Advance the cam?

[ssdurango]

In the book titled Street Supercharging by SA Design they say a good 'blower' cam is one that has no overlap, advance the exhaust event, retard the intake event (to get lobe separation), then advance the whole cam 2 -4 degrees to compensate.

Does anybody agree with this?

Also has anybody tried advancing there cam?

Thanks,
Tom

[Joseph Upson]

In the book titled Street Supercharging by SA Design they say a good 'blower' cam is one that has no overlap, advance the exhaust event, retard the intake event (to get lobe separation), then advance the whole cam 2 -4 degrees to compensate.

Does anybody agree with this?

Also has anybody tried advancing there cam?

Thanks,
Tom

I don't believe it is as simple as that. The best cam for later engines has been said many times to be the stocker for a base line to be used with boost, because it usually has the best all around driveability characteristics.

As far as lobe separation is concerned, the need should be based on the engines efficiency as it relates to the level of boost you are running. The concern with overlap is the possibility of forcing your intake mixture out into the exhaust while both the intake and exhaust valves are open at the same time or visa-versa if you have a poor exhaust system with lots of back pressure resulting in a contaminated combustible mixture due to residual combusted gases remaining in the cylinder after the exhaust valve closes.

Simply put, at your maximum boost level you want your exhaust back pressure and your intake boost pressure as close to a 1:1 ratio as you can get it. This will differ between a turbocharged engine vs. a supercharged engine because of the turbo being driven by exhaust gases in which case as boost levels climb so does exhaust back pressure so I believe the SC engine would be more likely to have fuel and air blown through the cylinder out into the exhaust due to experiencing much less exhaust back pressure as boost level climbs.

Increasing the lobe separation from my dyno simulations takes away from maximum performance, and I believe the trend is to increase the LSA by default because often motor heads are thinking along the lines of high output blown engines which require that kind of cam profile due to the double digit boost levels being run. I doubt it's needed for our motors since 9 out of 10 of us aim for practical levels in the 10 psi range.

http://www.turbomustangs.com/turbotech/main.htm

[ssdurango]

Thanks for the response, makes sense. I'm not as horny for a new cam now oh well

Tom