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Discussion Starter #1
Hi everyone.
66 nova i purchased back in the fall came with what looks like a rebuilt 350. The carb is a new holley, new mallory breaker less distributor, roller cam, headers and flowmaster mufflers without any x pipe or cross over pipe that turn down at the axle.

The question is the engine seems to idle very rough. I set timing at 12 deg but am getting some pinging under hard load there. Today I did a compression test to make sure engine was ok. Got 205-210 PSI on all cylinders cranking until highest pressure and got 175-180 PSI with the 4 puff method.

I am not sure if the timing is correct or not. If I advance more pinging will get worse and any less and Idle gets worse. Do not have any specs on cam other then I know it is a roller cam. Not sure of the compression ratio other then the pressures listed. I have been running 93 Octane. Anyone have any ideas ? Thanks in advance for the help !!!
 

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The idle mixtures can greatly affect the idle quality. I know when mine are going out of tune because the idle gets a little rumpity and sometimes it'll after run.

If you can, use vacuum advance, it smooths out the idle tremendously. Because sitting there at idle it can be a lot more than 12° but it takes vacuum advance to get it there, the engine load timing... or vacuum advance. No load at idle, more advance can be brought in, then at the same time the centrifugal can be brought down keeping pinging away.

If you don't want to mess around with vacuum advance you're only option is to bring down the initial and re-check for ping at launch.

There so much more that goes into timing than setting initial and total it's beyond the scope of what can be described here, and without pizzing off the "total is best" crew.
 

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There are a bunch of good articles that can help you set up the timing. As far as my understanding is the higher the compression, the less advance in timing needed because a higher compression ratio can be more efficient at mixing the fuel and oxygen in the cylinder. So it takes less time for the mixture to burn to peak cylinder pressure which you want to occur around 10-15 degrees after TDC for best performance. So, you will have to play with it to see what yours likes. Just read the plugs and make sure you are not getting any detonation. However, the best way to see how much total timing it likes is to go to the dragstrip and play with it a bit.

As far as the compression ratio, I couldn't tell you. If we had the Duration of the camshaft we might be able to guess. Your psi readings are at a very low rpm. At that low of an rpm with a high overlap of the camshaft, it will actually allow some of the intake charge to exit the exhaust and go back into the intake and not completely fill the cylinder. But at higher RPM in the camshafts power band air is rushing into the cylinder and becomes more efficient at filling the cylinders. So, the psi reading you have are really only good to check if the rings are sealed well and and consistency across all of the cylinders.

Sorry if I rambled on a bit lol, I kind of started and couldnt stop.
 

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With the compression numbers u gave, it seems like you have somewhere in the 11:1 compression maybe higher depending on the duration of the camshaft. If this were my car i would shorten the centrifugal advance as much as possible, or lock it out. If the cam has a lot of duration its going to want a lot of initial timing, but with a long advance curve it will have too much total timing and thats why its pinging. If you have a good starter lock out the ign curve, set timing at around 28 degrees, go test drive and if it dont ping start adding more timing till you here it ping again. Then set it about 4 degrees below that. Dont bother with a vacuum advance if the cam is not making enough vacuum. Jmo.
 

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first you need to determine total timing.... cant do a thing with out it....

Initial timing is what makes the car easy to start.... Total timing is what make it run like a scalded ape!:devil:.. and also leads to detonation...

lots of factors contribute to detonation, combustion chamber shape, squish/quench, piston shape..... craning compression is due to intake opening point and closing point... not so much as how the lobe separation is. but can be a factor.. Usually high cranking compression is a wider lobe separation due to earlier intake opening point and earlier closing point... with later exhaust closing point...


you must determine total timing first and what total timing is @ 2500 to 3500 RPM depending on gears, converter, cam, etc.....
 

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The cranking psi numbers are good. A lot of people don't understand that an engine is basically an air pump and cranking psi is relevant and informative.
It would be helpful if you knew your cam specs but it's not critical for tuning your engine.

What's your total timing? You may need to limit the total advance so you can advance the initial for better driveability.

