chamber yet and a huge improvement in performance. Then you run a touch less timing and a less aggressive timing curve to run pump premium.
 
Wow, 13:1! I don’t know your intended purpose but if you have bottom end reinforced and are wanting to make big power... do you have access to e85? Change the fuel system (still pricey) and then change the cam and heads when time and budget allows it.
 
My 9.5 comp 340 with Ported J heads will knock on Winnipeg pump garbage...I don’t think you should jerk around with any of these half *** fixes, put pistons in it or run AV or race gas.
 
Well, unless the heads have been milled substantially, it’s not 13:1.
Typical stroker flat top piston, zero deck clearance, .040 gasket.......even with the heads as small as 60cc works out to only 12.14:1.
(13:1 would require a 54.5cc head)
If you added 10cc worth of gasket(19cc total) it would drop to 10.83.
That’s if the heads were as small as 60cc.
If the heads were a more “normal” 65cc, it would drop to 10.28.
69cc, it would drop to 9.89.

Zero deck, 5cc pistons, the .098” Cometic gaskets, 64cc heads...... puts it at 10.08.

Add some cam duration and you’ll be a lot closer to pump gas operation(might need octane boost for WOT operation).

My suggestion is pull a head off and get some measurements before you order any parts.
 
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Mopar204 said:
I know I messed up. I really want to avoid redoing the engine, especially changing out pistons. I’ll test the compression again with a quality tester. What I used was a cheap old one
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PRH said:
Imo, the “right” way to do it is...... do your homework first.
For this application, a set of closed chamber heads, inverted dome pistons selected to provide 10-10.5cr with zero deck height along with the heads of your choosing, and a big enough cam to keep the cranking compression well within the safe zone.

I think B3’s solution is a cheap way out for the time being.
I’d probably go up a bit with the cam as well, while I had it all apart.
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Not the best way, but fits the OPs above statement.
 
67autocross said:
My 9.5 comp 340 with Ported J heads will knock on Winnipeg pump garbage...I don’t think you should jerk around with any of these half *** fixes, put pistons in it or run AV or race gas.
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Lots of good info mentioned but if you hear pinging that is bad.... You can easily lift a ring land and other things. Take it apart and figure out what you have compression wise. It's best to stop now and save your parts (hopefully.)

JW
 
For street use;IMO,reverse dome Pistons, is the cheapest best solution. Then co-ordinate the heads and cam to keep the pressure in line for your chosen gas.
But with a little investigation and math, you might find yourself not far from, heaven forbid;having a sharp 3.58 stroker. Or perhaps a 3.79.
In any case; off with the heads! Time for some serious measuring. Then some math, then some decision making.
_________________________
Not dissing you 67autocross;or any other fellow Manitoban
but,IMO; there's nothing wrong with Winnipeg gas: my combo burns 87E10 and goes 93 in the Eighth on that crap@3467 pounds raceweight. At 367 cubes, do the math:IIRC, that's over 430 horsepower....... on 87E10. Please don't you guys blame your problems on the gas.
I'm a big fan of tight Q and aluminum heads, but I understand that not every one has the budget for that. But, I and others have run close to or a tic over 200psi, on pumpgas with the aluminums. I'm a lil more conservative at up to 185psi, cuz I was set up for 87E10, from the beginning. Over 100,000plus miles I have saved alotta coin, and honestly 430hp on the street is ridiculously overkill. I can never floor it for over 5 or so seconds without risking jail-time,lol. By 8 seconds I'm knocking on 100mph. Top of second is over 80mph with 3.55s.....You know how hard it is to stay out of third gear? Hyup, darn near impossible.
Sometimes I miss my smogger-teen. Put it in 3rd at 10 mph,nail it, and listen to the TQ moan for IDK; seemed like forever. Stop, turn around and repeat. Repeat, Repeat,lol.
Look; I'm no rocket scientist, but if I could figure Dcr out, back in 1998, I'm pretty sure a lot more of you guys should also be able to. I shoulda aimed higher; On the street (all I know) 185psi is nothing for aluminum heads,Tight-Q, and 87E10. The guys running 200psi, are claiming still on pump gas,American pump-gas mind you,lol

