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Discussion Starter #1
I am hoping to get some information from you guys here that know a lot more about engines than I do (street engines at least). What is the highest/safest compression ratio you can run on pump gas and on a complete daily driver? Is 9.5:1 a bit too high or even 10:1? I would love to stick to 8.5 or 9:1 but with the pistons that are in my 350 block and the heads I am wanting to go with I doubt it's going to happen. The heads on this block now are junk GM crate/truck emissions heads that are odd 75cc's (yes 75 not 76). That makes the TRW Speedpro's I have at 9.5:1 compression I believe but I have been looking at some plain PBM Thunder heads from Erson cams and trying to figure out my compression ratio. I am thinking about going to a roller cam setup and run either vortec heads or these $600 PBM Thunder heads that I think will be better than what I have now and flow easier which in turn will help mileage and power.
Oppinions or suggestions would be greatly appreciated!
Thanks in advance!
 

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depending on your piston "dome" volume and the cc of the chamber plus the gasket thickness will determine your compresion. as far as highest and safest... i guess this is going to be a long discusion, but speaking from experience i have been running 11.3-1 with Eddy rpm heads on my 355 for the past ten years with KB pistons. and running good ol 91 piss gas the whole time and 36* of timing in it. so from my POV i would say with the alluminum heads you should be fine at 10.5-1 with no problems what so ever. with an iron head i would run nothing over 10.5-1 just to be safe.
 

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usually with aluminum heads you can go 1 point higher than iron with the same octane, so if you had 9.0cr with 91 octane then 10.0 will be safe.
 

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You can detonate with 8.5:1 compression ratio and people have successfully run pump gas with 12:1, so compression ratio alone isn't the sole determination of octane sensitivity.

Best of Powertrain has the FAQ on the subject.
 

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Discussion Starter #5
Well the heads I am looking at are not aluminum. They are plain cast iron and only a step up from stock heads really. They are not the best but I cannot afford nice $1,200 aluminum heads like I'd like. As for the pistons they are TRW's (Speed Pro) L2256 .030 over Forged pistons. The pistons are supposed to have 9.72:1 compression ratio with 64cc heads I believe. The heads I have are 3998993 which show as 75cc heads NOT 76cc and that should bring the compression ratio of these pistons now to 9:1 I think (I could be wrong and I know the gasket also determines this as well). Only reason I am changing the heads (and there is a SLIM chance I wont) is because I need a new cam and lifters, springs, retainers, etc so I figured I'd buy these PBM heads with SS valves and little bit better porting job done on them than factories. I was going to go with Vortec heads but do not want to change the valve covers, intake, etc. Supposedly these PBM Thunder heads have similar specs to Vortec heads and were recommended by a machine shop guy I know on a hot rod site. He recommended going with the roller cam setup even though it's more expensive and these heads.
Again, this wagon will be my daily driver and I am trying to get good mileage and reliability out of it. Sorry I am all over the place here, this is all up in the air and I am STILL trying to figure out what the heck I am going to do and which way I am going with this engine so I can get the drivetrain part of my build out of the way.
 

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I would have to totally agree with Paul.


In my opinion there way to many variables to say what compression ratio is "safe".


Just different brands of the "same" octane fuel can make a tremendous difference.
 

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I'd have to agree with everyone else so far. EVERYTHING effects an engines knock sensitivity. Even things nobody ever mentions like average relative humidity, ambient temp and altitude.
Then there's quench distance, cam timing, combustion chamber shape, piston top shape, fuel quality and of course perhaps the most important, your ability to TUNE!!!!


Certainly WAY too many variables to just make a blanket statement about what works and what doesn't.:yes:
 

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Well the heads I am looking at are not aluminum. They are plain cast iron and only a step up from stock heads really. They are not the best but I cannot afford nice $1,200 aluminum heads like I'd like. As for the pistons they are TRW's (Speed Pro) L2256 .030 over Forged pistons. The pistons are supposed to have 9.72:1 compression ratio with 64cc heads I believe. The heads I have are 3998993 which show as 75cc heads NOT 76cc and that should bring the compression ratio of these pistons now to 9:1 I think (I could be wrong and I know the gasket also determines this as well). Only reason I am changing the heads (and there is a SLIM chance I wont) is because I need a new cam and lifters, springs, retainers, etc so I figured I'd buy these PBM heads with SS valves and little bit better porting job done on them than factories. I was going to go with Vortec heads but do not want to change the valve covers, intake, etc. Supposedly these PBM Thunder heads have similar specs to Vortec heads and were recommended by a machine shop guy I know on a hot rod site. He recommended going with the roller cam setup even though it's more expensive and these heads.
Again, this wagon will be my daily driver and I am trying to get good mileage and reliability out of it. Sorry I am all over the place here, this is all up in the air and I am STILL trying to figure out what the heck I am going to do and which way I am going with this engine so I can get the drivetrain part of my build out of the way.
More like 8.6 with 75cc heads, compression as a rule of thumb goes up or down .1 with every cc difference, also +.1 with every .010 overbore, and + .1 with every .010 milled off the heads. Use this as a ballpark figure only, so with the +.030 pistons you should be in the park with 8.9 +/- .1
 

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More like 8.6 with 75cc heads, compression as a rule of thumb goes up or down .1 with every cc difference, also +.1 with every .010 overbore, and + .1 with every .010 milled off the heads. Use this as a ballpark figure only, so with the +.030 pistons you should be in the park with 8.9 +/- .1
8.9 with what thickness head gaskets?
Pistons in the hole how much?

Those two together can have almost a full point of compression difference.:yes:
 

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8.9 with what thickness head gaskets?
Pistons in the hole how much?

