if i rebuilt my motor on my vortech g and used the 9 to 1 ratio for my pistons would that cause the boost to not reach its max of 7 to 8 lbs on the stock pulley. Because i did this and for some reason it was only pushing a max of 6 lbs and only putting out 311 whp where before the rebuild they had it tuned at 400whp then for safety to 375.

 

I don't know about the boost, but I doubt it.

However with a lower compression motor you will get less power than with the higher compression.

 

you mean b4 boost right?

 

No. I believe with the same amount of boost on a lower compression motor you'll make less power. No idea why he's making less boost though.

 

Shouldn't change the boost with a SC. Something else must be wrong.

 

they put too small of compression pistons in, they were 8.5 to 1 when they sould of been 10 to 1 i think so i was missing about 120 hp

 

who is your tuner? you a lil power when you go lower compression but can be compensated in the tune itself.

 

Power loss is minimal (few percentage) going 10.3 to 1 to 8.5 to 1 (from compression change alone).

 

^damn thats a small pic lol

 

^ It is for my monitor.

 

+1

I think there has been much misunderstanding with respect to compression ratio. For what it is worth, here is my perspective in order to shed some light on this subject.

At the most basic level torque, and hence horsepower, comes from the amount of air/fuel mixture that is in the cylinder when the air/fuel mixture is ignited. Now, there are several factors that can reduce that power, for example improper air/fuel ratio, low compression ratio, improper ignition timing, detonation, pre-ignition. (Side note, in previous posts I referred to detonation and pre-ignition as pre-detonation, but that was incorrect as they are two different events, even though they are somewhat related).

The graph posted by RCDash shows the percentage of power loss for various compression ratios. As the graph clearly shows, there is approximately 3.5% of power loss with a 10.5:1 compression ratio. At a 9:1 compression ratio, the power loss is approximately 5.5%. To put that in perspective, the difference for a 300 hp motor is approximately 6 hp.

The difference is not that much, but when somebody is running a naturally aspirated motor, there is only so much they can do to increase power and every little bit helps. In other words, if you can pickup 6 hp by raising compression, another 8 hp by porting and polishing your heads, 12 hp using headers, etc., it all adds up.

Moreover, in a N/A motor, one can safely run 11.0:1 compression, and sometimes even higher, on pump gas. The reason for this is that a N/A motor only has as much air/fuel mixture in its cylinders as can be sucked in by the vacuum created during the piston down stroke. A forced induction motor requires some different considerations. Instead of being completely dependent on the vacuum from the down stroke, the forced induction motor forces additional air/fuel mixture in the cylinders (hence the name "forced induction").

Now, the following is very important. Assuming the same compression ratio, since there is more air/fuel mixture in the cylinders in a forced induction motor, there is more fluid (air and fuel) to compress in the same volume. Thus, per the combined gas law, in the forced induction motor there will be much higher pressure and much higher fluid temperatures at the top of the compression stroke. This means that the air/fuel mixture is much more prone to detonation and pre-ignition. For this reason, we typically lower the compression ratio on forced induction motors in order to keep detonation and pre-ignition at bay.

By running a lower compression ratio, we lose a couple of percentage points in engine efficiency, and thus a few hp, but we more than gain the hp back by being able to force much more air/fuel mixture into the cylinders. In other words, given the above example, the 6 hp lost due to using lower compression can be readily picked up merely by increasing the boost.

In summary, in a naturally aspirated motor, you typically want to increase compression ratio to maximize engine efficiency, and thus gain hp. In a forced induction motor, the opposite is true to an extent; as you increase your hp goal, you want to lower your compression ratio (to a certain point) so that you can force more air/fuel mixture into the cylinders without experiencing detonation or pre-ignition.