91 Octane in a 340?

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Rusty ! I thought that your toilets run backwards!
My 410 has what I thought to be 11to1 compression. It has SRP forged pistons with a 28 thou head gasket and 61 cc trickflow heads. I measured it for comp pressure with a elcheapo Compression guage and it said it was 150 psi . I purchased a Snap On guage and it showed 205 psi , which is more likely. Irun my car on premium grade fuel and it doesn't Ping!!


 
Not sure if this helps you or not, but I ran a 340 w TRW forged 10.5 to 1 replacement pistons (actual comp probably mid 9s as already mentioned) 272 455 purpleshaft 230 @ .050 or so, performer intake, headers, mopar performance electronic ignition 35 deg all in at 2500, and j heads. Engine would not run on lower than 93 octane. Hottest part of the summer I had to run octane boost or it would diesel when shut off. Admittedly I was not sharp on tuning 30 years ago. I also live it Georgia and it can get 95 degrees here pretty regular in the summer.
 
iam 9.99 legit 218@50 110 lsa 169 cranking psi power timed needs 93 to run full timing
just run 93 even 93 94 is skunk piss
91 never
 
AJ/FormS said:
Cylinder pressure is a much better indicator as to if you need to run a hi-octane or not. This is what the engine actually sees versus an abstract number that has very little usefulness in reality.

The Compression ratio formula reads;
(CV +cv)/ cv , where CV is the swept volume, and and cv is the total chamber volume. all are in EITHER cc or cubic inches, Generally ccs are used, do not mix units.
So a stock340 has a swept volume of 696.364cc
And the typical total-chamber volume, with an .039 headgasket, is gonna come in at around 81cc, so that makes her a 9.6Scr engine.

In Edmonton, at some 2130ft elevation, you can take a 10.5Scr 340engine, which in reality is likely closer to 9.6Scr as shown above, , and just by changing when the intake valve closes, vary the pressure from say 152psi, down to 124psi, or even less.
>With open chamber iron heads;
At 140psi or lower, you should be able to run 87E10 in it no problem.
at 150psi, probably still on 87, but the low-rpm/hi load may say otherwise at your early all-in timing setting.
at 155psi, for sure needs 89 (or better)
At 160psi, 91 is getting iffy. The power-timing may have to be delayed or reduced. Your engine is not likely gonna like the current 33* @2500, unless the stall speed is over that, and the chamber temperature is kept in check.
at 165psi, everything has to be just right to get away with 91, and more likely is that the timing is gonna take a hit.
<But notice that it's still a 9.6Scr engine.>

In Edmonton, at 2130ft elevation, your stock Hi compression 340 longblock, with a 268Plus roller cam, is gonna be down on pressure, so
I very much agree with those who recommend to do a compression test.
Pressure makes heat, makes power...... so long as full-timing can be used, which on an iron-headed 340 is usually 36>38 degrees, after 3600rpm.

I wonder if you are fixated a lil too much on the power numbers.
On the street, using just two gears, 250 hp is plenty to have fun with, because you can adjust the output with gears and stall. However 2130ft elevation is already about a 10 psi handicap from sealevel, which you can definitely feel.
On the street, you are very seldom gonna see max-power in Second gear, unless you are running an automatic, and 3.91s or better, and don't mind speeding.
IMO, with a noisy exhaust system, it is as good as impossible to hear detonation. But you can devise a test for it, and if severe enough you will for sure feel the hammering going on under the hood.

As for the test:
The test is just a time trial over a measured, distance or speed range. But you gotta load the engine up to prevent tire spin, and at some rpm after the mechanical timing is all-in, and your method has to be repeatable, To that end, if an automatic trans is used, use Second Gear, all by it self. I use the speedOmeter, from whatever speed 3500 in Second gear is, to at whatever rpm the cam peaks at, plus 200 or so.
Say you have 3.73s; 3500 will be ~45 mph, and 5600 will be ~75 mph. So make the test from 45 to 75.
For the following tests,
>if at any time, the car slows down, abort the testing. A slow-down can only mean ONE thing, namely, that the engine lost power. And that would most likely be, but does not have to be, because of detonation. But in any case, we don't want to go slower.
> also, repeat all tests at least twice and average the results.

