I was watching a rerun of "Trucks!" the other day and Stacey made the comment that having a supercharger on a higher compression motor would be pointless as they would cancel each other out.....WHY!?!?! I have thought and thought and thought about this and cannot figure on any logical reason why they would even affect each other. I understand that a blower packs in more air and high compression means the air gets compressed further, but why would these cancel each other out, if they even do? I'm thinking that he may have been mistaken, but I can't be positive because I've never built an engine with a supercharger. Theoretically more air is better and further compression is better, so why would this combo be bad?? Please help!!!
Scott

 

It'd work good, if you can afford the race gas.

 

When your engine moves the piston up just before the power stroke begins (ignition event), the pressure drastically increases inside the combustion chamber because the contents (air, fuel) stay the same, but the space its contained in, decreases based on your compression ratio (10:1, 8:1, etc).

When you supercharge (or turbocharge) an engine, your pumping in more air (and fuel) than naturally aspirated, THEN compressing it based on the the piston moving up, based on your compression ratio.

So with a supercharged engine, you have a might higher effective compression ratio than what the engine is mechanically set up for.

This extra pressure, especially when the ignition event occurs, drastically increases the downforce on the piston head, the wrist pin, the rod, and finally the crank, which converts the downward motion to a rotating motion.

To support the higher cylender pressures, you need more octane, in order to avoid having the gasoline ignite before oyu want it to, which would create an opposing force against the piston head while the crank is trying to push the piston up.

Thats when things break.

 

But, would still theoretically produce more power- correct????
Scott

 

It would if you could keep from detonating a piston it would produce a lot more power. More likely you'd only be able to run a small amount of boost, gains of which could probably be obtained more effectively through a cam/header combo. Unless you dropped the compression or intercooled/aftercooled the compressed air charge. You would need to run MINIMUM 93 octane, more likely race gas or combine with alky/water/propane injection to keep knock down.

 

Shoot, I run only 9 psi and 93 is a stretch.

 

I think What Frederic and Stacey are trying to point out is that you would have to retard the timing so much under boost to prevent detonation that you would not be gaining any significant amount of HP and still have the possibility of damage.

 

Exactly...High discharge temps can be a blown motor's worst enemy. Intercoolers, aftercoolers, and alky injection work very well in lowering these temps and will allow you to run more timing.

 

A small shot of nitrous (not enough to affect HP) helps cool things down very well too. I believe I saw a system called the N-tercooler a while back.

The only drawback I can see is having to keep the bottle full, and avoiding the temptation to use it to affect HP.

 

Yeah, boy that would cost alot to run everytime you put the hammer down. Seems an intercooler might be cheaper.

 

Well it's all about the amount injected. Whereas with using Nitrous for power gains uses quite a bit, when using it for the cooling effect you inject very little, and it lasts a lot longer. You can use CO2 also. See here.

I would use it in addition to a conventional intercooler, after the intercooler, to make it even cooler allowing you to turn up the boost at the track.