BC

http://www.motorsportsdigest.com/forced2.htm

Seeing as how the 928 has been found a few times in a row to be closer to 9.4 or so instead of 10:1, then we are perfectly situated.

ViribusUnits

The artical is BS.

mspiegle

I don't buy it. Compression points do not add as much power as they are claiming. You're getting your power by cramming more air and more fuel in there.

BC

I understand that Lag has repeatedly given us the math on the heat issues associated iwth compression, and that is a main issue. My point with linking this article was to give some perspective on what supercharging does to the compression ratio under boost.

I think that this article, while possibly a bit loose with facts, cannot be called BS off the cuff.

I Think Lag said each point is 4%?

ViribusUnits

I didn't call it BS off the cuff. I read the entire thing.

The author has limited, if any, understanding of pre-ignition, or detonation. No where in the article are the words "octane", or "resistance to detonation" used. No where was this used as a concept to limit the boost, or compression ratio.

The author has a limited or non existent understanding of charge density. No where is the concept of charge density mentioned, or used. Charge density is what makes a supercharger, or turbocharger so powerful.

No where is an after cooler or intercooler mentioned as a device to control detonation, or increase charge density, something it does very well.

The paragraph that states "8:1 with 18 lbs boost and 9.5:1 with 12 lbs boost) will have almost the same effective compression and about the same overall power" is BLATANTLY wrong. Ask any of the turbo folks.

Also the statement that "In my opinion, anything less than 8 lbs of boost is a waste of a supercharger." is eighter stupid, or ignorant. MANY kits are sold with 4-5 pounds of boost. These kits result in 30-40% increases in the available power of the engine. That is WELL worth it in my opinion.

Now for compression ratio's effects. One effect the CR has is that it increases the speed at which the fuel is burned. To a limit this is a good thing. The quicker the fuel is burned, the high the efficiency of the engine. Empirical testing indicates about a 3 to 4% improvement per point of CR. This is limited at the top by detonation.

However, it goes deeper than that. If you've ever tried to pick an aftermarket cam, you know that one of the factors taken into consideration is the static CR of the engine. A higher static CR engine can run a cam where the intake valve closes at a later time, AND still develops a high enough effective compression ratio to deliver enough power to remain derivable. This can make a big difference in the performance of the car, while keeping the car derivable. I hate to use effective CR after reading that article, but I don't really know how to say it otherwise. The effective compression ratio concept is an important concept, just not in the way he says.

BC

.....((((And in THIS cornah, we have the undesputed champion of backyard logic-bashing trashtalk))

Uh - Dood - I said he was loose with the facts. We all know that 18pis will give more power then 12psi in most conditions, irrelevant of CR (within certain specs).

Alot of religions are based on the bible, but they don't take it LITERALLY.

I mean, Gearing doesn't SPECIFICALLY turn HP into ftlbs. I mean we UNDERSTAND what you are saying, but its not an exact interpretation of what happens.

rob rossitto

my head hurts..

ViribusUnits

If that is the case, then what is the point of the article?

Lagavulin

Brendan, thanks for the link, that was a good article.

Everything he states is sound and accurate.

ViribusUnits

Lagavuin, are you trying to tell me that an engine running 12 psi boost and 9.5 CR, will make as much power as an engine running 18 psi and 8:1 CR?

Or that under 8 psi of boost isn't worth the trouble?

I don't buy it.

I will agree that much of what he says is accuret, BUT the conclusion he comes to isn't exactly correct in my opinion. IMHO 8:1 is low compression, or 9.5:1 is still fairly low. 12:1 or 14:1 is high. You need at least under 10.

Anyways.

SteveM928

I'm with ViribusUnits and mspiegle. There are some definite flaws in some of what the guy says in that article and in some of his ideas.

2V4V

Brendan, thanks for the link. Interesting, if annoyingly fast and loose with the years of experimental and racing data, reading. I do wonder who the author is. I know his name is not Whipple or Bell.

Lag,

Did we read the same article?

There is some truth to what he says, but he treats static compression as if it were the main power god in a supercharged app, which ignores all the other factors than make a blown app work.

Sure, on the same boost you get more power output with higher compression to a point. But the numbers he uses are soooo far off (at least according to a quick run of the formulae that seem to work in everyone's experience here) it's just hard to take any of it seriously. That and he takes none of the other really important variables into account. which makes it all the more fishy to me. But, maybe I'm missing something, it's gettin' late.


Greg

Lagavulin

I agree that his numbers are off, but I'm specualting that he threw those numbers out there to help make a point to those less informed. If you can listen to what he's saying and not fixate on those numbers, his message is fundementally correct.

Here is what I think is a good summation of his article (..from his article):

"Once again, the compression -vs- boost issue. For a car that will see the streets (actually for most applications), the best thing to do is start with a motor compression that is high enough to make the horsepower you want for normal driving. Don't rely on your supercharger to make all your horsepower. With a good motor compression, add as much boost as is safe for your particular application. Decide on a final effective compression, and work your way back through the formula to find your maximum boost level..."

That is *****-on. In other words, there is no need to run 8:1 cr when you're only going to run 12 psi of boost; instead, run 9.5:1, and NA operation will be much crisper/stronger as well as more power production when under boost, versus the sluggish NA 8:1 engine.

I think he covers that briefly here:

"With the proper fuel system and related engine components, an effective compression of 16:1 to 18:1 should be more than workable."

Remember, this isn't an article about intercooling, fuel systems, blower types, pulley ratios, etc, etc. He's focusing solely on the relationship between compression ratio and boost, that's it.

ViribusUnits

But you can't do that.

It's a ballanceing game. The better the intercooler, or the fuel grade, the higher a CR you can run for a specific level of boost. Or visa versa, better the intercooler and octane, the higher boost you can run for a specific CR. Also, it depends on the blower type. The higher the effency of the blower, the lower the charge temp, and the higher boost, or CR you can run.

Please tell me your not going to try to run a 18:1 CR with a non intercooled blower. That would be running 13 pis boost on a motor with a CR of 9:1. Everything I've read seen and heard indicates that will blow the motor. I've never had the unfortuante experence of seeing someone do it, but I really don't want to eighter.

The practical limit for the effective compression ratio for a non intercooled motor on 93 octane fuel is around 12:1 or 14:1.

For an intercooled motor on 93 octane fuel, on an intercooled engine, you might see 18:1, depending on the set up. But on an intercooled engine, I don't belive that effective CR is a valid concept. Basicly, it's not transferable between two engine. An engine with a CR of 8:1 and 18 psi of boost has a CR of 18:1, as does an engine with a 10:1 CR and 12 pisg boost both have an effective CR of 18:1. Assumeing all else is equal, an engine with the 10:1 static CR however has a MUCH higher peak combustion temp.

Or even better, certain engines with a static CR of 8:1 can run 21-26 psi of boost on 93 octane fuel. That works out to a CR of 19-22:1 For an engine with a 10:1 static CR, thats roughtly around 13-17. I'm sorry, but thats not going to happen.

The higher the peak combustion temp, the higher the chances of detonation.

Lagavulin

Have you stopped and considered for just one moment that just *maybe* you don't understand what the author is talking about? Your response suggests that you completely missed the point.