Set your power-timing to 33*@3500.
Get a helper with a stopwatch in the backseat looking over your shoulder, then motor up to 40mph in Second, then floor it. When the Speed-O needle crosses 45, the helper starts the timer and when the needle crosses 75, the helper stops the timer. You watch the stinking road, lol. This will be your baseline.
>Now reset your Power-timing to 31*@3500, and repeat the test. If we're losing power, it will take longer. This is to be expected at just 31*..
Next, reset your Power-timing to 35*@3500; and repeat the test . If the car is making more power the time will be less, good news, we can move forward.
One more time; reset the Power-Timing to 37* at 3500 , and repeat the test. If the time is longer, do not repeat the test for a second time. Go back to 35*, and call it done.
BUT
If the time is again less, repeat it a second time and average the results. If still quicker than it was at 35*, Then Stop the test, lets not push our luck, ......37* is your magic POWER-TIMING number, on this day, with whatever gas was in the tank, at this location.
Now;
I want you to compare the averaged times very carefully. because the times should be very short, and closely spaced, within a second or so from quickest to slowest not including the one at 31*..which is why you need a helper for consistency.
But the point is this;
compare the times at 33*, to the times at whatever best, that you got. Now ask yourself this:
Is it worth it to you, to run on the ragged edge of detonation, to get that portion of a second quicker? If you answer yes, then, IMO you need to reconfigure your 340, or you need a different engine, lol.

Ok then, now that your power-timing is figured, you can tune the timing from idle to 3500........ And then the Vcan, and ten the cruise-timing....... lol
If at any time you change mainjets or PVs, or Scr, or pressure, or your heads, etc, then you gotta revisit your timings, all of them, starting with the Power-Timing..........
Happy HotRodding.
Click to expand...
Really good posts guys, thanks. I will start with measuring the PSI. So you are saying at my elevation i will run 10 PSI lower than at sea level?
 
More like 12psi:
so if you build to 180 psi with alloy heads, and make no changes, then the Wallace calculator predicts 192psi at sealevel. You can run ~87gas up in Edmonton, and 91 down at the coast, or maybe 89 on the prairies which are around 1000ft or less. Same engine; no problem.
But
if you build to 165psi with iron heads, at Edmonton, and then drive down to sealevel, the Wallace predicts a rise to ~177, which it gonna rattle the engine pretty hard every time that you lay into it.
Here's the kicker;
when you get to 5000ft crossing the Rockies, Your Edmonton pressure of 165psi, will have dropped to 150 and your 340 is getting a lil lazy.
Highwood pass is over 7000ft, so now yur down to 140psi. You might need a supercharger....... lol.

A change in elevation requires a change in mainjets (as does temperature), to the tune of about one size per 2000 ft, IIRC.
Therefore if you plan on traveling vertically, you really need to consider the size of your engine, and various pressure changes that it will go thru
Your 340 will get you over Highwood pass, but 7000ft requires 2.5 jet changes on the way up and 3.5 on the way down to the beach, lol., and 140psi is getting sluggish.
This elevation-business requires a careful co-ordination of parts, including the size of the engine.
if you plan on spending any significant time at sealevel, on the coast, then, IMO, you gotta set your pressure close to the max for the lowest expected elevation, and you get what you get in Edmonton, else install EFI, or, leave the car at home. This is the reality of it.
BTW
if yur thinking the 340 is the bees knees, for this kindof application, I cannot agree. Well, it could be..... with a stroker crank in it...... but then...... it wouldn't actually be a 340 anymore........ altho, only you will know it.
Well and the guy you just blew off at 85 mph! lol
 
American V8 engine, factory iron heads, 9.9:1 CR, 4 bbl carb. Had to run 98 octane [ about 94-95 in USA ] & could still get pinging under light load on a hot day.

Completely new induction system. NOTHING ELSE CHANGED, JUST THE INDUCTION SYSTEM.
Runs on 91 octane, has done for 15 yrs, unable to get it to ping under any conditions.