Your engine may like 12-14 initial but wants no more than 38 total. You may have 30 degrees of mechanical advance so if you advance the initial from 8 to 12, the total advances from 38 to 42.

If you get 42 total with 12 initial that would mean you'd need to limit your mechanical advance to 24 degrees.

I wouldn't lock out the advance and the "advance it until it pings and back it off" is simply bad advice.

There should be several informative threads on spark timing in Best of Tech archive above.
 

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Discussion Starter #9
Can I assume by the cranking pressures that I am ok with pump gas ( 93 ) ? They seemd a bit high to me is why I didnt want to be chasing my tail trying to get timing correct to stop pinging if it I was never going to be able to get there. What is the best way to check total timing, a adjustable timing light or degree tape on the balancer? Thanks for all the help everyone !!
 

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Octane sensitivity has many factors but compression pressure is certainly one of them. I would say you should be OK with 93 if you tune the fuel and spark "curve"*. A degreed balancer, timing tape or dial back timing light can all be used to check total timing. Your distributor should have a way to limit the mechanical advance. You'll need to find the instructions (online here).

*graph of advance vs rpm as plotted on cartesian coordinates X and Y.
 

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Discussion Starter #11
Paul is the degree tape or the dial back timing light more accurate or easier ? Seems like it may be hard getting tape to stay on balancer but then again how accurate are dial back timing lights? Thanks
 

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Most of what you want to know is in this thread.

Total Timing Indicators and tape tricks

Mike Goble points out all you need is a to make a mark on the balancer and use a degreed pointer. If you make a 36° mark on your balancer, you can see timing from 24° to 52° BTDC using the 36° mark, and using the TDC mark you can see from 16° BTDC to 8° ATDC.
 

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and dont for get how much cheaper our fuel has gotten in the past few years.

Ive been having vapor lock issues i never had in previous years
 

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Bad advice? really? Care to explain why its bad advice?
Im not replying for Paul, he has too much engine knowledge for me to do that. But I saw this post and this question.

IMO. Locking down the dist. advance at "before" detonation is a subjective stance. Detonation that can be heard is going too far. When you hear it you already have pre-ignition before that point. Our fuel these days is stuffed with so many additives for the new cars and EPA needs. New cars have knock sensors that many times will sense the knock well before we can hear it. So the computer retards accordingly. And fast. Gotta love EFI.

So by locking the mech advance down at a supposed "safe" area you may be thinking you have the spark curve well within a safe limit. But in reality yer using pump gas which varies from load to load and environmental conditions change. Yer still subject to PI.

I dont see it anymore "unsafe" as running on full springs. I think the dizzy needs to be checked for correct springs and if they are then I would look at getting some stiffer springs.

And I see 210 PSI as a good reading.

Im thinking the springs are floping over and stuffing some serious advance in too quick and maybe too far.

Scope it, I like the dial scopes. JR
 

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put 5gallons of race gas in it and see how it is. Your cranking compression is too high. Good luck!
x2
you can chase timing issues all day but if you have bad gas or not enough octane for the compresion your running (which you said you didnt know) it is a easy test to just add good fuel first and if it still dose it a pinging problem can also be a broken piston skirt. I have seen this first hand and hope its not your problem but 12 deg of initial timing and any more it gets worse and any less it dont want to run says something else is wrong not timing just my opinon. Try the gas and see what happens. Good luck!
 

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Discussion Starter #18
Well I got around to checking my total timing like every suggested. I put a mark on my balancer at 36 deg. Initial timing at 12 deg and total was right at 36 deg. Still am getting some pinging under hard load. I checked the specs on the Mallory distributor I have and it said it has 24 deg of advance which would be correct with what I am getting. Should I go lower still with the total and adjust distributor to keep initial up?? It really seems like I like I could advance the initial a little further also. Also I found the distributor install instructions but did not know how to adjust the advance anyone have any ideas. It is a Mallory model # 5048201. Thanks for the help !
 

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Discussion Starter #20
My question with the racing fuel is? Will adding racing fuel with a higher octane content cover a timing issue and eliminate the pinging or will that only work if the compression being too high is the issue ? Not sure if I worded that right hope everyone understands. Thanks
 
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