Would 12to 1 be o.k. on pump gas in an LT1....I can get 93

Pulled my engine out and would like to get more out of it.....Planning on more comp. and new cam

 

From what I have learned here: Depends mostly on the cam intake valve closing point. Search DCR (Dynamic Compression raitio) for more info. Also affected by altitude, air temperature, coolant temperature, and other factors related to air density and heat transfer in the heads.

Others who know more can tell you if a particular cam will be ok with it but reading up on DCR is a good starting point.

 

i'm with this guy, 12:1 may be low even with the larger cams

 

well I was going to get a new custom grind.....I want to wait til my heads are done, I just didn't know if I could go that hi and still run pump gas, I can't afford 100 unleaded or race gas for a street car.

I need the car to be able to travel 35 miles.

So with 12 to 1 and a custom cam I will be good to go? I have searched on the DCR... But I have no Idea how far I would be able to push it.......I will read more on it and try to talk to a few people.

 

You can definately go 12:1 on pump gas, even with a relatively small cam (early IVC). So yes, with that compression there will definately be cams that work and are streetable. I think I am going to be near the high end at 12.5:1 with an LE3 cam in the works.

Are you planning to spec your own cam or have somebody do so for you? IMHO the extra $100 or so to have one professionally designed to your needs is SO worth it unless you have tons of experience or like trying a bunch of combinations.

I think it's generally recommended to pick the cam first, then set up the CR to match, but there are only so many piston volumes to select from, so I worked it the other way. Smallest cam to support 383 with Mahle flattops and 55cc chambers. Or, rather, Bret spec'd the cam for my combo. I just gave the details and paypall $. Nice how that works. He's even trading it out for a different grind at no extra cost since I changed my combo midway, and the cam was still new in box. Awesome. (purchased with LE package).

 

I'm not going to spec it out....Brett is going to do that

I would like to see 430 from a hyd. cam and 3.48 stroke.....and run on pump gas.

I'm sure it can be done but we will see what happens

 

well there ya go, brett is the guy to talk to, he will be able to reccomend a static compression based off of the cam he selects for you.

 

12:1 is not a problem on most of these motors Mike.

 

Many different ways to skin the cat here IMO.

I am in the same process and pistons will be one of the last items I buy. My heads are getting machined right now and after that you can determine combustion chamber volume, and then decide on a head gasket to get the optimal quench, also the deck height will figure into all of this aswell so might want to have the blocked cleaned up before piston selection to determine the correct compression height.

Determine your plans with the car and what cam you are going to run and set the DCR accordingly and then determine the piston needed.

Kinda lenghty when written out but you should wind up with the correct piston this way.

 

I quess I'm going to date myself again, but at one time this would have been faily high compression, requiring leaded high test.
So, is this good across the DA range or would we be seeing problems at higher levels? Also are we completely outside "emissions friendly" here?

 

I would like to know that also.....12:1 in most peoples eyes is race only fuel.....

emmissions is no concern to me, I would like it to be able to drive....maybe 30 miles b/c I would like to get it into some street classes......other than that its not much of a street car, just DOT tires and mufflers

 

The LT1 has reverse flow coolant (cooler combustion chambers) along with aluminum heads and a decent chamber when contoured a little bit. This along with a good cooling system can take alot of compression compared to a gen 1 with iron heads. Also, with a late IVC from a bigger cam your DCR can be pulled into the mid to low 8s which will be pretty safe on 93 octane.

12.5:1 on pump gas is doable, you just need to get alot of things right and working with Bret will get you there.

If E-85 is available around you, consider converting over since it has an effective 105 octane with the added benefits of Ethanol, along with it being considerably cheaper than 93 octane premium gasoline. With E-85 people are pushing 13.5+:1 in the right setups.

 

People are hung up on static compression ratio.....the engine actually SEE'S dynamic compression ratio...lets' just say I am north of 12.0 to 1 static on pumpgas.


9.0 to 1 dynamic compression is the "general" recognized limits on pump gas.



David

 

where is the limit going to be on a stock casting head without going to dome pistons and getting really exotic? I would like to get as much as I can out of it.....without getting into lots of one off parts, I'm not wanting to dump a ton of money in this car. It has too much in it now

 

As long as you keep the quench "around" .035 to .040, you can do that with a flat top and a 0 deck and a .039 Fel Pro gasket, Cometic makes a .035 compressed with a 4.060 bore. You just gotta call some cam manufacturers and get some cams on paper and play with a dynamic compression ratio calculator. When you call them get the intake valve closing event on said cam.

8.5 to 9.0 to 1 dynamic is "ideal".


David