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Old 07-06-2011, 02:19 PM
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locked out timing or timing curve?

The distributor can easily be locked out and the ignition system has the ability to retard timing by 20* during cranking.

How do I decide it I want to use a timing curve or just lock the timing out?

Under what conditions is lock out timing most desirable?

I would imagine that certain combos work better with lock out timing than others, I am not clear on which combos that would be though.

Thanks

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Old 07-06-2011, 02:34 PM
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Quote:
Originally Posted by z-adamson
The distributor can easily be locked out and the ignition system has the ability to retard timing by 20* during cranking.

How do I decide it I want to use a timing curve or just lock the timing out?

Under what conditions is lock out timing most desirable?

I would imagine that certain combos work better with lock out timing than others, I am not clear on which combos that would be though.

Thanks
Big cams and race engine's are best suited to locked in timing advance a manual transmission makes living with this arrangement a lot easier. Cams getting into the range of 300 degrees of duration and over a half inch of lift are getting into this category.

Electronic systems have the ability to both control advance and retard eliminating the need for vacuum or mechanical advance within the distributor. These are great choice as the engine can be very closely tuned for RPMs and loading.

Bogie
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Old 07-06-2011, 02:46 PM
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I take it a street car with an auto trans with a low stall speed and an engine with a low powerband is not suited to locked timing? This is basically the combo I will have.

I have two choices with my ignition system:

1) Centrifugal advance with vacuum advance

2) Locked out timing with cranking retard that sets back to the locked setting above 800rpm.
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Old 07-06-2011, 03:16 PM
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a street car, especially a car with a mild cam, needs vacuum advance. As the vacuum advance adds to the mechanical advance during light loads (high vacuum).

here is a nice writeup about timing and has some good timing recommendations at the end.

http://www.corvette-restoration.com/.../Timing101.pdf
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Old 07-06-2011, 03:19 PM
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Quote:
Originally Posted by z-adamson
I take it a street car with an auto trans with a low stall speed and an engine with a low powerband is not suited to locked timing? This is basically the combo I will have.

I have two choices with my ignition system:

1) Centrifugal advance with vacuum advance

2) Locked out timing with cranking retard that sets back to the locked setting above 800rpm.
A mild street engine with a low stall converter would be best with mechanical and vacuum advance systems. These balance the advance requirements across a broad range of RPMs and differing gear loadings with available pump fuels.

A locked out system that's all in by 800 RPM is likely to experience sever detonation as with typical street gears high ratios netting low RPMs against high loading on the crankshaft will make a very ping prone engine. This can be made to work by lowering the compression ratio, but that kills efficiency very quickly which will having you buying a lot of gasoline for the power produced and miles driven. Assuming the detonation doesn't bust something expensive to replace like a piston beforehand.

20 degrees of retard may prove to be too much leading to great inefficiencies as stated above. This systems are used on race cars to get them started. A proportional advance is eliminated on these as the engine is not expected to operate over a wide power band, it needs to idle and scream, no other pesky requirements. So advance systems are eliminated to reduce a possible failure point. Retard is also used with blowers and laughing gas as these things increase mixture molecular density and a peculiar thing about combustion in an engine is that the speed of the burn is proportional to mixture density. This is why advance systems peak out in a range of 2000 to 3000 RPM beyond that point the burn speed naturally increases and no further amount of advance adjustment is required. In the case of supercharging and nitrous it is common to have a situation where the burn happens so fast that it becomes necessary to pull it back to keep from getting into destructive detonation.

An engine cannot be operated with everything maximized, the best we can do is find sets of optimal solutions which consist of a selection of less than maxed out events. Some events have bigger impacts than others and example would be that an optimal path to power and efficiency would within a confined range trade some compression for more advance. Now this is a dangerous concept so don't go running with it. It's just an example that if you have an engine with a detonation problem compromising on compression ratio will have less a negative effect on power than would reducing timing.

For the most part where street engines are concerned there is a lot of data for best designs and results so I would recommend books like How to Build Max Performance Chevy Small Blocks on a Budget by David Vizard or Small-Block Chevy Engine Buildups: How to Build Horsepower for Maximum Street and Racing Performance by Chevy High Performance Magazine, and there are many others out there.


