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Discussion Starter · #1 ·
Just looking for thoughts on timing.
I have a pump gas 540 Chevy (10.2:1), hydraulic roller (.640 lift int & ex, lift is .243/.249 @ .050, 112 LC), mechanical advance only, 707 HP at the crank on the dyno per the builder (crate motor).

This is in a 3400 lb car with 355 gears, converter flashes at around 3500.

I know this question can have dozens of answers and there are endless variables, just looking for general recommendations on initial timing and where full advance should come in.

Right now I believe initial timing is at around 12 degrees, total timing is around 32 and full advance doesn't come in until about 4000.

I recently bought the car and based on several discoveries on the build, I have doubts about their expertise.

I feel a shorter curve with less total advance (more initial timing) would clean up the idle a bit, but maybe I'm wrong. Reaching out to others that may have a similar combo for experienced advice.
 

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True Hotrodder
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Ha - yep nobody reads.

I would go for 15-16 initial, 34-36 total, all in by 2500-2800. And yes there are a lot of variables but that should put you in a better ballpark and then you can make minor moves depending on what the combination is telling you it wants.
 

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Without vacuum advance which probably wouldn’t function with whatever idle exists with that cam but with a milder yet still fairly hot cam you usually have enough vacuum advance to crutch a lower base setting that makes the engine easier to crank.

Mechanical advance only with your cam is going to want a lot more advance. Off the top of my head 18-22 degrees to start but you’ll have to plat around to find what it likes. This with your compression ratio will be hard to crank up because the early spark will put a lot of load on the crank thus starter. You may find it advantageous to put a separate switch on the ignition so the starter can spin it up before facing early burn cylinder pressures as it tries to shove such early firing pistons over TDC.

Generally any base advance beyond 10 maybe 12 degrees needs to be subtracted from the mechanical to prevent high rpm detonation by over advancing the total. Certainly it should all be in earlier than 4000 rpm. But this is something you have to fish for in terms of exactness as all sorts of variables come to play including your geography and weather beside gearing and other vehicle specifics. Just under the detonation threshold is where max power is to be found, this you’ll have to tease out.

Bogie
 

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Ported vacuum? You lost me? Are you referring to vacuum advance? If so it only has mechanical advance.
Ha - yep nobody reads.

I would go for 15-16 initial, 34-36 total, all in by 2500-2800. And yes there are a lot of variables but that should put you in a better ballpark and then you can make minor moves depending on what the combination is telling you it wants.
Yup I missed 'mechanical advance only'. Forget the 'ported vacuum' comment please - sorry.

You're looking for timing recommendations on a built motor that's already been on a dyno. The place to optimize the timing settings is on a dyno. No one is going to give you a specific number that is correct, but rather it will be a guess. I wouldn't chance a build like yours to guesses (probably close to $10k build imo).

I would start by contacting the dyno that ran this motor and find out why the settings are where they are at currently, or where they set them to. If the numbers are correct that you supplied in the 1st post, then there a number of ways why they might be there. The engine builder was looking for a number, 700hp, and then detuned it for longevity/safety; the dyno operator tried more timing and it fell on it's face and left the timing where it is at currently; the balancer isn't set at TDC or is failing giving a falsely low number with your timing light.


When I see 'mechanical advance only' I assume race only with no street driving, or as Bogie mentioned, a cam that develops little to no vacuum at idle. Your cam is not a giant cam for a 540. Assuming a normal ICL/LSA cam grind, if this is in something that will run on the street, then get a vacuum advance dizzy ad use it imho.

How do you intend to use this motor/car? Can you track down the dyno operator and builder?
 

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True Hotrodder
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OMG.

Okay there's misinformation right off the bat regarding the cam if you didn't notice. A .243 number is not going to give you .640 unless you have one hell of a rocker arm involved. Those are duration numbers, not lift numbers.

He just purchased the car and has whatever information the previous owner passed along to him. And unless he has "the" or "a" dyno printout in hand, I doubt that this engine ever got bolted to one. This is probably a combination engine that a builder put together - maybe dyno'd the first one built - and is selling copies of it. Its a common practice and there's nothing wrong with doing that - the previous owner probably didn't know either. The cam is relatively mild as they go for a 540 BBC.

So this brings up the timing. Obviously he has driven it and realizes that it's not making the power where the vehicle needs it and after looking at the current timing setup has asked for suggestions. As all of us have mentioned there is no "timing setting" that is a fits-all deal. We throw our 2-cents out there, he picks something that seems reasonable and plugs it in and tries it out. Nothing recommended that I read is going to destroy an engine. He can fine tune what he has at this point and be happy with it as a street cruiser and occasional stoplight blast. Given the current cost of fuel it's not going to be the family grocery getter.
 

