Have a 91 rs 350 tbi with all mods....
Im planning on milling my vortecs down and putting in a thinner head gasket..
I want to raise the compression to around 10.5...
The car is a 5 speed and has 3.73 gears out back also has lost roughly 300 lbs from factory weight
Could I run cali 91 octane all day...
I run 91 on my 11 compression LT1 with no detonation problems but ofcourse the LT engines can handle higher compression ratios..
Should I go a little lower or could I get away with 10.5???

 

Do you really need the 20hp and 20ft-lbs that the ~1 extra point will give you?

Seems like a waste of time and money- if the ECM detects knock/detonation then it will pull out advance (retard timing) to null any gain.

 

Are you vortec heads aluminum? Your LT1 heads, were aluminum of they were from an Fbody. With aluminum heads you can usually get away with an entire point of compression more. like 9.0 -> 10.0 and so on.

But if you are having to pull apart the engine just to do the head decking, and that is the only reason you are doing it, then yeah I would just not bother. Personally don't think it's worth it. It also depends on your cam how much compression you can play with.

 

Stock LT1s get away with 10.5:1 because of the reverse-flow cooling. And the iron-head LT1s in the Impala SS weren't running 10.5:1, either. Only the aluminum-head versions.
The true difference between iron and aluminum isn't a full ratio, it's half a ratio. And each full ratio is worth only 4% in terms of power.
Best to just run 9.5:1, unless you're on top of a mountain. Elevation allows and needs more compression.
The practical purpose of running high compression is to compensate for late intake valve closing. The more compression you have, the later the intake valve needs to close.
The late intake closing is for high RPM.

 

I think you can swing 10-1 comp with a fast chamber and iron heads.
Im running 11.75 to 1comp.
I do have aluminum heads even w/no fancy pants reverse flow cooling.. runs 160-165 all day in the heat w no pinging or over heating.

 

I have a 10:1 Vortec headed 353 and have no trouble with detonation although I use 94 octane fuel. That said, I've paid attention to the dynamic compression ratio with respect to the cam I'm using and I'm under the limits of what an iron head can tolerate.
Have you read this article?
http://www.empirenet.com/pkelley2/DynamicCR.html
That said, a high compression and the 16 degrees of initial advance I run makes it a little tough on OEM starters, especially when the engine is hot.

 

But you don't have California emissions laws to worry about.

 

The heads are iron im also going to go wit bigger valves 2.05/1.6 better springs and get a full port job
Have a mild cam 210/220 @.50 .500/.510 lift 114lsa but i might step that up as well to somethg like 224/232@.50 .510/.520 lift 112lsa
Just wanted to boost the compression to complement the rest of the mods
So could I do 10 flat on shitty 91 lighter fluid?
Im sure i'll get a nice power jump... how much do you think 30-40hp?
Also dont have any emission **** on the car...
California BAR can suck it!!! hahaha

 

Do you know what the piston dish cc is on that engine? IIRC, it's something along the lines of 18cc.
If that's the case, you'll have a static comporession ratio around 8.75:1 with 64cc heads and a .025" head gasket. That's assuming a piston below deck of .025" (which is about the OEM spec), standard bore and stroke. A 7cc (aftermarket) piston will push the SCR up to 9.75:1.

 

flat top 6cc pistons...
Built the motor myself the compression ratio is roughly 9.4.....
I think i'll mill the head down enough to yield a 58cc chamber combined with a .028 head gasket should give me close to 10.3 compression

 

WTH?
Look, stock deck is 9.025"
Half the stroke, plus the rods and pistons, is 9.000"
So use a 0.015" gasket for best quench, add a flat top piston with valve reliefs, and a 64cc head puts you around 10.3:1
There are plenty of good online calculators for static compression.
But better yet, there are also online calculators for figuring dynamic compression. Some of them even account for altitude.

 

I am using a static compression calculator...

 

Sounds like you have target value for your SCR. Use that value and work out your DCR based on the two cams you've spec'd. Pick the one that yields the most favourable DCR.
10:1 means squat if you have a cam that bleeds off compression pressure because of a late intake valve closing. The same works in reverse. 10:1 will be undrivable on 91 octane if the cam has small duration numbers.

 

Did the math...
sorry ronnjonn had the deck height clearance wrong

So these are the numbers with the head milled to 58cc and .028 head gasket

Small cam(what its running now) SCR= 10.7:1 DCR= 8.39:1

Bigger cam SCR= 10.29:1 DCR= 8.18:1

 

Why is your SCR different when comparing cams?

 

Yes.
The DCR is a better tool than SCR. But there's an even better tool: cranking compression. 200 psi is a good practical upper limit for pump premium. DCR can help you get close, but once the engine is assembled, see what it cranks.
Then you can change gaskets, or move the cam, or whatever, to get right to the goal. If you find 215, then retard the cam, or be prepared to need premium plus octane booster sometimes. If you find 185, then advance the cam, or tighten the quench to 0.035", or both. Or be glad to not need premium gas.

 

ooops typing foul both SCR are 10.7

 

I agree on the 200 psi cranking pressure (at sea level) for a reasonable target to shoot for. At 205 psi, I was still able to run 94 octane without any hint of detonation (keeping my temps in check). Like I said before though, that's hard on starters!

 

Thought so.
At or around 8.0:1 DCR I've found to be quite streetable with pump gas. There is little margin for error though. Not being able to get decent premium or having a cooling issue (and I can attest to both situations) leaves you vulnerable to engine damage. I can't speak for getting up to 8.5:1.
I've heard it's a smart thing to cc the Vortec heads as they're notorious for being off on advertised chamber volume. If you do get them milled, you will want to double check anyway. At least you have the option of different head gaskets should you find you've gone too far (at the expense of optimum quench).

 

Now that I think about it, I seem to recall reading that a stock ZZ4 was found to be at 205 psi.

 

With 10:1 static compression and the little cam (208 degrees @ .050" intake), I wouldn't be surprised.