jackc

Hi guys,
I have heard that a high vaccum engine is a good running one.

My Engine builder states with his street port that his engines
produce only 12 - 13 hg. of vaccum @ 850 rpm. Is this kind of low?

What would cause such a low Vaccum. Maybe the street port is too large?

Thanks for all replies... jc

altrdego

OK first I'm not a GURU

I know that with piston engines aggressive valve timing or valve overlap, the engines need to run at a higher Idle.

Valve overlap is what happens when intake and exhaust valves are actually open at the same time. In an aggressively cammed engine it is manifested at low idle by a rough idle.

I know this may sound strange but it improves the air fuel mixture flow at higher rpms when the actual amount of time spent open simultaneously is very small. At lower RPM's it is inefficient. Which is why detroit won't start making super high performance engines for the family sedan. Fuel economy.

Extending this reasoning to street porting and other aggressive ports, yes at low RPM you'll have poor Vacuum due to for lack of a better term port overlap or less efficiency at lower RPM's.

I've read some warnings on some sites that a race port motor would be very street unfriendly due to the high idle RPM required.

RexTech

I think that your vaccum is Ok because my engine is street ported with 3 mm apex seals and it has 12-13 Hg too. It runs fine. As altrdego said, our engines are not good at low RPM but very nice at high because the timing. The porting is like put a high performance overhead camshaft in a piston engine, it will sound like **** at low but like a fine wath at high.

Maestro

Im not sure on my FD as my boost gauge got stolen before my engine blew and the engine was ported.

But on my FC which had a very large Street port mine sat a little over 10 Hg. (11 Hg Max.)

When I had the Power FC it said about -460ish kg/mm

Maybe someone running a Power FC with a stock port could tell what there vacuum is ?

suganuma

im going to repeat something said way too much.....do a search first and then if that doesnt help make a post

i made a post about this a while back b/c i was getting info both ways. someone replied confirming by linking a steve ciriani's site

something to the account of:

stock 16-18
mild port 14-16
street port 12-14

with vacuum getting lower as the porting is more aggressive.

do a search and see what you find. i think rich (goodfellafd3s) actually made a poll concerning this not too long ago as well.