94vetteboy

Finally have enough money to rebuild my LT1. I keep seeing different setups that one can use. Thinking about a stroker 383 or a 409, and am just wondering if 91 octane will work with a 11.3:1 compression. I know that once you start hitting the 12.5:1 you start needing a 110 octane racing fuel. So, just wondering if it will work and if any of you guys are running around an 11.3:1 on pump gas.


I was thinking about going with a motor that I saw in the magazine "High Tech Performance."
Block Bored .1 over
callies 3.875" crank
Manley 4340 5.85" rods
ross 11.2:1 pistons
Porting the heads, 2/1.56" valves
TPIS Miniram intake, 58mm throttle body, 30lb. injectors
409 cam 225/225 at .050 .525/.525 lift ,112deg. sep

REDC4CORVETTE

Depending on the end result ,you need to find out what cc the heads will be and the end compresson.
I started with 12 to 1 but with the AFR's it turned out to be 10.9 to 1.

CFI-EFI

Engine knock (detonation) is a function of the octane of the gasoline and the cylinder pressure. Besides the compression ratio, the point that the intake valve closes is a major factor. Your cam isn't wild enough to get away with using ultra high compression ratios. Aluminum heads dissipate heat faster than cast iron, which helps to keep the pressure and detonation down. You should be fine with your 11.2-11.3 target. Keeping a tight quench area goes a long way toward suppressing knock under any given set of conditions.
Good luck getting a production block to accept a one hundredth thousandths over bore.

RACE ON!!!

JCAIRE2

Octane also depends on your elevation. If you plan on using the car only in CO then you could have a higher compression and run pump gas than if you were down at sea level.

94vetteboy

Thanks for the help. I'm hoping that my block can handle the .1 bore. If not I am going to go with the .060 bore 383 setup that lingefelter recromends in his book on modifying small block chevy's.

C4Ownr24

wait, how on earth are you going to get away with a .1 bore? that's a tenth of an inch. At that point you'd have completely sacrificed integrity in the cylinder. Overboring is somewhat overrated anyways. A .60 bore is sufficient, and honestly I suggest you go with a .30 or .40 to leave yourself room for a future rebuild. Just my 2c.

94vetteboy

I've heard of people running a .1 bore depending on their block and assuming the machinest is capable. And it seems to me that it is about the only way you can get around 425-450rwhp without laughing gas.

olman

Nathan Plemons

Remeber the LT1 has reverse flow cooling which seriously helps with detonation problems. I've seen many high performance LT1 builds that run 12.5:1 on pump gas. Granted, pump gas around here is 93 octane.

Regardless though the static compression ratio only tells part of the story. Your camshaft will have a lot to do with detonation. 12.5:1 on a stock cam and it goes boom. 12.5:1 with an aggressive cam and you'll be just fine.

BTW, you mentioned the MiniRam intake. That's a TPI intake and actually makes less power than the LT1 intake. It's a step in the wrong direction.

LT4BUD

[QUOTE=Nathan Plemons]Regardless though the static compression ratio only tells part of the story. Your camshaft will have a lot to do with detonation. 12.5:1 on a stock cam and it goes boom. 12.5:1 with an aggressive cam and you'll be just fine.
QUOTE]

You know I have heard this, but I don't really understand it though.....

It seems that the idea of an aggressive cam is to get more air into the cylinder, which should lead to more pressure on the compression stroke........?????????

Maybe at some rpm, probably low, the cylinder pressure will be less than at higher rpm, but should ultimately be higher with hotter cam????

Also cylinder pressure will be function of throttle position as well as rpm

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89'Bowtie

My LT1 is running a static compression of 11.1 to 1 with no problems on 91 Octane. As a matter of fact, the car's performance noticably goes down when I've put 94 Octane in the past.

Remember, you want to run the lowest octane possible without preignition.

What does it all boil down to? I'm sure with the correct tuning and pump gas you can run the car happily in the 11-12 to 1 compression range.

JUAN J SANCHEZ

Camshaft- bigger cam relieves cylinder pressure so it can get away with more comp, quench- go with "0" deck height and a .050-.040" head to piston clearance (this may vary with bigger bores and piston to wall clearances). If you can go to a flat top with a small chamber to achieve desired comp its better than a large chamber with a dome piston (flame travel). It's tough to figure out cranking cylinder pressure before assembling the engine but once it's all assembled you'll want about 180-190psi anyting below 180 will give you less than optimum and 200psi will require sharp tuning. I agree 11.3:1 should be fine!

FD2BLK

Exactly correct Dynamic CR is what will determine detonation characteistics.
Here is a calulator for DCR
http://www.kb-silvolite.com/calc.php...4e446cacce7bd5

Here is a great explination of how a cam actualy affects detonation
http://www.davemiranda.com/dmhotrods.htm

94vetteboy

Thanks for all the info fellas, I know fell safe running the compression that I am looking at. Now it is all up to deciding on if my block can handle the .1 bore. If not I know that another person on this forum is running a 409ci by going with a .060 bore and using a 4" stroke instead of a 3.875" stroke.

scorp508

Going by the mathematical way 11.111111111:1 is supposed to be the limit, but environmental factors obviously can change that either way.

cv67

IF your block could be bored over that much, it wouldn't be worth the risk of losing ring seal. I'd be concerned about paper thin walls flexing, especially on a stroker. I'd go with a 30 or 40 over, or go aftermarket block and huge cubes.Remember, the rod angle on most strokers has a tendency over time to "egg shape" the cylinder from the rod trying to"push" the piston through the wall.

CFI-EFI

A very large percentage of late model blocks are not able to safely take a .060" bore. A .100" over block would be rare indeed. Regardless of the overbore you decide on, the money to have the block sonic tested would be well spent.

A typical "383" is a .030" bore with a 3.75" stroke. With a 4.060" bore you end up with a 388.

RACE ON!!!

siggy_freud

IMO the power gain from that sort of bore isn't worth compromising the engines integrity.

You are also going to have the factor in the new bore into your final compression. More bore into the same CC cylinderhead is going to bump that up.

No Go

I have 11.9 to 1 measured CR in my 381 CU IN LT4 engine. (.020 overbore with 3.75 stroke)

I also run the GM 847 camshaft that has 296/304 duration and .575/.595 lift intake and exhaust repectively.

I run the highest octane I can fine...92 around here and 91 on the road at times. No issues for last 14,000 miles of very hard running.

If you are going whole hog to a 409, then you are leaving power on the table by not pushing the limits-bigger cam and more compression otherwise just build a bread and butter 383 and be done with it.

Also, my cranking compression averaged 227 PSI with a high of 232 on one cylinder...still returns 26 mpg on the road.

JUAN J SANCHEZ

Good info FD2BLK!