Does anybody know what compression rate to build my engine when I will have 15psi max boost and running pump gas 95 or 98 octane?
I will also use the MSD module that sets the timing back at boost.
I don't want to loose to much compression as the car will be street driven and will be in NA mode or low boost for the most of the day.
Anybody knows???????

 

the stock 9:1 will be fine. IF your building a bottom end I would go with an 8:1 or a 8.5

 

The more compression you run the less boost you will need to run for your power target. Octane has to do with head design, Air intake temps, compression and boost.

 

I would just run stock compression if I were you.

 

how the hell do you get 95-98 octane "pump gas" thats not typical pump gas... i understand few places run it b/c we got one here in town pumping 110 but you gotta sign waivers and such saying you use it on the street. but i dont consider that pump gas....

 

In the end, on street fuel, talking 91 octane, will you get more power by lowering your compression and being able to run more boost?
Don't care about lag, just the above question.

 

Ok, so run 14:1 compression and 2 pounds of boost. See where this logic is going? Less compression + most boost makes more power, especially if you're running pump gas.

 

He is getting 95/98 at the pump b/c he is in Europe. However, he is probably paying ~$6 a gallon for it.

At least that was the price when I was in Belgium this summer...

 

14-1 compression with 2 lbs of boost would make a hell of a lot of power..

You ever wonder what kind of cylinder pressure Alcohol Funny Cars must be making... Seeing how they run like 30 lbs of boost on 12-1 compression motors.. WOW...

 

The top fuel guys run 6 to 7:1

 

From what I've read..

You guys are going the wrong direction with this. Horsepower with a given displacement is about getting more air fuel in there to burn. You want the minimum compression and maximum boost you can get away with and still have decent off boost response. Yes, higher compression gives you more volumetric efficiency, but you're talking like a few percentage points VE dropping from 9:1 to 8:1. How do you think that compares with the horsepower increase from the boost you can run at 8:1 vs. the boost you can run at 9:1?

For a real world example look at the buick guys running 15-20 psi @ 7.5:1 compression, some of them would smoke your 5.0's w/ their 3.8L V-6's!

 

:withstu:

 

Re: From what I've read..

If you're running pump gas, low compression makes more sense.. You're trying to get the most out of only 91 or 93 octane.. But when race gas is in the formula, things change.. I read the spec's on one of the top Pro-5.0 cars, and he says it has 10-1 compression..

And those top fuel cars that run 7-1 compression, you need to remember.. Naturally asperated fuel cars run over 270 mph in the quarter.. Faster than those 12-1 compression, 30 pound of boost, blown alcohol dragsters.. My point is, nitro make "un-godly" cylinder pressure by it's self..

 

His 95 or 98 octane is most likely MON, not our U.S. standard of RON+MON/2. His 98 MON is equal to our 93 (R+M)/2 stuff. Not that this changes anything in this discussion, other than build for lower compression

 

I thought top fuel was running about 5:1 compression and about 100lbs of boost

Thats what I read in a magazine a couple months ago.:shrug:

 

that's quite possible, I found that info on a web site the other day. Who know's how old it was.

 

Bottom line, if you have a stock block, then 9:1 compression is about ideal for pump gas, because the block is the limit. If you have an aftermarket block, rods, and pistons then you might consider 8:1. I have run 20psi on pump gas with 8.25:1 compression and water injection. Imagine the power you can make with 20psi on pump gas.

 

Pump gas overhere is 95 or 98 RON octane and yes is costs about $6,20 a gallon!
In Germany they run 91 RON but in Holland 95 is the lowest what you can buy.

 

Race gas and octane

Well folks, I just had to register and reply to this whole thread...it doesn't look like any of you folks have ever done any serious research on race gas.
The octane number is merely a relative number that indicates how hard it is to actually get the fuel to ignite. Ignition occurs because of heat due to compression and or an ignition source.
Too much compression (ie heat) and you auto ignite...detonation.

Go read VP fuels excellent website on fuels and recommendations. They break it down very simply...x-amount of boost and up to x-points of compression.

I have tuned and helped tune numerous Hondas that take 15 lbs. of boost on a 10-1 motor on 110 octane...we routinely get 550 plus hp from 2 liter hondas on 110 or less octane.

The simple mathmatical formula is to determine how much the boost pressure (actually "air density") increases the compression and then just go by this new "computed" compression ratio.

example: 15lbs boost @ 70% effeciency = a new density of approximately 71% more AIR...add this to your original ratio...

9-1 yields 15.39-1 new compression ratio. (9 x 1.71)

you just choose a fuel that is rated for 16-1 compression (like c110) and DON"T mess with the timing.
If you cant get hi enough octane, then you start backing off timing to compensate.

its simple when you have all the facts.

 

Gee, I have to say...

did you really see it necessary to register just to post several irrelevant opinions?

DutchMustang said nothing about racing gasoline, nor did anyone else that I can see. Jeff mentioned nitro, which I fail to see the relevance or either to the original post.

There again, wrong octane and more importantly wrong bore. I've heard of much compression under boost with motorcycle engines too, but the one in question is a Ford Mustang automobile.
Smaller engines are less prone to detonation, this is not one of them.

I'm sorry to be an @ss toward you on your first post here, but I doubt many of your statements. Particularly that efffective compression is C.R. X density ratio. If that was the case you could not even run 8 psi @ 9:1 (on pump gas) because that would equate to 13:1 compression.
I've never even seen hondas run 13:1 on pump gas.