96lt4c4

I am ordering a stroker kit....

With SRP flat tops it will put Comp. to 11.8:1. Will this thing run on pump gas? I am going to run the Comp 467 XFI cam. 93 octane here in Louisville.

If I go dish it will be about where it is stock 10.8:1

What should I do?

ODIVETTE

I have a 383 L98 that I built. I have 10.8:1 compression and, believe it or not, runs perfect on 87 octane and fusses with 93 octane. I could not (and still can't) believe it. Your concern was mine as well. I figured 10.8:1 was about as high as one could go and still run pump gas. If 10.8:1 runs perfect in my car with 87 octane...maybe 11.8:1 will work in yours with 93 octane??

Or maybe 87 octane?!

rklessdriver

11.8 will most likely not run on pump gas. Your Dynamic Compression Ratio (DCR) actually determines this - but since part of calculating the DCR figures in your SCR obviously your cam would have to bleed off alot a lot of cyl pressure in order for your engine to run on pump gas. Without the opening and closing numbers of the cam I can't figure your DCR for you. However from my experience, high compression engines, pump gas and long L/S camshafts do not play well together.

These are the specs I came up with for the Comp 467 XFI.
Operating Range: 2000-6000 RPM
Duration Advertised: 280° Intake / 288° Exhaust
Duration @ .050'' Lift: 230° Intake / 236° Exhaust
Valve Lift w/1.6 Rockers: .576'' Intake / .570'' Exhaust
Lobe Separation Angle: 113°

Where the intake C/L is installed (degreed in to) will make the most difference because that will control when the intake valve closes (in releation to the pistons position in the compression stroke). Retarding the I/C (there by closing the intake valve later in the stroke) would help lower your DCR some but...

IMO at 11.8 SCR this thing would detonate itself to death in short order on 93 octane. Order the dished pistons and try to get the the SCR around 10.5 with this camshaft to be safe.
Will

CFI-EFI

The amount of overlap has ZERO to do with the cylinder pressure. The overlap period is when the exhaust valve is closing into the intake stroke and the intake valve is opening during the exhaust stroke. The compression stroke doesn't even think about starting until 180° after the middle of the overlap period. Overlap does NOT bleed off pressure during the compression stroke.

RACE ON!!!

96lt4c4

Anyone else? I know the LT1/4 heads run cooler than a regular small block, and that helps with knock.

Also SRP calculates there advertised compression with zero deck height. I might have the block decked just enough to square it up, but not taken to zero. Does anyone know what the stock deck height is on one of the LT motors?

rklessdriver

CFI - You are correct, I was not thinking about the cycle when overlap occours. The only thing that effects cyl pressue is the intake valve closing point.

I have revised my post.

His engine still will not run safely on pump gas.
Will

d48mclain

Usually the stock blocks are decked .022 to .025. I got unlucky on my 97 SS and it was a Monday morning car......0.40. For sure have it zero decked and probably use a .039 gasket to get your quench tight for better efficiency. Right now mine is .036-.037 and frankly I'd like to have it just a little tighter even. FYI...Lingenfelter liked his cars quench at .043 but that's a little conservative by todays standards.

Not sure where everyone is getting their information regarding compression and pump gas but my current LTx combo is running 12.8 to 1 static and when I ran the numbers it came out to be 8.4 dynamic. Want to double check the dynamic as it sounds low to me but in the past ran as high as 13.3 to 1 static with no issues.

Get a good program off the internet like:

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

DCR Calculator with VB6 Runtime files


And follow the directions to a T as it's a bit complicated and you will need a cam card as you need to plug in duration numbers other than .050.

edcmat-l1

Not true. We build almost all our stroker LT motors with that hi of compression. Thay run just fine. Granted, you have to tune them right. Incidentally, we build most all our street motors with what would be considered compression "too high to run on pump gas". Some motors respond better than others. The LT motors seem to like the higher compression. The LS motors the same. L98s, not so much.

edcmat-l1

Yep. We've done a few at 12:1. I personally have done some big blocks over 12:1 that run just fine on pump gas.

d48mclain

I'm with you all the way on this compression stuff and honestly wondering how these "rumors", "wife's tales" or bits of misinformation still keep hanging around.

For reasons I'm not quite exactly sure right now, but when going from 12.4 to 12.8 I did have to pull some timing out below 3000 rpm in the mid kpa areas but a former engine was 13.3 and didn't even have to pull any out of it.

Dunno....my heads n pistons are thermo coated vs not on the other engine but I'm thinking because of the thermo coating on the heads, I may even be running a bit more compression than I think. The deck height was measured with the piston coatings so that's out of the equation.

