[generationGT]

What's the best compression for high hp applications' i.e. turbo's and s/c's and nitrous. With my engine build/ purchase i want an engine that will eventually be capable of upwards around 600+ hp. What's the best compression for an engine such as this?

I've seen crate engines with 9:1 compression and i am wondering if this is good enough for the beast i want to build, and what exactly is compression anyway, i understand the word "compression" but how does it relate to a power adder application? Thanks.
G

[systronic]

im probably not the BEST person to explain this, but i think i understand it properly.

compression ratios are a comparison of the volume occupied by your air/fuel mixture in each cylinder at both ends of the rotating cycle.

i.e. if you have a 9:1 motor, when your fuel and air first enter the cylinder, the volume occupied is 9 times the volume of the mixture when it is compressed by the piston before firing.

i think i explained that right...

im not much into the power adding business (yet), but i know that higher compression motors (10:1) are not good for running any type of boost.


perhaps someone else could leave some info as well, maybe as to how safe it is to run boost on a 9:1 motor, or 10:1 etc.

[GTRaptor]

That was a good explanation for compression, now, a SC or Turbo will INCREASE that compression further, they will compress the air and feed it into the combustion chamber.

So you end up having more compression !

That is why most will recomend a low compression ratio where there is a ton of boost. Typically we are talking about less than 9:1 depending on boost and other factors such as fuel, altitude, timing.

You can achieve 600 WHP without using too much boost and 9:1 to 9.5:1 compression, but you have to build a very strong engine.

[albroswift]

You don't have more compression with boost, the compression ratio remains the same. What you do have is more pressure, as you are compressing a larger volume of fuel/air mix with the same compression. Boost is better then more compression, as no matter how high your comp ratio is, you are only compressing the displacment volume of the cylinder. with boost, you are starting the compression stroke with more fuel/air mix, so at equal pressures, you have more fuel air in a lower compression boosted engine then a higher compression N/A engine.
Sample math Sample motor:

[GTRaptor]

This is from Motorsports digest:

All motors have a static compression ratio. This is the amount that the air inside the cylinder is compressed. It is a ratio of the cylinder volume at BDC to the volume at TDC. When a supercharger is added, additional air is forced into the cylinder effectively raising the compression ratio. The result of this is called effective compression. The formula for finding the effective compression is very easy:

((boost psi / 14.7) + 1) x motor compression = effective compression.

The effective compression allows a supercharged motor to be compared to a normally aspirated motor. For the most part, a supercharged motor with the same effective compression as a (similar) normally aspirated motor with the same static compression should have about the same overall power.

[GTRaptor]

If you use that formula:

9.5:1
((10psi / 14.7) +1) x 9.5= 15.96 effective compression
8:1
((10psi / 14.7) +1) x 8= 13.44

Now lets move boost 9.5:1 and 8psi
((8psi / 14.7) +1) x 9.5 = 14.67

Thats why you need to reduce timing and increase octane.

[albroswift]

"effectively" being the key word. The compression ratio does not change in a boosted state, but manifold pressure is increased, resulting in a denser F/A charge
at ignition. The best part (of a boosted setup) is that fuel detonation is a function of the final actual compression of the F/A, not volume, so with a boosted engine you can burn a larger volume of fuel at the same compression (effective compression ratio) as a higher compression ratio engine, octane and timing being equal. Big block feel out of your small block!

[Redrocket9.0]

From what I understand from all the reading I've done on the subject, the main limiting factor of compresion in any given motor is heat. As the air/fuel mix is compressed, it heats up. The compression in the cylinder will MULTIPLY the charge temp by a given rate, not just add certain amount of heat. So the temp of the compressed mix will directly reflect the intake charge temp, and since a power adder such as a blower compresses and heats the intake charge, you have to lower the compression of the motor to compensate or you'll end up with so much heat in the charge that it will detonate. thats why so many high perfomance turbo
and supercharged cars have intercoolers, its not because cooler air is denser and adds power, it's because with the cooler intake air, you can run more boost without detonation.
If you really want to get into the numbers of it all, go to the automotive section of Barnes and Nobel and buy the book call "Superchaged". It's an awsome book, but it's to much for me to memorize so I cant get into the math like that does.