BTW, nice looking car
 
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No worries... I was just stating that my J headed 340 with 9.5 comp can barely get way with pump gas. It is good if I fill up and use the gas within a few weeks... if the car sits for 3 weeks or more I can get it to ping a little under load.
I think if he wants to run decent with J heads around these parts 9.5 comp or around 160 psi cranking compression is the limit
 
PRH said:
Well, unless the heads have been milled substantially, it’s not 13:1.
Typical stroker flat top piston, zero deck clearance, .040 gasket.......even with the heads as small as 60cc works out to only 12.14:1.
(13:1 would require a 54.5cc head)
If you added 10cc worth of gasket(19cc total) it would drop to 10.83.
That’s if the heads were as small as 60cc.
If the heads were a more “normal” 65cc, it would drop to 10.28.
69cc, it would drop to 9.89.

Zero deck, 5cc pistons, the .098” Cometic gaskets, 64cc heads...... puts it at 10.08.

Add some cam duration and you’ll be a lot closer to pump gas operation(might need octane boost for WOT operation).

My suggestion is pull a head off and get some measurements before you order any parts.
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Your right on that. I quickly just pulled that number from my last stroker build where the piston was .010 above the deck and it had 55cc chamber. But it has 230-240 cranking compression. That’s way above the 180 psi pump gas limit. And for the guy who’s motor pings at 9.5 to 1 something is very wrong. My street 360 has 11.14 to 1 and doesn’t detonate at all even with a 100 shot of nitrous. Mopar small blocks have a very good chamber design that is very tolerant to detonation. Putting better heads on a motor is not a “half *** jerk off” solution. It’s killing two birds with one stone.
 
I’m just giving the guy my real world local experience with what I believe is his limit on our 91 pump gas with J heads.
Sure I could tell him that my other car with aluminum heads and 11:1 sitting in the next bay of my garage runs great on the same gas... which it does, but I don’t think that would do him a lot of good.
I think changing out the pistons his cheapest solution...or if you only drive the car a few thousand km per summer leave it alone and run race gas.
 
So I’m going to try to do this correct. Will be pulling the heads off tonight most likely. Get the heads CCed and get some more measurements how far my pistons are coming up to the deck. At the very least even if I dont end up doing it this summer I will know what I need to do for a winter project in between next season. I will keep you guys posted once I get all my numbers, thanks again guys
 
I hope your deck and head surfaces are smooth enough for Cometic type gaskets. If not we will be seeing a "Why am I leaking water out of my head gasket" post.
 
67autocross said:
My 9.5 comp 340 with Ported J heads will knock on Winnipeg pump garbage...I don’t think you should jerk around with any of these half *** fixes, put pistons in it or run AV or race gas.
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Half *** fixes! Well, I guess I'll close the doors and put the shop up for sale. I'm sure Cometic will want to follow suit for making a .098" thick gasket for absolutely no good reason. Some of you guys are right ready to spend someone else's money, when they are desperately looking for an easy solution to a mistake they made. Sometimes it can't be done, but in this case it can. There is nothing half assed about adding chamber volume to a head to lower compression, which is all a thicker gasket does when there was no quench to begin with. Stop beating this guy up over his mistake, and have a little compassion for his dilemna. This is where a good engine builder actually SAVES you money by avoiding these problems in the first place.

I have one of these problem child's in the shop right now. A 496 stroked big block with Stage 5 iron heads and supposedly 10.5:1 compression. The cam was 302/308 advertised duration 244/244 @.050" on a 108 spread (custom grind btw), and couldn't tolerate more than 20 degress of timing. Turns out it was actually 11.6:1 and even with the long seat duration, it wouldn't run on pump gas, even with an octane booster. Once everything is fixed, it will have more power and driveability than it could ever gain from the ridiculously high static ratio. Unless the motor is injected and the timing is controlled by a computer and knock sensors, pushing the limits is just asking for trouble, imo.
turbodart68 said:
Your right on that. I quickly just pulled that number from my last stroker build where the piston was .010 above the deck and it had 55cc chamber. But it has 230-240 cranking compression. That’s way above the 180 psi pump gas limit. And for the guy who’s motor pings at 9.5 to 1 something is very wrong. My street 360 has 11.14 to 1 and doesn’t detonate at all even with a 100 shot of nitrous. Mopar small blocks have a very good chamber design that is very tolerant to detonation. Putting better heads on a motor is not a “half *** jerk off” solution. It’s killing two birds with one stone.
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You assumed he had "well over 13:1 CR" based on anecdotal evidence of your own motor, without knowing if his specs matched yours? Assumptions are what caused his problem in the first place.

