Whatsup everybody. I'm going 383 and just wanted to know what is the maximum compression that should be run on a street car with pump gas. I'm definitely going to ask my builder but just thought I'd see what range of compression people here are thinking.

 

well a stock lt1 is 10.4:1 and runs fine on 93 I know when I rebuild mine next winter ('09) im going with 11:1 and I'll probably run sunoco 94 as much as possible.

 

This is why you have to be careful who you listen too.
The LT4 was 10.8:1.

The cam comes into play with dynamic compression being a consideration in how much static compression you can run BUT, I run 11.2-3:1 on a stock shortblock no problem on 93 octane, a lot of guys go 12:1. AGAIN though cam comes into play.

The overwhelming majority of engine builders are not familiar with the LT1 and are not going to be willing to build it with proper compression, we run more than most other applications. Matter of fact a lot of ignorant builders have LOWERED compression when rebuilding LT1s not realizing how high it is stock.

The b-body iron head motor was 10.0:1 stock and came tuned for 87 octane.

 

Good read
http://www.empirenet.com/pkelley2/DynamicCR.html

How big of a cam are you running ?

 

Thanks for the link James I had always heard about DCR but never knew too much about it. Definitely a good read...still trying to take it all in.

My cam is 224/236 574in/572ex 111 lsa

 

An LTX engine with a static compression of 12.5-1 with a wide LSA cam (i.e., 113 degrees) will be just fine on 93 octane.

WD

 

Yep, run a SCR of 12.48 to 1 on my 401. Dynamic of 9.1 to 1. No problems running in summer heat with air on. Air temp, engine temp, altitude, engine VE and tuning/timing are just some of the considerations...Bigger radiator helps (mines stock size) along with a nice cleaned (hot tanked) block. I run 32 degrees max timing only because it doesn't make any more power going higher for my combo. I have tried up to 35 degrees max and gained nothing on the dyno or at the track.

Engineer....
I see you have went with a PI3600 over your old 3200 Did you notice much of a difference?

C Rules...
Not knowing your ABDC on the intake lobe (at base duration) along with your actual combo. I can only guess that somewhere aound 11.5 to 1 SCR will get you 8.5 to 9 DCR with that "size cam"....This is only a guess.....!!!!! put your numers in Pat Kelly's DCR calculator and see what you come up with!!!

 

Thanx.....

 

I was working the calculator and got stumped on a few parameters the
Combustion chamber volume:
Is this the chamber volume of my heads. If they are stock units ported and polished they would be 58?

Piston to deck clearance. How can i find this out?

Also it looks like my cam card says the cam centerline is 111. Is it possible for the cams centerline and lsa to be the same?

Thanks for your help.

 

who said anything about an lt4 ????????
what i was saying- is im going with 11:1 on my rebuild.. thats that.

the OP asked what we "thought" so I merely told him what I will be going with..

 

No guess on chamber volume....It has to be measured...a few cc's off either way can make a big difference.

Your cam card should have base duration. And, 111 can be lsa and intake centerline as long as the cam is not retarded/advanced (eg. 109 intake 113exhaust=two degree advance get it...) If your cam card has ABDC of intake (at base duration) then you can skip the cam section on the calc..It's only their to help find ABDC with cam cards without ABDC on them...

Piston to deck clearance can only be measured once crank, rods and pistons are installed. A machinest can get a good idea but to get the numbers right your need to install parts, then check...Then pull parts out, and deck the block again to get your correct figure you want. Mine is .011 (measured on top of my piston centerline on piston pin centerline to deck surface)... I was shooting for .010 so, I consider that really good. With a .029 gasket (measured crushed) I get .040 quench. Quench is a major piece of the puzzle to running higher compression. .040-.045 Is easy with good pistons/rods (they don't rock back and forth in the block that easy)...In fact many run .03-.035 with iron blocks but, I consider a good area on the street (having to deal with carbon build up over time) .040 ish.. You can also due a zero deck height and run a FELPRO gasket (.039 gasket). LS1's have a positive deck height (meaning they stick out of the block). Their factory gasket is around .048-.054 measured aand the pistons usually are .008 above the block. So, they have decent quench from the factory. My factory combo was measured at .036 down the hole..and with a .054 gasket I basically had no quench. Our blocks pistons to deck heights are all over the map so you never know until you measure them. This is another reason (besides compression and ring gaps) why a guy with a (pistons, rods) 350-355 combo will make great power over another similar combo on a stock short block... It's all in the combo.....!Starting with a good bottom end... Oh, yeah....don't listen to me look at my sig..

 

Yes, I did change from a Vig 3200 to a 3600, you're very observant. I've been to the track one time on opening day this year (Feb 3) for T-n-T. The track wasn't too good and there were a lot of street tire cars there, so my 60fts were really bad. It wouldn't hold off the line.

Typically I'm around 1.48 to 1.50. That weekend my best was 1.57 with most in the 1.60s (ouch). So, my new converter didn't get a very good evaluation the first time out spinning off the line.

WD

 

I mentioned the LT4 because it was 10.8 from the factory, meaning 11.0 is conservative for a performance build.

Your comment about trying to use 94 octane as much as possible on a 11.0 motor says you thought that was high compression, and for a LT1 it is not.

 

im going with 12-12.5 to 1 with my new build

 

I've always thought going over 11.0 on pump gas is pushing your luck.. so you're saying 12-12.5:1 will run fine with no miss or break up high in the rpm range on pump gas ?

 

Depends on the cam as we are talking about with DCR.

I am running 11.2-3:1 without a problem and quench on my motor is about .052 not anywhere near ideal.

I do not know the DCR, I just trusted Phil to steer me the right way.

On a gen 1 motor few guys will manage to run even 11.0, the LT1 is fairly unique in the compression we can get away with, not just because of the reverse flow cooling but because of the injection.

 

12:1 is very possible on pump gas with the LT1 because of the reverse flow cooling heads. The heads get cooled first so the temperatures in the combustion chambers stay down and allow for the clean, smooth burn at high compression. Very true about the ignition too Caprice. With the opti reader, timing is almost spot on.

 

Static compression ratio has very little to do with running on pump gas. This is part of the reason why large cams tend to kill down low torque. A large cam bleeds off a lot of compression and lowers your dynamic compression ratio, but gives you more power up top. The same is the opposite for small cams. Tony Shephard runs 12.8:1 on 93 octane.

EDIT: I left out a few things. You also have to take into account quite a few other things. Timing, quench distance, combustion chamber design and the cooling system of the vehicle have a lot to do with how much you can get away with.

 

Well after plugging in the advertised duration of my cam 274/288 I get an ABDC of 68. I was messing around with some numbers and I was looking at around a 9.1-2 DCR with a 58cc combution chamber. After reading what was on pkelley's site it sounded like that DCR was fine for race engines. I don't know how great that would be for my street engine running on 93. Jimbob you say 9.1 works fine for you on the street so go figure. That website makes it seem like 9.1 is something that should be left only for full race builds. Maybe kelly was being too conservative. Either way I was weighing out some options:

1) Hope that my combustion chambers are larger than 58 since they were ported...if not maybe port them to 64 if possible to get the DCR down to around 8.5.

2) Switch my cam out and go with something nitrous friendly which also gets me in a lower DCR range

3) Get some pistons with much larger reliefs

4) Use some thick head gaskets

5) Run lower compression (trade off higher static compression for a better DCR)

6) Build the mottor with high compression and run less timing

Anybody got any thoughts.

 

I can't say a lot specifically about LT1 setups, but LS1 setups 8.8:1 DCR is about the liimit on 93 octane.