Drag racing question:

[BillyGman]

This might be elementary to some of you, but hey, I'm really still a novice drag racer anyway. I'm well aware as most of you also are, that cold air allows your engine to make more power, but what about humidity? I've heard that humidity also effects your engine's power, but does high humidity cause more power or less power? And my assumption is that either way, it doesn't have quite as much of an effect on power levels as the air temperatures do. Right?

And BTW, if any one of you have raced your car at the same dragstrip on two different days, which saw similar temperatures, but very different humidity levels, then please share with us your findings of how much your ET's were effected one way, or the other. That is, providing your car wasn't tuned nor modified any differently for either one of the two days in question.

[SergntMac]

It's been my experience that high humidity costs power. Probably not as severely in cooler air temps, but high temp and high humidity, handicapps performance.

Just my .02C...

[snowbird]

This might be elementary to some of you, but hey, I'm really still a novice drag racer anyway. I'm well aware as most of you also are, that cold air allows your engine to make more power, but what about humidity? I've heard that humidity also effects your engine's power, but does high humidity cause more power or less power? And my assumption is that either way, it doesn't have quite as much of an effect on power levels as the air temperatures do. Right?

And BTW, if any one of you have raced your car at the same dragstrip on two different days, which saw similar temperatures, but very different humidity levels, then please share with us your findings of how much your ET's were effected one way, or the other. That is, providing your car wasn't tuned nor modified any differently for either one of the two days in question.The colder, the dryer, the better. I don't have documented facts thought.

I read somewhere that each lowered 10 degreeF give you 1 % more HP and TQ. So where i live, from the height of the summer +- 95F to the winter +- minus 25F, we can have 120F degree difference. ( I know i know, why the hell i live here ?!!) A 400hp engine in july would then give +- 448 hp in January. It seems a lot but certainly feel that way. Alll the locals tuned on the edge blow their engines in january and february with the combined effect of edgy tune and poorer fuel offered in winter. I did blow my Probe GT Turbo in the early 90s. The wastegate had frozen and, well, boom at -20 degree.

[Wags]

Yes, humidity does have an effect on performance. To put it simply, (there are alot of formulas to apply to this), the more humidity in the air, which is water vapor, takes up room that would normally be occupied by dry air. And the more dry air, the more power that you get from a combustion engine. This is just a very simplistic explenation of it. Let me know if you want the equations to figure out "density altitude", which combines air temp, humidity, and elevation. For instance, if you are running a lean air/fuel ratio for a pass, and the density increases (more oxygen in the air), you will not have the added fuel for combustion to take advantage of the extra air, so no added power for that pass.

Wags

[Bradley G]

This topic came up a few weeks ago at Norwalk OH.(Fun Ford weekend)

Jerry Barnes, Mikes Merc, MI2QK4U,Kanako & I.
I can attest the fastest times achieved on this given day, were in the early morning.
This event was where the current record (best ET/trap) was attained.
I counted only a few runs, prior to Jerry Barnes' blistering 11.2 pass.
Jerry mentioned, as we all sat around the trailer later, he was going to get a weather station, to record the other weather factors on his time slips.
My best pass with my car (14.2), was attained in a cool mid fourty degree weather.
I am not sure about the humidity on that day, but, it did rain earlier in the morning.

[sailsmen]

I beleive High humidity has as much of an effect as high heat.

One of the auto shows had a top bracket racer giving tips who said humidity was a bigger factor than heat.

[MikesMerc]

I beleive High humidity has as much of an effect as high heat.


This is true.




Yes, humidity does have an effect on performance. To put it simply, (there are alot of formulas to apply to this), the more humidity in the air, which is water vapor, takes up room that would normally be occupied by dry air. And the more dry air, the more power that you get from a combustion engine. This is just a very simplistic explenation of it.

And this explains it.

[Tinaree]

On the 4V humidity seems to make a good deal of difference. Wags has a good breakdown of why; water is lighter than nitrogen and oxygen molecules (H2O=18, N2=28, O2=32). According to Boyle's ideal gas law, a given quantity of air at a set pressure and temp will contain a specific number of molecules. The more of those which are water, the lighter (less mass) the whole volume is.

I read a BMW tech article somewhere which suggests a maximum impact of 5% HP difference due to humidity. In any case, it is a factor in determining the density altitude. The less humid, the lower the DA.

[Bluerauder]

This is true.

And this explains it.
And, it also explains why cooler air is better as well. As air heats, it expands. Cooler air is more dense. Thus, there's more oxygen available for the combustion process per any given volume. :D

[Tinaree]

Temp and pressure are far more important than humidity. Although it makes a difference, it doesn't kill your times.

Here is a quick and dirty DA calculator (http://www.csgnetwork.com/dadtcalc.html)

Try it out and see what a difference the relative humidity makes for a given temp and pressure.

[Smokie]

All else being equal humidity slows you down, I keep a spreadsheet on timeslips, it includes temperature and humidity, same track. Humidity rises and ET's also go up.

[ckadiddle]

That explains why Blue Thunder seems a bit perkier in the winter months with cool, dry weather down here.

