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locked out timing/any downsides on street cars?

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locked out timing/any downsides on street cars?

Postby rabbit » Mon Jun 07, 2010 1:07 am

hi, while fiddling with the stock HEI distributor i wound in a heap of initial advance just to see what would happen. Idle vac went from 10" in park to almost 15" with 26 odd degrees (instead of the usual 18).

I never bothered messing with the idle carb tune to truly see the effects but my question is this:

What are the downsides to locked timing on a street car (typically low speed/low load/plenty of idling etc)?

In the region of 30 degrees will the engine be hard to start, or run hotter than usual etc at idle?

edited (added to original post): Vac advance is long gone, so no joy there.
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Re: locked out timing/any downsides on street cars?

Postby GREG K » Mon Jun 07, 2010 3:20 am

When you increase the timing at idle the speed of the engine goes up.It goes up because the cylinders start to complete the burn earlier on the power stroke and that puts more energy into the crank instead out the exhaust pipe.When the piston is going down at the end of the exhaust stroke,it sucks in lots of atmosphere that has O2’s in it and this mixes with fuel from the intake and the rpm goes up.See,the throttle plates are shut so you don’t get much air in from them,but you get it in from the exhaust.The exhaust becomes an intake on the overlap.
When you increase the amount of timing at idle you change the amount of vaporisation time that the fuel load in the cylinder sees.The risk that you have when you do this is that it will generally force you to wind down the idle screw,due to the idle increase and this will upset the t-slot setting that is required to get a afr into the cylinders,so you end up with a lean condition in the cylinders and the engine cant take a load correctly.The fuel to run an engine on idle and off-idle before the mains come on ,comes from the transfer slot.

You need to get the timing correct together with the fuel settings.Timing and fuel settings go hand in hand,you just dont do one without the other.
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Re: locked out timing/any downsides on street cars?

Postby Dodge Freak » Mon Jun 07, 2010 8:10 am

The motor may kick back while cranking over. You could put a flip switch in the ignition and turn the ignition off until the motor is cranking over strong then flip the ignition system back on.

Good chance it will kick back when warm and could even bust some teeth on the starter or ring gear
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Re: locked out timing/any downsides on street cars?

Postby falcongeorge » Mon Jun 07, 2010 10:48 am

or the starter nose. Been there, done that.
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Re: locked out timing/any downsides on street cars?

Postby F-BIRD'88 » Mon Jun 07, 2010 10:54 am

This is very effective on motors with big racey cams with lots of overlap. long duration and tight LSA.

It makes the the engine idle much cleaner while idling in gear with an auto trans
Without drilling throttle plates 99% of the time.
It allows correct throttle blade opening position at idle, idle throttle transfer slot exposure for correct idle and off idle circuit function and throttle response
on motors with big cams.
It increases/ maximizes and stabliszs idle manifold vacuum especially while idling in gear allowing the PB/s to operate.
The bigger the cam duration and overlap, the more the increased idle base timing or locked mech advance will help.

This is not for motors with small/moderate cams or needed for mild/ moderate street cams with lots of idle manifold vacuum and Wider LSA's.

It corrects fuel burn timing at idle , The motor will run cooler cause the fuel is burning on time at idle.
The "EGR" exhaust remix effect of the valve overlap at idle slows down the relative fuel burn speed requiring more idle spark advance.

A spark power interupt switch allows easy hot starting with locked timing.

This is not a replacement for vacuum advance, but works with a functional vacuum advance for street driven cars with big racey cams.

If you got a big racey cam with duration over 246@.050 and tight LSA (lots of overlap) and auto trans, and like quick throttle response, consistant idle without exhaust odour, clean spark plugs that stay clean
(don't "Load up") Power brakes that work, you can probabily benefit from locked timing.

If using the GM OEM starter motor make sure you have the end support bracket/brace installed.
The two starter motor bolts alone are not enough to support the GM starter.
With the correct starter bolts and required end support bracket/brace installed you will not bust the starter motor, ever. Its a (must have) $10 dealer item.
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Re: locked out timing/any downsides on street cars?

Postby PackardV8 » Mon Jun 07, 2010 10:59 am

Yes, many downsides, as related above.
No, don't do it on anything other than a pure race application.
Maybe, you should read up on all the reasons why the OEMs went to so much trouble and expense to add centrifugal and vacuum advance curves. Bottom line: ease of starting, off-idle transition, part throttle response, fuel economy, top end power. Care to live without all of these?

thnx, jack vines
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Re: locked out timing/any downsides on street cars?

Postby F-BIRD'88 » Mon Jun 07, 2010 11:26 am

PackardV8 wrote:Yes, many downsides, as related above.
No, don't do it on anything other than a pure race application.
Maybe, you should read up on all the reasons why the OEMs went to so much trouble and expense to add centrifugal and vacuum advance curves. Bottom line: ease of starting, off-idle transition, part throttle response, fuel economy, top end power. Care to live without all of these?

thnx, jack vines


99% of the OEM motors do not require locked out timing cause the OEM factory cams don't have the duration or overlap that would require it.
A 1969 427 L88 corvette motor (264°-273° duration @.050" cam) with automatic trans is a example of a OEM (one of the exceptions) that definatly can and will benefit from locked out timing.
With big cams like this one manifold vacuum drops when idleing in gear which take the idle spark advance with it if/when you depend on vac advance and quick mech advance curve to get the required idle timing.
The (manifold vacuum dependant) timing drops as soon as you put it in gear. The timing drops off as soon as you punch the throttle, right when you need the advance. The engine loads up from lack of idle timing and flairs up when you go from in gear to neutral cause the vac advance fluctuates wildy.
The engine will heat up from lack of idle timing. The power valve fluctuates at idle creating a over rich idle and killing the plugs.
This was one of the few cars factory offered with suck a radical cam and auto trans.

