[G.Siewert]

What is the best curve vs rpm for a 75 455?
Also what should be the vacuum part vs mechanical? Engine is at 9.8 to 1 comp.
Cam is 230 @50, 490 lift, 1.5 rockers
Ported, lightly polished, over 30, cast pistons, automatic.

------------------

[G.Siewert]

What is the best curve vs rpm for a 75 455?
Also what should be the vacuum part vs mechanical? Engine is at 9.8 to 1 comp.
Cam is 230 @50, 490 lift, 1.5 rockers
Ported, lightly polished, over 30, cast pistons, automatic.

------------------

[Rocky Rotella]

Your combination sounds a lot like mine. Have your distributor spun on a Sun machine to ensure proper callibration. I tried to do mine with a dial back light and and Mr Gasket kit, I was way off.

Anyway, these numbers are at the crank.

Have it set for 24 degrees mechanical advance starting at 1200 rpm and all in by 2600 rpm.
We've found our cars are more responsive without the vacuum advance connected. You need it in place to lock the avdvance plate down, just don't hook a vacuum line to it. The numbers above would be cut in half if going by distributor numbers as the dist spins twice what the crank does. Setting your initial to 12 degrees will give you 36 total. 36 seems to be a magic number for iron headed Pontiacs.

[Will]

I would agree with Rocky that 36 will generally be very close to what you want. However, the exact numbers are very combination specific, and the amount of initial advance is even more so than the total advance.

There are a lot of opinions out there about how to setup distributors and what is "best." The bottom line is, best is whatever works best for YOU.

I will disagree with Rocky about vacuum advance. I run it on all my vehicles and the only difference I can tell is that my cars get better gas mileage in normal driving with it hooked up. It is my strong opinion that this is the way it should be. Vacuum advance should have no effect on WOT operation, and I personally don't like to hook it to manifold vacuum as it interferes with my ability to setup the initial advance where I want it. Incidentally, not having enough initial advance will hurt part-throttle, low speed, or off-idle responsiveness.

If you don't have the luxury or inclination to put your vehicle on a chassis dyno or flog it down the 1/4 mile regularly, you will not be able to achieve the "best" timing curve, but you can get pretty close.

Pontiacs with iron heads seem to like anywhere from 32 - 38 degrees total timing, all in by 2600-3000 RPM. 34-36 is probably the best bet. Initial timing is a total crapshoot as it depends on compression ratio, manifold type, cam specs, carb, etc. Best way to set it is to hook up a vacuum gauge and shoot for the highest vacuum reading at your preferred idle speed. Could be anywhere from 10 - 20 degrees initial depending on how the engine is setup. My old 400 liked 14 - 16 initial for best response and idle.

Good luck!
-Will

[Rocky Rotella]

Will, I've heard that about the vacuum advance from a few people. I'm not sure why but our cars feel more responsive when driving around town without any vacuum advance. I have an adjustable unit and limited travel to the least amount with the highest vacuum and it still didn't feel right. Like you said, each combination is different.

[goathead68]

Guys, I have a question. If you run your car on the highway quite a bit, wouldn't you want a little more timing-say 40 to 45 degrees, which is what the vacuum advance would give you if it was operating? I had problems with my car running hot ( on the highway) with the specs set as you suggested, and the car runs a lot cooler now that the vacuum advance is set up. I'm running 12 initial with 20 at the distributor, all in by 3000, with about 15 with the vacuum advance, for a total of 47. Thanks. Oh yeah, I have a '68, basically stock 400, automatic, 3.23 rear. At the strip, I disconnect the vacuum and run 36 total.

------------------
Ken

[This message has been edited by goathead68 (edited 06-04-2001).]

[Will]

goathead68, your experience mirrors mine. Can't explain why Rocky's had problems with vacuum advance. The only time the vac advance should be doing anything is at part throttle cruise, such as on the freeway. At that point, it gives better cooling and better fuel economy.

I tried running at the track with the vac can hooked up and unhooked. Didn't make a lick of difference.

Rocky, how much initial timing are you running, and were you running the vac can from manifold or ported vacuum?

Cheers,
-Will

[Rocky Rotella]

Will 12 degrees BTDC and from a ported source. The car seemed ok, but it was a bit soggy down low. It is kind of strange as I had it set for the highest vacuum and the lowest amount of advance. Maybe, I'll play with it again.

[Rocky Rotella]

I just came from out playing with the timing. I started fooling with the adjustable vacuum advance and found it was set a bit too high to do any good. I adjusted the vacuum setting down adn it feels great at part throttle. Also, I bumped my initial to around 15 and the idle smoothened out considerably. It's a lot better around 1500 than before. It still pulls hard on the top end but I'm kind of speculative of 39 degrees total. I need to find a way to limit that to 36 or so. I'm not getting any predetination so I may leave it. Thought maybe you guys would want to know.

[goathead68]

Rocky,
Glad to hear you got the vacuum working! After my last post, I rechecked mine and came up with 12 initial, 36 total @ 3000. It rattled a tad, so I went to the corner and put in some 104 Cam 3. Runs real good and smells even better. I'm using a 50/50 mix of the Cam 3 and Amoco Ultimate. The car runs about 190 degrees on the highway with this set up, plus the vacuum advance. Thanks for getting back to us.

------------------
Ken