Hydrolift

A 502cid (8,2l) engine needs 2.17 US gallons (8,2liter) of air pr. 1 rpm., correct?
How do you then calculate how big the air vents need to be to not starve the engine for air?

stevesxm

the volume of air is a function of the displaced volume ( as you note ) and the rpm.... as you note again... but the bottom line is simple.... if normally aspirated, you don't NEED any opening larger than the anullar opening of the throttle body/ carb (s) .. thats the choke point. however there is more to it than that.... cold inlet air and engine compartment temps are crucial to making power.... scoops into the inlet solve the cold inlet air issue... good engine bay entrance air volume and circulation in the engine bay should be accomplished at all costs...

on my top gun, i had two 2 .5 holes each side... for inlet air...or about 20 sq inches.... barely enough for the motors... i figured...

so i got two small pcs of 2x4 and set the hatch down on them such that there was now about 100 sq in opening abover the rear seat.... instant 200 revs , lower oil temps a great deal.... and i am certain the revs were NOT from better BREATHING but the fact that the intake manifold went from being about 200 degrees to ambient and the air inlet TEMP went down about 90 degrees

so i went to the local chandlery and bought some linear rail type air inlets... about 90 sq inches per side and problems solved. as far as air circulation goes... more is better

Hydrolift

This make sense. But what a bout a supercharged engine, how would you go about to calculate that?

stevesxm

i just looked up my formulae for calculating air flow thru an orfice w/ a differential pressure . it was, as i recalled, very long with a lot of variables and keyboard characters that i don't have... but if you type that phrase into google you can see it for yourself... but again... the bottom line is... the MINIMUM you need is the the size of the annular opening that feeds the unit... that is the choke point... BUT the same principle i described earlier simply applies to a greater degree... and i would suggest to you that here is the bottomm line.

if you can conveniently run an air scoop... do it. if you can not then you get as much fresh air in and out of your engine bay as you humanly possibly can. the more you do that, the better off you will always be. always. every single solitary time.
heat is the enemy of every single componet that lives in there.

it kills your power, and it wears things out and makes them work badly.

it really is as simple as that.

tomcat

stevesxm is on the right track when he says it's the drop in temperature of engine intake air that has the benefical effect. The math is simple, when it's hot under the engine hatch the air entering the engine is less dense, therefore less power. You have to compare the absolute temperatures, (ambient versus under hatch), so you add 460 to each.

eg. (77F + 460)/(150F + 460) = 0.88

That's right ... you just lost 12% of the HP.

Now you have a choice:

- do you add air to the engine compartment and try to cool it down so that the engine gets cooler air

- or do you direct ambient air directly to the carb? (right answer)

I don't think it matters to the engine that it is 150F under the hatch, everything on the engine is water cooled. And if you do the math, (a little more complicated) you find out that it would take a huge amount of cooling air to prevent the engine compartment from rising above ambient temperature. Engines radiate a LOT of heat, 8-10% of the fuel energy is lost in this way.

Here are some numbers to put this into perspective:

engine compartment volume = 120 ft3
2 @ typical bilge blowers = 120 CFM (1 air change/min)
2 @ 450 HP engines = 1200 CFM (10 air change/min)
cooling air required to limit engine compartment to +20F
over ambient when these engines are at WOT= 12,000 CFM (100 air change/min)

This last number is based on the BTU/min radiated per HP according to the type of engine and extent of water cooling. I admit to making an educated guess of 5 BTU/min/HP.

Hood scoops, linear vents and other holes can move a lot of air, but never enough to reduce engine compartment temperature to ambient. However, if they are located immediately adjacent to the carbs, they can supply unheated ambient air directly to the engine intake.

Island Time

I am trying to get more cool air in also. Question I have is was just increasing the side vents enough? I thought about also adding a vent (Exit for heat) in the back.

The only concern I had with ducting to the carbs was water/salt getting fed directly in.

James

We put a data logger "spider" on the engine of a 36 skater w/ 525's - boat has big scoops on the hatches - looked like the "cold"/fresh are was blowing in on the intake - Wrong - the air was circulating was actually blowing over the top of the engine - rolling underneath and then back up to the intake - the hottest part of the engine bay was the intake and that was 28 degrees hotter than the intlet air - this air flow also changed with speed - @ 75 the temps were all pretty good - @ 100 it was terrible @ 120 it was moderating - mid range temps are important if you have detination issues

Look for a data logger for temps - also cool tool to monitor drive temps @ various speeds - header temps etc...

Unit I use is called: Datapaq " EasyTrack" Model TB0225A and has seven leads that can be used to log at one time

Kidnova

Interesting that you post this. When I winterize I prop the hatch up a couple of inches, in hopes that it might allow for a little air circulation and cut down on condensation. As I was doing this at the end of last season it came to mind that, this might be a good thing to try when running as it should allow for cooler air in the engine compartment, and a few more R's. Lazy, so I didn't try proppong the hatch and running it that way this season. Your 200 RPM increase is encouraging. I have a couple of weeks left before winterizing so I'll give it a try. Just need to figure a way to prop the hatch, and keep it from blowing open/off at WOT.

stevesxm

like i said... mine is hydralic so i just closed it tight on a couple small pieces of 2x4 ... quick and dirty and the answer was immediatly self evident... as for tom cats numbers, he is essentially correct in all respects... especially the air inlet temps... the one thing i think he misses or simply underestimates is the effect the inlet manifold temp has on pwer and air fuel density... and that is VERY important... now i used the word " ambient" and that was just me trying to exaggerate for clarity... my mistake... and he is right... no way on the planet you are getting the maifold to ambient... BUT if you use a scoop and seal it such that all that air goes into the intake ONLY ( as opposed to just using them as ventilators)
AND you ventilate effectively and get the manifold temps down even 40 or 50 degrees, you are absolutely going to see a significant difference in performance... especially on an efi boat that has the sensor array nec to deal with the changing parameters... on a carb boat you will find yourself re jetting substantially ill bet....

and folks.. this is a no brainer. this is " free" power and longer service life on every single boat regardless of type or application... cold inlet air is ALWAYS better... cool inlet tract is ALWAYS better... and cool FUEL is an added freebie as well... i think the way mercuriser has done that is ... well ... the weirdest and most complicated i have ever seen for a simple process.... but the bottom line is, it works as well...

SB

Terrific info everyone - thank you.