Cranking compression means nothing, stayed the same, above.
 
Bewy said:
Cranking compression means nothing, stayed the same, above.
Click to expand...
I agree with most of the things you say, but not on this.
I think that if you delve into this, that you will find a different conclusion.

>>Detonation is almost always the result of a runaway chamber temp;
control that chamber temperature/eliminate the detonation.

>>I would say, that for a streeter, SCR means as good as nothing.
Scr is just a tool to help us achieve certain pressures/DCR targets that are known-to-be-detonation-free.
>>By empirical testing, we know that we can run a DCR of 8.0 on our pumpgas with iron heads. History has shown, that it almost doesn't matter how poorly you screw that 8.0DCR thing together, you'll be able to tune it to run on pumpgas.
>>And also by empirical testing, we know that we can run a DCR of ~9.0 with alloy heads, detonation-free on pumpgas; again, almost no matter how poorly a guy screws that together.

Lemmee flip it back to you, in a couple of different ways.
1) After I installed the AirGap intake, onto my alloy-headed 367, and routed fresh cold-air into the airhorn, I was able to up the minimum coolant temp to 205* from 185, and add two degrees of Power-Timing (to 34*), without detonation, on 87E10(R+M)2.
Same 11.3Scr/9.0DCR/195psi,engine. The difference was the reduced chamber temperature, due to the reduced induction temperature.

2) Using your example;
I will use the example of a [email protected], for ease of calculations, and set the elevation to 500ft.
Install a small cam with an Ica of 60*; the Wallace calculator predicts a pressure of 165psi/DCR of 8.16. With iron heads, this is on the upper edge of the pressure limit for our 91 gas. With open chamber heads, a standard non airgap intake, and hot under-hood air, this could be a tough tune.
But it's not the pressure by itself that sets the limit; rather, it's the in-chamber temperature-rise, caused by the pressure-rise. Reduce the inlet air temp, or the coolant temp, or some combination of the two, and it will run detonation-free.
The problem is this; most guys will not cut a hole in their nearly impossible to replace hoods, which then makes it very difficult to reduce that inlet air temperature and/or the temp of the intake manifold.
>Now take the above 9.9Scr/360 engine, and put the next bigger cam into it which will increase the Ica by ~3 degrees. Now the Pressure is predicted to drop to 160psi/DCR of 8.0. This will reduce the tendency to detonate, with the hot intake air. Or, just re-time the earlier cam to 3* later, same result.
I'm pretty sure you know all this,
so IDK why you would say
"CCP means nothing",
If that was true, then we'd all just run 8/1 engines no matter what cam is installed...... and so never have detonation issues.......am I right?
 
AJ/FormS said:
More like 12psi:
so if you build to 180 psi with alloy heads, and make no changes, then the Wallace calculator predicts 192psi at sealevel. You can run ~87gas up in Edmonton, and 91 down at the coast, or maybe 89 on the prairies which are around 1000ft or less. Same engine; no problem.
But
if you build to 165psi with iron heads, at Edmonton, and then drive down to sealevel, the Wallace predicts a rise to ~177, which it gonna rattle the engine pretty hard every time that you lay into it.
Here's the kicker;
when you get to 5000ft crossing the Rockies, Your Edmonton pressure of 165psi, will have dropped to 150 and your 340 is getting a lil lazy.
Highwood pass is over 7000ft, so now yur down to 140psi. You might need a supercharger....... lol.

A change in elevation requires a change in mainjets (as does temperature), to the tune of about one size per 2000 ft, IIRC.
Therefore if you plan on traveling vertically, you really need to consider the size of your engine, and various pressure changes that it will go thru
Your 340 will get you over Highwood pass, but 7000ft requires 2.5 jet changes on the way up and 3.5 on the way down to the beach, lol., and 140psi is getting sluggish.
This elevation-business requires a careful co-ordination of parts, including the size of the engine.
if you plan on spending any significant time at sealevel, on the coast, then, IMO, you gotta set your pressure close to the max for the lowest expected elevation, and you get what you get in Edmonton, else install EFI, or, leave the car at home. This is the reality of it.
BTW
if yur thinking the 340 is the bees knees, for this kindof application, I cannot agree. Well, it could be..... with a stroker crank in it...... but then...... it wouldn't actually be a 340 anymore........ altho, only you will know it.
Well and the guy you just blew off at 85 mph! lol
Click to expand...
Ma mopar had a thermoquad that adjusted tuning for altitude up to 7 or 8 thousand feet. If someone doesn't run fuel injection it might be worth seeking one out.
Click to expand...
 
