Bigger throttlebody with cai?
07-23-2006, 02:44 PM
#3
1st Gear Member
Join Date: Jan 2006
Posts: 70
RE: Bigger throttlebody with cai?
I've had problems getting working tunes from SCT for the aftermarket TBs, plus the functioning of the TBs. Your factory TB flows more CFM than the engine requirement.
STAY with the factory TB.
STAY with the factory TB.
07-23-2006, 03:05 PM
#5
4th Gear Member
Join Date: Apr 2006
Location:
Posts: 1,313
RE: Bigger throttlebody with cai?
Actually there is a formula for computing exactly what flow your engine needs in CFM.
For a street motor with 85% volumetric efficiency which is about normal. The formula is
CFM= rpm x displacement x .85 / 3456
So, (6500*281*.85)/(3456)= approx 450 cfm
That doesent mean that the engine will not make more power with more than 450 cfm, in the real world it will. But in theory, that is what the engine would pull in. And it gives you some handle on what you need to feed any given motor.
But look at it this way, if this was a carburated motor, I would probably be using a 650-700 cfm double pumper holley. Which would be oversized technically but make good power.
I dont know what the stock throttle body flow is, but IMO you dont need more than 650-700 cfm on a NA 281ci motor. If you even need that much.
For a street motor with 85% volumetric efficiency which is about normal. The formula is
CFM= rpm x displacement x .85 / 3456
So, (6500*281*.85)/(3456)= approx 450 cfm
That doesent mean that the engine will not make more power with more than 450 cfm, in the real world it will. But in theory, that is what the engine would pull in. And it gives you some handle on what you need to feed any given motor.
But look at it this way, if this was a carburated motor, I would probably be using a 650-700 cfm double pumper holley. Which would be oversized technically but make good power.
I dont know what the stock throttle body flow is, but IMO you dont need more than 650-700 cfm on a NA 281ci motor. If you even need that much.
07-24-2006, 12:17 AM
#6
5th Gear Member
Join Date: Jan 2006
Location:
Posts: 4,333
RE: Bigger throttlebody with cai?
ORIGINAL: classj
Actually there is a formula for computing exactly what flow your engine needs in CFM.
For a street motor with 85% volumetric efficiency which is about normal. The formula is
CFM= rpm x displacement x .85 / 3456
So, (6500*281*.85)/(3456)= approx 450 cfm
That doesent mean that the engine will not make more power with more than 450 cfm, in the real world it will. But in theory, that is what the engine would pull in. And it gives you some handle on what you need to feed any given motor.
But look at it this way, if this was a carburated motor, I would probably be using a 650-700 cfm double pumper holley. Which would be oversized technically but make good power.
I dont know what the stock throttle body flow is, but IMO you dont need more than 650-700 cfm on a NA 281ci motor. If you even need that much.
Actually there is a formula for computing exactly what flow your engine needs in CFM.
For a street motor with 85% volumetric efficiency which is about normal. The formula is
CFM= rpm x displacement x .85 / 3456
So, (6500*281*.85)/(3456)= approx 450 cfm
That doesent mean that the engine will not make more power with more than 450 cfm, in the real world it will. But in theory, that is what the engine would pull in. And it gives you some handle on what you need to feed any given motor.
But look at it this way, if this was a carburated motor, I would probably be using a 650-700 cfm double pumper holley. Which would be oversized technically but make good power.
I dont know what the stock throttle body flow is, but IMO you dont need more than 650-700 cfm on a NA 281ci motor. If you even need that much.
you sir, are a genius. thank you for explaining that. i just thought it was issues with the tune, and indexing the spring correctly, but apparently theres more to it.
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