I don't care what head is on the motor, putting a bunch of compression in a pump gas motor is not wise. I hope your 11:1 motor lives a long and happy life, but I certainly wouldn't want to be an internal component in that motor. Just because you don't hear it rattle doesn't mean it isn't rattling. Ever driven a late seventies 8.2:1 Mopar and listen to it sing it's death song?

A good chamber doesn't require 35-36 degrees of timing to make the best power, and that's what most SBM motors with factory heads need.

And one more time, volumetric efficiency is what determines cylinder pressure at any given rpm, not DCR.
 
pittsburghracer said:
I hope your deck and head surfaces are smooth enough for Cometic type gaskets. If not we will be seeing a "Why am I leaking water out of my head gasket" post.
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Agreed John, but I always spray them with copper coat anyway, just as a precaution. Good call though. I forgot to mention it.
 
In my opinion anything other than changing pistons is half ***, and likely they cheapest solution in the long run if the engine has that much squeeze. Some of the replies are telling him to change the heads to aluminum and the cam... now that is spending money.
Your head gasket idea is fine but if you have to cut the intake or buy push rods to make them work you might as well put the money into pistons...you will likely be ahead in the end. You will also have a set of pistons you can sell and likely get half about half the price of the new set.
As far as closing your shop because I gave someone a bit of different advice on the internet on how I would tackle the problem that’s up to you...
 
67autocross said:
In my opinion anything other than changing pistons is half ***, and likely they cheapest solution in the long run if the engine has that much squeeze. Some of the replies are telling him to change the heads to aluminum and the cam... now that is spending money.
Your head gasket idea is fine but if you have to cut the intake or buy push rods to make them work you might as well put the money into pistons...you will likely be ahead in the end. You will also have a set of pistons you can sell and likely get half about half the price of the new set.
As far as closing your shop because I gave someone a bit of different advice on the internet on how I would tackle the problem that’s up to you...
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Ok, so tell me, what does changing the piston do that the head gasket doesn't? You're still working with an open chambered head that has no quench, so there is nothing to be gained by changing the piston versus changing head gaskets. The intake manifold would not need cut because the head is raised. It would actually need a thicker gasket. As far as pushrods go, is that really going to be more expensive than pulling the motor out of the vehicle and completely redoing the rotating assembly, balancing and all (yes, it will need a rebalanced with a heavier dished piston)? Replacing the heads would at least afford quench, but it would still need a different piston to keep compression down.

The problem here is that you didn't just give a different opinion. You specifically called a solution I gave to the OP as half assed. That, my friend, is an insult to someone who does this work every day for a living. I don't mind a different opinion, and welcome it, but I can back up my approach with scientific fact. If you can tell me in theoretical terms how volume in the gasket is different than volume in the piston, when quench is not in the picture, I'd be open to hearing it.

As far as closing my shop because of someone's opinion, that isn't going to happen. I was facetiously alluding that I might as well close up shop, and Cometic as well, if our work and products are half assed. Fortunately, my business is thriving because of exactly the opposite.

I am having a bit of a "Monday" here, so I apologize if my tone is a bit edgy, but calling my work, and knowledgeable, experienced advice half assed is sure to get a sharp response, no matter what my mood may be.
 
turbodart68 said:
Thanks for assuming my motor is rattling. Actually its going on 15 years and its runs perfect. I know you think everyone is an idiot by your tone but some of us know what we are doing. And yes a small block Chrysler does have a good chamber compared to what else is out there. And no I didn't say "well over 13 to 1" I said it could easily say approaching 13 to 1. I also said he need to do some measuring and figure out exactly what he has before coming up with a solution.
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Nope, we both got it wrong. You said "easily 13:1" which is a bit synonymous with "well over". "Approaching", eh, not so much. Now if you had said "barely", I would agree that "well over" doesn't work.

I'd be very surprised if at teardown your engine didn't have severe cap walk, and unusual bearing wear. When the time comes, post up the results. Right now neither of us can prove anything, but experience tells me where to place my bets. That being said, I would never wish harm to anyone's engine, so I hope your are the exception, and not the rule.
 
