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Old 03-12-2017, 10:31 PM
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Default Carb help please

Wondering if I should upgrade my 750 holleys to 850s. I run a 502 with a 420 mega chiller and aluminum heads I jetted them pretty heavy the last couple years.The boat is a heavy 28 sportcat. Never seems to have that punch . Do I have enough cfms with the 750s ??? Also I run at 4000 ft

Last edited by 28ftsportcat; 03-13-2017 at 10:58 PM. Reason: Better title
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Old 03-13-2017, 02:58 PM
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your 750s are as small as i would use,you really can,t over carb a blower motor.have you tuned the engine with a wide band o2?
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Old 03-13-2017, 10:56 PM
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No I haven't I want too don't know where to bung a hole I have lightning headers where will they not get wet?
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Old 03-14-2017, 03:47 AM
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i don,t know if you can but bungs it the lightning headers.
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Old 03-14-2017, 06:18 AM
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what are you running for boost and compression? At that altitude I would think you could run a little more boost than us peeps running at <1000.
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Old 03-15-2017, 04:58 PM
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Originally Posted by 28ftsportcat View Post
Wondering if I should upgrade my 750 holleys to 850s. I run a 502 with a 420 mega chiller and aluminum heads I jetted them pretty heavy the last couple years.The boat is a heavy 28 sportcat. Never seems to have that punch . Do I have enough cfms with the 750s ??? Also I run at 4000 ft
If I recall Mild Thunder went from 750's to 850's on his 468's with 420 mb's and I'm pretty sure he made a statement that he wishes he would have went even larger 4150 1050's. He would have to verify this for 100% certainty.
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Old 03-15-2017, 06:46 PM
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For what's it's worth I ordered Lightning Headers years ago and had them install bungs in the collectors.
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Old 03-15-2017, 07:58 PM
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Originally Posted by JRider View Post
what are you running for boost and compression? At that altitude I would think you could run a little more boost than us peeps running at <1000.
Absolutely. I would think at least 2 more pounds of boost, at 4000, than at sea level.

This is where a map sensor with the daytona ignitions could help, (or efi datalogging).

Absolute pressure measurment, vs gauge measurement.
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Old 03-15-2017, 08:12 PM
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Originally Posted by MILD THUNDER View Post
Absolutely. I would think at least 2 more pounds of boost, at 4000, than at sea level.

This is where a map sensor with the daytona ignitions could help, (or efi datalogging).

Absolute pressure measurment, vs gauge measurement.
What about carbs?
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Old 03-15-2017, 08:24 PM
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Originally Posted by getrdunn View Post
What about carbs?
They are certainly on the small side. I have no experience with high altitude, but, my mind tells me, bigger carbs would help.

Roots do not draw air nearly as well as they move it. Just like its harder to breath at higher altitudes when you are used to sea level. You have less atmospheric pressure helping to fill your lungs. On the engine , there is also less atmospheric pressure, to help move air into the carbs.

Its very likely, that larger carbs alone, would net a boost psi increase for him. I know it has, without even taking in consideration, the altitude concept.

Before I bought new carbs though, or recommended them, I'd want to know 2 things.

What kind of "absolute" pressure he is getting , and what the air fuel ratio is. '

My gut tells me, that the engine may like a bit more ignition timing at higher altitude, from the lack of oxygen in the charge. The more dense the charge is, the faster it burns. The faster it burns, the later you can fire the plug. The less dense it is, the slower it burns, and therfore, you'd need to light the fire sooner.

one would need to jet down for high altitude, not up. Less oxygen, means less fuel. But who knows where we are starting from, so an "oxygen" sensor, will tell the story.

Just shooting from the hip here.

Last edited by MILD THUNDER; 03-15-2017 at 08:26 PM.
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