I had a 396 rebuilt professionally by a locally respected (and from all appearances, highly skilled) engine builder. They have an engine dyno and it dyno'd at 330hp vs a rated 310hp (L-47), and the motor performs admirably in all respects. It's bone-stock except for the cam, which is still very close to stock (214/218).

Except for vacuum - it only has 11" of vacuum at 750rpm. That's optimizing for ignition advance too, running it wherever produces the highest vacuum.

I talked to him about it briefly today, and he said that if it were off a full tooth on the timing set it'd run very poorly and wouldn't have dyno'd as well as it did (one tooth is like something over 20 degrees, it's a big amount).

They smoke-tested the engine looking for an oil leak (gallery plug) and the smoke machine said the engine was one of the tightest they've tested, which really only tells me there's no crankcase/valley vacuum leak.

Somewhat oddly, if you pull the vacuum booster line (creating a huge vacuum leak) it speed up a little. That's also stumped me a little.

What would you do next? My last resort is to strip the front of the motor so I can degree the cam and be sure, but since there are really no symptoms other than a number (on multiple gauges to be sure), I don't want to go that far.

Would a leakdown test tell me much? I was thinking if valve lash were too tight, perhaps I'd see that show up.

Cam specs are 214/218@0.050, .496 lift, 110 intake centerline, which should make a lot more vacuum I think!

Thanks,
Dave

 

I assume you have already capped everything off that is vacuum related at the intake manifold to eliminate vacuum leaks the car will have?

(brake booster, PCV, trans modulator, vac tank etc etc)

I would definitely expect 14-16" at vacuum with that small of a cam.
A leak down test is always a good idea but...I'd be more suspect of cam timing as well.

I would double/triple check the carb gasket/insulator, plug absolutely everything that gets vacuum from the carb/intake and go from there.

I've had one with a cam/crank gear a tooth off and it did run rough, but ran.

They may have the cam/crank gear lined up correctly at dot to dot, but the cam or gears could be ground wrong. A friend of mine recently caught this on an LS motor...dot to dot was 8* off... Changed the new timing set out for another one and it was 1* off.

 

I'm no cam expert but for me looking at the peak HP it seems kinda high for a cam the size yours is suppose to be.

BB's with stock cams seem to run out in the 4500 rpm range, (I guess that is relative to what the engine came out of stock).

when I look up a cam with the quoted numbers the recommended rpm range tops out around 4500 rpm.

maybe the cam don't match the cam spec's quoted

Neal

 

I agree with everything Allen said, double check all vacuum lines and carb gaskets

check the easy stuff first

Neal

 

FWIW, I have a basically stock LS5 engine with a Comp Cams 11-208-3 camshaft. Grind #CB 280H-10. 280/280 duration, .520/.520 advertised lift, 110 degree lobe separation, 230/230 degree duration at 0.050 lift.

The compression ratio was dropped from 10.25 to about 9.5 back in the 80's to run on pump gas available at that time.

I was getting about 14" - 15" vacuum when I last checked.

Your vacuum does seem to be a bit low.

 

What kind of intake are you running?

If idle increases by adding a vacuum leak this usually indicates it's rich at idle.

 

How much initial timing and total timing are you running, and how is your vacuum advance hooked up? If you have inadequate initial timing, and if your vacuum advance is not hooked up correctly, you will get vacuum numbers exactly as you describe...

Lars

 

I've tried just maximizing vacuum with timing, which I normally scorn, but wanted to see what was possible. With either the vacuum connected to manifold (incorrect for this year) or just a boat load of initial timing, that's how I can get it to 11. So ignition timing is already maxed (at least it doesn't ping!).

Since vacuum is low, I checked compression this morning, about 160psi cold cranking.

It does appear rich at idle, since a vacuum leak speeds it up. But the mixture screws =, if turned in, will eventually cause it to stumble, but not speed up. So I'm a tad confused.

The mixture screws do have an effect, you can kill the motor with them, so I haven't got the throttle blades open a bunch (or they'd be ineffective). But how could I lean out the idle then?

 

1 tooth = 20 degrees? that is not accurate. 1 tooth on the cam gear is only 8 degrees, or 16 if you count the whole 720 deg. cams are routinely bumped fore/aft a tooth or more, it may run better or worse....

the builder should have degree'd it anyway and documented it when it was built. I don't always myself, but if I paid a pro, you can bet he would have.

I agree with the others, do a lookdown test, and verify the cam and ignition timing.

and maybe try a different gauge. they are not always accurate.

 

More air below the throttle blades will make if it leaner.

Check your PCV valve. Make sure it is functioning. when the engine is running there should be a vacuum if you pull the pcv out and put your finger on the end of the valve. The PCV is baisically a calibrated vacuum leak. If it's not functioning you can get a rich idle.

Other way is to provide more idle bypass air, that's a tad more complicated. You've probably heard of guys putting holes into their primary butterfly?, they are essentially providing more bypass air (air that is introduced below the throttle blades without pulling more fuel through the idle circuit). I'm not suggesting you do that though. One step at a time.

Are you running a standard dual plane intake? Or is it a single plane or an airgap style intake?

 

The same shop that built this motor drilled holes in the primaries of my ZZ502 dressed as a 427. So I know the drill, pardon the pun, but I just can't see that it should be required on a bone stock 396 with a Q-jet.

The PCV works, as if you pull it out and cover it with your thumb the engine also slows down (again indicating it's rich at idle).

I wonder if it's time for one of those adjustable PCV valves? I hate throwing parts at a problem, but there's zero chance the random parts store PCV valve you pick up matches the calibration of any particular carb very closely....

 

Do you actually have any engine problem other than the fact that you don't like the number on the vacuum gauge...? I don't believe you really have a problem if the engine is running well. PCV won't cause poor vacuum - if you suspect it, just plug it and see if it does anything - I bet it won't. Every engine I own idles at 11 inches or worse (my 455 Pontiac idles at 7), and they all run frighteningly well... If it runs good, pack the vacuum gauge back in your toolbox and drive the car. I think you're trying to find a nonexistent problem.

Lars

 

I might be chasing a non-problem, but maybe it'd run EVEN BETTER if this was fixed... I guess if I understood the reason, I could be content, but not knowing is bugging me.

I got a new gauge to be sure, and ignition timing is maxed. Is there any way to sanity check the valve timing without stripping the motor down far enough to get a degree wheel on? Like verify TDC with a piston stop relative to the timing tab? Would that prove anything?

 

PCV was in regards to rich idle not low vacuum. Just suggesting how rich idle can be handled if normal efforts fail.
Is the vacuum a lot lower than before with previous cam with similar specs?

 

I never checked vacuum before the rebuild. I did check cranking compression which was 156-175, and is now 160 (only quickly checked one).

 

Harold Brookshire's (Ultradyne Cams) Quick N Dirty' method to check if cam is installed correctly:

1.---Pull off the driver's side valve cover. Turn the engine to TDC, and check #1 cylinder's valves. If both of them are closed, you are at the ignition point. Turn the engine over 1 time, back to TDC, and both valves should be off the seat.

2.---Check the height from the top of the retainer to the spring seat, first on the intake, then on the exhaust. I recommend something like a Machinist's 6" steel rule. Make sure you write them down, because you need to see which one is closest to the head.

3.---If the cam is in the engine right, the intake retainer will be .030" to .060" CLOSER to the head than the exhaust retainer is. If the cam is retarded, the exhaust will be closer.

This happens a lot, and a sure giveaway is very rough idle, very low vacuum, needing a lot of ignition advance to run, and coming alive about 35 mph.