Anyone know how much heat and humidity reall effects performance. I assume its worse on a carb motor. Ive been comparing my post exhaust install performance in high 90 degree / 80% humid. With my stk exhaust at 60 degree /low humid weather. The results are not so pleasing and im wondering how much the weather is messing me up. Thanks sj

Both heat & humidity hurt performance big time. I do not know %. Carb motors should have bigger lose than EFI but only if EFI computer system works off air temp / density. Blower motor lose greater % verses natural due to the blower packing in more air when cooler. Looks like I have lost 10-14 mph from 95 & high humidity verse 65 & low humidy.

10-14? holy crap.. theres hope yet. I have done a pile of work and have gained 1.5 mph. But im comparing to testing days where it was 60ish. thanks turbo. sj

Contact a shop with a dyno that corrects for weather. They should be able to tell you what the correction equation is.

Temp and humidity increase "density alltitude"...on NA motors it makes em think they are running at a higher elev above SL. As temps go up you need to lean out the carb. Get more air and less fuel. Puters on an efi motor do it for you automatically, but I still prefer a carb motor.
In a NA airplane you can manually lean out the motor as you climb and watch an EGT gauge to find the optimum setting...I need to invent something like that for a boat.
Dan

A combination of Temp., Humidity, and Barometric presure give you an adjusted density altitude, and an air correction. This correction will correct your HP numbers back to SAE standards 60 Degrees, 0% humidity, and 29.92 on the barometer.

Let's say your engine made 500 HP at SAE, and now we are at approx 4,600' with a correction of 1.11 it would be like you made 450 HP.

Your engine is an air pump, and the thinner the air the harder it has to work. The advantage of a supercharged engine is that you can chance your amount of boost to regain this power loss.

Now to a loss of 10 - 14 MPH ???? Since most say it's approx. 15 HP per mph that would have to be a loss of 150 - 200 HP ? That would seem very extreme.

I've lost 3-4 mph

I ran 115 at Havasu in December and 108 last week in Phoenix. Basically the same setup and water conditions. Just add another 40 degress and a bunch more humidity!

Craig

So what you guy are saying is when I was prop testing my boat with a blower motor the other day when it was 95 degrees and humid was not a good thing. If my prop calculations are right it would seem that I was somewhere around 520 hp instead of 570 hp. would that seem right for a loss in HP on a 454 with a 177 blower in those weather conditions????

I was always under the impression that humidity increased horsepower assuming the same temp. and pressure. It had something to do with more oxygen available. I lose lots of horsepower with temperature increases though. The difference between 50 and 90 is amazing.