Raylar

Good rule of thumb, but its also affected by humidity. About 1.5% loss of power for every 10 degree increase above 70 degrees F. So 40 degree increase equates to about a 6% loss of power or in a 496HO case about 26HP, not enough to srub off more than about 2 mph at WOT. I can tell you from expierence that a problem that is also appearing on a lot of new boats is not enough air intake into the engine compartments at WOT. Remember a 496HO uses about 700cfm at WOT= 700 cubic feet per minute, or a block of air 10 foot by 10 foot by 7 feet every minute! Most new boats should have the engine compartment checked with a sensitive pressure guage to see if they are pulling a slight vacume at WOT. This is just like pulling back the throttle! Not enough air in not enough power out! Check your vent sizes and engine compartment air intake feed size. Remember although not as attactive as flat billet intake vents, scoop vents will inject more air at speed than the flat ones. Also on 496HO's you should prop for about 5000 rpms at Wot, this is where they produce the best prop speed versus max HP. As for most deck boats in the 25 -28 foot range there are very few that will hit 70MPH speeds with stock 496HO's especially in Havasu heat.
I know what the salesman who sells you the boat tells you your deckboat will do 70+ mph., but a little test drive with real loads in real conditions will make honest the real proof for the buyer!
Just some thoughts and info.

Ray @ Raylar

onesickpantera

iTrader: (1)

Just had this discussion on another board about making some kind of sealed intake system on my 502 mag(I don't want to cut up the sunpad!). Only two 16"x2" vents and my engine compartment seems to get very warm.

It actually got started because I was asking about running an intercooler on a NA engine to keep the intake temps low. Most agreed that the cooler air would be offset by the airflow restrictions of the intercooler on a NA engine so the converstion turned to allowing the engine to suck fresh outside OUTSIDE air.

It was 95* here and very humid this weekend. I lost about 200 rpm and 2-3 mph when compared to a 75* day with lower humidity.

onesickpantera

iTrader: (1)

Really? I'm at 1300 feet and I never really considered that I was losing that much hp compared sea level?

CigDaze

Pressure at 1300' is 14.06 psi, or a 4.4% drop in pressure from sea level.

CigDaze

Here's a neat calculator.
Start with 77 degrees, 29.235 inches mercury, 0% Rel Humidity, and 0 feet altitude, for a typical standard atmosphere at which engines are tested and rated per SAE J1349

Going to 600 feet, you can expect a 2.5% loss in horsepower, at 1300' expect a 5.4% loss in power.

At 600 feet and 110 degrees, expect a 5.9% loss.

Check it out:
http://www.csgnetwork.com/relhumhpcalc.html

I'm tellin' you guys, it makes a big difference.

Havasu Cig

Depends on the hull. My DCB's lost a considerable amount of speed from spring to summer in Havasu (about 8-10 mph). My Skater is not effected nearly as much.

PhantomChaos

Probably not this year......

PhantomChaos

No....AAA baby! I was inside with the AC on watching a movie!