Bogie
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Old 07-06-2011, 06:30 PM
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I figured a curve with vacuum advance would work best.

No experience with locked out timing though so I figured I would ask.

Couple more more questions though:

1) If high gearing as commonly found in street cars causes low rpm / high load conditions, and droping compression could keep this from being a problem when using locked timing, couldent lowering the gear ratio make this work as well? If so, would the gearing have to be so extreem that it wouldent even be streetable? I imagine that low gearing in conjunction with a high stall or a manual trans would be plausable, but then we are getting into a high rpm / high powerband application again. But hypothetically speaking, what about a low stall, low gear combo such as 1500-4500 powerband, th350 auto w/ 2krpm stall, 29" tires and 4.56:1 gears?

2) How could 20 degrees of retard be too much? Total timing would normally around 35* in most applications as far as I have seen. If the timing were locked in those same applications that are 35* total w/ a curve then it would be locked at about 35*right? With a 20* cranking retard that would mean 15* while cranking and 15* till 800rpm. I don't see how 20* retard would be problematic.
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Old 07-06-2011, 07:21 PM
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Quote:
Originally Posted by z-adamson
I figured a curve with vacuum advance would work best.

No experience with locked out timing though so I figured I would ask.

Couple more more questions though:

1) If high gearing as commonly found in street cars causes low rpm / high load conditions, and droping compression could keep this from being a problem when using locked timing, couldent lowering the gear ratio make this work as well? If so, would the gearing have to be so extreem that it wouldent even be streetable? I imagine that low gearing in conjunction with a high stall or a manual trans would be plausable, but then we are getting into a high rpm / high powerband application again. But hypothetically speaking, what about a low stall, low gear combo such as 1500-4500 powerband, th350 auto w/ 2krpm stall, 29" tires and 4.56:1 gears?

2) How could 20 degrees of retard be too much? Total timing would normally around 35* in most applications as far as I have seen. If the timing were locked in those same applications that are 35* total w/ a curve then it would be locked at about 35*right? With a 20* cranking retard that would mean 15* while cranking and 15* till 800rpm. I don't see how 20* retard would be problematic.
Steeper gears help a lot to let you get away with higher compression. But your ending up laying with the whole machine, which I thought you said was a street machine.

The 20 degree retard leaves only 15 degrees out of 35. The amount of advance is sensitive to RPM, throttle position and load on the crankshaft. while 15 degrees might be fine for idle it will be insufficient in the mid RPM range where most street vehicles spend most of their time. The trade to stiff gears puts a pretty heavy load on your wallet for gas. There are solid reasons why cars have gone to high ratio rear axles with overdrive transmissions that include a really deep low gear combined with electronic fuel injection. The same approach will work with a carb it just takes more upkeep to stay on tune than injection does.

But I'm pulling straws here as I really don't understand what it is you're trying to do. For the engine you described and the use of the vehicle locked out timing just doesn't make sense. So what's really going on?

Bogie
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Old 07-06-2011, 07:53 PM
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If you're building a stock type engine do what GM did. They knew what they were doing for the most part.
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Old 07-06-2011, 08:34 PM
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Quote:
Originally Posted by oldbogie
The 20 degree retard leaves only 15 degrees out of 35. The amount of advance is sensitive to RPM, throttle position and load on the crankshaft. while 15 degrees might be fine for idle it will be insufficient in the mid RPM range where most street vehicles spend most of their time.
With the timing locked and the start retard activated anything above 800rpm will be more like 35* or whatever the timing is set at. So the mid RPM range will be 35*. So as I understand it, 20* retard with locked timing shouldent be too much unless I am missing something.

Quote:
Originally Posted by oldbogie
But I'm pulling straws here as I really don't understand what it is you're trying to do. For the engine you described and the use of the vehicle locked out timing just doesn't make sense. So what's really going on?
I have a project that basically fits this description...

