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Discussion Starter · #1 ·
I have an assumption that i'm either getting valve float, or the distributor is adding timing in the upper rpm range. I wouldn't assume there is a very easy way to check valve float, but I would assume there would be a way to check timing? This is a 6al msd box and a summit distributor. Initial timing set at 26 degrees and "all in" 34 degrees by 3,000.

Motor is equipped with some 200cc Promaxx maxx series heads so I can't imagine its running out of air. I have the rev limiter set at 6250 rpm, but it really seems to pull real hard between 3-5,000 rpm but struggle from 5-6250 rpm. Stock spring package on the heads.


EDIT: I GUESS I COULD JUST PUT THE LIGHTEST SPRINGS, OR NO SPRING AT ALL, AND THAT WOULD MAKE THE ALL IN COME IN SOONER LETTING ME CHECK THE TRUE TOTAL ADVANCE? I guess no springs would probably cause the distributor to get stuck at full advance though lol.
 

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Race it, Don't rice it!
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Wrap the throttle up to 5000 and watch the timing. Its that simple.
Also, Look closely at the MSD trigger wires. Look at the colors closely because it's not as obvious as one might think. When they are reversed, the box will still work but also add some problematic things at RPM. I've also heard/seen them reversed INSIDE the box so the best way to check them is check timing at 4000, Note the total, reverse the trigger wires, note that, use the one that gives the most retard and reset the dist to 34 at 4000.

What cam?
What spring?
 

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No spring at all would allow the fly weights to move randomly and never likely to return to their resting position, they would most likely move to fully advanced and just stay there.

The only way to test the curve when installed on the engine is with a dyno to load the engine. Other than that you need to find someone with a distributor machine that spins the distributor off the engine. You guess at an acceptable curve put it on the engine and try it out on a dyno or length of track, if you don’t feel it’s at its best you back to the machine to recurve and try again. It’s a way bigger PIA to do than you get off the antics of Power Nation or other YouTube mechanics.

26 degrees static foretells a damn big cam, don’t remember your specs. Really serious race motors may actually back the peak advance down but you still have about 10 degrees to what typically shows best power with modern chambered heads. Where the curve brings that in with regard to RPM is going to depend heavily on compression ratio, gearing, and weight. The detonation limit is your guide for max power anywhere along the advance curve. This will vary to a great extent with crankshaft loading, thus the relationship to gearing and weight and unfortunately without a computer program controlling the advance by transmission gear ratio the best you can do with a mechanical advance system is an average that pulls hard in the lower gears without blowing pistons in the upper gears where the engine is loosing mechanical advantage and forcing burn temps up. The other dependencies being environmental and geographical so there is no one size fits all when you’re out on the competitional edge.

Another way is with a detonation sensor that both informs you of the event and gracefully backs timing out to subdue the detonation without shocking the crankshaft. This is one of the elements that let the OEMs get the power they advertise as the end user are safely covered without even knowing the engine pinged. But there needs to be a computer in the loop to invoke some grace to the retard event.

Bogie
 

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Discussion Starter · #4 · (Edited)
Wrap the throttle up to 5000 and watch the timing. Its that simple.
Also, Look closely at the MSD trigger wires. Look at the colors closely because it's not as obvious as one might think. When they are reversed, the box will still work but also add some problematic things at RPM. I've also heard/seen them reversed INSIDE the box so the best way to check them is check timing at 4000, Note the total, reverse the trigger wires, note that, use the one that gives the most retard and reset the dist to 34 at 4000.

What cam?
What spring?
Yea I know I could, for whatever reason sticking my head in the engine bay and revving it to 5k plus makes me worried about my pretty little face getting smacked by some flying parts though! I just need to nut up or shut up!

I'll double check the MSD trigger wires. My assumption was if they were wrong, it wouldn't run. That doesn't seem to be the case.

Cam is a Crower 236/240 .555/.559 with a 110 LSA. And sorry, initial timing is actually 24 degrees.

Springs are stock hydraulic roller Promaxx springs
Hydraulic Roller .600 max lift
Closed …………………… 145 lbs. @ 1.800”
Open …………………….. 325 lbs. @ 1.200”
 

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Well, if it's gone to your set limit of 6250 before, it likely will live there again without any issues. And if any issues do come up, you were going to find those eventually anyway. Do us one favor though, make sure you have the wheels chocked and do not stand in front of the car - make your checks from the side of the vehicle.
 

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first problem is hydraulic roller. around 6400 they are all finished. The rollers are very heavy compared to solids. Second (or before first) check your fuel pressure and fuel flow at high rpm. Make sure the fuel line is big enough and your regulator is set properly for your fuel management system?
 

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More for Less Racer
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Using one or two small zip ties, lock the mechanical advance closed and set the timing locked at 34-36° and see what the performance is, just for a test.
This will eliminate the chance of it being the mechanism takes off on it's own and adds more advance above your 3000 rpm all-in spec.

You may be experienceing retard at high rpm instead of more advance, this is a more likely scenario.

My view is this is likely valve float.
I would have preferred to see 380-400 lbs open pressure, 325 seems pretty light to me.

Current spring works out to 300 lb/inch rate, that also seems too low to me. At .559" lift you are only seeing 313 lbs open pressure.....that's flat tappet territory.
 

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Yea I know I could, for whatever reason sticking my head in the engine bay and revving it to 5k plus makes me worried about my pretty little face getting smacked by some flying parts though! I just need to nut up or shut up!

I'll double check the MSD trigger wires. My assumption was if they were wrong, it wouldn't run. That doesn't seem to be the case.

Cam is a Crower 236/240 .555/.559 with a 110 LSA. And sorry, initial timing is actually 24 degrees.

Springs are stock hydraulic roller Promaxx springs
Hydraulic Roller .600 max lift
Closed …………………… 145 lbs. @ 1.800”
Open …………………….. 325 lbs. @ 1.200”

Watch out, that dipstick might blow out!
 

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The real danger is if running a mechanical fan is a blade coming off while your head is nearby.

The other is if you have vacuum advance hooked up you will get a false reading as unloaded it takes very little throttle to gain high RPM so there will be vacuum application to the timing that would not happen out in the street/strip world, so that system if used needs to be disconnected and the vacuum source plugged.

Bogie
 

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Discussion Starter · #10 ·
Watch out, that dipstick might blow out!
This actually did happen! Dipstick came out and so did a little bit of oil. It’s never done it under load at all? Can anyone explain why? This is a really fresh 383, is that something to worry about? I ran it down the road at full throttle through a few gears and rechecked. It didn’t move, so again it doesn’t happen under load.

I’ve been told a couple times, and now once more, that the springs are pretty weak for the cam I’m running. Other than a dyno, is there anyway to confirm valve float?

Also, how would I loose timing in the upper rpm range? I’m trying to figure that out logically but not smart info to do so!
 

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Race it, Don't rice it!
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Without getting super technical it’s the way the module and coil work. They need a certain amount of time (milliseconds) to trigger and the coil charge, discharge and recharge. The higher the rpm the less time there’s is so the ignition system as a whole begins playing games trying to keep up. Eric is right, usually regards timing when it can’t keep up and eventually even skips firing cycles. IE. High Speed breakup. You have a CD box though, I wasnt thinking that when I posted earlier, which is basically two heavy duty modules firing sequentially so each has more time to do it’s thing. A zip tie is a good idea. Quick and easy with the engine off. You can add the zip tie running but you’ll gonna have to be pretty fast to get the tail cut off spinning around in there.

most people aren’t used to sitting on top of a running engine at rpm. You get used to it.
 
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