Vacuum for this 228/236 duration cam? 455

I've got a 455 I am tuning, with a custom cam. The whole thing came to me assembled professionally, with receipts.

9.5 compression, Moly rings, hi comp pistons,
Sp1 intake,TA headers, big valve heads mild porting.
HEI distributor, 12 deg initial timing.

.489 lift intake&exhaust
228/236 duration @ .050
Lobe Sep 112

What type of idle vacuum should I expect, and what would be an appropriate idle rpm?

yes it has a modified qjet and is getting enough fuel in the idle circuit.

Thanks for your thoughts/experiences.

 

So I have been messing around with this engine and the timing and carb.

It likes a LOT of timing. And fuel. And idle air bypass with idle fuel.

19 degrees gets less than 10 inches Hg vacuum at 850Rpm in P

32 degrees gets 12 in Hg at 850 rpm in P

36 degrees gets 12 in Hg at 900 Rpm in P

It sounds feels and responds best 36 degrees 950Rpm idle in P getting 13 in Hg. This corresponds to a strong idle in D at 750Rpm.

big idle air bypass, big idle and transition fuEL.

7042240 ELECTRIC choke conversion FTW!

thoughts?

 

Ken, Maybe you should have someone look at the carb for you. :moonu: :laugh: ..............Bob H.

 

hah!

 

Does not sound all that unusual to me. Yes it will want a lot of timing at idle. I have never been able to consistently start a motor at close to 30* but low 20's are no problem with a quick rise off idle. My 413 cammed motors pulled about the same vacuum as you. I even tried Rhodes lifters and could not get more than 13 inches of vacuum. I it runs good don't worry about it.

So in conclusion it must be a carburetor problem.

 

So the 72 is on the road.

I had to go with 93 octane to stop singing under load.

I've got the timing set at 22 initial plus 13 mechanical.

I am playing with adding 10 or 12 vacuum advance on manifold vacuum.

35ish does seem to be the magic number it likes. At idle and full throttle.

 

Would not consider running anything but 93 octane(unless I could get more the pump.) So what kind of cheap SOB would not put premium in their Buick? :spank::laugh:

 

Vacuum advance?

 

I did not run vac adv at first, when I set the 35 at wot.

Now I have a vac can restricted to 12 degrees, on manifold vac, and I am playing with the preload spring to lose the pinging I get under heavy throttle.

Went up on my main jets too, and that made a big difference in power.

 

yes it's heading that way. It's. Difficult to get rid of the ping with12. I'll have to modify the canister again. Thanks

 

Does it ping without the vacuum adv hooked up. Possibly it has a spun balancer ring??? Or more compression then they thought?

 

Rings broken in yet?
Possibly sharp edges left around the chamber?
The porting take too much swirl from the port?
Exhaust valve seat width narrow or poor contact pattern, such as an interference angle?
Gasp...too hot of a plug?
You get an infra-red on the exhaust temps yet?

It does seem like a slipped ring.

 

Ken,

Get a cranking compression number on it..

JW

 

did a quick check on cylinder #1.

187 was the reading

 

This would be a good candidate for a low temp thermostat, if not already.

 

I am skeptical about the 9.5:1