Vacuum and horsepower

[Guest Viper488]

A new 'free horsepower' source in many forms of racing is creating a vacuum in the crankcase.

 

Benefits are reduced load on the bottom side of the piston rings, and more importantly, a less dense atmosphere for the crankshaft and pistons to operate in.

 

My question: All I can pull is 10in Hg at idle, and 7 in Hg at WOT (wide open throttle). Since this appears to be a substantial vacuum already, how much more is there to be gained by higher vacuum than 7in Hg in the crankcase?

Meaning, is there an appreciable and workable amount of more air to be drawn out above 7in Hg to say, 14in Hg?

 

Or are the pistons and crankshaft already operating so thin an amount of air at 7in Hg that fretting over finding the vacuum leak that drops an electric pump that will pull 22in Hg deadheaded, but only yields 7in Hg at WOT when connected to the crankcase probably not worth the trouble?

 

THANKS

[Glider]

I can't really see how a crankcase vacuum helps at all. Whilst it may reduce load on the rings of descending pistons, ascending pistons have to overcome the vacuum to reach TDC. Thus, some of the power generated on the ignition stroke of one piston, will be wasted on evercoming the vacuum pulling on the other.

 

In my engine, a build up of crankcase pressure would force oil out (a bad thing) and a vacuum would suck oil in through the feed, but fight against the scavange part of the system and things would get messy (so I have a timed breather system). In sealed crankcases any gain in the reduction of load against the downstroke would be lost in increased load in the upstroke. So, I don't get it.

[Guest Viper488]

So it would seem. But none the less, gains over 30hp are common. Daimler Chrysler found gains of 35hp when testing the idea of a dry sump oil system on their prototype Gen III Viper engines.

A tuner shop recently claimed 40hp as a by product of their dry sump system.

Do a search for Hot Rod Magazine's 381 SBC (small block Chevy) test and you'll see 25-30hp 'across the board' gained from a belt driven vacuum pump pulling 10in Hg.

Secondary tests showed driving the pump took 6-9hp . So the gross gain was well over 30hp!

The facts are in! Vacuum pumps are used in many forms of racing...F1, both car and motorcycle Pro Stock drag racing, etc

So, I still wonder, what's the air density difference between 7in Hg and 14in Hg? Functionally speaking.

[Cap'n Refsmmat]

So it would seem. But none the less' date=' gains over 30hp are common. Daimler Chrysler found gains of 35hp when testing the idea of a dry sump oil system on their prototype Gen III Viper engines.

A tuner shop recently claimed 40hp as a by product of their dry sump system. [/quote']

Dry sump is not the same as a vacuum. Dry sump means no oil in the sump, whereas normally the area has loads of oil in it..

[Guest Viper488]

The sump is dry because the belt driven pump is sucking the oil out and air as well. In this particular case, creating 14in Hg of vacuum in the crankcase...

 

My question is just about vacuum and how much in a practical sense the remaining air density in a confined space is diminished at levels higher than 7in Hg compared to 14in Hg in that same space.

 

How much less air (as a percentage?) does each additional in/Hg more of vacuum on a gauge represent? Can the air reduction per additional in/Hg be simply descibed?

[Guest Viper488]

I found my own answer at a woodworking website explaining in depth about vacuum clamps.

 

A perfect vacuum is just under 30in Hg.

 

Atmospheric pressure is 14.5psi.

 

So for approx. every 2 Hg there's 1psi of atmospheric pressure. If I have 10in Hg of vacuum in my crankcase, that's about a 5psi or 1/3 (approx) reduction in the air density in the cankcase.

 

And so on..