Amazing the stuff you read at times.

jeffyc

when i did my 383 stoker (its a l98 not a ltx) i used dished kb pistons, as my heads have 54cc combustion chambers (they are tpis afr heads)

when we figured out the cr at the machine shop the cr was 10.4:1
i was going to put on thicker head gaskets cause i thought it would have been higher

at the risk of starting a discussion, a more sherical the a/f mixers is compressed in the more power and less prone to detonation. ie the hemi motors

with the compression being the same, the motor with a hemispherical (dished pistions in this example) compressed a/f mixture will make a little bit more power and be less prone to detonation

Caboboy

This is perfect advice, it's all about dynamic compression; not static. As an example, I'm running an Erson version of the LPE219 and it was ground with a 4* advance. I ran Pat Kelley's program and found my DC would've been 9.5 had I installed the cam straight up, whereas if I retarded it back to 112* my DCR was a much more manageable 8.9. One of the purposes of my build was to be able to idle through parking lots like a stocker.......I wouldn't have been able to do that had I not retarded my cam timing DCR is maybe the most overlooked of the several important parts of engine planning

rklessdriver

Going over the specs for the Comp cam he has specified. I have already listed thoses specs in my previouse post. I inputted the camshaft specs in my DCR calculator and it tells me tha the intake closing angle with the Intake C/L degreed @106 is 66 degrees ABDC. Using his specified 11.8 SCR (which is also what I get calculating it with a 3.75 stroke, 4.030 bore, ,035 head gasket , 6cc valve relif and the piston .004 in the hole) his DCR is 9.3567 and that will not stand being run hard on pump gas. I would not ever build a motor like that for the street, too many variables with gas.

Now if he degrees the cam in at 108 intake C/L that closes the intake valve at 68 degrees ABDC. DCR is 9.1999 and that also would not run on pump gas.

Hell why don't we back it up to 112 intake C/l and see how big of a dog we can make it
??

The generally accepted standard is that 8.5 DCR is the max for pump gas for most engines. Of course some will stand a little bit more but that is the edge and why go to the absolute edge on a street car. The differences between the two engines in reality won't be much - and once your 12.1 pump gas engine detonates one time you lose 50hp instantly and detonation spikes the cyl temp horribally more detonation. It is going to take a while for the knock sensor to get the timing pulled back enough to stop it - less timing adv now thers some more HP you just lost, if it didn't burn a piston.

Mean while my 10.5 compression motor is just moving along making max HP (which is about 5-10 less than your high compression motor on its best day) at all times because it never detonates.

Why risk it?? We are talking about a car that someone has to rely on for transportation and the fact that he probally will be only able to afford to build this engine once without strain on the fiances why risk it when a set of lower compression pistons are so easily obtained? Maybe he just wants to build it 2-3 times -if you got that much, more power to you.
Will

96lt4c4

Roger that, I was thinking that it would be OK to run it that high. I think a lot of people think about the older style small blocks when compression comes up. 10.8 on an aluminum headed gen 1 motor would be pushing it. The main reason gm made the change to the LT block was for better cooling to the heads, which means you can run more compression. So would it be unanimous that I have it decked to zero?

5abivt

My LT4 has 11.8 compression. Not sure what the important cr is I'm sure its less that that. I do run 32 degrees max timing.. perhaps that's why it doesnt run well at the standard 35-6 degrees...

LT4BUD

Stock compression with LT4 is 10.8.....

I am running Felpro 1074 which compresses to .039 vs stock .049 giving me a compression of 11.3.

I am pretty sure others have used the Impala gasket which compresses to .029 giving a compression of 11.9

Far as I know these LT4 engines have been able to run on pump gas...

BTW I am not totally sure of these numbers, but I think they are correct

STL94LT1

cv67

Better to run a little less compression and more timing than too much vcompression and not enough timing.

Wether the car could or could not run at that SC for a street driver its a waste to go that high.

edcmat-l1

First and foremost, you need to take some measurements and see where you will end up. 2 or 4 CCs can mean a big difference in the static. The deck height plays a big role. Theres a HUGE difference in compression from .020 down in the hole, to zero. So, with that being said, I would measure as precisely as possible, which includes CCing the heads, make sure you end up with something in the neighborhood of 11.5, and the have a cam ground to keep the dynamic around 8.5 or 8.6. And its not the overlap thats gonna make or break the dynamic, its the intake closing. The cam you're looking at, the 467, doesnt have enough intake duration to get the dynamic where it should be. So, to get the intake valve closing at the right time you would have to retard it, and you dont want to do that if you can help it. So, drop the compression slightly, tailor the cam slightly, or run the compression high, and put more cam in it. The one thing that WONT work is high compression and a mild camshaft.

edcmat-l1

I do agree with that. And I have seen where dropping compression slightly can yeild more power. But thats more the exception than the rule.