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Why a Cold Engine Runs Poorly

10K views 10 replies 6 participants last post by  automotive breath 
#1 ·
Ok

this is crazy but I will ask it anyway. why is it that a cold engine, especially a rebuilt modified hi performance V8 engine like my BB Chevy runs poorly until it warms up. Yes I know this is "basic engine 101" type of discussion but I am asking it anyway.

I can think of 2 or 3 things that would make it run poorly when cold. First is the air/fuel mixture when it is cold doesn't vaporize well and results is poor combustion. Second, is the motor oil is cold and has a higher viscocity when cold. However, I though colder air is better for combustion because I believe it is denser and contains more nitrogen and oxygen............?, Third, perhaps the cylinder walls and pistons have more friction when they are cold and don't move as easily.

All this came to my mind just a few minutes ago when I took my car for a ride. It's a 72 Nova with a recently rebuilt 468 Chevy. It has a huge hydraulic cam (comp cams extreme magnum), iron oval port closed chamber heads (ported with valve job), 10.5 to 1 comp ratio, roller rockers, dual plane intake, Holley 4BBL carb - single pumper - vacuum secondaries - no operational choke, MSD Ignition and distributor, 2" headers, Holley high flow mechanical fuel pump @ 8 psi, no emissions or warmup controls whatsoever.

The first 5 minutes or so that the engine warms up it runs rough. I don't care because it's just a warmup period. Once it's warmed up the engine runs very well and strong - I'm guessing near 550 HP and really smokes the tires. but why is it that those first 5 minutes or so the thing runs like crap.

thanks.

Lee
 
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#2 ·
Doc here, :pimp:

I think you've answered your own question..

"No operational Choke"

The choke allows extra fuel to the engine during that warmup period to help it run better during warmup.after things warm a bit, it begins to open..(about 3 minutes on a standard electric choke.)

So If you want it to run better During warmup, connect and adjust the Choke properly, It should run fine after.

Doc :pimp:
 
#3 ·
The main reason for a choke, and the reason why it run so poorly when cold is from a phenomenon called fuel puddling or fuel shear. The carburetor vaporizes fuel and mixes it with air in a proper ratio. All things being perfect, the fuel stays in suspension. When the intake and heads are cold, often times the fuel comes out of suspension and wets the walls of the runners. That leans the mixture that the cylinders actually see, so the choke richens it up to compensate. Since the choke richens the mixture (which would normally cause the engine to die) the choke mechanism also includes a little cam or pin that holds the throttles open a little for fast idle.

During this cold period, the wetted walls will from time to time give up a drop or two which causes the rough performance. When you go to accelerate when its cold, more of the same happens. An unpredictable amount of the fuel that is metered through the carb will actually make it to the cylinder and stumbling and stalling might happen.

Once the intake is hot, any fuel that comes out of suspension and touches the runner walls is quickly evaporated. Intakes often have an exhaust crossover that speeds the heating process.

I don't think the cold oil or friction has anything to do with it. Those two factors alone might reduce performance, but not cause poor running properties. You could tune the carb to run better when cold, but then you would give up the proper tune when its hot. Since engines spend 90% of their time at operating temps, it makes sense to tune them for hot running.

EFI mostly cures these issues since they inject the fuel directly at the valve. There is almost no chance for fuel to puddle or come out of suspension. Cold enrichment can be much less drastic and high cold idle speeds much less of an issue.
 
#4 ·
curtis73 said:
The main reason for a choke, and the reason why it run so poorly when cold is from a phenomenon called fuel puddling or fuel shear....................................................... Cold enrichment can be much less drastic and high cold idle speeds much less of an issue.
That was beautiful, Curtis. :thumbup:
 
#5 ·
great explanation Curtis!

can only add in this case, the "big" cam = lower Hg (12hg?) at idle = much less velocity to keep it suspended....1200rpms idle when cold will increase the Hg/velocity (18+hg?) and that helps reduce wetting

a stock motor will pull approx 18hg/600rpm, 22hg/1200rpms choked for more velocity/less wetting cold just for reference

and to help appreciate (?) what the Hg values and velocity translate to: a 2HP wet/dry shop vac can only pull about 4Hg max!
 
#6 ·
docvette said:
Doc here, :pimp:

I think you've answered your own question..

"No operational Choke"

The choke allows extra fuel to the engine during that warmup period to help it run better during warmup.after things warm a bit, it begins to open..(about 3 minutes on a standard electric choke.)

So If you want it to run better During warmup, connect and adjust the Choke properly, It should run fine after.

