|12-11-2011 10:09 AM|
OK well to get it correct, finding true TDC first following the link cobalt provided is essential. The timing marks most likely only go up to about 18 Deg BTDC so unless you have a dial back timing light you should make a timing tape as well, follow the other link directions.
Since your engine is stock you can try about 12 deg BTDC initial timing, yes then add another 22 with mechanical advance for about 34 total all in by 3000 is a good place to start for a heavy truck. These two setting are made with the vacuum advance disconnected. Then add in another 14 with vacuum advance for a total of about 48.
First off after you are confident that the timing marks are OK check and see what you have now as far as total mechanical advance and total vacuum advance. Then you can determine if you need to tweak the distributor. It is best to find out these values first then you can go from there. Post some results after you check em out.
It is a good idea to pull the distributor and check it out for wear and clean it up. If you find it to be worn out then that skip white 6500 is an excellent value, I would not bother trying to rebuild the OEM when you can get an new one for less than 50 bucks, thats just me. Some more links:
Generally speaking with your stock 454 you should not need allot of modifications to your distributor, I am not sure that an adjustable can is really necessary in your case but it certainly won't hurt to have one in there and doing so will give you a chance to inspect the other working functions of the HEI as well as ensure your timing is setup accurately and to your liking Take some timing readings and post them if you are so inclined, good luck.
|12-10-2011 06:10 PM|
Its a stock 1974 454 with 1978 781 heads in a 1979 truck, probably about 8:1 comp, 4:10 gears, 4800# truck. Only thing not stock is the headers and exhaust (dual 2.5" magnaflows)
36 degrees total timing sound good? All in by what? 3000 rpm? Maybe I should just buy the advance kit, get the thing tuned and be done with it. This truck sure could use more MPG. As i'm sure you can imagine. Any little bit helps.
|12-10-2011 05:04 PM|
Thanks for that cobalt I have learned it all by simply doing it over the last couple years and reading posts from senior authorities like yourself, cheers!
454 there are no absolutes but there are common factors to consider. As cobalt mentioned this timing discussion is relatively simple in its subject matter but it is hard to describe via postings cause there are so many little differences in eveyones application.
There are 3 main aspects to timing an HEI distributor. Then there are other settings and tweaks that help fine tune it.
Since you are asking about the vacuum advance,,,,you must first dial in the other two main aspects that being initial or base timing and then mechanical advance and the rate that the mech adv comes in vs rpm (the curve). Only then can you really know what you need to do with the vacuum advance.
For starters post what the engine is made up of, is it stock? if not what are the modifications including the cam and compression ratio, stall convertor, gears, vehicle weight/type.
To answer your question if this is a stock HEI then it will usually provide too much vacuum advance but it all depends on where you have set the other two aspects of timing (base + mech)
a couple repeats on my part mark, was typing while you posted
|12-10-2011 04:55 PM|
What could keep you from using the stock vacuum advance can w/a [B] #99619-1limiter plate would be the point where the stock can advance starts and stops. Very briefly, this will depend on what the specs are on the stock can and what the engine requires.
Crane adjustable vacuum advance can and limiter plate kit- p/n 99600-1.
The exact vacuum an engine makes depends on several things and the cam is a major player when it comes to vacuum.
|12-10-2011 04:26 PM|
Great idea about finding true TDC to verify timing.
Just one thing I want to clear up. Are you guys saying that the stock vac advance will provide all the advance that I need, and that I should retain the stock vac advance and just limit it with the limiter plate (and recurving the distributor with the springs)?
|12-10-2011 12:58 PM|
I've posted the following about a hundred times, so once more can't hurt I suppose:
DETERMINING TDC will allow you to be sure the timing tab and damper are correctly indicating TDC.