AJ/FormS said:
I agree with most of the things you say, but not on this.
I think that if you delve into this, that you will find a different conclusion.

>>Detonation is almost always the result of a runaway chamber temp;
control that chamber temperature/eliminate the detonation.

>>I would say, that for a streeter, SCR means as good as nothing.
Scr is just a tool to help us achieve certain pressures/DCR targets that are known-to-be-detonation-free.
>>By empirical testing, we know that we can run a DCR of 8.0 on our pumpgas with iron heads. History has shown, that it almost doesn't matter how poorly you screw that 8.0DCR thing together, you'll be able to tune it to run on pumpgas.
>>And also by empirical testing, we know that we can run a DCR of ~9.0 with alloy heads, detonation-free on pumpgas; again, almost no matter how poorly a guy screws that together.

Lemmee flip it back to you, in a couple of different ways.
1) After I installed the AirGap intake, onto my alloy-headed 367, and routed fresh cold-air into the airhorn, I was able to up the minimum coolant temp to 205* from 185, and add two degrees of Power-Timing (to 34*), without detonation, on 87E10(R+M)2.
Same 11.3Scr/9.0DCR/195psi,engine. The difference was the reduced chamber temperature, due to the reduced induction temperature.

2) Using your example;
I will use the example of a [email protected], for ease of calculations, and set the elevation to 500ft.
Install a small cam with an Ica of 60*; the Wallace calculator predicts a pressure of 165psi/DCR of 8.16. With iron heads, this is on the upper edge of the pressure limit for our 91 gas. With open chamber heads, a standard non airgap intake, and hot under-hood air, this could be a tough tune.
But it's not the pressure by itself that sets the limit; rather, it's the in-chamber temperature-rise, caused by the pressure-rise. Reduce the inlet air temp, or the coolant temp, or some combination of the two, and it will run detonation-free.
The problem is this; most guys will not cut a hole in their nearly impossible to replace hoods, which then makes it very difficult to reduce that inlet air temperature and/or the temp of the intake manifold.
>Now take the above 9.9Scr/360 engine, and put the next bigger cam into it which will increase the Ica by ~3 degrees. Now the Pressure is predicted to drop to 160psi/DCR of 8.0. This will reduce the tendency to detonate, with the hot intake air. Or, just re-time the earlier cam to 3* later, same result.
I'm pretty sure you know all this,
so IDK why you would say
"CCP means nothing",
If that was true, then we'd all just run 8/1 engines no matter what cam is installed...... and so never have detonation issues.......am I right?
Click to expand...
AJ what ignition system are you using for this setup?
 
trivium91 said:
Im wondering if I can run 91 octane, so far I've just been running 94 but it's expensive and harder to find in my area. The engine is mostly stock from what I understand, so 10.5: 1 pistons pre 1971 with X heads. Though im not sure of the cam but I believe it's a COMP XE284 high lift but might be XE268 aswell. I have a Edelbrock AVS2 800 Carb I believe aswell. I just checked the timing and its set at about 33 degrees @2500RPM. The cam definitely has more duration than stock though and it's lumpy at idle. Would a cam with a longer duration bleed off more cylinder pressure and allow advanced timing or lower octane gas, or am I not understanding something?

Given the fact that these stock 340's were underrated in the power, I've heard that the stock 275HP was rated at a lower RPM and the actual power was actually around 330hp. Regardless, these engines came with 10.5:1 compression from the factory according to specs, what octane were these cars running with from the factory? I can't imagine it was over 91.

