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thanks Raff, It looks like it's almost pointless to run a header evac system when running a exhaust system.
the good part is you just saved me some time and money

I'm sure a pump is the way to go for the serious racer with an engine making big HP, but I'm not sure you can't have your cake and eat it too with an exhaust evac system. I will have my own answers to that question when I'm able to run my car instead of my snow blower - hopefully in a month or two. For now, I know I can pull 10" of H20 at idle with my exhaust system in place. Assume I can pull more vacuum than that with open headers at all engine speeds. Don't have any numbers yet though. Also no info on what will happen at speed with the exhaust system in place - crankcase pressure will likely go positive. I understand the benifits of running a vacuum in the crankcase, but not sure if there is a measureable HP gain between running 5"HG and 15"HG. The only thing sucking around my garage right now is the weather



First, in order to realize any significant power gain the ring pack hass to be optimized for the vacuum-usually a real low tension oil ring. Second, at wot, even on a mechanically sound motor, there will be some blow-by that will quickly overcome the evacs initial vacuum. This even happens with a good pump but the pump has much more capacity than the evac and can handle more blowby before the crankcase gets pressurized. Look at the pro stockers and comp cars. They all use vacuum pumps even though there is a small amount of parasitic loss. For maximum HP there just isn't any comparison.