NEway, to answer your question, the most common compression ratio of most boosted cars I've seen making from 400-900 Hp is between 8.3 and 8.7:1 depending on the octane rating of the fuel. If you go to turbomustangs.com you might find some more help.

I hope thats of some help, and I hope I didn't just invent a whole buttload of crap thats totally wrong, cuz I see that happen now and then. Good luck!

[albroswift]

All things being equal, the same cylinder pressure achieved by blower or high C/R will result in the same F/A temp at ignition. Modifying factors being intake air charge cooled by an intercooler, heated due to compressor or centrifuge inefficiency, ambient intake air temperature, etc. etc. Again, all other things equal, a boosted lower compression engine will be able to run more F/A mix at the same ignition pressure, temperature, timing, and octane as a N/A high compression engine. More F/A = more HP (And yes, I've been known to invent B/L's of crap also!)
PS My vote was for Kenne Bell, although I see a lot of HP ratings from the centrifuge type blower guys that would most likely kick my butt.

[systronic]

so my big question: is it safe to build a 9:1 motor and still add boost. say its a coast high performance street fighter gt 306 block, with a main girdle, probe forged flattop pistons, forged steel crank, moly rings, trickflow heads / intake / cam / rollers / rods / lifters..., etc... (my motor is in the works for this)

would something like 6psi be okay to add?
i would like to build a motor that is solid on its own, but someday i might get a bonus at work and decide to spend it on an S/C

also: does running nitrous have any type of effect on compression ratios?

[Redrocket9.0]

Absolutely. I see no problem with 6 psi on a 9:1 (stock) compression motor depending on how that boost is delivered. If that's intercooled boost, then you should be perfectly safe, and even if its not, you'll probably still be fine. I doubt you'll be making your desired 600+ ponies with that though. At 6lbs boost, you would need about 450 N/A hp to get around 600 boosted hp.
Nitrous makes the F/A mix burn faster and hotter, so it's more succeptable to detonation, I believe. so, yes.

That's my 2 cents, but I'm not a pro, so dont quote me.

Someone please tell me if anything i say is totally off the wall, cuz I'm learning about all this stuff, too. I'm very open to constructive criticism.

[systronic]

im not the one wanting 600 hp lol. i just want a fast, fun to drive mustang. im not looking to break any records or become an nmra champ or something. just love my mustang, and love the weekend track atmosphere.

if someone could help me figure how much compression my desired motor would be making, it would be appreciated. im not really sure on how to figure this stuff out. i know it depends on my pistons (ie. flattop, dished, or raised) but im unclear about how my head selection (cc's?) will effect this.

['95.0 stang]

Originally Posted by Redrocket9.0

Nitrous makes the F/A mix burn faster and hotter, so it's more succeptable to detonation, I believe. so, yes.

That's my 2 cents, but I'm not a pro, so dont quote me.

Someone please tell me if anything i say is totally off the wall, cuz I'm learning about all this stuff, too. I'm very open to constructive criticism.

Nitrous oxide is an oxidizer, it adds loads more oxygen to your mixture than is normally in the air and your car has to be tuned to add the appropriate amount of fuel. Also, the expanding gas cools the intake charge. People have problems with blowing engines with nitrous when the cars aren't properly tuned for the oxygen that they're adding and basically the fuel ratio goes out the window.

[Redrocket9.0]

Ah, see, there you go! Like I said, nobody's ever to good to learn a little more. I've never been a big NOS kinda guy and I have now been edgumicated.lol thanx!

to systronic, I think just about any mustang you build like that is going to be well into your goals of "fast, and fun to drive mustang". I personally think most stock stangs are just that