Doc :pimp:
thanks buddy
 
#7 ·
curtis73 said:
The main reason for a choke, and the reason why it run so poorly when cold is from a phenomenon called fuel puddling or fuel shear. The carburetor vaporizes fuel and mixes it with air in a proper ratio. All things being perfect, the fuel stays in suspension. When the intake and heads are cold, often times the fuel comes out of suspension and wets the walls of the runners. That leans the mixture that the cylinders actually see, so the choke richens it up to compensate. Since the choke richens the mixture (which would normally cause the engine to die) the choke mechanism also includes a little cam or pin that holds the throttles open a little for fast idle.

During this cold period, the wetted walls will from time to time give up a drop or two which causes the rough performance. When you go to accelerate when its cold, more of the same happens. An unpredictable amount of the fuel that is metered through the carb will actually make it to the cylinder and stumbling and stalling might happen.

Once the intake is hot, any fuel that comes out of suspension and touches the runner walls is quickly evaporated. Intakes often have an exhaust crossover that speeds the heating process.

I don't think the cold oil or friction has anything to do with it. Those two factors alone might reduce performance, but not cause poor running properties. You could tune the carb to run better when cold, but then you would give up the proper tune when its hot. Since engines spend 90% of their time at operating temps, it makes sense to tune them for hot running.

EFI mostly cures these issues since they inject the fuel directly at the valve. There is almost no chance for fuel to puddle or come out of suspension. Cold enrichment can be much less drastic and high cold idle speeds much less of an issue.
thanks curtis

it's funny sometimes how we can build race cars and often forget some of the basic "engine 101" theory.

Lee
 
#8 ·
red65mustang said:
great explanation Curtis!

can only add in this case, the "big" cam = lower Hg (12hg?) at idle = much less velocity to keep it suspended....1200rpms idle when cold will increase the Hg/velocity (18+hg?) and that helps reduce wetting

a stock motor will pull approx 18hg/600rpm, 22hg/1200rpms choked for more velocity/less wetting cold just for reference

and to help appreciate (?) what the Hg values and velocity translate to: a 2HP wet/dry shop vac can only pull about 4Hg max!
thanks mustang

you are right about the manifold vacuum. It's only about 10" at idle if that.

Lee
 
#9 ·
curtis73 said:
…During this cold period, the wetted walls will from time to time give up a drop or two which causes the rough performance…..
Curtis, this is a great explanation. After years of dealing with this problem and how it becomes more pronounced with a camshaft with a long overlap period, it’s easy to overlook the reason this happens. I’m not disagreeing with what you are saying but hope to take the discussion a little further.

It’s important to think about what happens during combustion in a cold engine.

You point out how EFI helps to cure this problem; in addition modern ignition systems have also done much to improve the warm up issues. I remember my first engine with a camshaft with a long overlap period, no choke and a point’s distributor. MSD and the like have done there part to light the lean mixtures encountered during cold start.

Another important point is when the air/fuel mix is compressed during the compression stroke and even further during the early phase of combustion the fuel has a tendency to drop out of suspension. This further leans the mixture adding to the problem; it also increases hydrocarbons in the exhaust to a point that it can irritate the eyes in a confined area.

I’m somewhat hesitant to say this but I have found a cure for this problem. The 67 Camaro in my avatar idles cold at 800 RPM with no choke.

Here are the engine specs:
AFR 190 heads with 67CC combustion chambers
Mini dome pistons - steel rods
Isky solid flat tappet cam 263/268 0.050” - 0.555” lift.
11:1 compression - 0.037” squish clearance
750 Holley - 83 jets square – Victor Jr. intake
RPM range 5200 – 7200
10.80 ET @ 121 MPH at 3200 Lbs.

The cylinder heads have been modified with grooves that aid mixture motion during combustion. The improved idle is one of the benefits that were realized with the modification. I realize this is hard to believe to most people, but for me seeing is believing.
 
#10 · (Edited)
auto breath,
not really a fair comparison....your at sea level with humidity and much higher early morning ambient "cold" temps....leejoy is in Chicago!

due to temp and humidity, it's the same here in FL, "most" motors do OK without a choke, if you do need one it's usually turn the cap full left for minumum time and (?) 1000rpm

leejoy,
there's your answer...move to New Orleans!
 
#11 ·
red65mustang said:
auto breath,
not really a fair comparison....your at sea level with humidity and much higher early morning ambient "cold" temps....leejoy is in Chicago!...
I agree you bring up a good point. I did not try to compare my engine to his or my location to his. I am very familiar with cold natured engines in moderate cold weather; we race in December and January!

The problem is even more pronounced with the huge BBC combustion chamber, flame time is increased making it even more cold natured than the SBC.
 
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