"low stall, low gear combo such as 1500-4500 powerband, th350 auto w/ 2krpm stall, 29" tires and 4.56:1 gears"

...and my ignition system + distributor (MSD) are set up to allow me to lock the timing and use a start retard. Although it can also use centrifugal advance w/ vacuum. I was contemplating wether to lock it out or not and as such I am trying to understand when to use locked timing and when not to. When locked timing can be made to work well and when it wont work well no matter what. This isn't something I have played around with before nor have I researched it much.

It is a heavier vehicle (~4200lbs) and I won't be flying down the freeway at 70mph so I don't mind the low gears. I am content with 50-55mph in the slow lane. Plus with the heavy vehicle weight and low stall the gears help reduce the load on the crankshaft....another reason for the low gearing.
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Old 07-06-2011, 09:02 PM
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Your mild near stock combo is not a good match to "locked out" timing.

You want a mechanical and a vac advance curve. Very similar to a stock motor.

Don't know why you are so confused. You don;t have a big racey cam in your motor. (245@.050 or more with a tight LSA)
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Old 07-06-2011, 09:31 PM
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Quote:
Originally Posted by F-BIRD'88
Your mild near stock combo is not a good match to "locked out" timing.

You want a mechanical and a vac advance curve. Very similar to a stock motor.

Don't know why you are so confused. You don;t have a big racey cam in your motor. (245@.050 or more with a tight LSA)
Confused probably isn't the right word.

My mind is made up as to what I will do with the ignition and that is to have a curve.

However, just to ease my own curiosity I am trying to get a better understanding of when locked timing can be effective. Some things in this thread made me believe there is more to it then JUST the camshaft. Maybe I misunderstood and the cam really is the only determining factor.
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Old 07-06-2011, 10:05 PM
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A engine with a long duration camshaft with lots of valve open overlap
has a lot of exhaust gas dilution in the air fuel mix when the engine is idleing or running at low rpm. (exhaust gas does not burn, it has the same effect on engine combustion as throwing water on a fire) This exhaust gas dilution slows down the burn speed of the air fuel mix in the chambers. Therefor the spark has to occur earlier in the cycle , when idling, to compensate for the slower fuel burn time.

The big cammed engine needs a lot more initial base timing to idle nice.

So much so that a mechanical timing curve becomes so short that its impractical. The engine want the same {or more} idle spark timing as it wants at WOT high rpm.

If the fuel burns too late in the cycle the combustion is lazy and tends to heat up the cylinder walls and exhaust ports instead of pushing on the piston.

A short duration cammed motor does not have the exhaust gas dilution problem when running at idle and low speed.

At higher running rpms when the motor is "on the cam" exhaust reversion in the intake track is reduced to near 0. Combustion burn speed is more normal. (fast)
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Old 08-04-2012, 10:18 AM
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This is good information.

So what about a shorter duration cam, say 218 @.050" but with a tight LSA 106? Is the burn still going to be sluggish at idle? I ask because thats what I have and I have my timing at 19 initial, no vacuum adv and total thats about 37-38 all in at 3k. I bumped it to 3,500 rpm but dont notice much of a difference. Btw I have a saginaw 4 spd with 2.73s for now. I haven't yet but was wondering if going to 2500 rpm would make a difference. Its a heavier car GMC caballero 78' (el camino). Also, would you recommend higher initial timing if the engine will tolerate it? thanks

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Old 08-04-2012, 06:41 PM
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You want to give the motor all the initial it needs to idle correctly
for that cam I would try 18 to 25deg at idle. Short mechanical advance curve.

The 273's must suck. That thing would fly with 4.10's

you need to get a vacuum advance too.
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Old 08-04-2012, 06:46 PM
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The more cam overlap the more exhaust dilution at idle, the slower the fuel combustion speed. So the more need for early spark at idle, so the fuel burns quickly and on time.

Late idle timing makes you have to open the throttles more at idle
moving beyond the sweet spot with just the right amount of idel fuel transfer slot exposure at idle. The throttles must be in the this sweet spot at idle or you get a flat spot off idle.
On a holley carb the idle is shared by all 4 throttles.
Should be more or less even.
A Edelbrock and a Qjet idle on on the primary throttles.
Correct idle transition slot exposure is critical.
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