Thread: 78 Trans Am getting some love

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Lol. The kit is just a adjustable vacuum advance kit from MSD. Base timing is with the vacuum advance disconnected, I plug it in, it goes to 24 which I can adjust with the adjustable vacuum canister. my biggest thing is with the mechanical weights at the end. I can swap out springs to speed it up or slow it down but how do you actually change the timing to advance it more or retard it more, If I turn the distributor to adjust it, it throws the base timing out of whack.Another thing I do not quite understand is manifold vacuum versus ported vacuum.If I use ported vacuum, wouldn't that completely make the mechanical weights obsolete because the vacuum increases as you rev the engine up therefore advancing the timing?. I feel like I'm 15 again lol

There is tons of info out there about manifold vacuum vs ported. Here is one good article.

https://www.hotrod.com/articles/ever...nition-timing/

The short version? Use manifold vacuum for best performance. It gives you a better, smoother idle, better throttle response, and the engine will run cooler at idle. Ported vacuum was a half baked idea used to improve emissions in the 1970’s.

I’m not sure if I understand your other question. Turning the distributer is how you adjust the base timing. Changing springs will affect at what RPM will the mechanical (centrifugal) come in. You probably want all of the centrifugal advance in by about 2500 to 3000 RPM.

This is what I don't understand. Let's say my base timing is 10° and my total advance is 20° If I want my total advance
To be at 30 then I would have to advance my distributor 10°. If I do that then my base timing will be at 20° instead of 10°.That is what I don't understand, is how you can adjust the base without affecting the total or adjust the total without affecting the base.

Changing the base timing is always going to change your total timing. To change the total timing separately, you have to physically modify the mechanical advance assembly. You can’t do it by changing springs.

Thanks, that's kind of what I was wondering but not what I was hoping.

Took it for the first test drive tonight. Holy cow did that thing wake up! I got a mess with the total advance, I got it set at 34 but my base timing is way too high now but for test driving down the road I didn't figure it bother. I think I'll have to get a little bit heavier springs for the secondaries, it seems to be a little bit too much at first but it does come out of it. for the first time in a very long time, I have to let off the gas in first gear or it won't stop spinning the tires, second gear is now more of a squawk than a chirp, and I actually got a chirp out of third gear lol. I'll get a few more things squared away on it like the timing and I think we're going to call it good for now. Sometimes I get sick of dingering with things lol.

Base timing you've got down pat! Great. Don't even think about the vacuum advance for the moment.

The total timing is somewhat adjustable, you can "manipulate" the weights to a point and squeak out more total movement which is to say the "total movement" of the cam advance at the top of the dizzy... which becomes the total number for your advance.. clear as mud yet? LOL... IF you have to much total advance you can damage the engine but you'll usually hear pinging and knocking / pre-ignition... I'm sure you know the sounds, and it sounds like you're not there just yet. If you're base is ten, and your mechanical total is 34 and the motor is still happy? I'd say it's a winner! The weights and springs will only affect how quickly you achieve the total, generally you want to reach total timing as quickly as you can. Provided it isn't pinging, banging bucking.

The vacuum advance will bring in or out advance depending on motor loading. As the throttle goes down, the vacuum goes down, at wide open throttle the vacuum advance does nothing. If your using manifold vacuum, and it shows 24 degrees btdc, great, once you open the throttle as vacuum drops the vacuum can becomes less effective but the mechanical advance is coming on as rpm increases. So one compliments the other is the easy answer.. HTH

I think for tomorrow night I'm going to get the base set back to 10 to 12 degrees, and then see what I can do with my total advance. The vacuum advance is no problem because I can adjust that wherever I need to. I'm not going to lie, at this point the 5.3 was the best thing I ever did to Rita , plug it in and go lol.

What is your base (initial) timing set at right now? If it’s not pinging, and it starts ok, you might just want to leave it alone. It sounds like the engine is pretty happy.

Set TOTAL timing at 4000 rpm, let the initial timing be what it ends up at (probably be 12-16°), if it idles too high (carb won't adjust down), move the vacuum advance from manifold to ported. Adjust your mechanical to be all in by 2500 rpm. AND HOLD ON....

Right now my base timing is close to 20° I would like to get it down to at least 16, but honestly it does start fine even when it's hot. You guys are making this easier for me, I was kind of wondering about leaving the initial timing alone, from what I understand, that's mainly for the starting ?

Originally Posted by 36 sedan

Set TOTAL timing at 4000 rpm, let the initial timing be what it ends up at (probably be 12-16°), if it idles too high (carb won't adjust down), move the vacuum advance from manifold to ported. Adjust your mechanical to be all in by 2500 rpm. AND HOLD ON....

If I set my mechanical to be all in at 2500, is it necessary to go all the way to 4000 with a timing light? Wouldn't it be the same as 2500?

If the mechanical advance is all the way in by 2500 RPM, the timing will likely retard about 2 or 3 degrees at 4000 RPM due to the timing chain stretching. I personally prefer not to rev a stock engine that high without a load on it.

If you're at 20°, either your vacuum advance is hook up while your checking or your idle speed is high and bringing in mechanical advance.

If your mechanic advance is all in at 2500 rpm that will be your TOTAL, and that's OK, BUT, check it at the higher rpm to be sure. Not unusual to see stock timing curves still advancing clear up to 4000 rpm. If that's the case, you'll leave a lot of horsepower on the table if you don't check it and fix it. You will only be at 4000 momentarily.