MAKE A TIMING TAPE to see what the total timing is, w/o needing to use a dial back timing light. You can also buy a timing tape, get one that matches the diameter of your damper.
|12-09-2011 11:19 PM|
|streetbruiser||Thanks for the schooling|
|12-09-2011 11:08 PM|
When using a vac gage inside the truck you can find the sweet spots for engine load VS engine vacuum. You can use these values to adjust your vacuum advance can. If you have a vacuum pump this can be used on the bench or under the hood to tweak the range of the vacuum advance can when it is connected to the distributor pickup. You can also use these vacuum levels to help choose and setup carb power valves or vacuum secondaries or choose step-up spring ratings for those types of carbs.
Vacuum advance has two things going for it. One is at idle the other at cruise. I use the maximum vacuum advance all in at cruise but just barely. Once you get into light throttle tip in you want that vacuum advance to start dropping out pronto. The adjustable can can help you do that as long as you are within the adjustable range of the can (typically from 5 to 11 "hg). This is all considering we are timed for max power. The engine is most likely to ping at highway light throttle low load tip in especially if the tranny does not kick down with little throttle and the engine vacuum only drops a few "hg.
Just to add, in my example the adjustable vacuum advance canister does not limit the total amount of vacuum advance, you need the limiter plate for that.
There are claims that some cans can actually change the range of when the advance is added as well as limit the total amount added, my crane adj can does not limit the total amount added it only allows one to change the spring range.
All in all with timing it is more important to have you initial and mechanical advance set up correctly, you can then set your vacuum advance up but it is more important to limit the vacuum advance than it is to adjust the diaphragm spring range as with the adjustable type.
|12-09-2011 10:02 PM|
|streetbruiser||ok. i was responding to the concept of driving around with vacuum guage attached. I'm ignorant to what that would indicate.|
|12-09-2011 09:52 PM|
Didnt use the vac gauge to set the timing. I was just saying that the reading I got when I did use the vacuum gauge was steady at 15". It didnt drop, swing, vibrate, etc.
It would have had erratic readings if the timing was off i would think.
|12-09-2011 09:51 PM|
|streetbruiser||i only use vacuum guage for carb tuning. A good timing light is a way better way to set your advance timing IMO|
|12-09-2011 09:51 PM|
|Torque454||i have no idea if i am getting too much or not. BTW 15" is at idle.|
|12-09-2011 09:45 PM|
|streetbruiser||actually what i said about adding lighter spring isn't right. that will only make it come in faster. I agree the limiter plate may be what you need if you are getting too much advance.|
|12-09-2011 09:35 PM|
Well why buy the skip white dist anyhow if your engine runs good? Buy the vac advance kit and give it to a bud that needs it,,,it's XMAS after all
The best part of the kit may be the vac adv limiter plate if it has one, that is the best 4 bucks you could ever spend. It is more important than the adjustable can imo. typically the can adjustments can vary the actuation of the advance by only a few "hg of engine vac.
Your motor pulls 15 "hg vacuum is that at idle what about cruise?, most adjustable cans cannot be adjusted above about 12 "hg. At 15"wg the vacuum advance will be all in no matter how you adjust the set screw. But if your motor pulls say 11" hg at idle or 8-10"hg at cruse RPM lets say then you can make better use of the adjustable can cause you can still adjust it within those vacuum levels.
take it for a spin with the vacuum gage connected and record whats going on,,,it will help you spend your money or not
|12-09-2011 09:34 PM|
|streetbruiser||I always lay down the money for an adjustble vacuum advance b/c most factory cans add up to 25 degrees extra timing, which is too much for me. However it is just one little detail that could probably wait, especially if you are happy with the way the car runs already. There are definitely things more fun to spend your money on, and there is no guarantee that you will notice much of a difference anyway. If you have a good digital timing light that shows rpm, or have a friend that has one you can borrow, and you find that you are close to about 35 degrees total timing, all in by about 2500 rpm; You can't really improve on that much. I usually like to set my initial timing 12-14 degrees btdc. You can pick up a spring set for a couple bucks and put one lighter spring on the mechanical advance. That might be enough to get you there too.|
|This thread has more than 15 replies. Click here to review the whole thread.|