Having 390 built with 9 1/2- 1 compression, comp 270h cam, edel heads, rpm intake, stans' headers, msd....and i want to put a speed demon carb on, but I don't know if a 650 would be enough for my altitude or if I should go with 750. I have read on here that demon's flow a little better than advertised. Any suggestions?

 

I'm surprised no one has replied.

I'm not making any recomendations, but I've never heard of altitude being a factor in sizing a carb. Equipment, set up, and intended use pretty much dictate how much carb is enough.

Altitude becomes a factor when you start tuning. Things like ignition timing and fuel metering unlock the potential you spent big bucks on.

Scouder, are you out there?

 

yeah same here it does not matter you size the carb to displacement and application and 650 for that engine is a little to small

 

thanks, had a guy tell me a 750 would be too much because the altitude, i knew i could dial it in with a 750, but this guy ( a mechanic) said 650.
thanks for info

 

Sorry amigos, but altitude makes a huge difference in every facet of building/tuning an engine. Incuding carb size.

I also live at 7000', and the amount of available air here is reduced about 23% from sea level. Which means that any normally aspirated internal combustion engine will have a 23% reduction in volumetric efficiency. Which is another way of saying your engine size is effectively reduced by that value. A 390, for example, becomes a 300 at 7000'. Whatever carb you would run at sea level you can knock 20% right off the top at this altitude. A vehicle that requires a 750 at sea level will run just fine here on 600cfm. Compensating for altitude with a smaller carb increases fuel charge velocity and helps with low end response.

Hope this helps.

-Scouder

 

thanks scouder...i was hoping you would chime in on this. BTW I live 100 miles down the road from you in R.S. So do you think based on info i have given, a 650 speed demon would be plenty of carb? Any other tidbits you have to offer I am also more than happy to listen to.
thanks again

 

Hey, that's cool that your just over the hill. We just finished rewiring the computer network for the school district over there.

I ran your engine through a spreadsheet that I wrote. Even being extremely generous with the volumetric efficiency (80%) I show a 650 being more than enough at this altitude. If you were running at sea level (assuming 95% VE) it would require a 750 if you wanted to spin it over 6000. You won't kill yourself either way, but there is no sense over carbing if you aren't going to hot rod it at sea level very often.

-Scouder

 

thanks again, yeah there is probably very little chance of me spinning it over 6000, considering it is going into a 4x4.

thanks again