[Fourth Horseman]

I hear a lot of people here (Salt Lake City, Utah) complain about the high altitude and thin air. I guess it makes sense, the thinner the air the less fuel you can mix in with it.

[BillyGman]

Thanks very much to everyone for your replies. It's much appreciated. I was just curious about this, as well as unknowledgeable too. But now I've been schooled thanks to you guys. ;)

[maraudernkc]

This is very true and on a supercharged motor in hot weather you loose boost. My car was making 16PSI on 80 degree Days now it is making 12 PSI on 95 degree Days with higher humidity.

Cool air is your cars best friend!


And, it also explains why cooler air is better as well. As air heats, it expands. Cooler air is more dense. Thus, there's more oxygen available for the combustion process per any given volume. :D

[Mike M]

This might be elementary to some of you, but hey, I'm really still a novice drag racer anyway. I'm well aware as most of you also are, that cold air allows your engine to make more power, but what about humidity? I've heard that humidity also effects your engine's power, but does high humidity cause more power or less power? And my assumption is that either way, it doesn't have quite as much of an effect on power levels as the air temperatures do. Right?

And BTW, if any one of you have raced your car at the same dragstrip on two different days, which saw similar temperatures, but very different humidity levels, then please share with us your findings of how much your ET's were effected one way, or the other. That is, providing your car wasn't tuned nor modified any differently for either one of the two days in question.

If the air is occuied with water there is less room for oxegen.

[BillyGman]

This is very true and on a supercharged motor in hot weather you loose boost. My car was making 16PSI on 80 degree Days now it is making 12 PSI on 95 degree Days with higher humidity.

.

I don't know why that would be. On my car, the boost gauge reads 9.5-10 PSI when I'm at full throttle regardless of the air temperatures. Can it be because I have a different type of intercooler? But then again, even if one type of intercooler kept the engine cooler than the other one does, I would think that even that wouldn't effect the amount of boost pressure that the supercharger yields. It would reduce HP readings, but not boost pressure to my knowledge.

Can it be that you have a leak in your intercooler system that you don't know about ? This is a sincere question Greg. Not a dig against your S/Cer set-up.

[sailsmen]

16psi to 12psi seems like a big swing, I have a Centrfigual that is in many ways similar to yours as respects making boost.

I am only making 10psi, but it seems to make 10psi in all temps.

[AzMarauder]

The colder, the dryer, the better. I don't have documented facts thought.

I read somewhere that each lowered 10 degreeF give you 1 % more HP and TQ. So where i live, from the height of the summer +- 95F to the winter +- minus 25F, we can have 120F degree difference. ( I know i know, why the hell i live here ?!!) A 400hp engine in july would then give +- 448 hp in January. It seems a lot but certainly feel that way. Alll the locals tuned on the edge blow their engines in january and february with the combined effect of edgy tune and poorer fuel offered in winter. I did blow my Probe GT Turbo in the early 90s. The wastegate had frozen and, well, boom at -20 degree.
I too can second the colder the better... but I don't have any read on humidity. In Arizona we have very little.

My 03 Cobra would run 12.70s in 90-100 degree weather... 12.50s in 60-70 degree weather. This difference remained consistant. It got to the point I could call my ET based upon the temperature. If I was driving worth a damn, I'd be with in about .003 with my ETs over the whole night. The only difference would be... what was the ambient temp... would I be in the 12.50's or the 12.70s.

[AzMarauder]

16psi to 12psi seems like a big swing, I have a Centrfigual that is in many ways similar to yours as respects making boost.

I am only making 10psi, but it seems to make 10psi in all temps.
My experience (albeit not with a Marauder) is what you describe here. All my boosted cars make the same PSI regardless of temp. My two turbos (Starion and Thunderbird Turbo Coupe) and my previous 03 Cobra all reached their max boost levels regardless of temp or humidity. The performance was drastically different as I mentioned in an earlier post, but the boost gauge didn't show any difference.

[BillyGman]

Since Greg has an air-to-air intercooler set-up, while most of us have an air-to-water one, I'm wondering if a leak in the air-to-air intercoolers would effect the boost pressures. I know that leaks in an air-to-water intercooler set-up like most of us have wouldn't in any way reduce boost pressures, but I don't know much about air-to-air set-ups, and therefore I'm wondering if they're in any way in the path of the compressed air of the supercharger, or of the intake manifold.


Because if they are, then maybe a leakage in the air-to-air intercooler can reduce boost pressures. However, this is merely a guess on my part, and not a very educated guess at that.

[TooManyFords]

When a ricer pulls along side of me and askes me "what's under there?", I tell them it's a vacuum leak!

Cheers!

John

[Tinaree]

Climate shouldn't have an impact on the volumetric efficiency of a supercharger. However, that efficiency is a percentage, and if you start with lower input pressure then you will make a lower output pressure, maintaining (generally) the same efficiency. In other words, if you spin the blower in a vacuum, you won't make any boost.

Maybe Greg was in a situation where he compared two days with extremely different pressures? That could have been an accompanying factor.