What does engine pinging/knocking feel like? Just a temporary loss of power for a split second, or similar to a misfire?
Click to expand...
You need to watch a Youtube by GT 350 Garage. Yes, I know he focuses on Ford, but he does a couple of tuning posts that are applicable across the board. One involves timing and advance curves and the other discusses at length fuels, modern fuels as compared to the muscle car era leaded fuels. He gets into oxygenated fuels and air/fuel ratios.
Pinging or spark knock is nnot really felt, it is heard. The 390 Ford 2V was supposed to run on regular fuel. They had a tendency to rattle twice coming off a street light. Sounds a bit like marbles in a can. In those 390's that is the limit on spark advance coming off idle and will not harm the engine. Now under cruise or full throttle for a longer time, yes the engine will be damaged.
The 390 4V requires a bit of 91 to control spark knock. I found 40 years ago that about $5 when filling up was enough. Now that is likely $20.
Modern fuels vaporize easier and combustion is cooler. The ethanol in the gasoline may be corrosive to old carburetor and fuel pump parts.
The ethanol changes the stoiciometric or Lambda fuel ratio. On the old fuels prior to ethanol, the air/fuel ratio for complete combustion was accepted as 14.7:1 With 10% ethanol, that is now about 14.1:1. The GT350 Garage video does a good job explaining this.
I am changing my Performer 600 primary boosters to the AVS2 650 annular boosters for fuel atomization on the Airgap manifold. Edelbrock Tech recommended starting with the AVS2 calibration, which appears to be richer. I did not understand this until watching this video. When I am able to assemble and install the engine, I will get a wide band 02 monitor to verify the fuel calibration. The distributor advance will need to be adjusted also.
 
mgoblue9798 said:
AJ what ignition system are you using for this setup?
Click to expand...
Very basic;
Accell Super Coil, (Square top) with it's recommended ballast resistor.
Jacob's Electronics Opto-Timer amp, with a dial-back timing option
Factory Magnetic-Pick up distributor with modified guts, and modified Vacuum advance, plumbed to the spark-port.
 
AJ,
Post #34. Nope, do not agree with much of your post. As I have stated before, DCR is useless when trying to determine reqd fuel octane to stay out of detonation. Do not understand why folks use DCR. One reason it is useless is the large number of variables that actually cause det, two of which are heat & A/F mixture.

Back to DCR. Can be the same for two engines, one will detonate, one won't. Why? Because DCR is a calculation that does NOT take into account the amount [ specifically: the weight ] of air drawn into the cylinder which can vary because one engine has a more efficient induction system [ better intake ports ] than another.

Detonation is often just one cyl, maybe a few, but not all cyls.
 
Bewy said:
AJ,
Post #34. Nope, do not agree with much of your post. As I have stated before, DCR is useless when trying to determine reqd fuel octane to stay out of detonation. Do not understand why folks use DCR. One reason it is useless is the large number of variables that actually cause det, two of which are heat & A/F mixture.

Back to DCR. Can be the same for two engines, one will detonate, one won't. Why? Because DCR is a calculation that does NOT take into account the amount [ specifically: the weight ] of air drawn into the cylinder which can vary because one engine has a more efficient induction system [ better intake ports ] than another.

Detonation is often just one cyl, maybe a few, but not all cyls.
Click to expand...
Just out of curiosity how would you calculate the octane requirement of an engine you build then? I get that Scr and Dcr are just measurements but you have to start with something.
 
I'll give you that,
but I specifically said
>> for a streeter<<,
Because generally the cams we streeters install, will have an ICA over a very narrow range of roughly ~60 to 66 degrees with the occasional foray to perhaps as high as 68/70*, and very few of us actually run the pressures up to the high end of the scale; too low being the norm.
IMO
most if not almost all, of out detonation problems, occur below 3500 rpm.
This is because, over that rpm, we can easily take steps to eliminate it; such as backing off the PowerTiming, or cooling the inlet air temperature, and nowadays, the AirGap style intake.
But from what I have seen and heard here on FABO, most everyone (streeter) just wants to throw way too much Power-Timing into their hotrods, in that idle to 3500 zone; and so, detonation is nearly inevitable.
IMO
This does not point to a deficiency in the DCR equation,
but rather,
points to a too-aggressive tune, including;
the failure of the factory-style distributor to provide a proper timing curve,
and a failure to properly tune the vacuum-advance system.
Bewy said:
One reason it is useless is the large number of variables that actually cause det, two of which are heat & A/F mixture.
Click to expand...
and these are tuning variables ....
 
mgoblue9798 said:
Just out of curiosity how would you calculate the octane requirement of an engine you build then? I get that Scr and Dcr are just measurements but you have to start with something.
Click to expand...

