00Vette

iTrader: (4)

I was just juggling the idea of getting some milled heads (sometime, maybe), and was wondering...."what would be safe to run pump gas with". I've heard of guys running well over 12.0:1 compression on pump gas (obviously with retarded timing) and not having any problems.

So what do you think would be the highest CR you could go with pump gas and why?

Mikey 97Z M6

iTrader: (37)

Your cam choice will play a huge role in how much compression you can run. Valve overlap can bleed off cylinder pressure, effectivlely lowering your dynamic compression and allowing lower octane fuel per given compression.

Mike

Silverhawk_02TA

Yep. It's all about the DCR, not the SCR.

jimbob

iTrader: (4)

I'll agree with that.....

With that said though
..do not go higher than 8.5 DCR with a non reverse cooled engine (LS1). To run right at 8.5 DCR in the Ls1 you better have your tuning cap on and have no oil enter the cylinders (otherwise detonation). A safer easier number to shoot for is 11.5 SCR and let the cam size change your DCR. Some of the stroker guys are using 11.75 SCR as there number of choice. For comparison my compression is 8.78 DCR with 12.48 SCR.

Nate_Taufer

iTrader: (1)

With larger cams that dictate fairly low Dynamic compression ratios (i.e. big splits on lower lsa's) you can run 12.0:1 with a GOOD tune. It's all in the tune. Something like a 23X+ would be happy with that big of SCR

Nate

00Vette

iTrader: (4)

Now do you think 23X would be pushing the LS1's limits?

Silverhawk_02TA

A cam with a 23x duration is pretty much a medium-large cam as far as a hydraulic roller in an LS1 goes.

WAHUSKER

iTrader: (4)

Several guys here run cams that big. My blower cam is 22x / 23x. But IMO a cam that size really likes / needs a good set of heads to work right......

00Vette

iTrader: (4)

Right, I agree. I've heard of 24X cam's in these motors........but with milled heads, pushrods, and rockers to match a larger cam for a high CR motor is what I was refering to. I wouldn't imagine putting something that big in a high CR application would work! But then again, I don't know, that's why I posted this thread...

Nate_Taufer

iTrader: (1)

I run a 233/239 on a 112 lsa with stock 806 castings and put down 418/395 to the wheels. With heads and a full tune plus my new clutch i'm shooting for 475 to the wheels. This cam (although a lot of people would say it's not streetable) is still fine for me to drive around town.

If i were to ever get off my *** and tune it fully it would be pretty easy to drive around.

I would shoot for around 11.5:1 to give you some head room in case you get bad gas or the like for your application.

Nate

00Vette

iTrader: (4)

Yeah...........that sounds good. I could probably just have my current heads milled and get a new cam/rods/rocker setup to match the heads.

But would I need to buy pistons with dishes to keep the valves from hitting the pistons?

WAHUSKER

iTrader: (4)

If your heads are milled more than about .020 you should fly cut the pistons, especially if you are running .600 lift & the duration you are talking about. I have around .600 lift with my setup and with -18cc dished pistons w/fly cuts I have over .200 PtoV clearance. You want at least .100 if you're going to spin it.....

One of us had heads that were milled .030 to .050 & with a .600 lift cam & all 16 valves made contact @ 7500rpm.

00Vette

iTrader: (4)

does the dish in the pistons affect the CR? I would think since there is a groove in the piston that it would lower the CR. But then again, its probably not a signigicant enough groove/cut to do that much!

I would prefer not to go into the motor. Too much $$$ that I don't have and won't have for a bit.

So the question is: "what cam, pushrod, rocker combination do you guys think would work well for a 11.5:1 SCR motor, using say milling my heads down to 58cc. And would 2.02intake 1.575exaust valves work with this setup? I currently have LS1 heads with 63.5cc's giving it roughly a 10.7:1CR.

NataSS Inc

iTrader: (9)

My MTI stage 2R LS6 heads have a 58cc chamber and I am running the 2.055 and 1.60 valves. The heads were milled .030 and with the cam I was running caused Mr. Valve to meet Mr. Piston on all 8. Just be careful with came size when you start milling the heads unless you flycut or add pistons with valve reliefs. I was this was for a long time on premium pump fuel.

Nate_Taufer

iTrader: (1)

If you build your motor correctly there will be no problems. If you don't measure p-v clearance though there most likely will be issues. Use some checker springs and a micrometer to measure. If you need you can cut valve reliefs in your stock pistons.

I shoot for .080" on the intake and .100" on the exhaust with a pretty tight quench you will make a lot of power.

With what you are wanting to do i wouldn't go larger than a 224/228 cam if you want to use a 112 lsa.

If you are running a 114 you may get away with a tad bit larger but that is a lot of variables.

You will also most likely want to pick your cam first, then mill to get a desired DCR. I'd shoot for a max of 8.5:1 DCR or so to be street friendly.

Nate

00Vette

iTrader: (4)

Thanks for the Inputs and Idea's...............

How much (roughly) would it cost to have cylinder heads milled? And do you guys think this road is worth the gains?

Nate_Taufer

iTrader: (1)

Very much so. Tight quench with a 224/228 on 112 with decent lift with 8.5:1 DCR or about 11.8:1 SCR would work wonders and with a decent head you should make 450 to the wheels with that cam and an ls6 intake.

Nate

TwoFast4Lv

iTrader: (8)

I personally would not cut the LSx heads if I did not have to

No one has mentioned making sure yoiu have proper travel over the valves Make sure you have that AND proper lifter "crush"

00Vette

iTrader: (4)

why wouldn't you mill the LS1 heads? I don't understand travel over the valves......... Do you mean keeping the valves from hitting the pistons? Wouldn't shorter rockers and pushrods aleviate that issue?

TwoFast4Lv

iTrader: (8)

The rocker must "sweep" across the top of the valve in a particular pattern. You want this pattern centered on the valve. This can be adjusted and or changed by push rod length, Shimming the Rockers, changing over all valve hieght, head gasket thickness or cutting the heads. The only things that will change piston to valve while keeping proper sweep across the valves and lifter crush is cutting the heads or using thinner head gaskets. Lets not forget bigger valves, cam lift and duration while we are there

I personaly perfer to keep as much of my head surface and strength by keeping the heads as uncut as possible. I also would like to see 0.100 on the intake and 0.140 on the Ext. My BARE minimum would be 0.070 and 0.100 for Piston to valve. With a Stick shift car I would not budge on those minimum numbers 0.001! You miss one shift or down shift the wrong point once and over run the springs,valve float, and many things can happen. Any thing from just kissing the pistons to complete engine failure

I have seen many things like broken rocker arms, exploded lifters, bent push rods, bent valves. Most of the time you see this stuff here you are having a P to V issue

Maybe we should get together and have a tech sesion on how to check P to V and the proper way to set up and LSx Engine