I am debating on making a supercharger my next mod. I was noticing all the blower makers say to install on your "stock" engine. I was assuming that means stock compression. If so, what would be too much compression? Stock my car is 9.75:1, and recently I shaved the heads bringing it up to 10.25:1. Anyone that would like to share some of their wisdom, please dont hesitate. Thanks.:hail:

well, it depends, you will see more gains on a high compression engine, with boost, however, you will run into the need for high(!) octane gas, very very quickly.

say you have 8:1 compression, you could run quite possibly 12psi, given specific cam/combustion chamber...

same engine, with similar burn characteristics, only 10:1 compression, would allow you to only be able to run 4/5 psi, on pump gas.

both combinations will make excellent power, however out of boost, the higher compression engine will be more powerful, and the lower compression engine will be more pump gas tolerant,

basically it depends to ME on what blower you are looking at, if it is a relatively small blower, i would look into slightly higher than recommended compression, maybe 9:1, because a small blower is only going to push so much air/make so much boost, so if you put a small blower onto a very low compression engine, you will have to spin the blower VERY high RPM's to make much boost/power. this is hard on the blower, and will lead to early failure.


adam

this is actually something that i have been wondering too. i am planning on building a boosted small block. i would like to run a d-3 procharger and run about 25lbs. would a set of 8:1 pistons be too much? would i have to run racing fuel?

you couldnt run 25 lbs on anything that i can think of without race gas.


adam

I recommend you contact the supercharger manufacturers and ask them what they recommend for your setup (higher compression). They might have already done some R&D on using their supercharger on higher compression engines. Usually when you build a motor for F/I you lower the compression and run more boost. The route you went (raising compression) is usually used when building a high output N/A motor. Not to say its a bad idea to install a blower on your motor, but because of the situation you're in you need to make sure you dont run too much boost and damage the engine. I would say its safe to run 6 psi (pretty common setup) on your motor if you run 92 octane gas, but its just a guess. I'm sure you want to be certain you're not pushing it so thats why I recommend you contact the supercharger manufacturer.

My car had 9.5 to 1 and I was running 11#s on pump gas and never had any problems with mine.
made 560rwhp!

Your air charge temps (into engine) are most important I think, not including head design/material and tuning. 9.75:1 and maybe 4-6lbs non-intercooled or ~10-12 lbs on a good intercooled engine with 93 octane. A good intercooler will allow you to run a lot more boost before detnation.

Not quite, there are 2 basic problems with this reasoning
- Usually, modifications that raise compression increase airflow, using a compressor which is just right to barely enough for the stock setup, if the engine is capable of more airflow the compressor will not be able to keep up.
- Second, most centrifugal compressors (most of the superchargers and turbochargers people run) get more efficient at higher boost pressures, so a more restrictive engine which will see a higher boost pressure when you’re forcing the same amount of air into it can get away with a smaller blower. In simple terms, assuming that you’re moving the same amount of air, it takes a larger blower to move it at a lower pressure

Remember, airflow = hp, boost = restriction and does not = hp.

well, i am going to use a d-3 procharger, which is ati's 3rd largest blower. so i don't want to use a very high compression motor. i was thinking of 7.5/8:1.

low compression/high boost will always make more power then the other way around, assuming you keep things otherwise equal.

only 11's?

with that much hp, i'd think you'd be seeing 9's.

my first SC car was a 5.0L with 10:1 compression and 8 lbs of boost. it went like a **** throughout the rpm range. Then, I thought I would be smart and lower the compression(i went 8.5:1) and raised the boost to about 18 lbs.... Not a very good car for the street, but in the upper rpm range, it was great. depends how much boost you wanna run and what you plan on doing with the car...... for the street car that it was, I preferred the first motor with less boost.

Here is a handy reference to go by, this is for fuel injected cars keep in mind, carb cars can take a bit more boost according to ATI.

I wish I could do 9's
but I am at 4500 feet elevation and still have basically stock suspension only have a torque arm so far.

Why is it that you can run more boost with carb than FI? Is that referring to direct port, or does that also apply to TB injection? Considering the similarities of carb and TBI is the reason I ask.

Another point to remember. An engine with more static compression will be more responsive prior to building boost. If it is a street driven vehicle, this should be a primary factor.

I would think going with more static comp and less boost would be a more reliable setup that is easier on parts.

At what point does this not apply? I've always had a hard time with this chart..In theory you can run 30lbs at around 7.1:1 on pump gas? Intercooled of course, but this just seems a bit nutty..At what point does it become in practical..I'm all about 20lbs+ on the street

ATI's website says that carbs can take more boost because carbs deliver the majority of their fuel still in liquid state, and that it acts like a mini intercooler, or something to that effect. I am not sure about throttle bodies honestly.