6-71 blower on a 340

Coyote Jack said
It is all good to quote the charts. They are a good basis to start from. The problem is they are not real life. There are a lot of factors when it comes to measuring real boost. Two of the more important are the cam profile and the flow ability of all the parts of your engine. Remember, when we measure boost we are just measuring the amount of resistance your engine is giving to the flow of air. If there is no resistance, even a 16-71 won't produce much boost.

My engine is a 340, +0.030 with heads that have lots of porting done and a healthy cam. When I run around the street, most of the time my top pulley is a 37 tooth and the bottom is a 33 tooth. That gives me a .89%+/- underdrive and my boost gauge tells me I see 6 lbs of boost. That is the reason I say at 1 to 1 my 340 will see about 7 lbs of boost. Another 340 configured differently will see a change in boost.

Hope this helps to explain my original statement.

Jack

I agree about charts being a good start. There's no hard and fast rule but there are guidelines.
These charts are put out by the manufacturers of the superchargers, such as hollley (weiand), dyers, littlefield etc. So they are indicative of typical results.

Your motor is way way... way outside of those guidelines. I've never known anyone to drop from 17psi to 7psi from a cam and some port work. so your results seem atypical to me.

I layed out all the basic maths involved in determining an approximate boost level to confirm it's relative accuracy within a pretty narrow margin. These equations can be applied to any positive displacement blower on any engine.

That being said, I would recommend anyone considering a blower swap contact a blower specialist/manufacturer when trying to determine drive ratios to suit their compression ratio, displacement etc.