I heard somewhere that higher compression gives a little better MPG. Is this true?
So here's my proposed plan. I'd appreciate all input!
Super Ram intake, 383 stroker crank, stock bore (to raise compression) with new pistons and rods of course, cc305 cam, AFR 190cc heads, on an LT1 with T56.
Opinions please.....sounds like a good plan? I'm looking for around 400HP and 15-17 city, and 20-25 highway.

This will be going in my daily driver '88 Trans Am, so MPG and streetability is a concern!

 

don't forget though...the higher the compression, the higher the octane you have to run
i guess you have an lt1 in your t/a, right? so i guess you're already using 93
just wanted to let you know if you're raising compression, you gotta get more octane
but, i'm figuring you already knew that

 

Well, I'm planning the mods for the LT1. I'll be installing it into my '88 T/A in a few months. So, if I run higher compression I'll need to run higher octane. So the MPG difference would probably be ofset by the cost of 93+ octane huh?
So would a S'charger be the better route for MPG and power rather than a stroker?

 

Static compression makes little difference in an engine. You need to pay attention to your dynamic compression. That is what will determine the required octane. Adding a supercharger will increase your dynamic compression, as will camming choice, heads, pistons etc

 

Using a standard bore will not raise your compression. Almost all piston companies compensate for this when they make overbore size pistons. So they stock piston will create the same compression ratio as a .030 over piston. So in order to get more compression you will have to mill the heads or deck the block. And back to your question, Raising a compression ratio will more than likely retain the same gas mileage. If any difference it may be worse. But I seriously doubt you woould notice a difference.

 

I was unclear about the heads, i meant chamber size, not the ports themselves.

As for SC affecting dynamic compression? You will be under pressure, so that volume from your intake close point will be greater in the SC engine than in the NA, will it not?

 

A supercharger works by forcing a greater volume of air into the combuston chamber. Compression ratio is the same whether the engine sucks in 100 cubic feet of air or is force fed 1000 cubic feet of air. A supercharger allows for greater volumetric efficiency because it fills the chambers better than what the engine could do by simply sucking in air. A supercharger or increased compression will not increase gas mileage because you must use more gas to keep the engine at stoich (more air requires more gas) and to keep the motor from detonating you will need higher octane fuel. An LT1 can be made to run at 11.5:1 or even 12:1 compression because of it's reverse flow coolant system and aluminum heads, but it requires you to select the proper cam and have it tuned to the hilt.

Almost forgot, boost pressure is not a reading of how much air is being forced into the cylinders, it is a resistance reading of how much air is being forced inside the intake manifold.

 

Dynamic compression is what you end up with in the cylinders when the spark ignites, with a supercharger you end up with more FILL, so therefore; your dynamic compression changes with a supercharger, I don't think dynamic compression is a measurement of before & after affects in the event, sure a cam is the only thing that changes that but you increase dynamic compression with:

Cam
Blower
efficiency of the heads depending on port velocity/valve job etc...

Static compression is also plays a role in dynamic compression as you will still have more pressure in the cylinder with more static compression, & thats all dynamic compression is, its what your engine builds for pressure in the cylinders, everything is pretty much accounted for there.


The theory of dynamic compression being the ratio of what you start off with & what you end up with isn't dynamic compression at all, thats called efficiency, a measurement of how much pressure was lost, yet anything listed will still give you a higher dynamic compression resultant.

Thats my understanding.

 

All else being equal, a larger bore will increase the CR.

Yes, in general, higher CR will increase efficiency and hence usually increase gas mileage. But the limit on static CR is determined (as has been posted already) by the cam choice. This, in turn, establishes the dynamic CR. For an aluminum headed OHV small block, the DCR should not exceed ~8-8.5:1 if you expoect it to run on pump gas.

Of course, the whole thing is a lot more complicated. Cam choice will also effect gear choice, and a lower (higher numerical) gear will tend to decrease mileage. A stroker LT1 4th gen optimized for gas mileage is kind of any oxymoron anyway. Why bother?

Rich Krause

 

Thanks for all the usefull info guys! I'd be interested in some of your combo's and what kind of MPG you're getting. Basically I'm looking for as much power as I can get without affecting gas mileage too much. THanks,

 

I get 200 miles to a tank city driving through traffic work & back, mods are in the SIG.

 

Well, I'm getting a T56 in a few weeks, and I'm going to put it in with a stock LT1 first. After I get everything running right, I'll start modding...so I have time to plan out my engine setup. I was thinking Lloyd Elliot heads/cam package, along with a 383 assembly, and possibly a Super Ram. Would anyone hazard a guess as to apporximate HP TQ and MPG?