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Discussion Starter · #8 ·
All good info and I appreciate it.
The motor seems to make good power as far as "seat of the pants". Have not been to the track yet but it will easily ignite street tires through all three gears. Yes it is a street cruiser, occasionally will see the drag strip.
Skip White is the builder. I've made no effort to contact them mainly because I've read they rarely reply and I've also seen mixed reviews. I have no idea whether or not the distributor in the motor was the one used on the dyno, I'm assuming not.

I'm fully aware there is no magic formula that can be applied. I'm mainly looking for a ball park on how much initial advance I can safely run for better idle quality.
One last note: the cam specs I initially supplied were from the cam card and this matches what is listed on the web (Howards p/n 120665-12). .243 duration is at .050.
 

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True Hotrodder
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Yep - I found that camshaft.

Again it's a trial and error deal with timing - take 500 pounds out of the combination and it's a different number.

I would make a change and see what it does, record what you do so you can get back if needed - have fun with it!
 

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I would go for 15-16 initial, 34-36 total, all in by 2500-2800. And yes there are a lot of variables but that should put you in a better ballpark and then you can make minor moves depending on what the combination is telling you it wants.
Came here to say this. 16 initial/36 total. If you have some really good heads with inspired chambers you might pick up a couple ponies at 14/34. Where you get it all in is pretty academic since you stall at 3500. Anywhere before that (I say 2800-3000), have it all in. You'll have to dance around the total to find what works best. Without knowing the chamber design or piston dome/dish it's hard to guess. Domes kill flame front speeds requiring more spark lead. Flat tops are better. Dishes may sometimes require more advance depending on squish/chamber.

Initial is totally inconsequential. Put it where it runs well. The only time initial can be too far advanced is if it's to far advanced for mid-RPM and heavy loads, but with a stall of 3500 you'll never see that. With 3500 stall (if you're never putting it on the street) you could actually probably lock the mechanical at 34 and call it a day as long as you don't get starter kickback (but with 10.2:1 you probably would)
 

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Race it, Don't rice it!
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Timing is a function of cylinder head shape and combustion efficiency. Basically, timing is just starting the fuel burn early enough so that the fuel is applying the most pressure on the piston for the longest period of time. Exactly where that is going to be is mostly a function of the head design.
For the range of advancement, for best drivability, it's going to vary depending on engine efficiency at part throttle.
You didn't say what head or what fuel your using but typically, 12-18 initial at idle and 32-40 fir most modern engine designs of pump gas. Trial and error, testing and notes is the only way to maximize it. Total timing is easlily done on a dyno and so can the amount of advancment needed but it's very time consuming a frankly a pain the rear end and still best done in the car, on the road.
 

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Discussion Starter · #12 ·
Been playing with the curve and I've drastically improved idle quality with more initial advance. Where I am now is about 18 degrees initial, 33 total, all in around 3000.

Going back a bit, what started all this (posted it in another thread) is the snap ring broke under the reluctor (MSD Pro Billet) leaving me with no spark.

Moving to the present, after making these changes, now I have backfiring through the exhaust starting at around 4000-4500 RPM, regardless of throttle position. I'm not seeing how any of the changes I've made would cause this? I've checked with a timing light and there is no change in total advance when it reaches the point where it starts breaking up.

Any thoughts?
 

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Discussion Starter · #13 ·
Found the problem. I'm too embarrassed to admit what it was, but I will anyway.
Those familiar with a MSD Street Fire know that the rev limiter is adjusted by two small dials. One for X 1000 RPM, one for X 100 RPM.

Apparently someone snuck into my garage late at night and changed it from 6300 to 3600.

Disclaimer: part of this story may be fabricated.
 

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Don't feel bad, I've seen MSD "pills" or chips limit 300 rpm below the number printed on them, and I've heard of guys finding them 500 or more off, one example even 3000 rpm below it's rating. Mistakes happen.

The 300 low a buddy fought with for 3 weeks until in desperation he put a new chip with a 300 rpm higher rating and instanly got it to rev 600 more rpm like it should have been.
 

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Race it, Don't rice it!
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Those MSD limiters have been a real problem in the class racers on dirt with chips and boxes all over the place. Guys are buying 20 chips at a time and testing them too see whats what. 6800 chips have been 6200-7200.
The chips and boxes are easy enough to open and fix it but then whats the point of class rules? Honest people honest I guess.

I had some tires done at a local shop on my hot rod. I'd forgotten about the 3200 chip in the box and usually shift at 2200 so I never even though about it until the tire tech guy reminded me by bangin the chip pulling it off the lift. That seemed a little unneeded.
 
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