Engine Starving for air?
07-25-2016, 05:19 PM
#31
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I will be documenting what I do and what the results are, including air temp at the air cleaner. However I think it's way more than a temp issue. It would take a little time to have a temp issue cause a problem, but when the hatch would bounce open and closed the engine would instantly react to the change. It was change you would really feel, and at WOT you need a lot of power to feel any difference. I certainly wasn't creating a 15 hp difference.
Last edited by endeavour32; 07-25-2016 at 05:22 PM.
07-25-2016, 05:20 PM
#32
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The amount of air flow you need to remove heat given off by the engine is much higher than the amount of air needed for combustion. You can get a feel for this by looking at performance data from marine engine builders like Cummins.
http://www.sbmar.com/cummins-marine-...rmance-curves/
As an example, scroll down the page and open the pdf for the KTA19 which is rated at 600 HP. On page 2 under Intake System it tells you the engine needs intake air of 1140 CFM. It also says that heat rejection to ambient (the engine compartment) is 1309 Btu/min. Let's say that outside temperature is 90F and you would like to keep engine compartment temperature no higher than 120F. This temperature rise of 30F is a typical design target for boat builders but it isn't easy to do.
This equation gives you the CFM required to remove the engine heat:
CFM = Btu/min
0.0175 X temp rise(F)
In this case the answer is 2493 CFM. Add the 1140 CFM you need for combustion air and you are at 3633 CFM per engine. Getting over 7,000 CFM into and out of an engine compartment isn't easy, which is why you can see air temps under the hatch much higher than 120F.
I agree with the posters above advising air temp measurements in the engine compartment. When you can keep the temp rise at a max of 30F you are doing well.
http://www.sbmar.com/cummins-marine-...rmance-curves/
As an example, scroll down the page and open the pdf for the KTA19 which is rated at 600 HP. On page 2 under Intake System it tells you the engine needs intake air of 1140 CFM. It also says that heat rejection to ambient (the engine compartment) is 1309 Btu/min. Let's say that outside temperature is 90F and you would like to keep engine compartment temperature no higher than 120F. This temperature rise of 30F is a typical design target for boat builders but it isn't easy to do.
This equation gives you the CFM required to remove the engine heat:
CFM = Btu/min
0.0175 X temp rise(F)
In this case the answer is 2493 CFM. Add the 1140 CFM you need for combustion air and you are at 3633 CFM per engine. Getting over 7,000 CFM into and out of an engine compartment isn't easy, which is why you can see air temps under the hatch much higher than 120F.
I agree with the posters above advising air temp measurements in the engine compartment. When you can keep the temp rise at a max of 30F you are doing well.
07-25-2016, 05:58 PM
#33
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Very old , but very accurate, easy math we did at the track for years was 10*F air temp change will be extremely close to 1%hp change.
07-25-2016, 06:52 PM
#34
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I say it's time to call the heating and cooling man up the street and have him start bending some sheet metal up or better yet get some theraflex hvac duct and fabricate some hoods to go around flame arrestors. Ram that fken air in any which way I could without being to ghetto. Ha.
Hey mike what is the size of the cowl vents behind the windshield. They are isosceles/trapezoid shape correct? The actual opening size?
Hey mike what is the size of the cowl vents behind the windshield. They are isosceles/trapezoid shape correct? The actual opening size?
Last edited by getrdunn; 07-25-2016 at 07:06 PM.
07-25-2016, 07:09 PM
#35
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I say it's time to call the heating and cooling man up the street and have him start bending some sheet metal up and fabricate some hoods to go around flame arrestors. I'd ram fken air in any which way I could without being to ghetto. Ha.
Hey mike what is the size of the cowl vents behind the windshield. They are isosceles/trapezoid shape correct? The actual opening size?
Hey mike what is the size of the cowl vents behind the windshield. They are isosceles/trapezoid shape correct? The actual opening size?
07-25-2016, 07:37 PM
#36
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If you want to find out if it temp issue just route you vents right to the flame arrestors. You will be semi ram air but you will gardener that you dropped your inlet temp without having changing the amount of air. It your do need more air and not temp then I would route some additional vents directly to flame arrestor and leave the factory ones for cooling/ air exchange.
The engine hatch popping open and you getting more power doesn't point to more air or to hot of air as the issue. As it pops open a but you let X cfm of outside air in. It comes in right on top almost funneled to the intake. So you bypass the hot air and give it more air in the compartment. Think of a building. At 1" of w.c a 6" round duct carries 100cfm. A door way half way open at that same pressure is 1500-2000 cfm. You moving at 60 is way more pressure then 1"w.c. so you for a lot of cooler air into the compartment.
The engine hatch popping open and you getting more power doesn't point to more air or to hot of air as the issue. As it pops open a but you let X cfm of outside air in. It comes in right on top almost funneled to the intake. So you bypass the hot air and give it more air in the compartment. Think of a building. At 1" of w.c a 6" round duct carries 100cfm. A door way half way open at that same pressure is 1500-2000 cfm. You moving at 60 is way more pressure then 1"w.c. so you for a lot of cooler air into the compartment.
07-25-2016, 07:59 PM
#37
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Many V boats with a windshield I've been in, the air moves backwards (towards the frt) at the rear seat/ hatch.
The shorter peoples hair blows towards the windshield, the taller people may have their hair go towards the rear.
If you sit on the very corners of the rear seat and stick your head out, your hair will go backwards.
And Yeh, I got geaky enough once on a certain boat (and a few cowl hooded cars) to use those air flow direction ribbons .
The shorter peoples hair blows towards the windshield, the taller people may have their hair go towards the rear.
If you sit on the very corners of the rear seat and stick your head out, your hair will go backwards.
And Yeh, I got geaky enough once on a certain boat (and a few cowl hooded cars) to use those air flow direction ribbons .
Last edited by SB; 07-25-2016 at 08:50 PM. Reason: changed 1st word to 'many'
07-25-2016, 08:51 PM
#38
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Many V boats with a windshield I've been in, the air moves backwards (towards the drt) at the rear seat/ hatch.
The shorter peoples hair blows towards the windshield, the taller people may have their hair go towards the rear.
If you sit on the very corners of the rear seat and stick your head out, your hair will go backwards.
And Yeh, I got geaky enough once on a certain boat (and a few cowl hooded cars) to use those air flow direction ribbons .
The shorter peoples hair blows towards the windshield, the taller people may have their hair go towards the rear.
If you sit on the very corners of the rear seat and stick your head out, your hair will go backwards.
And Yeh, I got geaky enough once on a certain boat (and a few cowl hooded cars) to use those air flow direction ribbons .
John, to really get some good info on this, I may need a partner to observe while I'm driving.
07-25-2016, 08:59 PM
#39
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K. I get the wheel and case of beer and tequila and you do what you gotta do under the hatch. Dont worry I'll be easy on it. Right. Lol. Will be around all weekend and ready to take a break from flat landers so let's go. We'll get right.
07-25-2016, 08:59 PM
#40
Laugh all you want but I actually built a hood around the flame arrestor on one of my boats and ran a 4 inch rigid hose to it and ran it to open air at the side of the rear bench seat. Worked great untill it sucked up a pop can. I then installed two 4 inch pipes to it and ran well with 100% outside air without being preheated. I work with a lot more than ductwork.. Did I log into Yellowbullit?
