frankgv

Hello,
How do you find where the best total timing is and at what rpm.

This is a new build 383 sbc.
I have read a lot of numbers but where is the best.
They say it is engine dependent , again how do I find out.

I've read where wrong "timing" burned pistons, I sure don't want that.

Can you help from perhaps your own experience.
THanks,

Frank.

Corkscrew

Check the FAQ sticky at the top of the forum page!

Westlotorn

First you have to tell us what heads you are running.
Factory stock?
GM Vortec
The head designs make a difference on what timing you run.
Factory Stock GM old style head and most aftermarket heads like 35 to 37 degree's total. 36 is usually safe.
Vortec type GM 1996 and newer heads seem to like 28 degrees total timing.
Some aftermarket aluminum heads like a max of 32.
Start with sharing your head type and maybe we can advise you.

Randy G.

Too much timing won't burn the pistons, but the resulting detonation will break them.

frankgv

Hi, not sure my previous info posted.

My heads are aluminum AFR180.
Is there a way to check for optimum total timing and at what rpm.
Thanks.

Westlotorn

Everything I know of AFR Heads say they will run similar to the factory heads so start with total timing at 36, fine tune from there.
I have a set of AFR 190's. AFR's are expensive but really nice heads.

Too much timing can certainly burn a piston, it makes the combustion chamber hot and once it gets hot enough the chamber starts acting like a spark plug with hot spots igniting your fuel charge.
Sometimes you get two flame fronts burning at the same time and it will make a furnace in there. The two flame fronts swirl and can melt right through a piston head or straight across through the deck into the next cylinder. I have seen pistons torched through the middle or even down one side with a very smooth hole melted down.
Lean burn or an intake air leak can do the same thing to a cylinder.
Detonation is an early explosion of your fuel charge while the piston is coming up with 7 other pistons driving it. The cylinder pressure caused by the early explosion is what fractures pistons, breaks rings and blows out head gaskets. Normal cylinder pressure at wide open throttle may be 600 PSI but in detonation it will jump to 3,500 PSI or more and break things.

frankgv

Thanks,
Call me %$@!# ?? , but HOW do you know what that magic total timing number is and at what rpm.

65tripleblack

65tripleblack

The WOT total spark advance is different for each engine/head combo based on what Westlotorn has said. It is accepted that a SBC with stock GM heads develops max torque with total WOT spark advance somewhere between 35 and 39 degrees BTDC. For just about any engine, maximum cylinder pressure should occur at about 14 degrees ATDC. This maximizes the mechanical advantage on the down stroke and extracts maximum torque out of the cylinder burn. Based on combustion chamber size, fuel octane, and piston type, empirical testing will determine the spark timing that yields maximum torque.

WHERE (at what RPM) "all-in" timing should occur is a function of your camshaft. What are your camshaft's specs?

Westlotorn

If you plan to use your distributor, with a timing light hooked up engine at idle, watching the timing mark. Rev your engine up slowly and watch your timing advance. Some continue to advance past 4,000 RPM and some are fully advanced at 2,800 rpm. Vac advance should be disconnected and the vac source plugged for this test. On the street with pump gas a dependable driver is usually happy at 35 or 36. Now read Lars posts on Timing and it will help you fine tune. For Max power to the cutting edge study up with Tripple Black.
Have a scribe handy and record how much timing you have at rpm from idle till it stops advancing. If you have any issues the notes will help fix them. Every 500 rpm record your advance. You will need a set back timing light to do this.

cardo0

Ya know it's almost funny no one has a "how to" book for this.

Some precautions are needed though. When u measure your adv in the driveway w/engine unloaded u don't want to rev higher than 3000rpm - no reason to either.

U really have multiple questions here Frank. I will try to help. Optimum timing and fueling are done on a dyno nowadays. They use an A/F monitor that reads out in Lambda. But I'm sure u want something to get u started so your stock 300hp or 350hp carb jetting should rich enough but if your worried u can install 1 step richer jets. If the car smokes black and fouls plugs u are to rich.

For initial timing w/o dyno u can start w/a stethoscope to listen for knock/ping and make your adv curve just 1 or 2 degrees below ping for every 500rpm.

Are u ready to time now? Is the new 383" in already?

Hope this can help.

