mechanical advance settings??
I just bought an MSD HEI distributor and it comes with several mechanical advance springs and bushings. I'm wondering which setting to use to get the best performance. I will be running an XE284 cam with 240 at .050 duration. 10.5. 1 compression and 93 octane gasoline with aluminum AFR heads. At what rpm should i have the timing fully advanced? its adjustable from about 2800 to 5500 rpm. Also, would i get better performance if i locked out the vacuum advance or should i leave it on? thanks for any suggestions
The rule of thumb is normally to START at 34 degrees (initial and Mechanical) all in by 3000 rpm. If you can you go more before detonation, great. 34 degrees is a good starting point. Do all of your testing with the vacuum advance disconnected.
Once that is done, then if you have an adjustable vacuum advance, START with adjusting to 48 degrees of total advance (initial, mechanical, and vacuum) and try it. If any detonation, back down, if none, then move up, but to no more than 52 degrees. You just have to test and find out what is best for your combo.
My set up
There are 360 degrees in a circle. There are 8 cylinders leaving 45 degrees between each cylinder. If you put 48 to 52 degrees of time in the motor you will be firing on the wrong cylinder.
I used the red bushing. It stops at 28 degrees.
I used the light silver and light blue springs. Advance comes in at 1300 rpm and is all the way in at 3200 rpm.
Disconnect vacuum advance. Set time at 10 degrees at 1000 rpm. The 28 degree stop with 10 degrees static will give you a total of 38 degrees. Reconnect vacuum advance and set idle to 800 rpm.
I found that if you use the 2 light silver springs the advance will come in around 800 rpm. I found that the engine would stay advanced at red lights and idle at 1200 rpm. I would have to shut the motor off and restart it to get weights to recoil back to 800 rpm.
Leave the vacuum advance connected. Later if you decide to buy a Digital 6 plus Multi Spark box, you can lock out the vacuum and mechanical advance and use the retard option on the box.
I guess everyone tunes their engine to fire on the wrong cylinder then.
HERE is the proof of what I was saying. Read the whole thread. I stand by what I stated. It is written in many articles. You need an adjustable vacuum advance to ensure you are not going over the limit of 48-52 degrees.
HERE is another article. I would not take the initial/mechanical settings as gospel, you still need to see what you can get away with, but your total (initial + mechanical) should be between 34-? degrees, the most your engine likes without detonation. The point of this article is the total as a whole (initial+mechanical+vacuum) should not be more than 50-51, and why you need so much. Just be sure to test to what is best for your application
There are a ton of threads on this board to back up what I stated, as well as many articles written in magazines, the net, etc. Just take a look. I run my setup with 52 degrees of complete timing, not a hiccup of detonation at all, but again, that is my setup.
It took me almost an hour but I read every posting and article that you suggested. The distributor can go up to 51 deg but not on a running engine. The vacuum loss retards the timing back to mechanical total.
14 initial + 21 centrifugal + 16 Crane adjustable vacuum advance with EGR = 51 total.
I understand that you set your distributor by it's maximum advance but I don't know why. According to these postings. If we were running at 38 deg total and both open up our throttles, you would drop to 35 deg and I would stay at 38 deg. I would be producing more HP.
Combining the 12 degrees of initial advance with about 8 degrees from the centrifugal advance and then adding 20 degrees from the vacuum can when operating at low throttle settings is just too much.. 40 degrees of total advance will probably cause pinging or even detonation.. That would be bad.
At 46 deg you would be on the next cylinder. That would be real bad.
Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
I disagree with the statement of seeing 50º advance if you were going down the highway, (10º initial, 20º - 25º mechanical and 15º from the vacuum) you stated elsewhere when the mechanical advance kicks in the vacuum is not adding anything.
I also disagree with seeing 50 deg of advance.
Under high load, WOT, performance conditions there is almost no manifold vacuum so the vacuum advance does nothing.
So if at say 2500 RPM, your total initial and centrifical advance is 32 degrees,and you are pulling a total of say, 36 with vacuum advance, when you start stepping down on the throttle, the vacuum advance will retard in relation to the loss of vacuum signal, to the point where, the 4 degrees of vacuum advance disappears, if enough vacuum signal is lost.
THanks greatly guys. The carburetor im running says to set initial timing between 14 and 16. so ill probably use one of the other stop bushings. Ill just have to see what the recommended total advance is for that cam. I doubt ill be running enough vacuum to work the vacuum advance completely. ill just have to work with it all when i get to that point. Thanks again for all the help.
If you start running hot. Move timing back and change stops.
I never had a carb manufacturer tell me where to set timing or I never noticed it anyway. What carb you have?
Its a Barry grant speed demon. It came with an application guide giving what size engines to use each carb with, what duration cams each carburetor is rated for, what transmission to use it for. And it gives the recommended initial timing setting. I havent even built the motor yet, im just kindof accumulating parts, and getting some advice. If it starts running hot, is that a sign of detonation? How can ya tell if youve got detonation problems. Ive tried and tried to tell, but every time i try to advance the timing real high on my truck, it just starts cutting out and missing around 3000 rpm, but i never can hear any pinging or anything. Sorry, im kinda turning this into an engine thread more than an electrical thread but thanks again for any advice.
I think they told you to use 14 deg because they assume you are using stock GM HEI. There is no adjustable stop. The 14 deg static plus stock HEI stop of 21 deg is 35 deg total. The stock HEI would require 17 deg of static to hit 38 deg of total.
You could also try the 25 deg silver stop and 13 deg of static for 38 deg total. This would be in the parameters of the carb manufacturers suggestion.
Do not use the blue stop or you will be back in stock GM HEI territory. Use the red or silver.
Too much time causes engine to run hotter. That's why I prefer 10 deg of static time. My car runs hot.
Detonation is a rattling noise, also known as ping. Sounds like your timing chain is rattling. Mostly noticeable going uphill. Very hard on the engine.
Keep us posted as to your progress.
sure thing, once again, i really appreciate all the help. thanks. It will be a while before i get it all done, i still gotta get some more parts and such. But i guarantee one thing, ive sure learned alot off this site. Thanks again and ill keep ya posted.
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