Well I am sort of confused...read tons of articles on vacuum advance on GM HEI SBC from reputable authors and thought I had it somewhat understood on the basics: base timing, mechanical, vacuum: ported vs full. Anyway totally confused in wanting to get a better tune on my recent rebuild of my 383 SBC 10:5:1 compression, XE262H Comp cam, 3.08:1 rear, 4spd (basic specs) just touched base with the engine builder who set up my initial timing of 10deg base and 20deg mech all in at ~2500 RPM (stock GM HEI). I told him of what I read about what I read about typical timing of SBC (initial, total, vacuum advance etc) and he disagreed. He said for my application (gear ratio, lot early cylinder compression with short duration cam, high static compression etc) that I want a total timing of 28-32 degrees for the street, that is ideal. Mentioned I just purchased an Acell 3025 adjustable (rate and amount) vacuum advance canister and I planned on experimenting with vacuum (ported and manifold) etc and we talked about for my application that vacuum advance is not ideal or necessary, if it was used it would not increase the total amount of advance but work with the mechanical advance to change the rate of advance which would help if I was running a lower gear ratio of like 3.73:1 or 4:11:1 (which I am not). I mentioned how typical SBC would run 32-38 total and at cruise would run timing in the 50s (basically to help gas mileage on cruise), which he said that amount of timing is no good for my application and it would "cook" my engine ,that that amount of timing would cause damage, especially since my RPMs are around the 2500 RPM range, basically my build is setup for low RPM, high torque applications. Can anybody help me understand the thoughts behind what I am reading (typical) and what the builder's opinion's are (based on his years of experience and old artlicles, manuals with distributer curve info etc). What am I missing ?
Well I am sort of confused...read tons of articles on vacuum advance on GM HEI SBC from reputable authors and thought I had it somewhat understood on the basics: base timing, mechanical, vacuum: ported vs full. Anyway totally confused in wanting to get a better tune on my recent rebuild of my 383 SBC 10:5:1 compression, XE262H Comp cam, 3.08:1 rear, 4spd (basic specs) just touched base with the engine builder who set up my initial timing of 10deg base and 20deg mech all in at ~2500 RPM (stock GM HEI). I told him of what I read about what I read about typical timing of SBC (initial, total, vacuum advance etc) and he disagreed. He said for my application (gear ratio, lot early cylinder compression with short duration cam, high static compression etc) that I want a total timing of 28-32 degrees for the street, that is ideal. Mentioned I just purchased an Acell 3025 adjustable (rate and amount) vacuum advance canister and I planned on experimenting with vacuum (ported and manifold) etc and we talked about for my application that vacuum advance is not ideal or necessary, if it was used it would not increase the total amount of advance but work with the mechanical advance to change the rate of advance which would help if I was running a lower gear ratio of like 3.73:1 or 4:11:1 (which I am not). I mentioned how typical SBC would run 32-38 total and at cruise would run timing in the 50s (basically to help gas mileage on cruise), which he said that amount of timing is no good for my application and it would "cook" my engine ,that that amount of timing would cause damage, especially since my RPMs are around the 2500 RPM range, basically my build is setup for low RPM, high torque applications. Can anybody help me understand the thoughts behind what I am reading (typical) and what the builder's opinion's are (based on his years of experience and old artlicles, manuals with distributer curve info etc). What am I missing ?
To be exact a better understanding of the cam timing would be needed. But basically your mechanic sounds right.
The rocket science under the subject of ignition advance is the time it takes for a burn to occur and the time in which it has to occur. These have some strong interelationships but each can be dissected.
The time it takes for a burn to occur is related to the molecular density within the cylinder. In general high compression increases molar density which reduces burn time, therefore, the amount of lead can be reduced as the speed of the burn is sufficiently fast to go to completion in the time available. The camshaft specs become the next important player and is a very large player. A mild cam increases the molar density within the cylinder
at lower RPMs while reducing it at high RPMs. Such a cam combined with high static compression doesn't need a lot of advance, because the burn proceeds quickly thru the thicker mixture. If you change to a more aggressive cam, then at lower RPMs mixture is lost out the exhaust during overlap and is pumped back into the intake manifold with a late closing intake valve until such time as there is sufficient velocity inside the intake manifold to overcome these effects and ram mixture into the cylinder. Typically below the torque peak of such a cammed engine, the molar density in the cylinder is below that of the otherwise equivalent short cammed engine and the less dense mixture needs more burn time simply because the molecules are spaced further apart and that slows the flame front. Therefore, there will not be enough time for the burn to go to completion and energy will be wasted as the exhaust valve will open on the still to be burnt mixture and it will be tossed out to burn in the exhaust system instead of pushing on the piston. This results in less power, poor mileage, and really hot exhaust headers/manifolds.