Two things to remember.

1. There is absolutely ZERO reason to run less than 91 octane (or whatever the highest octane available in your area) ever. Low and mid grade fuels have far more fillers and junk in them than premium grade fuels do, and the regulations are much tighter for premium grade fuels.

Proclaiming one can run their stuff on 87 octane is a fools errand. The money you save is pissed away in less fuel efficiency and dirty combustion.

2. Most guys can run way more compression on pump gas than they think they can. You have to control some things but it’s not hard to do once you clear your mind of all the bullshit that gets pumped out in the media and on forums.

I have been running higher than orthodox compression ratios on pump gas since the mid 1980’s. It‘s crazy to piss away power and driveability from fear of detonation.
 
mgoblue9798 said:
Not sure if this helps you or not, but I ran a 340 w TRW forged 10.5 to 1 replacement pistons (actual comp probably mid 9s as already mentioned) 272 455 purpleshaft 230 @ .050 or so, performer intake, headers, mopar performance electronic ignition 35 deg all in at 2500, and j heads. Engine would not run on lower than 93 octane. Hottest part of the summer I had to run octane boost or it would diesel when shut off. Admittedly I was not sharp on tuning 30 years ago. I also live it Georgia and it can get 95 degrees here pretty regular in the summer.
Click to expand...
Yes it's helpful, we get 95 degrees in summer here aswell, than -40 in winter, and no thats not a typo lol. I guess even if it doesn't ping when its cooler, it might when its that hot outside. I think I will still measure cylinder pressure though. Though it looks like the car is already set at 34 degrees all out, the previous owner was a Mopar guy and says all molars love 34 degrees. Though its sounds like 34 is slower a bit which will allow running 91? Though I do have functional hood scoops, which should help to cool the engine.
 
Rat Bastid said:
Two things to remember.

1. There is absolutely ZERO reason to run less than 91 octane (or whatever the highest octane available in your area) ever. Low and mid grade fuels have far more fillers and junk in them than premium grade fuels do, and the regulations are much tighter for premium grade fuels.

Proclaiming one can run their stuff on 87 octane is a fools errand. The money you save is pissed away in less fuel efficiency and dirty combustion.

2. Most guys can run way more compression on pump gas than they think they can. You have to control some things but it’s not hard to do once you clear your mind of all the bullshit that gets pumped out in the media and on forums.

I have been running higher than orthodox compression ratios on pump gas since the mid 1980’s. It‘s crazy to piss away power and driveability from fear of detonation.
Click to expand...

Idk, I put a decent amount of street miles on my car and there's a big price gap between regular and premium gas nowadays, close to a full dollar at some stations. When I put together the 360 in it now I didn't want to spend the money to swap out the stock 5.9L Magnum pistons and hone the cylinders for new pistons and rings. With Edelbrock open-chamber heads, .027" Cometic head gaskets and stock pistons (shallow dish) .050" down at TDC my static compression is right at 9:1. I've run only regular gas in this engine since I installed it in the car and it's going on 5 years now with about 10,000 miles. I've accidentally had the ignition advance as high as 44 degrees initial+mechanical (at 5000' elevation) and it never pinged, just made slightly less power than when I backed it down to 40. Since moving to sea level it's now at 36 degrees total, not including vacuum advance. I know premium fuel is "cleaner" but I also run a few ounces of Lucas ethanol fuel treatment with every fillup and the car never sits for more than 2 weeks without being driven. I had the carb off the intake yesterday and the plenum looks spotless, no varnish or any weird stuff. Inside of carb is clean too, it's only the outside where some raw fuel spilled and then slowly dried where there's some yellow/brown varnish residue.
 
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