B3RE said:
Ok, so tell me, what does changing the piston do that the head gasket doesn't? You're still working with an open chambered head that has no quench, so there is nothing to be gained by changing the piston versus changing head gaskets. The intake manifold would not need cut because the head is raised. It would actually need a thicker gasket. As far as pushrods go, is that really going to be more expensive than pulling the motor out of the vehicle and completely redoing the rotating assembly, balancing and all (yes, it will need a rebalanced with a heavier dished piston)? Replacing the heads would at least afford quench, but it would still need a different piston to keep compression down.

The problem here is that you didn't just give a different opinion. You specifically called a solution I gave to the OP as half assed. That, my friend, is an insult to someone who does this work every day for a living. I don't mind a different opinion, and welcome it, but I can back up my approach with scientific fact. If you can tell me in theoretical terms how volume in the gasket is different than volume in the piston, when quench is not in the picture, I'd be open to hearing it.

As far as closing my shop because of someone's opinion, that isn't going to happen. I was facetiously alluding that I might as well close up shop, and Cometic as well, if our work and products are half assed. Fortunately, my business is thriving because of exactly the opposite.

I am having a bit of a "Monday" here, so I apologize if my tone is a bit edgy, but calling my work, and knowledgeable, experienced advice half assed is sure to get a sharp response, no matter what my mood may be.
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If you think putting thick head gaskets on it and playing around with thick or stacked intake gaskets is the way to go that’s fine by me. If I was doing the same to my own engine I would still call it a half *** fix...to me the correct fix is lower compression pistons. No need to feel insulted.
Keep in mind I only replied to the original poster because I have honestly put on hundreds of thousand miles on small block dodges in the same city as he lives over the last 25 years. I could take him for a ride in one of my cars and show him exactly how much timing or cylinder pressure you can get away with here on our pump gas with a set of J heads.
 
Just throwing my 2 cents in again. Some of you guys don't understand BUDGETS and sometimes the shortcuts needed to stay on one. Is it the right way? HECK NO!!!!!!! But sometimes we do things we gotta do to enjoy our rides. New pistons are not cheap and We are now pushing towards April and for those of us that live in cold areas is the start of car season, not rebuild your engine season. I personally would call and talk to Jim Dowel at Racer Brown (after 3pm eastern standard time) or any highly recommended cam manufacturer and tell him whats going on compression wise and see if he can recommend a cam to "bleed off" some compression and spec a head gasket from there to see if you can make it drive-able to enjoy the cruising season. I wish you nothing but the best of luck so you can enjoy your new found horsepower.
 
OP, please list the SCAT kit number so we can be sure we know what you are working with. And please re-run a cranking compression test before pulling the heads... I'd advise that you borrow a 2nd gauge and compare.

There is a lot of guessing going on, and the way forward will vary with what you actually have for static CR and dynamic CR. I am pretty solid at working out the numbers, and I am not seeing the high CR's that are being thrown about, with your use of J-heads, unless those heads have been milled a whole bunch or the cam timing is advanced a tooth. So something is not jiving between flat tops in open chambered J-heads, and your reported compression readings; I am getting low 10's for SCR and DCR's in the low 8's (at sea level), just running the first sets of numbers with typical J-head chambers and flat tops .012" below the deck.

The cranking compression numbers will be a good indicator of the DCR that you have right now. It may be as you say, but building a good baseline of data is what is needed right now.
 
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The verification that the cranking compression is too high is the fact(from what’s being described) that it won’t even come close to running on pump gas, even with octane boost added.

Sure, double checking with another gage is fine....... but whatever the CR is...... it seems to be too high.

I’d plan on at least pulling one head for some measurements.
 
PRH said:
The verification that the cranking compression is too high is the fact(from what’s being described) that it won’t even come close to running on pump gas, even with octane boost added.

Sure, double checking with another gage is fine....... but whatever the CR is...... it seems to be too high.

I’d plan on at least pulling one head for some measurements.
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I agree as the heads will probably need pulled anyway for a gasket change. It’s been a long time since I’ve played with factory stuff. While the head is off I would lay a head gasket on the head to see how much chamber work (metal removal) is available to gain a few cc’s. Heck the head may even flow better and be more efficient.
 
OBTW, if you do run compressions, run the engine, if possible, for a minute before compression testing, to make sure the lifters are pumped up. Collpased lifters will make the compression numbers invalid.
 
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