Carbs
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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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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
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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.
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.