My car picks up a lot of HP when it's cold out. Too rich?
 

1970 T with webers. PMO spec'ed jetting. Not sure what size the mains are.

The car is a lot faster when it's cold out. Is it a jetting thing or is that just what happens when the motor is able to run cooler? By the way the car never runs hot on even the hottest days. If I could get it to run like that when it's warm out that would be great.

Ideas?

Thanks

Alex

Well, colder air is denser that should account for the increase in performance but im not sure if that means anything is wrong.

Nearly every car has more power when it is cold outside (especially turbos!). Years ago, I took my 930 to Death Valley. It was 100+ outside and the poor thing was really struggling after 5+ hours of the torture... I love running during brisk 40 F nights!

Not only does the engine make more power, but the tires spin more easily which makes it seem like even more power.

Let's not get out of hand here. It's still only a 2.2T! So just because it runs better when it's cold does not mean I cold lean it out and gain anything in regular weather? Does this open the door to another cold air induction thread?

I was just about to say that! After all, that's why every car design chases down cooler outside air for even a "granny car"-to get better performance (or mileage).

After the first cold front in Florida passes each winter I think, I don't remember this car being this quick.

Jack, 87 Carrera

I have a problem answering this issue. A text answer will give a theoretical hard answer and won't be able to take into account the air temp after it hits the carb and winds down to the combustion chamber.. meaning warm intakes will warm the air, etc, etc.

So cold air is denser and contains more oxygen. The prob with dealing with this is that a carb is air volume metered instead of mass metered. In theory again it would be nice if we could feed it more fuel the colder the air becomes etc if we tuned it at a warmer air temp to start with.

Another ball park number is that a 1% increase in density = 1% potential increase in power/torque.

Another ballpark number is that a 10deg decrease in temp = a 1% increase in density in real world circumstances. I forgot what the text numbers are.

Again another guess is that a 40deg decrease in temp = 4% increase in power. Kinda like cold air is a mini supercharger.

please take the above a bit of logic without pro hard numbers.

Interesting topic. I noticed the same thing with my 3.2L that on a cold morning that it seems that the car has much more power.

Outer cooler - as opposed to Inter cooler. ;)

If your carburated engine makes more power when it is cold out, that proves your mixture is waaaay too rich when it is warm. Depending on how rich it is, it could even be too rich when it is cold out.

As was stated above, the carb doesn't react to air density, only air flow. If you are driving on a cold day, more O2 is being ingested. But O2 doesn't make any hp on it's own, it has to have the necessary amount of fuel to go along with it. Since your engine does make more hp when it is cold out, that proves you are dumping in the necessary extra fuel at all times.

With the same jets, when it is warm out, there will be extra fuel passing through your engine when there isn't the needed O2 to burn it. This extra fuel just goes along for the ride and passes out the exhaust as unburned HC. Since the fuel has considerable mass, what it does is put out the fire, absorbing the heat of combustion, reducing hp. By leaning out the jets, you will regain some of the lost hp you notice when it is warm out.

A few of you made the same observations with your FI cars. Those of you that are smart enough to have your O2 sensors hooked up are having your A/F ratio properly adjusted to compensate for the colder air. If you were to moniter the pulse width when it is cold out, it would be much wider than when warm.

This being the case, how much leaner should I go? Does anyone know what jets I should have? I'll check and see what I do have. With six venturies, re-jetting costs a bit, particularly if you don't get it right the first time.

Alex

So, when it's cold outside and the air and fuel are both more dense a carburated car won't make more power unless it's setup to be rich when the weather is warm and the air and fuel are less dense? Seems like when the gas is more dense and the air is more dense you get more of both into the engine which will help regardless of the metering method, but I'm far from an expert.

The reason that it doesn't re-open that door is that cold air induction only brings in ambient air. It won't actually cool the air to lower than ambient. So, bringing in ambient air when the temp is 95* outside is not going to benefit like when the temp is 40 or 50.

When the temp is low the engine is cooler, the gas in the tank is cooler, the air is cooler. All of these things together come together to provide a benefit. I think it's been determined that the air in the engine compartment isn't 40 or 50 * higher than the ambient temp, so the potential benefit of a cold air intake is not as great as the benefit of a cold day.

If you want that sort of benefit you'll likely have to get into some sort of water spray system, not just a CAI.

By comparison to the density change of the air, the fuel's density change is microscopic. It can be neglected.

Originally posted by dvkk

If your carburated engine makes more power when it is cold out, that proves your mixture is waaaay too rich when it is warm.
Since your engine does make more hp when it is cold out, that proves you are dumping in the necessary extra fuel at all time. ------- Sounds good but that hasn't been my experience
My EGT's are about 25degF hotter at 70degF than at 40degF. If I lean out 70degF operation EGT's will become hotter and will have greater deviation between hotter and colder EGT.. in other words the spread of piston power will become greater. It's as lean as I could make it and be easily streetable for my act.


Since the fuel has considerable mass, what it does is put out the fire, absorbing the heat of combustion, reducing hp. By leaning out the jets, you will regain some of the lost hp you notice when it is warm out.
---------- it does put out the fire but it still feels stronger at night. I have hard numbers on that.

A few of you made the same observations with your FI cars. Those of you that are smart enough to have your O2 sensors hooked up are having your A/F ratio properly adjusted to compensate for the colder air.
-------- FI is another story somewhat because FI doesn't have the atomization critical issue that carbs have. A cool carb and intake allows the mix to be dense as possible.

I don't really have a text answer on what's happening but i think I could get away with 1/4 idle mix leaner at night and make even more power. That 1/4 turn is worth about the 25degF in EGT. Then I'd have to reset for daytime hotter ambiant. I'm figuring the above on 30-40degF ambiant difference.

My intakes are cool to the touch on warm days probably because the heat shield gaskets below the intakes. If I could feel warmth that would start at about 120degF. There is a thermal coating for inside the intakes and the head intake chamber to keep the mix cool. I'm not considering stop and go driving.

Re: My car picks up a lot of HP when it's cold out. Too rich?
 

Hi Alex..
Did you do the carb install and adjustment ?

What is the car used for ?

Do you have any carb specs.. venturi, jets, etc ?

"I don't really have a text answer on what's happening but i think I could get away with 1/4 idle mix leaner at night and make even more power. That 1/4 turn is worth about the 25degF in EGT. Then I'd have to reset for daytime hotter ambiant. I'm figuring the above on 30-40degF ambiant difference."

"Idle mixture settings have no effect whatsoever on max hp when you are at WOT. You need to play with the main cct to affect peak hp."

ps: I should have added that all above is during street operation and that the idles affect my engine significantly below 4k rpm.

I found this.

"Correction factors are sometimes used to find the correct carburetor settings for changing temperatures and altitudes. The chart in fig 8, shows a typical correction factor chart. To use this chart, jet the carburetor and write down the pilot and main jet sizes. Determine the correct air temperature and follow the chart over to the right until the correct elevation is found. Move straight down from this point until the correct coreection factor is found. Using fig 8 as an example, the air temperature is 95š fahrenheight and the altitude is 3200 ft. The correction factor will be 0.92. To find out the correction main and pilot jets, multiple the correction factor and each jet size. A main jet size of 350 would be multiplied by 0.92 and the new main jet size would be a 322. A pilot jet size of 40 would be multiplied by 0.92 and the pilot jet size would be 36.8. "

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