[lust4speed]

If you're trying to compare the an iron headed RA2/3/4 style head to whats available in aluminum, I think you're going to lose that debate.

You pulled that assumption from where?

[Flyin TA]

Originally Posted by lust4speed

First things first. find out why your vacuum is so low. As Poncholvr said, that isn't that large of cam - especially with 488ci to have that low of vacuum. Anything else you do will only be a band aid masking the main problem.

Possibly a vacuum leak, or valves too tight? Solve the vacuum problem and you might not have any other problem after that.

I unhooked all the vacuum and capped them off. No differences. Best vacuum I can get out of it is 11" with or without everything hooked up.

I did get it to idle with the AC on with the idle set at 1000rpm and no vacuum advance. Drops to 750rpm but still runs.

I am thinking I need to to switch from manifold to ported vacuum. Thoughts?

What do you mean valves too tight? Springs pressure? Or rockers?

[Ollie]

Rockers, which rocker arms do you have stock or after market, with adjustable roller rockers.
stock rockers are adjusted tight against the bottle neck.
After market are adjusted 1/4 turn past where they stop clicking. if they are adjusted down too far the valves will be held open resulting in a loss of vacuum.

[Flyin TA]

Originally Posted by Ollie

Rockers, which rocker arms do you have stock or after market, with adjustable roller rockers.
stock rockers are adjusted tight against the bottle neck.
After market are adjusted 1/4 turn past where they stop clicking. if they are adjusted down too far the valves will be held open resulting in a loss of vacuum.

Comp 1.65 rollers. Those are fine. We checked them and head bolts after the engine dyno.

I think this cam with my 10 to 1 compression just doesn't make much vacuum. The recommendation from cam motion was 224/230 and I talked them up to a242/248 by saying I didn't have power brakes. I ran a 248/252 cam in my 10.5 to 1 496 eng. brakes worked fine then and now so didn't think much of it.

[lust4speed]

For clarification, the type of vacuum leak I'm talking about would be between intake manifold and head(s), or to a lesser extent, carburetor to intake manifold.

[Flyin TA]

Originally Posted by lust4speed

For clarification, the type of vacuum leak I'm talking about would be between intake manifold and head(s), or to a lesser extent, carburetor to intake manifold.

I did check intake carb and spacer as well. Hopefully note the heads.

[lust4speed]

Originally Posted by Flyin TA

...I am thinking I need to to switch from manifold to ported vacuum. Thoughts?

Sometimes manifold vacuum can worsen idle problems with A/C. Engine definitely idles more efficiently with manifold vacuum, but flip on the A/C and you sometimes get a triple whammy. The A/C draws the engine RPM down a couple hundred as expected, but then this small drop in RPM also reduces available vacuum slightly and that relaxes the vacuum advance lever. So instead of a few hundred RPM drop the idle drops a little further. Then at this point the few degrees of mechanical advance that were present drop out and the idle falls off a cliff.

It's pretty much a given that you will have to turn in the idle set screw farther in to maintain the previous idle speed, but there's a good chance the engine will be more stable when dropped in gear and/or the A/C is flipped on.

What do you mean valves too tight? Springs pressure? Or rockers?

What I was referring to is that a valve might have been adjusted a little too tight after being re-set after the dyno run and you might be hanging one or more valves slightly open which definitely kills vacuum.

[Skip Fix]

My 469 HO w/a RA4 cam had pretty wide lobe centers, stock heads, a stock intake, and a quadrajet and 10.75 compression. It was a recipe for detonation

But many folks here say you can do that on pump gas no problem! Just not tuned right

[Flyin TA]

I hooked up the holley high idle solenoid today and it makes running the AC much easier. Took me a little time to get it adjusted just right.

I am running manifold vacuum advance and and idles around 800rpm with the AC off and 1000rpms with the AC on.

Thanks for all the info. I am learning a lot from this project.

[Erik_H]

Flyin TA, can we get a picture of it? I am still considering changing my throttle bracket setup to allow for a solenoid. Thanks.

[Flyin TA]

[Flyin TA]

[HWYSTR455]

Interesting thread, and interesting posts. Seems there's some confusion on this topic, but all I can comment on is what's worked for me. And basically, it's a combo of more initial advance, a little higher idle speed, manifold vacuum for the can, and slightly leaner idle mix. Set the idle mix with the AC running, cut the AC off, and tinker with it until you find middle ground. Believe my intial is in the 16+ range, plus vacuum on top of that. The initial helps prevent the 'cliff' previously spoken of. With an adjustable can, with more intial, you can back off the can, adding more tension to the can spring, preventing the cliff.

The driver has 11-13" at idle, and a fairly health 248/254 roller, AC, alluminum heads. The clone has '69 #5 iron heads, and slightly over 10:1 static. Both idle fine in and out of gear, with or without the AC on. The only complaint I have is that out of gear with the AC off the idle is a touch high for my liking. (@ 950-1050 on both).

The fast idle sol one would think is an easier solution, though you have to be careful not to get the carb out of the idle circuit. I've always been able to get satisfactory results without them, so I don't use them. (the sol)

.

[Stuckinda60s]

Originally Posted by poncholvr

Geoff. I think you just missed something very critical to pinging and detonation regarding aluminum headed engine/combinations which incorporates "quick burn" technology (late model "shaped" chambers) vs. the old, rough casted iron technology of my iron headed 455 HO. 1st, when early iron headed engines were being produced...

You must have a pretty unique set of H.O. heads since all Pontiac engines, with the exception of the 303 RAV, had fully machined combustion chambers. That's why Pontiacs could run lower octane gas than Chevy and others did, with less danger of pinging and better emissions.

[Cliff R]

"Who is this Geoff guy? And why doesn't he post in the forums regarding his positions about ported vs. manifold vacuum advance?"

He used to post on here, but pretty much just regirgitated old information over and over again, and links to it, etc. He absolutely HATES me, because I've tuned many thousands of carburetors and distributors, and don't advocate MVA for ever single application. We bumped heads a few times, these days he just sneaks around and privately helps folks out via PM's and such.

I'm also able to run 11 to 1 compression on 87 octane fuel, with zero issues anyplace, and that engine HATES MVA to the distributor, runs FLAWLESSLY hooked to a ported source.

But none of that needs to be argued any more, folks have their opinions, some folks "google" up most of their info (you know everything you read on the Internet is true), I base EVERYTHING that I post on these Forums on FACTS, from over 3 decades of tuning experience. I do this full time, every single day, for a living, so I get my learning curve tested on a daily basis.

MVA for this application may just not work because with a manual transmission, there is very little load at idle, and the throttle plates are nearly closed. Introduce a slight load and the "signal" to the advance may fall off enough the timing retards, or the carb just isn't pulling hard enough on the idle curcuit to compensate. Most likely a product of both of the above.

In any case, I'd try going to ported or no timing at idle, richen up the carb some, and see if you can get a stable idle AC on and off. Sure, the rpms are going to drop off some, but from 1000rpm idle speed it may hold in there around 700-800 with correct tuning.

If not, a fast idle solenoid is a quick fix for the deal, even stone stock low compression tiny cammed engines used them, they work perfectly in just about every application I've tried them with as well......Cliff

[zeebo]

the strange thing is is that some cars have that solenoid, some dont...even in the same model year, and all my cars have ac..(ugh..lol)..my 73 lemans doesnt have it, but my 73 olds does..go figure..i think later cars had it installed about 100%...