Those two together can have almost a full point of compression difference.:yes:
How? maybe .4 difference, my info was to get in the ballpark for an idea of were he's at based on the information given, Some of you guys get way to techical on a general question on a street engine, ive research part differences years ago using head cc, piston shape/cc, bore size and gasket thickness along with comparing parts from different engines and cross checked info all the time to get an idea. As for my car in the past i ran 11.0 on ultra 94 octane every summer day to work and back.
 

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9 to1 with 87

11 to1 with 92/93 premium not a issue.

11 to 1 is the max, the very limit. No room for error.

I have 10.6 to one, use only BP/Amoco white gas.

Al
 

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11 to 1 is the max, the very limit. No room for error
I had a few too many adult beverages one night and got talked into racing a Hayabusa--turned on the bottle (200 shot) 36° timing, 91 in the tank, made a curb to curb pass blowing the tires off--I think the only thing that saved my 14.5-1 compression, iron headed engine, was the fact it didn't hook and never loaded the engine very hard.
 

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9 to1 with 87



11 to 1 is the max, the very limit. No room for error.

I have 10.6 to one, use only BP/Amoco white

gas.Al
I agree with the 11 to 1 is max on a pump gas iron head engine, my car wouldnt pass the shut down test with 93, would run ok but would have run on when i shut it down, and when using 94 it simply shut off, so without backing off on timing i ran ultra 94 90% of the time.:D
 

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How? maybe .4 difference, my info was to get in the ballpark for an idea of were he's at based on the information given, Some of you guys get way to techical on a general question on a street engine, ive research part differences years ago using head cc, piston shape/cc, bore size and gasket thickness along with comparing parts from different engines and cross checked info all the time to get an idea. As for my car in the past i ran 11.0 on ultra 94 octane every summer day to work and back.
gasket bores, piston down hole, gasket thickness, piston shape depressions, and or chamfering. Actual chamber CC's not advertised..

gasket bores from as little as .010 larger than bore size or as much as .200 larger, thickness from .015 to .051, advertised heads @ 64 that were 66+cc.. how far down the ring pack starts... is the piston @ 0 deck or is it ..025+ down hole? piston recesses.. 5cc? 12cc? 7cc? 20cc?

all these variables can account for vast compression differences..

in my book as much as 2 full points with all the variables in opposite directions..

Engines are technical.. every aspect.. you can throw together any thing you wish and guess what ya have or measure.. and know what ya have.....
 

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gnaw....info is free and was long before modship... we are here to disseminate truth and dispel falsies so you have the tools to make important decisions and informed decisions...


I paid steve 4.2 mil for the privileged of modship:no:
 

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How? maybe .4 difference, my info was to get in the ballpark for an idea of were he's at based on the information given, Some of you guys get way to techical on a general question on a street engine, ive research part differences years ago using head cc, piston shape/cc, bore size and gasket thickness along with comparing parts from different engines and cross checked info all the time to get an idea. As for my car in the past i ran 11.0 on ultra 94 octane every summer day to work and back.
I think Veno covered it the possible differences. I'll just add that TRW pistons are known to boast a higher compression ratio than reality because they typically are down-hole quite a bit. Anywhere between .015" to .030"!!:eek: That alone is quite a bit of difference!!:yes:

I too have ran 11.3 on iron heads and 92 octane Chevron fuel.:yes: Daily driven in the South East Texas heat! :eek:Plenty games to play but not anything I'd recommend for a novice.:no:
 

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Discussion Starter #19
As far as knowing how far down the pistons go, etc, I do not have the tools to check this. Most people will just say, quit worrying about it and just throw the engine together and run it but I am OCD when it comes to building these kinds of things and would like to KNOW what I have. In the end I want to try and achieve 9:1 or 9.5:1 at the most and keep it running on medium grade pump gas if possible. If not then I may scrap the entire engine idea I have and just buy a roller crate motor, throw a mild cam in it, a decent carb and intake and let it go. I'd like to use this engine since I already have it and a lot of parts for it. If I could do it all over I would have gotten a roller cam motor with vortec heads, thrown a mild cam in it, a nice intake, carb, headers and have it running around 9:1 compression and throw it in. It's not really too late for that now but then again, my money is already thrown into what I have and I dont have much comming in to restart on another engine/tranny combo.
Building cars isn't for the poor I can tell you that! :D
 

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As far as knowing how far down the pistons go, etc, I do not have the tools to check this. Most people will just say, quit worrying about it and just throw the engine together and run it but I am OCD when it comes to building these kinds of things and would like to KNOW what I have. In the end I want to try and achieve 9:1 or 9.5:1 at the most and keep it running on medium grade pump gas if possible. If not then I may scrap the entire engine idea I have and just buy a roller crate motor, throw a mild cam in it, a decent carb and intake and let it go. I'd like to use this engine since I already have it and a lot of parts for it. If I could do it all over I would have gotten a roller cam motor with vortec heads, thrown a mild cam in it, a nice intake, carb, headers and have it running around 9:1 compression and throw it in. It's not really too late for that now but then again, my money is already thrown into what I have and I dont have much comming in to restart on another engine/tranny combo.
Building cars isn't for the poor I can tell you that! :D
You can mearure it on the cheap with just a couple cheap things.:yes: You'll need a piston stop, a STRAIGHT edge, and some feeler gauges. Use the piston stop to find TRUE TDC, mark the balancer. Remove the piston stop and turn the engine over to your TRUE TDC. Then use the straight edge and feeler gauges to make the measurement. Make sure and push down on the "Valley side" of the pistons and the outboard side of the pistons so you can also check what kind of "piston rock" you have.

It won't be a PERFECT measurement, but it will be within a couple thou if you're careful.

All of the tools required will cost less than $50.:yes: Especially if you can make your own piston-stop out of some scrap metal and a bolt.:D
 
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