I just finished building my engine which is flat tapped 305 (030 bored) with LT1 heads (50cc after miling) and LT1 intake. My cam spec: Lunati Bracket Master II Cam 235 int./240 exh. @050, 0.490 lift, Lobe separation 112. 1,5 RR
Now i installed it with 4 deg. retard, it idles at around 1000 RPM, and vacuum is only 70 kpa. wonder if i would advance that camto 2 or 4 deg. will i get more vacuum or even less?

best regards

 

Vacuum is determined by vlv overlap which in turn is determined by LSA and vlv duration. Adv or retarding the intake centerline should not affect vacuum.

BTW it get about 20"Hg = 70KPa which is very good vacuum. Actually its higher than I've ever seen.

 

ah i used wrong logic in giveing vacuum at idle, 70Kpa MAP reading equals to 30 Kpa vacuum, so that is actually only 8Hg, i'd love to see around 11-14Hg (16 would be perfect) but from what You say there is nothing else that i can do.

best regards

 

Advancing the cam will generally increase vacuum, since it causes the intake valve to close sooner as the compression stroke begins rather than the beginning of the compression stroke reverting into the intake.

None of this has ANYTHING WHATSOEVER to do with "overlap", which is the period at the OTHER end of the intake stroke where the exh and int valves are both open (their operation "overlaps") except to the extent that "overlap" can serve as a proxy or "indicator" for the overall "size" of the cam.

The ONE SINGLE VALVE EVENT TIMING that makes the most difference to EVERYTHING about how an engine runs, is the intake valve closing point. Which you can readily adjust by twiddling the cam. Be aware however that cam phase affects EVERYTHING; not just vacuum, but also the engine's entire behavior across the range of RPMs. Precisely because, it tweeeeks the ONE SINGLE MOST IMPORTANT EVENT.

IMO that is a poor cam choice; a 40-yr old design from back in the day when gas mileage and various driveability problems such as low vacuum (power brakes were not common yet when that cam was introduced) were considered less important than they are today. You'll just have to deal with the consequences of an obsolete cam.

No way you'll get 16" of vacuum with that cam, even at 1000 RPM. You might be able to get it up to 13 or 14" by advancing it 4° instead of retarding it 4° but even that is a stretch. A modern cam would have been a better choice if vacuum is a concern.

 

thanks sofakingdom, power brakes are not a pronlem as i run electric vacuum pump for them. i could live with that low vacuum casue with Megasquirt i could go Alpha-N mode and don't care, but actually i run LPG also and this is why i seek for more vacuum at idle, LPG on BLOS mixer does nor run well with low vacuum

well i'll try with advancing that cam and see what happens.

best regards

 

There are so many factors which can determine the manifold vaccum but like Sofa said, advancing will cause the intake valve to close sooner. Manifol vacuum reached will also depend on your altitude. An engine that gets 17" of vaccum at sea level may only get 14-15 at 3000-4000 feet altitude.

For "most" engines, installing the cam straight up is best especially if you are not going to check piston to valve clearances. Advancing or retarding changes the valve timing in relation to where the piston is in the cylinder.

Not sure about the Bracket Master cam but a lot of cams designed for the street already have 4* advance ground into them. Normally it will say so on the cam card. For a street car, advancing a cam moves the powerband lower to where you'll use it the most. Retarding a cam will move the powerband higher where a race engine will use it the most.

The camshaft in my race car is installed straight up and I shift at around 7400 rpm. If I retarded the cam, I could bump my shift point even higher but don't want to. My camshaft is designed to make power to 8000 rpm and the engine is built to go that high also. No power brakes so manifold vaccum isn't an issue.

 

Please gives us feedback on what you find. I expect vacuum will not change or very little as advancing the intake centerline closes the intake valve sooner and increases compression pressure but valve overlap controls vacuum.

Good luck.

 

well i advanced it to 4 deg and it gave me 17kpa (5 inHg) more of the vacuum, bumped timing to 28 deg on idle and another 5kpa (1,4 inHg) added too. real cool

best regards