Pontiac gurus, I need your help!

[starterguy]

Need some advice from the Pontiac faithful, my brother built a 400 for his 28 roadster and I could use some help tuning it. It is .030 over with mildly ported 6X heads, has about 9-9.5 comp with flattops and a stock crank and rods (good bolts) He installed a Comp hyd roller cam with .502-.510 lift, 276-282 duration 224-230 at .050. Its ground on a 106 intake centerline with 110 lobe seperation.
He has a Offy dual quad intake on it with two Edelbrock 600 carbs on it, ign is MSD billet with vac advance, street replacement (no 6al or the like).
What kind of timing do these motors like? I have heard not much initial and like 30 degrees total at low rpms? I know I will have some jetting ahead (ok lots of jetting and rods and....) and the carbs have progressive linkage so that will help. but some ideas on what the motors like or if someone has built a similar combo what have they found? Thanks guys will post some pics soon.

[DanStokes]

I SUCK at tuning but have found the cure for tuning issues. Drag the car over to a good shop with a dyno and an AFR meter and work with their tuner. You can do in an hour what would take you a year at the track. The trick is finding a good tuner to work with but they will have done this hundreds of times and can help you focus right in on timing, mixture, etc. It'll sound expensive but is cheaper in the long run.

If the car has an AFR meter you can do this on the road but it takes longer and there's the chance of getting tickets from the local authorities.

Dan

[Barry Donovan]

I only see two carbs, short duration.

that one may be a toughie to get what you want right away.

tuning carbs, get a vacuum reading at idle..go from there.

[DanStokes]

I'd send a PM to Thumpin455. He is at least one of Da Mans (or is that "Da Men?) on Ponchos and may be able to help get you in the ballpark. I stick with my dyno advise, however.

Dan

[OHC 6 Sprint]

I'll take a stab at it ... I had a similar build, but using Holley carbs with non-progressive linkage. So, IMHO, start with:

9 degrees advance, 34 total, all in by 3000 - 3200 RPM.

It would help to know the advance on the cam as installed - usually 4 degrees.

I had to run a cranking retard to get it to fire when it was hot. Pontiacs are notorious for being a little snot-nosed when hot starting with headers or long branch manifolds.

Your results may vary.

[OHC 6 Sprint]

Also ...

He has a Offy dual quad intake on it with two Edelbrock 600 carbs on it...

If it's not too late, check the match of the manifold to the ports. I have seen several Offy DQ manifolds with core shift problems new out of the box. Some more than others.

Just my $.02 worth.

[Loren]

Also ...

I have seen several Offy DQ manifolds with core shift problems new out of the box. Some more than others.

Just my $.02 worth.

It was a long time ago but I bought an Olds Offy w/ core shift and machining off-index so bad, it was unusable as sent. The local machine shop couldn't get a grip on the concepts of where it was "off", pardon the pun, and made it worse. When finally corrected so it could be installed, it wouldn't run worth crap. Finally fixed it by finding and installing a factory intake...built by guys who knew what they were doing. The brand-new Offenhauser went to the scrap yard and their "tech team" should have been sent there as well. A bitter experience after having had a good one with a different engine, I wouldn't trust anything that says "Offy" if you can avoid it, i.e. at least check that it looks like other manifolds and seems to fit right.

[starterguy]

Thanks guys this means alot, I have done Chevys and Ford but no Pontiacs. OHC 6 I will take your advice and start there at least i have a decent start. Dan, i like your idea (to proud to think of it myself lol) the nearest chassis dyno and tuner is 2 1/2 hours from me though it sounds like it may be the way to go if this proves to troublsome.

[Thumpin455]

The 6X head isnt prone to ping even with crap gas on a 400 so dont be afraid to advance it more if the engine wants more. If it surges at cruise, then its too much advance. I dont think it will idle with 9 initial, it might but the carb will be open and it will probably burn your eyes. All my 400s and 455s liked around 11-15 initial, and they all liked 34-52 total. The 52 was a 455 with 670 heads and 11.3:1, so its a bit of an anomaly, but the closed chambers sometimes want that much advance. There is variance between same size engines with the same heads and cam on how much timing they want, lots of variables involved.

I like to time them by ear first, make sure it starts easily, then go from there. Since you have changed almost everything from stock, you can throw the book out in regards to timing specs. It will want something different now so even giving you numbers to try might not do much for you.

You want the throttle closed as far as possible, the most vacuum you can get at idle, and it will probably need about 800-900 rpm to idle ok. You want the advance to come in quick, but make sure it isnt going too far. If you cant get it to idle down and not surge then you will have to limit the advance travel, just like anything else. I havent run into that yet, but the MSD distributors I have come close.

The dual quads will have to be set up to run together, but it should be done at the same time as the timing. I advance the initial until I can really close down the carbs, as long as it doesnt start hard. Keep track of where you have the timing set, how many turns on the idle screws, and the idle RPM. It will give you a guide for later. Make sure the float levels are low enough that gas isnt dripping from the boosters, then make sure it doesnt bog or lean pop when you whack the throttle or ease into it. If it surges or does anything screwy, the floats are too low. If it burns your eyes the floats are too high and or the timing is not advanced enough.

I would invest in a wide band O2 for it, and a vacuum gauge. You are going to need to keep track of where it is while you adjust things.

If you arent too far from Lake Superior, you could come by and I would get it running right for you.

[65longroof]

Correct me if I'm wrong, but, aren't total and initial timing settings somewhat dictated by the centrifugal built into the distributor? For example, my Pontiac HEI has 18 deg of centrifugal and I want to run around 34 deg total. Therefore, I need to run 16 deg of initial to get my total of 34. That's the way I've been doing it anyway and haven't had any problems.

[OHC 6 Sprint]

Correct me if I'm wrong, but, aren't total and initial timing settings somewhat dictated by the centrifugal built into the distributor? For example, my Pontiac HEI has 18 deg of centrifugal and I want to run around 34 deg total. Therefore, I need to run 16 deg of initial to get my total of 34. That's the way I've been doing it anyway and haven't had any problems.

Yes, but it's not "built in." You tune the distributor's weights and springs to get the correct centrifugal advance curve.

[Thumpin455]

Correct me if I'm wrong, but, aren't total and initial timing settings somewhat dictated by the centrifugal built into the distributor? For example, my Pontiac HEI has 18 deg of centrifugal and I want to run around 34 deg total. Therefore, I need to run 16 deg of initial to get my total of 34. That's the way I've been doing it anyway and haven't had any problems.

Also distributors have different amounts of total advance from mechanical and vacuum, the HEI that I ran in the 11:1scr 455 would reach easily from 15 to 52 employing both vacuum and mechanical advance, and would slow down a full second in the quarter if total was limited to the 34-38 range. Another HEI that I ran in the 8.9:1scr 455 maxed out its timing at 40 from 15 initial, but it surged a bit so I ran it about 12-13 initial, it also had a very fast curve, it was all in by 2300 rpm. Timing is something that needs to be checked to see where its at so you know if you need to limit the advance or not. If the engine likes it, then you dont have to worry about it, but if its surging and burning your eyes out from a rich condition at idle, then you need to limit it.

Admittedly I go about it in a rather backyard/shadetree/redneck way, but I like to see what the engine wants and then fine tune it after finding out what its doing. Sometimes you can throw it together, set it at 15 initial and it runs great, sometimes it has issues that you can end up chasing if you only look at one aspect of tuning it.