LFarr

I'm running a 500" BBC at 5400rpm I'm reading 10-11"s boost in the carb enclosure, with an 800cfm unmodified Holly 4780-4.

My question what pressure difference have you seen at WOT above and below the throttle plates, and what would you guess mine to be ? i.e. if its 11#'s in the carb enclosure, what's likely to be below, "what's my engine seeing?

I'm giving some thought to instaling a waste gate ?

This whole question has come up with "high speed lean-out " above 10#'s boost. Someone recommended larger needle & seat which I have ordered from summit. If that works then I will just continue to monitor plugs for evidence of detonation & excess heat.

Thanks......Larry

ps if you'd rather talk than write provide your phone number & I'll call.

articfriends

I tested my efi motor for pressure drop acrossed the throttle body and it was around 1 to 1 1/2 psi BUT I still had prochargers restrictive screen in inlet,Smitty

LFarr

Thanks Smitty,

Yours is the first data I ever seen on this subject.

I can't makeout the numbers on your guages. Can you provide the actual readings.

I'm blowing thru an 800 cfm Holly, with Dart (HP500) manifold. Maybe the % drop in pressure will be of some help in determining what my drop is likely to be.

Any idea what the dimentions of your throttlebody are ? Maybe I can calc the area of your throttlebody versus my 4-barrel throttle plates and come up with a comparitive guestimate.

Thanks......Larry

cobra marty

Throw a boost guage on the intake and know for sure.

tomcat

Your carb is rated 800 CFM @ 1.5 " Hg, the standard test pressure for four barrels. At 5400 RPM and 100% volumetric eff. your 500 CID engine needs 780 CFM so you are using it all. Therefore the pressure drop across the carb will be about 1.5 " Hg which is equivalent to about 0.75 psi. Since entry losses into the carb are greater when air flow to the air horn is not ideal, such as in a typical carb box, you might see a 1 psi pressure loss.

If you are going to the trouble to measure pressure at various places, make sure you measure compressor discharge pressure, immediately after the blower, and compare that number to intake manifold boost. You will be surprised at the amount of difference (loss) from blower to intake.

SB

Most blow thru 'carburetor' modifiers like to use small carburetors to fight off the higher speed lean out that becomes a reality as boost goes up. Believe it or not, many even use 750's on bigger cid and/or bigger boost applications.

Carb's are made for our atmospheric conditions, not boost. Therefore, carb's must be modified by knowledgeable people with this application, and even still, as boost goes higher and higher problems/issues still present themselves.

If you haven't sent that carb off to a blow thru expert like Nickerson, CSU, or etc, I would stop what your doing, give them a call, and get that carb sent out.

We are concerned more about fuel metering in blow thru carb applications than pressure differences a smaller carb may present.

BTW: Blowing thru a carb or t-body will substantially raise it's airflow rating.

SB

Oops, jst re-read it.

Get that carb sent out now !!!!

Don't be mislead by some claims (usually ATI makes these) that carb's just need rejetting. They need a whole ton more than that to 'work under pressure.'

As mentioned few minutes ago, stop what you are doing, pick up the phone and get that carb sent out !!!!!

hellbents10

CSU makes a great blow thru. A 750 is good for 1100hp on a blow thu application. They use smaller carbs for a stronger signal and ultimatly more fuel.

articfriends

A couple years ago I took to very accurate 4" boiler gauges that read vaccuum and pressure,calibrated them so they matched and checked pressure drop at various ponts on my motor. It was a 502 then and I turned the boost waaay up for the test so numbers would be close to 10 psi. I found that pressure coming from a few inches from blower discharge to pressure after inercooler had about a 3/4 psi drop,pressure from after innercooler to manifold acrossed throttle body including ati's back fire screen had a pressure drop of 1-1 1/2 psi. This was at about 12 1/2 psi to 14 psi which greatly exaggerated the drops because I don't run that much boost. With the 9-10 psi pulleys I hardly could read a pressure drop from blower discharge acrossed innercooler (aw504) and 3/4 psi drop acrossed the throttle body,Smitty

EYEBURN

shouldnt a boost guage always be positioned below the carb on the intake?
boatn70