What is "Normal" Vacuum - 1975 L48*

[Tommy F.]

I have just fired the freshly rebuilt L48 in the '75 coupe. Internals are stock, with the exception* of a Comp Cams 151H grind, which is supposed to the a knock off of the 327 / 350 hp hydraulic. Fresh original Q jet is in place.

Idle set @ 850 RPM, Initial timing 10 btdc, warm motor, vac advance tied to constant vacuum port....

Lumpy idle, eratic speed, smells fat, 12-13" vacuum. Idle mixture screws are useless....I am assuming that at this vac level, the power valve is opening, thus the lack of idle mix control.

I can't tell if I am dealing with a massive vacuum leak, or another case of "bigger ain't better" in the cam department.

Duke, don't fuss at me, I am planning on working Sunday to get more info, but was looking for anyone else's experience with the 151H Comp Cam.

What is an expected vacuum level?

Help!

Thanks

[Duke W.]

Re: What is "Normal" Vacuum - 1975 L48*

On a 327 the OE ...151 cam pulls about 14-15" at 750-800, so I expect that what you have is not an exact duplicate of the OE ...151 cam. It may have similar duration, but probably a narrower LSA, which means more overlap and less idle vacuum and less low end torque.

I don't know what "151H" means. I have a circa 2000 Comp Cams catalog. They list cams by "part number" and "grind number", and the grind numbers look something like the number you gave, but the part numbers are in the formate xx-xxx-x.

What is the actual part number of your cam?

This catalog lists the 3896151 on page 212 titled Muscle Car Cams Factory Muscle and they show part number 12-106-3 and the basic specs they list are the same as the OE ...151 cam.

So what do you really have?

Duke

[John H.]

Re: What is "Normal" Vacuum - 1975 L48*

Idle mixture screws are useless....I am assuming that at this vac level, the power valve is opening, thus the lack of idle mix control.

Tommy -

If there's no response to the idle mixture screws, the throttle plates are too far open and it's idling way rich on the transfer slot instead of on the idle mixture orifices. Q-Jets don't have power valves.

[Duke W.]

Re: What is "Normal" Vacuum - 1975 L48*

The idle vacuum/speed spec I gave for the L-79 is with full time vacuum advance. With ported advance it may be less, but probaby not that much less.

You should go hunting for a vacuum leak before you pass judgment on the cam.

I looked up grind number "151H" on their Web site, and it corresponds to the part number I mentioned previously for what they essentially say is an exact replica of the OE ...151.

Similarly, grind number "929H" is supposed to be an exact replica of the ...929 '67-up base engine cam.

I was on hold on their tech line today - finally gave up and emailed them. I am looking for a vendor who can grind a cam with the ...151 lobe on the inlet side and ...929 exhaust lobe on the exhaust side. If their "clones" of these two cams are, indeed, exact duplicates of these OE cams, then they have the lobe masters to grind my "Special 300 HP" cam.

Was the cam Parkerized? That would be indicated by a dark gray color on the lobes and the rest of the cam except the journals, which are polished to remove the coating. Parkerizing helps protect the lobes during the first few minutes of operation, which is critical to long lobe life. OE cams are Parkerized, and the two Special 300 HP cams made by Crane were Parkerized.

So does Comp Cams Parkerize their camshafts?

Duke

[Tommy F.]

Re: What is "Normal" Vacuum - 1975 L48*

Thanks to Duke & John...

Duke,

The Comp website does address "nitriding" AS AN OPTION for a custom cam....the 151H grind is part number 12-106-3.
I did not see the cam before installation, but I get the impression that nitriding is optional, so I don't believe that it is hardened.
I think that you are right, and will remove the carb, and work to eliminate any possible vac/leak source....
I did notice delayed and hard shifts from the th400....maybe the modulator gave up, although no evidence of trans fluid getting sucked up in the motor.

John,

The Q-jet does have a "power piston" that is directly below the bowl vent, and if I understand correctly, is a vacuum sensitive enrichment valve...
I have re-jetted more Holleys in the dark than I care to remember, but the Q-jet is still a little mysterious to me, but I can see the advantages of the carb.

Back to basics....fuel/compression/spark....

Thanks for the help guys,

[Duke W.]

Re: What is "Normal" Vacuum - 1975 L48*

Thanks for the info. Nitriding would be overkill with OE lobes and valvesprings. I'll be talking to them next week and ask about Parkerizing.

...a little more info on their 151H and 929H "grinds". According to the specs both of these cams are ground four degrees retarded relative to OE indexing. Either that or they are symmetrical lobes in which case they certainly are not exact replicas of the OE asymmetrical lobes.

By the time I'm done with them next week they may not even want to do business with me.

Duke

[Timothy B.]

Re: What is "Normal" Vacuum - 1975 L48*

Tommy,

My experence with L79 with manual transmission is approx 750RPM idle and 14"vacuum.

You made lots of changes at once so start with known simple things, OJet carburetor idle screws not effective. Also, plug all vacuum fittings and isolate to engine to be sure there is no leak.. Check the vacuum advance control for part # and tell us what it is because at 13" vacuum that control is not working at idle. If you get one that's fully at advance stop with engine idling you will pick up another 15* idle timing.