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Could someone please explain to me the relationship between timing and engine temp? How does the timing cause an engine to run hot?
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Re: Engine temp vs timing
If the ignition timing occurs later than it should, the mixture continues to burn as it exits the exhaust port. Since you have water cicrulating around the port, you're using this residual combustion energy to heat water instead of pushing on the piston. Beyond creating a potential overheating problem, you're waisting energy that could be going to the rear wheels and giving you a more pleasurable driving experience. It's actually a little more complex than this, but you get the picture. Late timing = potential overheating. -
Re: Engine temp vs timing/Gerry
Gerry
Are you saying that retarded timing can causes heat problems? What if the timing is advanced?Comment
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Re: Engine temp vs timing/Gerry
You need to understand what timing is all about. It's lighting the fire at a point in time such that maximum cylinder pressure occurs just after top dead center.
When the engine rpm is low there's lotsa time to fool with so you can light the fire later. Also, it takes a finite amount of time for the charge to burn. When the charge is dense with hydrocarbons it takes less time to burn than when the charge is lean.
Therefore: 1) an rpm advance mechanism is needed (centrifugal or mechanical advance, and 2) a charge density related advance is needed. (manifold vacuum).
As rpm comes up more advance is introduced to compensate for the shorter time necessary for the piston to reach TDC. At wide open (or heavy footed) throttle, the vacuum disappears (or diminishes) and advance is removed in porportion to charge density by the vacuum advance. (retard).
At idle when the rpm is low and the charge density is lean, maximum advance occurs and is the sum of vacuum advance and initial mechanical advance. For instance a 16 degree vacuum can and 12 degrees mechanical advance adds to 38 degrees total. At 3000 rpm and very light throttle the total advance might be as high as 48 degrees or so.
If you don't light the fire soon enough the charge is still burning and trying to reach maximum cylinder pressure when the exhaust valve starts opening. This process burns the exhaust valve while dumping a lot of heat (mechanical energy) into the exhaust system and water jacket which results in overheating.
Lighting the fire too soon causes maximum cylinder pressure before TDC and makes the engine want to run backwards. The inertia and other cylinders have to overcome this effect. It's very wasteful and places enormous stress on the compression rings and piston tops. This premature ignition is called PRE-IGNITION as opposed to DETONATION which is raising the charge pressure and internal heat until the charge burns. Either malady is detectable by "pinging or knocking" noises.
Dale PearmanComment
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Re: Engine temp vs timing/Dale
Good explination. I follow that. How do I know what my max advance should be?Comment
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Re: Engine temp vs timing/Dale
A good starting point is to recreate the timing characteristics that GM used for your engine. If we are talking a smog era engine here, some of the parameters change, but the OEM setup is still a good place to work from. The actual specs are available in your shop manual, or I can look them up if you provide the year and engine combo.Comment
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Re: Engine temp vs timing/Bill
Unfortunately, this is a replacement engine for my 327/350. It is a 350 bored .03 with dart sportsman heads, the original intake and carb. and headers. The distributor is from my 327/350 also. I am trying to "tune" the engine for best operation. I am thinking of the 69? 350 cu in/370 hp engine.Comment
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Re: Engine temp vs timing/Bill
SB chevy engines with stock type combustion chambers make the most power with 34/38 degrees of total timing at 3000/3500 RPM. measure the circumference of you damper and didvide by 10 and put a mark at this measurment on the advance side of the TDC mark,this will be 36 degrees. remove 1 spring from the advance mech.,rev the engine till the timing mark stops moving and then move the dist. till the 36 degree mark you added lines up with the "0" mark on the timing tab. this is done with the vacuum advance disconnected. if this turns out to be about 3000/3500 RPM do not reinstall the spring as you will have a good starting timing curve. reconnect up the vacuum advance. if you reach the max timing movement before 3000 RPM you will need to experiment with springs to get to the 3000/3500 PRM range.Comment
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Re: Engine temp vs timing/Bill
I'd go with a stock 984 Delco distributor. The timing will be all in around 2800 rpm and you should adjust the timing to 36 degrees at 3000 rpm and lock it down. (disconnected and pluged vac line). Where-ever the initial timing falls os OK. (six to twelve degrees). If there's a problem with the mechanical advance it'll show up with this approach to setting timing.
Dale.Comment
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Re: Engine temp vs timing/Bill
look for a vacuum advance that gives about 12 degrees at 12 inches of vacuum, similar to 350/350 or 350/370 engine used. Vacuum advance should start to come in at about 7 inches of vacuum.Comment
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Re: Engine temp vs timing/Bill, Dale, Clem
Gentlemen I thank you for your help. I think with this information even I can set the timing and check the advance. As always, I appreciate your help.Comment
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Re: Engine temp vs timing
Hello,
This may not be the place to ask this question as the car I'm dealing with (my son's) is not a Corvette, but there is so much expertise here I'd like to give it a try anyway.
My son's car (recently purchased) is a 64 Nova with a stroker 383 in it. The engine was out of a 79 Chevy truck originally. The distributor is hooked to what looks to be ported vacuum on the Holly carb instead of manifold vacuum. When we connect the advance line to vacuum at the base of the carb, it appears to retard the timing instead of advance it.
My questions are:
Were there distributors that worked this way vs what I'm used to where more vacuum on the advance can actually pulled more timing advance?
What is the optimal advance setting for an engine to get it to pass an emissions test? I've been told that setting the timing as close to tdc as possible at idle is best for low emissions.
Thank you,
WaltComment
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Re: Engine temp vs timing
There is no OEM vacuum can that is set up to retard under conditions of vacuum. There are combination pressure-vacuum systems that retard the tining under pressure (corvair turbo) but no V8 units that I am aware of.Comment
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Re: Engine temp vs timing
Hello Bill,
Thank you for the response. I double checked, and this does not work in reverse like my son thought he saw the timing mark move. Vacuum applied to the distributor will indeed advance the timing. Problem is, I don't think an engine is made that can apply enough vacuum to make the thing move! I tried moving the breaker plate by hand and I had to apply so much pressure to it that my fingers hurt... I disconnected the vacuum advance can and the breaker plate moves freely by itself. I guess I need to investigate the different advance cans that are available that I've heard others mention here that apply full advance at a relatively low vacuum reading. Duke was one I thought had the numbers for different vacuum advance assemblies.
I'm still wondering about the timing setting for the least amount of emissions so I can tweak this for the upcomming emissions test we need to pass.
Someone did tell me about a product called "Guaranteed to Pass" that you can buy from auto parts stores (Checker Auto is one here in Colorado). I'll be picking up a can of that today as well to get ready for the emissions test.
WaltComment
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Re: Engine temp vs timing
It would take very severely retarded timing to cause incomplete combustion so that the mixture was still burning as the exhaust valve opened, however, retarded timing will increase EGT because of incomplete expansion.
The optimum timing for any operating condition (assuming no detonation) is the timing that creates minimum EGT.
As a rule of thumb, you want 25-30 degrees of idle timing, which is the combination of vacuum plus initial - and maybe a few degrees of centrifugal if you have a quick curve. Idle and part throttle operation, especially at low revs need more timing because a low density exhaust gas diluted mixture has a lower flame propagation speed than a dense mixture typical of WOT operation.
Another rule of thumb is to select a vacuum advance can that provides full vacuum advance at idle, so the can should offer full advance about two inches blow the engine's idle vacuum. The '65 SHP/FI can provides 16* of advacne at 8", which is just right for a SHP or aftermarket high performance can.
Initial plus centrifugal should total about 36-38* and you want to get it all in as soon as possible, the limiting factor being low speed detonation.
DukeComment
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