i spun a bearing on my stock bottom cam car so i desided to go all out and be one of those high end stroker guys, so what i need to know is what is the max compression ratio these engines can handle on pump gas. (no emissions if that matters)

i was thinking about 12.3 to 1 or so. but i will still drive this car on pump gas and dont want to overkill.

any help or suggestions will be appreciated as i dont want to mess around and get something that runs only on jet fuel.


thanks, brad

 

No set ratio. Depending on cam selection you can run up to 14 probably. Run a cam with alot of overlap and duration and you can run higher compression. anywhere from 11:1 to 12.5:1 sounds good though, even more importantly is your quench height than compression, but normally they go hand in hand.

 

well i am going with xe 230 236 112, maybe not the best stroker cam but its what im doing

as far as the quinch hight i'll let my engine builder do that, he just hasnt built many street lt1 strokers so he was concerned on the compression.

i had in mind decking the block also to help square up everthing and get the tolerances where they need to be.

so i shouldnt be concerned about pump gas on that much compression?

 

Recommended quench height is between .035-.045, shoot for around .040 or .045 to be safe, stock is around .070 off top of my head, so that is quite an improvement.

If that doesnt make any sense, all it is is the space between the top of the piston and the deck of the head.

Depending on piston selection. Say you use a 4cc dish piston, standard valve relief CCs. If you deck the block to say .005, meaning the top of the piston is that far down from the deck of the block, which should be about 9.005" from the centerline of the crank, but it will differ slightly.

Then use a .039" head gasket.

With a 4.030 bore and 57cc combustion chamber, that puts you at 12.5 static compression. That cam will likely eat that up no problem and run just fine on 94, I wouldnt go any less than 93 though and you shouldnt see any problems. No matter what, get the block decked, how far you go is up to you or the machinist, I like the idea of having more to deck in the future, thats why I say .005 in the hole, not zero deck.

 

I just went through this about 4 months ago and my machinist suggested I go with 11:1 becasue it was an everyday car. I don't regret it.

 

thanks alot thats what i wanted to know
again thanks

 

Curious how you dont regret it? Sounds like you speak like its on the verge of something?

Im not arguing for compression or anything, since you will likely never feel a boost in compression unless its from 6:1 to 18:1 on alky, but why did he suggest to stay at 11?

IMO, forget completely about compression, get that quench height in the range i spoke of and you will be happy.

 

Well the reason is that I for one wont have to worry about always having to run high octane gas. Another thing is that If later I wanted to go with N2O or a supercharger it wouldn't be out of the question. 11:1 is about as high you wanna go on standard super 92 gas.

 

I dont mean to be rude, but it seems like you have done a lot of research, I have seen you posting. Static compression means close to nothing. You could probably run 14:1 on 93 no detonation with the right cam. No one but the "smart" people know how to figure that stuff out, I myself consider myself a very intelligent person that is a quick learner, and looked at how to figure dynamic compression and quickly forgot about it.

Just to explain slightly what it means, if the overlap of the intake and exhaust...where the compression is leaking out, the static compression can change severl tenths of a point to even points depending on how much overlap there is.

So saying that 11:1 is the "MAX" you want to go on 92 pump gas is misleading and untrue, 11:1 is the max to go on 92 pump gas with a cam that doesnt allow any more is the more true statement. 11:1 is a good solid number to shoot for, you will never have problems with 11:1, not even on a stock cam, but you can go more, and if its not setup right, 11:1 can hurt you in cases.


16cc dish piston
4.020 bore
.005 deck height
.039 gasket
52cc chambers

11.1:1 quench height .045

-2.4cc dome piston
4.020 bore
.025 deck height (stockish)
.049 gasket
58cc chambers

12.0:1
quench height .064

See how it can hurt you depending on piston selection, i took it to a slight extreme to show why it works like that, but you get the point.

oh, forgot, nitrous would LOVE 12:1, and 11:1 is too much for much past 4psi of boost

 

Cool Jona...you weren't rude at all man. I definately don't know all the specs and reasons. . But thats basically what I was saying......you'll be safe with 11:1.

 

That works! I guess i might have wasted my breath, but maybe he or others learned something, assuming I know what Im talking about!

 

You didn't waste your breath, some of us paid attention.

So what ratio did your engine end up with? Does it have a detonation problem?

 

i'm a general mech so i know just enough about technical stuff to get into trouble.

i apolagize if this is stupid

so what you are saying is my cam has enough overlap to bleed off or scavenge x amount of pressure leaving the psi to a tolerable level?

so if i were to run a cam with less overlap i would get more actual psi with lower ratio

then is that were the overcammed engine comes from, a huge overlap with low ratio that bleeds off more psi then the engine was built for leaving you lower actual psi?

 

Dynamic CR (DCR) is largely a function of when the intake valve closes as the piston is heading up towards TDC (IVC point). An earlier IVC raises the DCR, later IVC lowers it. More overlap will increase peak cylinder pressure by raising the VE, but this is not the same thing as DCR.

As has been stated, unless you specify the cam you are using, talking about static CR is largely meaningless. If you tell me what cam you plan to use I can give you a calculation of the DCR. For pump fuel you want it at 8.5:1 max, assuming everything else is perfect. Most people should aim for 8:1 though, as everything else is seldom perfect. Detonation robs power at the best and breaks parts at the worst.

Rich Krause

 

well i'm going with the xe 230 236 112

the reason i'm asking is a lot of you know more then i about this stuff and i want the most i can get with what i'm doing and still run the 93 pump gas.

thanks for your help