Target Air Fuel ration N/A engines.?

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08-12-2016 | 10:26 AM

#91

Bill 3

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Joined: Mar 2001

Posts: 417

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From: Table Rock Lake, MO

Quote:

Originally Posted by Black Baja

This is the only thing that I could thing of. On the exhaust end of it it's no mystery. A motor with a double swap cam sounds like a lame duck compared to a standard firing order. We've seen 16" of vacuum over 3,000 rpm with some double swap motors. Which can be trouble especially when you have a 6.5 PV. Try to get the boat on plane and she starts snap crackle and popping because the carb will not flow any fuel. Put in a 10.5 PV (which are a pain to get) and she's good to go.

I have the C-fire double swap on a fairly aggressive cam (240/248, 629/643). It idles good but only pulls about 10" of vacuum at idle. I run into the problem of still pulling around 9" of vacuum at 3500 rpm. Many of you have tried to help me with my carb jetting. I never thought about the c-fire cam potentially causing my issues. Maybe this is contributing to part of my struggle.

08-12-2016 | 10:44 AM

#92

MILD THUNDER

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Joined: Jul 2004

Posts: 11,332

Likes: 73

From: chicago

If this vacuum concept is true, does it also lower boost levels when supercharged?

08-12-2016 | 10:46 AM

#93

Baja Rooster

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Joined: Oct 2013

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From: SF Bay Area

Quote:

Originally Posted by MILD THUNDER

Keep in mind , going down jet in primary, is also gonna lean wot unless you add fuel somewhere else.

Correct. I considered going back up on the secondaries if the primaries are the problem.

Quote:

Originally Posted by MILD THUNDER

All this fuel tuning talk. And some guys still think they can call a carb shop, give them cubic inch, cam specs, and get a carb that will have perfect afrs out of the box??

I gave my carb to a shop to check out and tune. Not even effing close. Going down two jet sizes gave me another 400+ rpm, and what *felt* like another 75hp. A joke.

[QUOTE=Eliminated572;4470210]

Quote:

Originally Posted by Baja Rooster

Since I'm super fat at 1k

Well, is the super fat condition at cruise due to the PV's opening? Or simply over jetted? Could be both. Have you checked vacuum under these conditions to know what is happening? Note: blown PV's would do this as well. Verify leakage before proceeding with changes..

Yea, I need to check vacuum before playing with the brass.

08-12-2016 | 11:10 AM

#94

Uberlord

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Joined: Jun 2016

Posts: 56

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From: Syracuse, NY

Question. Why does a boat engine like to be run rich? As opposed to stoic at idle and cruise. And fattening up at wide open? I'm comparing the way my cars are tuned as opposed to marine applications.

08-12-2016 | 11:21 AM

#95

Baja Rooster

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From: SF Bay Area

Quote:

Originally Posted by Uberlord

Question. Why does a boat engine like to be run rich? As opposed to stoic at idle and cruise. And fattening up at wide open? I'm comparing the way my cars are tuned as opposed to marine applications.

It's a cushion of margin for varying conditions, and boats error on the side of being rich to cool the cylinder temps because the probability of melting Pistons is super high as the engines are under constant load. There is no such thing as cruising in a boats. If they did boats would need brakes.

I've seen a difference of half a point in my AFR just from the afternoon into the evening. Ive seen people running closed loop EFI keep it in the mid 13s. I have no opinion of if that's ideal or not.

Last edited by Baja Rooster; 08-12-2016 at 11:24 AM.

08-12-2016 | 11:31 AM

#96

Uberlord

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Joined: Jun 2016

Posts: 56

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From: Syracuse, NY

Thank you. Makes sense. It's interesting trying to compare car to marine applications. As similar as the engines are. They are worlds apart in their application.