98Z28CobraKiller

iTrader: (17)

.

DAPSUPRSLO

iTrader: (14)

A pro stock motor is only said to require about 1300cfm of flow too, but they run two dominators which exceed 1300cfm by quite a bit. Hmmmm..., wonder why that is

I've linked to this thread before but this may make things a little clearer for some so enjoy!

http://www.speedtalk.com/forum/viewt...161&highlight=

Just like that chart said, that is only a starting point and in my honest opinion, not such a good one.

The idea of the bigger throttle body to crutch the small plenum is to aid in combatting turblence which occurs in a smaller plenum. A larger throttle body will allow less vacuum in the plenum during wot operation which may help to feed the cylinders better through reduced turbulence. For all practical purposes, you can't over throttle body a motor with our fuel injection. You can over carb one as the signal/vacuum pulled through a carb is responsible for its fuel atomization capabilities but a direct port fuel injection set up such as ours doesn't have that problem. The runners of the intake and ports of the head become responsible for the shape of the power curve predominantly it seems in an injected setup.

HALLZ

iTrader: (1)

Wouldn't it have a bit to due with the velocity vs volume?

The volume that is needed is one thing, the velocity it receives for particular applications and combinations such as max effort acceleration with a small open plenum maybe effected differently than say a road race or street driven mild set up. I would think that the runners of the intake and ports of the head would definitely have a lot to do with the power curve. Is that not one of the places people manipulate for "hidden hp"?

MadBill

A few points re CFM requirements, as posted by HALLZ and mentioned by DAPSUPRSLO:

1.Typical carb/TB CFM ratings are determined at 1.5" Hg. pressure drop, which is in the ball park for factory carb sizing for a low performance V-8, to ensure good air velocity and thus atomization of the fuel at low RPM.

2. Sizing a carb by using the RPM/displacement formula ensures that, if its volumetric efficiency is 100%, the engine will pull 1.5" of vacuum at peak RPM. This means the engine is getting (30 - 1.5)/30, or 95% as much air as if there was no restriction.

3. Unlimited race engine builders don't much care about low RPM operation, but they do care about that 'missing' 5% air/HP and so use at least twice as much "CFM" of carburetion than the formula dictates, resulting in a peak vacuum reading often less than 0.5 "Hg.

4. Since a TB has no need for atomization, the sky is theoretically the limit for the 'best' CFM rating, but practical considerations like over-sensitive throttle response and diminishing returns set in as the TB flow rises.

5. Flow restrictions are cumulative, like resistors in series, so if the displacement formula says you need say 700 CFM and your air cleaner, MAF and TB each flows 700 CFM, the flow at 1.5 "Hg of all three together will be much less, and the resulting engine vacuum and power loss will be much more.

Examples to follow...

MadBill

Further to my previous post re CFM, here's some calculations from a recent dyno session with an '04 Z 06 fitted with open LG long tube headers, port-matched FAST manifold, stock TB, a very small K & N filter, GM Performance CNC heads and ASA cam. It pulled 462 WHP @ 6,800 RPM:

Per the EFI Live's tap into the ECM, MAP sensor reading showed a baro of 29.5 "Hg.
At 6,800 RPM, WOT MAP dropped to 28.2 "Hg., a delta of 1.3 " Hg. A good 2 X 4 tunnel ram shows only ~ 0.4"Hg. at peak RPM, so targeting the same delta for this application would give a MAP of 29.1 "Hg. @ 6,800 RPM.
Therefore, reducing the intake restriction via a larger filter and/or throttle body and/or MAF should give 29.1/28.2 x 462 = 477 HP, a 15 WHP increase.

Example 2:
French Grimes in a SpeedTalk interview described a test of a sheet metal manifold EFI engine with a bolt-on lid and a top-mounted TB. Discussion arose as to whether a larger TB would increase power. Instead of sourcing one and porting the manifold to suit, he replaced the lid with a piece of 1" thick plywood, with the TB mounted to it. With the engine at full song, a brave helper slowly slid the lid to one side, effectively creating a 10" x 20" rectangular "intake opening". The power gain was less than 10 HP, barely 1%, suggesting that the current TB was a negligible restriction. (Personally, I would rather rely on a MAP sensor or vacuum gauge...)

GIGAPUNK

iTrader: (2)

a 6.85 inch cube has more volume than the LS6 plenum!
math is good