I have a 1974 Barracuda with a 1969 340 with 323 gears and 727 tranny. The 340 has a 10.5 to 1 compression and a mild cam. I've been running 91/92 octane pump gas and haven't heard any pinging. I blew a head gasket and after digging into it the engine has to have an overhall after 200 miles. Not sure that it was the gas. The crank was cracked and there are other problems from the original engine builder. A new engine builder (Total Engine Repair)is telling me to lower the compression with a thicker gasket or I should be running a combination of 30% racing gas and 91 octane gas. Any thoughts or past experences with pump gas on high compression engine?

10.5:1 was the "advertised" compression on the early 340's. In reality, most Mopars from the muscle era had 0.5 to 1 point lower compression than they were rated at. My stock 69 340 runs fine on 91 octane and regular head gaskets.

The proper way to determine your compression to actually measure the CC's of each combustion chamber, and how far the piston sits above the deck, the thickness of the head gaskets, then do calculations to determine your ACTUAL compression. Your cam choice can also play into this. Some grinds can bleed off compression so that it is not as big an issue.

The "rule of thumb" is 9.5 cr max with iron heads. With a mild cam and no quench, a true measured 9.0 would be safer. I agree with your engine builder, compression was high for pump gas. In some ways, a street driven car has higher detonation risk than a track only car. The engine is drawing engine compartment air which may be 150-160F on a hot day and it can be run for extended periods of time which heat soaks everything.

The only mystery is why no apparant detonation. The only thing I can think of is that it happened at higher RPM and was masked by the noise and power of the engine.

To fix it you have lots of choices. The ideal fix if you keep iron heads and the same cam would be quench pad pistons, matched to the chambers to give .040" quench with the setup dialed in for 9.5 or just a bit higher CR. This likley means new pistons and labor to dial in the quench.

The gasket deal may drop compression at lowest cost but make sure its enough to get you under 9.5 since there would be no quench.

Depending on the pistons you are starting with It may also be possible to mill the tops to lower CR. You would need to consult the piston manufacturer to determine how much if any can be removed without hurting piston structure.

NO piston damage ? .... You will see-that before blowing a head-gasket IMO.

I think we need more info on what you actually have. Is that 10.5 compression actual calculated compression or just what the factory service manual said for stock pistons or just what the previous owner said? Everyone's definition of "mild cam" is different, and cam makes such a huge difference on how much compression you can get away with.

Nacho,

I saw your post and voted for the minimum CR... which may still be too high depending on cam/quench as one comment suggested. At the same time, I know a race car with 160 stat, hood scoop, hot cam and cool down time between rounds can go a lot higher. It depends a lot on quench, cam and application of the engine.

As posted above, some more info from the OP would help.

Well I can't tell you much more then I did about the engine specs. I didn't building the engine, it was done 10 years before I bought the car. The engine has been sitting for 10 years without being broke in and that may have the engine to wear as fast as it did. I talked to engine builder again today. The cam numbers are 27? we can't read the rest. He's going measure it to find a comp cam that is close to what I have now. He did cc the heads and determined it has a 10.5 to 1 cr with cast iron heads. He recommends to keep it the same and burn 30% racing fuel with 92 octane gas. That way I can keep the HP I currently have. I fugure I will only put on 500 miles in a year and this would be the best solution. Does the 9.5 rule of thumb apply to the cast iron heads if you burn a higher octane gas or is that 92 octane? Will this work or should I still bring down the cr with thicker head gaskets?

It should run fine with the higher CR and 30% race gas. The "rule of thumb" is for pump gas.

I have a 71 340 10.5:1 engine and run it regularly on 91/92 gas without problem. Your problem is likely related to something other than detonation. If you do not see piston damage, your not detonating.

If you want some safety margin, put aluminum heads on the rebuild. With proper quench and aluminum heads 10.5:1 should be easily supported with 91/92 gas.

Ensure you are not running lean. I suspect your engine problem may be related to hard running the engine when it was too lean. How did you set the carb up?

Astrobuf

I be perfectly honest I relitively new to engine building as you can tell. I took the engine to a local dyno shop and they set the timing and rejetted the carb. We did about 8 pulls on the dyno and he changed out the jets a couple of times and set the timing to 35.

I'm running a 10.25:1 440 with aluminum heads. It will run on 93 but pretty doggie, had to retard the timing slightly. I put the timing back and now run 50% 100/93 mix.

Let's not forget the Skyactive direct injection and a hole in the top of the piston that looks like it was stolen from a Diesel. Compression ratio is actually 14:1. Also the new engine has a 4-2-1 header to reduce exhaust reversion. Ignition is retarded mainly to keep enough heat in the exhaust to allow the catalysts to work.
It's a really neat system, is as far ahead of MPFI as a Hemi is ahead of a flathead. Most of the technology isn't transferable to our engines, though.

R.

SkyActiv Mazda pr's surprised me by saying that it will keep the originally advertised
14 to 1 for Japan, Australia and Europe,
but drop the USA models to 13

maybe our gasoline lags

brought back memories of what happened with the USA version of Michael May's Jag V12 HE