Optimum air / fuel ratio for marine engine
 

What is the optimum A/F at WOT for a naturally aspirated EFI gasoline marine engine running 87 octane to get maximum HP with out detonation? The 496 versus 502 BBC are the two engines of primary interest.

Re: Optimum air / fuel ratio for marine engine
 

From Torco Fuel's website:

Air/fuel ratios vary with load conditions (those that are ideal for fuel economy are not ideal for maximum performance). Because of small losses during the vaporization process and other losses that occur when the residual exhaust gases in the cylinder dilute the fresh incoming charge, a more realistic air/fuel ratio for maximum horsepower on gasoline will probably be a richer 12.5-to-13.5:1. When the mixture ratio strays from the optimum, either excessive air or fuel will be present in the cylinders and the power output of the engine will deteriorate.

Re: Optimum air / fuel ratio for marine engine
 

If you have aluminum heads you can get away a little leaner. But over all I like to see 12.8 to 13.2 to stay safe. we run are blower motors at 11.8 to 12.0. You could also look at cylinder temps with the air fuel to double check yourself.

Re: Optimum air / fuel ratio for marine engine
 

Thanks for the info.

How does one check the cylinder temperatures and what are the temperatures that indicate bad fair good best?

Re: Optimum air / fuel ratio for marine engine
 

You'd need an EGT to check cylinder temps. Usually when running an engine on a dyno, each cylinder can be checked because there is an EGT sensor in the exhaust of each cylinder.

Re: Optimum air / fuel ratio for marine engine
 

Livorsi makes a EGT gauge.

Re: Optimum air / fuel ratio for marine engine
 

I had gaffrig/livorsi egt gauges with temp probes on my lightning headers and they read 200-250 degrees HIGHER than actual temp,hopefully they have straightened that out but mine were about usueless,Smitty

Re: Optimum air / fuel ratio for marine engine
 

What should the exhaust temperature range be?

Re: Optimum air / fuel ratio for marine engine
 

1150 to 1250 is what I have always heard,1330 is melt down. My gaffrig egts read 1500-1600 degrees all the time on my old blower motor so it was obvious they were useless,Smitty

Re: Optimum air / fuel ratio for marine engine
 

On a closely related topic, does anyone know the formula to determine the amount of fuel needed for a specific horsepower? Trying to determine GPH of fuel pump. I have alway just gone overkill in the past.

Re: Optimum air / fuel ratio for marine engine
 

EGT Temp. vary wildly depending on how far the prob is away from the exhaust valve.On a dyno the probes are usually 6 inches from the valve.This is hard to do on a marine water jacketed header. I had mine in the exhaust port next to the header flange,i saw temps in the 1700*range.The dyno showed 1300*with the probs 6 inches downstream !

Re: Optimum air / fuel ratio for marine engine
 

To be blunt, given the variability of EGT measurements emntioned as a function of the distance from the combustion chamber, it would seem that all it is good for is cylinder to cylinder EGT balance check. Are there EGT numbers for that?

Re: Optimum air / fuel ratio for marine engine
 

Some helpful information regarding EGT and tuning parameters;
http://www.maxracesoftware.com/egt_probes.htm

Bob

Re: Optimum air / fuel ratio for marine engine
 

Incredible .. the stuff you come up with ....great info ...m

Re: Optimum air / fuel ratio for marine engine
 

Thank you gentlemen all!

Re: Optimum air / fuel ratio for marine engine
 

I figure about .5lb/hp*hr. You can add 10-20% margin and that should work out fine. So for 100 hp do:

100*.5= 50 lb/hr

+ 20% = 60 lb/hr

You could just multiply that by hundreds and that should get you there. So, 700 hp = 420 lb/hr or about 60 gal/hr or 1 gal/min. Just be careful when sizing your fuel pump to know the operating pressure range for the pump. A pump curve is always best to know that your fuel delivery will be adequate at your operating pressure.

Rene

Re: Optimum air / fuel ratio for marine engine
 

Im running a Revolution Marine aluminum exhaust manifolds. Is there any way to mount a EGT sensor in this type of exhaust?

Re: Optimum air / fuel ratio for marine engine
 

Yes,PM Nordic95 here on the board.He had a post on how he mounted an O2 sensor in his Revolution Manifolds.

Laken

Re: Optimum air / fuel ratio for marine engine
 

If the flange is thick enough where it mounts to the head you can drill there.Most EGT probes are 1/8 pipe thread,about 5/16". Be sure you don't get into the water jacket , or you're S______!

Re: Optimum air / fuel ratio for marine engine
 

Thanks Bob, Good Read!!

**** (alias****)

Re: Optimum air / fuel ratio for marine engine
 

Anyone?

Re: Optimum air / fuel ratio for marine engine
 

Mine also ran high,The stock 500efis would run 1500-1550 on the gage at wfo .High 1400's from 3000 up.

Also remember that pump gas will read a higher temps than race fuel.

Re: Optimum air / fuel ratio for marine engine
 

Bob some more of Larry Meaux's knowledge:eek:
Just think he's just some dyno guy in a corn field in La:cool:

Re: Optimum air / fuel ratio for marine engine
 

TYLER or ?? Question..I have a portable air/fuel monitor with dual probes. I want to put one on each engine to see ratio at different rpms. what cylinder woud be best on blown/cooled/572? and also, how far from the motor is acceptabe? in a spacer or drilled into the head itself? does the distance matter by going to the collector of the header?
Thanks for any info.

Re: Optimum air / fuel ratio for marine engine
 

The only reason I see to put it a single tude is if you can tune each hole sepretaly.All that ive seen are in the tail pipes on marine headers

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http://www.offshoreonly.com/forums/s...ighlight=bungs