Would this cam cause my lack of vacuum??

[72RS]

I am running a 383 stroker in my 72 Camaro. I am only pulling 8" of vacuum at idle. Timing is locked at 42* and am running a Comp Cams HYd Roller with the following specs.


Adv Dur 294 300

Dur @050 242 248 lobe seperation 110

Lift .540 .562



Looking for input on if the cam is the culprit or is something else at play here.

[TheJDMan]

Holy crap, why so much timing?

[72RS]

Holy crap, why so much timing?

when they had it on the dyno that is where they got the best numbers...

[andrewb70]

That's a lot of cam, and is certainly the cause of your low vacuum. That's a lot of timing too, but with that big cam and a lot of overlap, I can see how it might be OK.

Andrew

[octane speed shop]

That's a lot of cam, and is certainly the cause of your low vacuum. That's a lot of timing too, but with that big cam and a lot of overlap, I can see how it might be OK.

Andrew

Hey Andrew
yea, it's your cam. keep in mind a carb can take a lower vaccum signal (110 lsa). also i agree that 42* is a bit too much total timing, maybe try for around 32-36*.

[ChevelleNV]

put a vacuum gauge on it, and adjust the timing till you get the best vacuum signal.... probably end up around 32-36*

[72RS]

put a vacuum gauge on it, and adjust the timing till you get the best vacuum signal.... probably end up around 32-36*

I thought backing off timing would decrease vacuum...I think I will back it off a bit anyways. I was never thrilled with 42* but the guy builds a ton of great motors so I figured who am I to question it.


Thank you for all the feedback..

[ChevelleNV]

I thought backing off timing would decrease vacuum...I think I will back it off a bit anyways. I was never thrilled with 42* but the guy builds a ton of great motors so I figured who am I to question it.


Thank you for all the feedback..

Put a gauge on it, I bet you will be surprised at what happens

[72RS]

Put a gauge on it, I bet you will be surprised at what happens

will try it...thanks...

[72RS]

This may be a dumb question but I won't do any harm to the engine or effect its performance by backing the timing off will I, being that it was dynoed at 42*

Like I said earlier I always thought 42* seemed like a lot.

[ChevelleNV]

This may be a dumb question but I won't do any harm to the engine or effect its performance by backing the timing off will I, being that it was dynoed at 42*

Like I said earlier I always thought 42* seemed like a lot.

Just cause it was dynoed at 42* backing it off is not gonna hurt the motor, worst that will happen is you will gain little drive-ability and loose a little performance. when they dynoed it, they set it up for maximum performance which almost always kills drive-ability and gas mileage.

[Later-A-body]

I had the exact same cam specs in my 464 Pontiac and it had rowdy idle, and only pulled about 12" of vacuum. With 80 fewer cubic inches that wont be real street friendly. If you were to run a cam with a wider LSA of 112 it would pull more vacuum and be more streetable.

[MonzaRacer]

OK, so yo guys are all wrong on timing.
Tuning for best power on dyno, you dial in initial for best power, then keep speeding up the rate of centrifugal advance till engine makes best power THEN hook vacuum advance to manifold vacuum.
As for total timing minus vac advance, if the heads have a slow burning chamber the may NEED more timing.
I have tuned hundreds of engines for street and track duty and soooooo many are low on timing.
36-38 deg is a cliche timing point used by so many. I have seen vortec heads require so little timing over regular heads. Also I have found some aluminum heads take some really messed up timing curves.
NEVER assume anything about a persons tune unless its tune is causing an issue.
Now if you really want more vacuum, try a little extra cam advance.
I really like using cam advance to build best cylinder pressure, thus making best torque.
Old trick but it does work well.