questions about carb sizing
 

hey all,

haven't posted in a while. anyway today i got the new hotrod magazine, and in it there is an article about how to turbo any car.

after reading through it, and trying their equations i was able to figure out a few numbers for flow rates. i am trying to figure out what sized carburators i need for my engine.

right now the build-up consists of a 427 small block with a dual quad intake and twin turbos.

with a V.E. of 1 i was able to obtain a unboosted flow of 865 cfm at 7000 rpms.

after running through the pressure ratio and figuring out the boosted florates i got 1747 cfm.

now my question is, which flow rate do i use to size my carbs? is two 450 cfm carburators able to deal with the boosted flow? or do i need two 850s to flow enough air?

thanks

ace

also..

what is better for my setup? mechanical secondaries or vacuum secondaries?

thanks

Well, after seeing the same article, I wouldn’t base any decisions on what they wrote. There were all sorts of technical errors in it, such as suggesting that actual airflow will be the airflow NA * pressure ratio (in real life you loose quite a bit due to the adiabatic efficiency of the compressor)…

are you doing a blow-through sytem?

yeah i agree, the article had many many holes in it, but it did give me a rough guestimation on what to expect. at least the guess is on the big side.

not only is there problems with the losees, but the basis for the flow rates of a boosted engine is based off of a perfect engine with 100% VE.

yes, it will be a blow thru system. The manifold is an offenhauser dual quad that i am going to work over with the grinder. for the carbs, i am wanting to use a pair of demon carbs.

andrew

Blow thru engines actually need smaller carbs than NA engines. If you are doing this you would only need 1 with mech. secondaries. here is a chart to help you decide.
600 cfm for up to 500 hp
650 cfm for up to 650 hp
700 cfm for up to 750 hp
750 cfm for up to 900 hp

What biggtime said. right on the money. I'm doing a twin carb blow through system also, but its alot easier to do a single carb.

does that also apply for supercharged engines? im not really familiar with any of this kinda chat..can someone explain to me what a blow thru system is? thanks

A blow thru supercharger system is Blowing thru the carb, as apposed to the roots setups that the carbs sit on top of the blower and get drawn thru.

You really only need to size it for the NA cfm, since under boost you'll essentially be forcing the same volume of air in, but it will be more dense so it will be a greater mass of air to make more power.

The only reason to go with a larger carb is if the smaller one just can't deliver the fuel you need for the HP you should be making.

That assumes we're talking blow through, draw through you size it for the airflow that you get under boost...

okay,

so lets say i were to get a pair of road demon jrs. say 625 cfm a piece. i would then probably change the secondaries to mechanical.

ace