Originally Posted by dmjlambert
I would like to thank everybody for your input on this thread. I think it is getting pretty interesting and I'm learning.
I did read the links. I recognized all of them and I've had them bookmarked before. I can't say I understand all of it, but I think perhaps I understand a little more each time I read them. I'm a novice, and I show it at the beginning of this thread where I didn't have much of a clue about how to use the timing light or the timing marks and didn't know how many degrees per mark. Good, I'm glad you read the links. I didn't realize how new you are to this, my apologies. I had to read it a few times and mess with the truck as well to fully grasp it. Once you understand it, it's actually very easy. Everything on these trucks is like that. I'd hate to see you learn it the wrong way right off the bat. You're in a great place to learn from a lot of great people.
I have a GM HEI distributor that a friend gave me. I don't know what vehicle it came out of. I've been looking for a part number stamp on it but don't see it. Perhaps it is inside. I will look some more. The vacuum advance can was damaged and it didn't work. Another friend shade tree mechanic put the current vacuum advance on it, and I don't know how it was selected, I just know it was new from O'Reilly Auto Parts. He set the timing, had trouble figuring out the timing light, and the engine was working OK but pinging sometimes while driving. Then I picked up from there and started this thread late last year. The HEI is a nice upgrade. Make sure your spark plug gaps are increased to 0.045 with HEI. Since the HEI is unknown, consider it an emissions stocker. So you’ll probably need a different set of springs and the weights and centerplate probably are incorrect too. The new vac advance can you have probably isn’t right also, but may be fixable. You’ll need to figure out how much mechanical advance the distributor has too. To do that, you’ll have to throw the timing light back on it. Unhook your vac advance and verify you still have it set to 7 degrees. It doesn’t sound like you’ve had the distributor cap off, so yank it and make sure the weights are lubed up and see what numbers are stamped into the weights and center plate. Then take the springs off. Be careful, they will shoot off on you and you don’t want to lose them. Throw a rag on top while you’re doing it. Put the cap back on, start it, hold it at about 1500 to 2000 rpm and see what the timing is at. Taking the springs out ensures the mechanical advance is all in and you won’t have to rev it up crazy high. Take the number you just got, and that’s your total timing. Now subtract your 7 degrees initial timing, and that gives you your mechanical advance degrees. Then plug the vac advance back in to manifold vacuum. Let us know what timing you see.
I am getting more involved with the mechanics of this truck. Now you guys are getting me interested in tuning. I don't have a vacuum gauge but now I'm interested in getting one. I don't know how much mechanical advance the distributor has. I don't know what type of heads the engine has. The casting number is 10147898. After reading all the great info on this forum that you guys offer up, I have recently moved my vacuum advance from ported to manifold vacuum. The 898 heads are standard stock crate engine heads, so they’re really not performance heads. They’ll work just fine for what it sounds like your purposes are though. Great, glad you moved it over to manifold vacuum.
When I brought this thread back to life the other day I was interested in knowing the technical details about: if most total advance you can go with your timing without causing detonation is desirable, why is higher amount of vacuum advance not better than smaller amount of vacuum advance? A higher amount of vac advance isn’t desirable because you can only go so far with advancing before it counteracts itself and starts pinging and causing damage. Vac advance is only meant to be supplemental, at idle and cruise, when there is high vacuum/low load on the engine and the fuel mixture is lean in order to allow for efficient combustion.
Please tell me if I figured out the answer from input from you guys and the links provided that I have been reading. I want the maximum total advance you can get at wide open throttle and heavy loads without causing detonation. If I have a lot of vacuum advance, and adjust the initial timing as advanced as possible to avoid detonation at idle and mid-throttle, it will be the appropriate amount of total advance at idle and mid-throttle. But the total timing will drop more than I want at wide open throttle, because all of the vacuum advance goes away at wide open throttle. On my engine, since I have 23 degrees of vacuum advance, I will lose 23 degrees of advance from my total at wide open throttle. Optimum performance may be realized if I "lost" a lower number of degrees. Do that by lowering vacuum advance and increasing initial advance by about the same amount. The engine with stock parts was designed for minimum warranty period repairs, pretty much without regard for performance, and severely limiting advance under load reduces the chances of my engine having any detonation problem, regardless of quality of fuel. Reducing the amount of vacuum advance is part of hotrodding the distributor. Sounds like you pretty much got it. You want your total timing (initial timing + mechanical advance) to be 34-36 degrees and your vacuum advance to be 10-12 degrees. Using my truck as an example, my dizzy has 22 degrees of mechanical advance in it, so I run 14 degrees initial to get to 36 total. Most aftermarket and GM Performance dizzys have 20 to 22 degrees in them. The springs you put in the dizzy should allow for full mechanical advance by 2500-3000 rpm. My vac advance is limited to 10 degrees and you’ll have to adjust the rate at which it comes in based on your vacuum level at idle. When you check your timing at idle with the vac advance hooked up, you should see around 20-24 degrees. That’s what the engine wants.
|