The modern race has been to improve power and efficiency with higher burn speed instead of more timing lead. This is because there is a point where the timing lead raises pressures too much too soon and this trys to drive the piston back the way it came from, thus lost power and efficiency, and a greater tendency to detonate. Modern high swirl, fast burn combustion chambers and ports are designed to get around the need for excess timing thus to reclaim the energy lost to trying to push the piston back from whence it came. Another trick in that bag is the movement to long Lobe Separation Angles (LSA) on the cam. This reduces overlap and helps boost lower end dynamic compression which is a different way of saying it improves molecular density of the mixture so it burns quicker. Timed fuel injection is another trick in that bag. Whether mechanical or electronic, the designer can with these high pressure systems hold off on putting fuel into the engine until the exhaust valve is closed so there are no fuel losses past the exhaust on overlap. Direct injection is here where the fuel will be injected into the cylinder after the valves are closed. That means right off the bat that the cylinder now has that amount of air that with carburetors, throttle body or port injection schemes was occupied by fuel. So when the fuel is directly injected into the closed cylinder the absolute pressure in the cylinder will rise by the amount of the fuel volume. It will be in effect like a mini-supercharger. Cylinder pressures will be higher, energy losses thru mixture loss curtailed, the fuel evaporating in the cylinder will cool the charge allowing higher compression ratios allowing reducing spark lead further to capture more lost energy, and making our hot rods further obsolete and slow by comparison.
Unless you are using fast burn chambered cylinder heads, I think your timing could be advanced.
I see no reason that a curve couldn't be tailored for your engine to give it the 34-36 degrees that many SBC engine combos like- but at the RPM that it's needed. If you cruise at 2500 RPM's you don't need all the timing in, but you would want it to be available when it IS needed, IMO.
And I also see no apparent reason not to use a vacuum advance. It helps in several ways, no need to go into all the details at this point, but it sounds like you already have a good grasp of the issue. Perhaps your mechanic needs to be brought up to date?
Fill in the details so as to see if there's a piece of the puzzle missing.
I don't see why you can't use about 30 degrees total for initial plus mechanical advance (10+20), combined with about 18-20 degrees of vacuum advance. That is fairly typical for an SBC 350 or 383. Most of them will tolerate an even higher initial advance, depending on the cam.
The vacuum advance only works under high vacuum, such as highway cruise, so it should not cause problems with pinging during acceleration (I assume that's what the mechanic meant by "cook" the engine). You could also set it up without a vacuum advance, but it would probably reduce your gas mileage significantly.
Bogie....Thanks for taking the time to post the detailed and very informative info. It further helps me understand what is going on...and the "whys".
Cobalt327 and Bruce...thanks for your insight. I am thinking I won't be able to "convince" my mechanic to change from what he knows to work, tried to emphasize the fact that I just wanted the additional advance under cruise since I was concerned about gas mileage. I guess if I want vac advance set up I need to experiment on my own since he defineately don't want me running timing in the 50s no matter if it is cruise or not.
I agree with Cobalt and Bruce,... your "mechanic" has a poor grasp of what vacuum advance does for the engine. Sounds like he is one of the many who know just enough to be dangerous, and lump vacuum advance in the "pollution garbage" category. You are on the right track in your thinking already, your engine will really benefit from a functional vacuum advance, and will like it to be up near the 45-50 degree range you listed, maybe more, and use a full manifold vacuum source. You will have to experiment with the adjustable can and find the highest it will tolerate without surge or ping.. High vacuum situation, cruise type mixtures are very lean and require the even earlier spark to fully burn and provide their highest cylinder pressure at the correct point in the combustion cycle.
Your spark timing is going to differentiate between your spark plug heat range and spark plug gap.
Using a stock GM Coil in Cap HEI requires a plug gap of .045, a stock external mounted coil requires a .035 spark plug gap. Both with a mid-range heat index.
I would verify this. Then with your vacuum advance hooked to a manifold vacuum source, set your total timing (all in)above 3000 rpm, at 47, or 48 degrees. Test drive it, and listen for any pinging. If you hear any, back that timing down by 2 degrees, and test drive it again. If not, run it up to 50, or 51 degrees total.
your "mechanic" has a poor grasp of what vacuum advance does for the engine. Sounds like he is one of the many who know just enough to be dangerous, and lump vacuum advance in the "pollution garbage" category.
Thanks for the insight ericnova72. I am going to stop by his shop this morning, but I am doubtful I will convince him to dial in additional timing. He is going to test drive the car after break in (I have a few hundred miles on it after install), read the plugs, adjust the carb (mixture, primary jets if necessary), tweak timing etc. My "mechanic" for what it's worth has been doing performance engine building for 30 yrs and after talking to him he seemed to really grasp what was going on and was explainingg the engine internals, cam, valves, pistons and what they were doing in relationship to my setup and was convinced that the additional 20 deg or so timing even at cruise will not benefit my build and that it defineately is useful for lower compression engines and emissions , but just don't want to use it for my build with my compression, cam, rear etc. He even gave me a history lesson of ported vs manifold vacuum etc..we talked about cam, compression and piston location at spark advance of 50 deg or so. It seems he bases his thoughts on a lot of the older setups maybe from the 60s or early 70s and was even looking at old repair manuals and articles from back in the day showing the actual distributer advance curves. Anyway I guess I will let him do his thing and on my own dial in some vacuum advance as it seems it is what I need from what I read and the replys to my post.