There is a few reasons why the OEMs didn't use the type of racey high overlap/long duration cams that require this much base idle timing.
( long duration and tight LSA )
1. 99.5% of the new car buyers would bitch about the idle quality.
2. Poor gas mileage.
3. High exhaust emissions.
4. Restrictive untuned exhaust systems with log style manifolds. Theses exhausts systems don't work with Tight LSA cams. The exhaust scavenging effect you can get from a tuned exhaust (long tube headers) is all but supressed making a tight LSA cam useless.
5. Air conditioning vacuum control and power brakes dependant on high manifold vacuum.
6. 99% of new buyers would bitch if they had to use two hands to start the car. (spark interupt switch)
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Re: locked out timing/any downsides on street cars?

Postby panic » Mon Jun 07, 2010 12:42 pm

In order to be valid as a recommendation, locked timing requires an amazing coincidence: that (except for start retard) the exact same ignition point is best for every engine speed, load, throttle position, temperature, mixture, gear ratio, and vacuum level.
I'm not aware of any engine in which these requirements are actually met by a single setting.
In some cases (such as those mentioned) the magnitude of the error (and it's always an error) is small enough that for some it's a reasonable compromise.
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Re: locked out timing/any downsides on street cars?

Postby Troy Patterson » Mon Jun 07, 2010 1:22 pm

Whether or not a given engine likes lots of initial advance, a very rapid timing curve or locked timing depends on the engine in question. Generally, the more radical the engine, the more timing it will like.

I've locked or provided lots of initial advance on engines I would never have expected to be able to handle it due to retarded cam timing, low compression and a number of other undesirable issues. I have been surprised at times also with how much timing good engines can handle.

What is too much timing? Too much timing will cause the engine to hard start when hot - after heat soaking; too much timing will cause "negative torque" appearing to be extreme responsiveness but in fact is destructive; too much timing will cause a nervous idle; too much timing will cause the engine to run rough, etc.

There may be one or more symptoms of excessive timing. The only way to know how much timing is optimal for a given engine is to advance it and see if it likes it or not.

As far as adjusting transition slot exposure, transition slot exposure should be checked and adjusted to the engine regardless of initial timing.

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Re: locked out timing/any downsides on street cars?

Postby 69-CHVL » Mon Jun 07, 2010 2:55 pm

Been locked-out for 4yrs on my street 502, and motor idles and drives much better. Motor doest struggle to start at all either hot or hold. I am running a ZZ502 starter, which is nothing special, just a GM mini-starter. Looks like something off of a pick-up truck.
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Re: locked out timing/any downsides on street cars?

Postby PackardV8 » Mon Jun 07, 2010 3:12 pm

One other downside to locked advance, if your engine must pass emissions testing, that will often cause it to fail.

thnx, jack vines
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Re: locked out timing/any downsides on street cars?

Postby Q-ship » Mon Jun 07, 2010 5:54 pm

By "locked timing" I am assuming you mean eliminating vacuum advance, or do you really mean locking the timing to best high RPM performance?

In either case, it is a detriment to a street car as you will give up substantial driveability, part throttle performance, at least 5 MPG, and easy starts.

Have you ever heard of anybody with a programmable, computer driven engine, EVER running locked out timing?

Vacuum advance in conjunction with mechanical advance is in effect, an analog computer, and is the best street performance mod that can be made if it's adjusted right.

Race cars can get away with it because they have converters that stall at high RPM's, and run from the start at high RPM's, but I have yet to see even one of these cars that is anything but a dog on the actual street.
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Re: locked out timing/any downsides on street cars?

Postby SKeown » Mon Jun 07, 2010 6:06 pm

I tried locking the timing at full advance on my mid ten second street driven AMX. I saw no benifit and it did fight the starter, so I went back to quick mechanical advance only, all in by about 2200 RPM.

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Re: locked out timing/any downsides on street cars?

Postby vincenelson » Mon Jun 07, 2010 6:51 pm

If the motor starts ok, and the fuel you use will allow it without pinging, then try it. We run all our hotrods with locked out timing, we like the throttle responce. Make your own tests to see if you like it or not.
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Re: locked out timing/any downsides on street cars?

Postby twinturbo496 » Wed Jun 09, 2010 7:16 am

I just changed from a MSD street distributor with vac and mechanical advance (12° initial, 22° mechanical, 20° max vac) to a crank trigger & crab cap with 30° fixed timing. I was very concerned about starter damage and hot start problems, the setup already had ignition switch separate from the starter, but so far it has started just fine, pump throttle twice, spin starter for 2-3 seconds and hit the ignition, even on 87 octane.

However, the engine is a 8.5:1 CR BBC with a 14-71 blower & a .680 solid roller. Perhaps big cam + low compression = acceptable option

That said, the only reason I did it was because I couldn't fit a real distributor under the blower, if I could, I would still go back to the street distributor setup.

One interesting side note: Even with all new parts, the conventional setup (with plastic thrust button) had significant spark scatter, but it is completely gone with the crank trigger, I wonder if the scatter was from the timing chain, distributor gear backlash, harmonics in the mechanical advance...???

I bet some people on this site know where the scatter really comes from, and probably know if it makes any significant difference in hp.
The VOLT should have had a diesel engine...
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