Westlotorn

Most Modern Distributors are fully mechanically advanced by 3,000 RPM but the older Vette distributors? some were not fully mechanically advanced till over 4,000 RPM.
He needs to know what his distributor does prior to setting his timing for driving.
He could always pull the distributor and take it to a shop and have it properly curved before install.

frankgv

the cam is compcam 8-423-8 XR276HR what specs are you looking for. Heads aluminum AFR180
When"all in" rpm is established (again, how) do you then re-spring or somehow limit the distributor from exceeding those rpm values.
Thanks.

frankgv

Thanks,
It is an aftermarket HEI with the mechanical tach drive.
I am learning, may be a little slow, but, I think it said it had a build in curve ?????? then what.
So if the curve is set prior to installation , how does that comply with the "unknown" engine performance, Im still learning. please have patience , this is my last hooray, lol.

I have been running it in my old 327 as is !! with about 12* idle timing, period. and was pretty good LOL.

frankgv

Thanks,
its not in yet, sitting in the garage getting measured for bell housing alighnment, lol. and putting on heads etc, measure PR length!!!!

I talked to a shop here in Chicago subs, and they will set the dyno curve but want to know at what degree and rpm ??????
isn't that what a dyno is for ??

talked with some other members here also, perhaps you can collaborate with them.

karkrafter

I really don't think this is all that complicated. GENERALLY you want ALL your timing in early..2200 rpm that would be tuning the advance weights & springs...generally, light springs will do the trick. The purpose of vacuum and mechanical advance is more a product of keeping the timing RETARDED to ease in starting and dieseling when you turn the car off. Adding advance in the distributor [changing bushing] is for the same reason...not to give it MORE total advance as much as to be able to RETARD the timing more to make the car easier to start. Most often this is not needed unless the car has ultra compression.


In the real world, set total timing to 35 degrees. If you had the advantage of a dyno, advance or retard at max hp. In the old days... no dyno, yes drag strip, advance and retard by time slip.


Seat of the pants..advance till you get spark knock then back off 2 degrees..but I think with aluminum heads you can't go by the spark knock method, I've heard you can't hear it with aluminum heads but it is still there.


Better to be 2 degrees too retarded than 2 degrees too advanced...


And make sure to set the timing with the gas you will be using everyday.

65tripleblack

Frank,
Vintage GM distributor advance program was determined by 4 factors: autocam shape, points cam advance slot length, advance weights, and spring tension. There were hundreds of combinations used "back in the day" for advance programs, and I'd be more than willing to bet that they only needed 5 to 10 of those combos.

In any case, it is not very complicated, as stated. The object of the advance map is to keep your engine out of detonation, while at the same time maximizing torque. Detonation is most likely to occur at part throttle operation and usually near the engine's torque peak.

The easy way for you to go would be to hook a timing light to your HEI distributor and check for full advance (with vac advance disconnected and plugged) at about 36-38 degrees, with advance "all-in" by around 4000 RPM. Road test and listen for detonation. If none is heard, then lighten advance weight springs to have timing "all-in" sooner. Road test. Continue to have advance come in sooner until detonation is heard, then go to next stiffer weight springs. It is OK to have two different tension springs on the weights in order to set "all-in" RPM.

With 224 degrees intake duration your cam, it is in the same ballpark as the L79 cam, but since your engine is a 383, the duration is somewhat milder and I'd treat it more like a L75 cam (3896929), since a larger displacement engine has a greater tendency to detonate. I'd have the distributor set up by a pro using a Sun distributor machine and I'd suggest using L75 distributor specs for the advance program, as a baseline. I have a feeling that this will be sufficient, but if you want to fine tune it, then there are three ways to do so, and each will require knowledge and hands-on skills:

1. Dyno testing
2. Racetrack testing
3. Road testing

Here are some specs for 1965. Use the 1111076 specs as a baseline.

65tripleblack

Setting timing your way will change WOT total advance. WOT total advance is a constant of 36-38 degrees. It is the "ALL-IN" RPM which determines detonation resistance.

frankgv

Thanks,
Not sure I can hear a ping / knock with my aluminum heads and aggressive exhaust.

GTOguy

For good, comprehensive information, search for 'timing by Lars' or 'Lars Grimsrud'. He has the best, easiest information for what you are trying to accomplish.