My "mechanic" for what it's worth has been doing performance engine building for 30 yrs and after talking to him he seemed to really grasp what was going on
It's not so much that what he's saying is wrong- only that there are different ways to achieve the same thing.
His methods are going to produce a safe, conservative timing curve that you will likely be able to run crappy gas with, pulling a trailer uphill in the middle of summer. He's making doubly sure you're not going to return w/any detonation damage! And I believe this concern of his is at the root of his recommendations vis a vis timing advance curves and no vacuum advance.
However, adding vacuum advance into the equation is where I have a differing opinion. I believe that vacuum advance can be safely added- it drops out ASA the engine load increases (and the manifold vacuum signal decreases). There is a brief period of "cross-over" where the timing can be high, during transition between engine speed and load, but this isn't generally thought to be a problem in a car and engine set-up such as yours.
Total timing may or may not need to be advanced from what he's proposing. AFA the "efficiency" of the Pro Comp heads- I really have no empirical knowledge. They may not require as much timing as an OEM cast iron head- at least I'd hope so. They're Al, which offers a bit of leeway in any event.
Bottom line for me, is that likely the mechanic's timing set-up (as far as mechanical and initial goes) will work fine. Possibly it will leave some MPG and HP on the table.
You might have issues that will need to be addressed when adding a vacuum advance to his set-up.
The possibility exists that using a vacuum advance operated by "manifold" (as opposed to "ported') vacuum will have the timing at idle a bit high. This could be a problem w/dieseling at shut-down. Likely it'll require the idle speed to be lowered and idle mixture to change some.
These are minor problems, though, and can be addressed when (and if) they come up.
Here is an update, just spent the better part of the afternoon tuning the car with the builder (several hours). Timing, mixture, pulled plugs, idle adjustments, test drives etc Hooked the Acell adjustable vacuum advance unit to the ported vacuum with it screwed all the way CCW (minimal vacuum advance) and long story short base timing is 10 deg, centrifugal 20deg (did not mess with the GM HEI curve), with vacuum ends up 37-38 total timing (at least what the timing light showed when you cranked the idle up and the vacuum increased)...I believe mechanical is all in around 2500 RPM or so. Also seems like the #68 primary jets (dropped 2 sizes from #70) worked out well. After tweaking timing, mixture etc and ending up with this final tune (for now) the car pulls w/o hesitation through all the gears (keeps on pulling even at high RPMs) with no pinging even in low gear up hill. He really got on it (embarassed to say more than I ever did) and it kept on pulling with out any hesitiations (no problems except for shredding an alternator belt), I am happy on the performance even with this 3600 lb car with 3.08:1 ratio rear (makes me wonder if I shoulld upgrade to 3:73:1 in the future ), cam selection I think worked out well for this build and my applications. Also to note I am running 93 octane gas. Also starting and shutdown was acceptable, no hard starts or dieseling/run on.
Is this the ideal tune ? I am guessing not, but I have much to learn and plan on experimenting more once I get a few more miles on the engine. Thanks for all the posts...been a help. This is a "hot rodder" forum so the goal I know is get increased performance and not be so conservative.
Your scheme to put some miles on it then do some fine tuning makes good sense to me. And that your guy was ready and willing to put his foot in it is a good sign, too.
The MSD box will make tuning easy, that's a real handy thing to have!
I see the vacuum advance is activated by ported vacuum. Don't be afraid to change this to manifold vacuum as you start testing, just be aware you will likely need to lower the curb idle.
Either ported or manifold vacuum has its place, but most performance engines "like" manifold vacuum- it tends to make the idle smoother and gives the engine a lot of response right off idle.
Also, don't hesitate to dial in more vacuum advance than the minimum. As long as it's not "surging" at cruise or pinging when you whack open the throttle (the "transition" I wrote about earlier), you're not likely to be over-advanced. And, because the vacuum advance drops out as the vacuum decreases, it won't have any effect on WOT operation, either.
I'd be real curious to see how the current set-up handled regular and mid-grade fuel. Might surprise you. But if cost is not an issue, using the 93 is fine.
3600 lb car with 3.08:1 ratio rear (makes me wonder if I shoulld upgrade to 3:73:1 in the future
I think u would like a 3.73 final rear end gear
or even 3.55 's :thumbup:
however drive it for a while and see how it pans out
dont let any of them off the hook until u get it how u want it.
have a good time with it
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