Re: 10.5 to 1 compression and gas octane
[Re: dnd0001]
#1041652
07/27/11 09:01 PM
07/27/11 09:01 PM
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Joined: Jul 2003
Posts: 6,167 Maryland
GO_Fish
master
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master
Joined: Jul 2003
Posts: 6,167
Maryland
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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.
Scott B.
"I'm a self-made man... I started with nothing, and I still have most of it!"
68 360 rusty B'cuda 'vert (GO Fish)13.59@ 98.72 mph
69 340 GTS stock 14.18@ 95.60 mph
01 5.9L Ram 1500 Quad Cab 4x4
01 3.5L 300M 16.23@ 86.97 mph
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Re: 10.5 to 1 compression and gas octane
[Re: RapidRobert]
#1041661
07/29/11 02:07 AM
07/29/11 02:07 AM
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Joined: Sep 2004
Posts: 654 MN
astrobuf
mopar
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mopar
Joined: Sep 2004
Posts: 654
MN
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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
So, are you really a Rocket Scientist?
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Re: 10.5 to 1 compression and gas octane
[Re: 63stabamatic]
#1041665
07/29/11 09:39 AM
07/29/11 09:39 AM
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Joined: Jan 2003
Posts: 24,562 Brookeville, Md
Mr.Yuck
Not enough dumb comments...yet
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Not enough dumb comments...yet
Joined: Jan 2003
Posts: 24,562
Brookeville, Md
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Quote:
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.
I'm running a 440 w/ 11.19:1 w/ Alumunum heads and use 93 w/o issues. I'm even going to step it up to 38* full in at 2200 but I also have one of these...
http://www.snowperformance.net/product.php?pk=31
No issues.
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Re: 10.5 to 1 compression and gas octane
[Re: dnd0001]
#1041666
07/29/11 10:14 AM
07/29/11 10:14 AM
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Joined: Jan 2006
Posts: 8,162 USA
360view
Moparts resident spammer
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Moparts resident spammer
Joined: Jan 2006
Posts: 8,162
USA
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Quote:
We did about 8 pulls on the dyno and he changed out the jets a couple of times and set the timing to 35.
Along with all the other factors already mentioned in above posts, the setting of the ignition timing throughout all the load and rpm points, is critical to avoiding detonation.
If you are willing to sacrifice some low to mid-rpm full throttle torque, more static/dynamic compression ratio can be run with limited octane.
The new productions engines like 11.7 CR in the Ford Escape Hybrid and 13 CR in the Mazda 'SkyActiv' engine depend on retarded ignition timing and a carefully mapped out timing over all load and rpm operation, to run on 87 octane.
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Re: 10.5 to 1 compression and gas octane
[Re: 360view]
#1041667
07/29/11 11:58 AM
07/29/11 11:58 AM
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Joined: Jan 2003
Posts: 16,376
dogdays
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 16,376
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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.
Last edited by dogdays; 07/29/11 12:10 PM.
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