Still learning the SC stuff. I'm trying to learn the theory of boost pressure vs compression ratio. Now, provided an engine is properly beefed up with forged internals, etc., which will provide more HP at the dyno...

A. An engine with 9:1 compression ratio and higher boost (say 8 pounds?)

B. An engine with 10.5:1 compression ratio with lower boost (4-6 pounds).

Is there a formula that one can use to calculate the HP for a blower on an engine?

 

Higher boost will put more O2 in the cylinders which allows you to burn more fuel to make power.
Higher compression will produce more cylinder pressure briefly at ignition, but the benifit diminishes.

The key of course is to balance the two for best power and detonation avoidance. An elementary way to think of boost is that it effectively makes the engine larger.

You need a decent compression ratio to maintain driveability and response at part throttle. Not to mention efficiency increases with compression and some emmisions drop.

An automatic car equipped with the right convertor can get by with less compression and more boost as the rpm is able to quickly jump into the boost zone assuming you have a turbo or centrifugal blower. Here its pretty straightforward, run less compression to allow you to turn up the boost (airflow).

A stick car really needs some static compression to be fun to drive at lower rpm, at least in my opinion.

But if you're just talking about a point of compression, say from 9.5 to 1 to 8.5 to 1 then don't go to great lengths to worry about either. If you're considering 10 to 1 or 8 to 1 or less then yes, there can be a great difference in power between them. There is always a tradeoff, especially when you're dealing with pump gas.

To confuse the issue even more, great power can be had from the 11 to 1 LS2 with 7-8 lbs of boost on pump gas. Even more is possible though if you were to drop the compression with some LQ9 heads and turn it up. It probably would no longer be a LEV certified vehicle though. I don't know for sure.

 

[QUOTE=markinkc69z] It probably would no longer be a LEV certified vehicle ...QUOTE]

whats a lev certified vehicle

 

Low Emmisions Vehicle. Something the LS1 has managed to accomplish, and make 400hp. Try that with a Honda! (naturally aspirated)

 

 

High compression will always yield more power than low compression, regardless of boost. The sole benefit of low compression is detonation avoidance. One of the magazines just did a test on a 400+ cid supercharged Ford with 8.5/1 compression, then switched to 11/1. At 1100+ hp, it gained something like 150 hp by raising the compression alone (on C16 fuel, of course).

 

Mike: that is not strictly true for forced induction. For a given engine, to achieve a lower CR, the combustion space must increase. Increasing the combustion space, on a forced induction engine, results in a combustion chrage with more mass and hence more power. What you have to keep in mind is that while with an NA motor, where a lrger combustion space does not mean a larger fuel air charge, if the intake tract is contantly under positive pressure, the intake charge will be larger (have more mass).

In fact, if you consider VE for a SC combo with a larger combustion space, you can see that it will increase compared to a smaller area over the top of the piston (combustion space) but it stays the same for an NA setup.

Sorry if my explanation sucks, I just woke up.

Rich

 

Rich, that's exactly what I used to think, until I read the dyno test and had to revise my theory. My new theory has to do with compression and expansion ratio's and goes something like this:

A naturally aspirated engine has roughly the same compression and expansion ratio. Assuming a single pattern cam installed at "0", the [dynamic] compression ratio will be around 8/1 as will the expansion ratio during the power stroke. When you lower the compression and supercharge the engine, the dynamic compression ratio becomes around 12/1 (2/1 in the supercharger then 6/1 in the cylinder). However, the expansin ratio is actually less than a NA motor at only 6/1, so you didn't expand the hot, high pressure gasses as much. Install a dual pattern "blower cam" and the expansion ratio drops even more and the situation becomes even worse. Alot of energy is lost out the exhaust pipe. If you could keep the expansion ratio at 8/1, you wouldn't lose so much energy, but hat would require running a higher 8/1 dynamic compression ratio also.

A turbocharger partially solves this problem because it expands the exhaust gas another 2 or 3/1 in the turbine and recovers some of this energy.

 

Interesting theories guys..anyone else?