65mm vs 70mm TB w/ 12-15lbs of boost?

[NothinButNotch]

I have a Edel. Performer Intake which has a 70mm TB opening. Currently I am running a 65mm Accufab TB. I am wondering if there is any HP/TQ to be gained by switching to a 70mm TB and/or if there is, would it be worth the expense?

I currently see 10-12 lbs of boost, depending on weather etc, and will soon be doing a pulley swap and running 13-14 lbs of boost. I don't have any accurate dyno numbers at this point but will soon after the re-tune with the smaller blower pulley but the car runs low 11's and I am shooting for high 10's.

Is it worth the swap or should I just leave well enough alone? Thanks, Ryan

[fanglemeister]

450hp at 6k rpm = 850 cfm, approximately. Being that the throttle body is blow-thru though, means it can flow alot more under boost than it can at atmo pressure.

fwiw, here are the atmo pressure approximate flow values at -1.5 hg:
65mm w/ no air filter = 670 cfm
70mm w/ no air filter = 880 cfm

In the real world, on your car, swapping would be worth 12-15 rwhp or so at 6k rpm.

[BlownNotch]

450hp at 6k rpm = 850 cfm, approximately. Being that the throttle body is blow-thru though, means it can flow alot more under boost than it can at atmo pressure.

fwiw, here are the atmo pressure approximate flow values at -1.5 hg:
65mm w/ no air filter = 670 cfm
70mm w/ no air filter = 880 cfm

In the real world, on your car, swapping would be worth 12-15 rwhp or so at 6k rpm.

What throttlebody ISN't "blow-thru"?

[fanglemeister]

What throttlebody ISN't "blow-thru"?

Any positive displacement blower setup is probably going to be draw thru. Turbochargers are setup both ways. I have seen a few draw thru conversions done on centrifugals too; it has it's advantages, chief among them being no need for a bypass/blowoff valve and less "work" heat acccumulating in the compressor portion of the blower/turbo.

[NothinButNotch]

I'm confused... :) Are we talking about throttle bodies or mass air meters?

[fanglemeister]

Talking throttle bodies, but the principle is same for both: The MAF and TB will both have more flow capacity on the boost (blow-thru) side than on the atmo (draw-thru) side.

You actually have one of each example! A draw-thu TB on the Lightning and a blow-thru TB on the Mustang. I was indicating that a slightly undersized throttle body does not have as big an effect on a blow-thru app like your Mustang. The Lightning is where you need to keep the intake path opened up wide!

Theory goes like this: flow potential across any restriction must account for the sum of the pressures before and after the restriction. If the restriction has nothing but atmospheric pressure on one side and the blower on the other side, it has less flow potential than the same part having the blower on one side and the engine's pistons drawing in air on the intake stroke on the other side. Also, when air is compressed, more molecules can fit through the restriction for a given airflow speed.