what size throttle body for adequate air flow?

[whittlebeast]

I was ignoring the throttle shaft size and for that matter, the air sample tube in the MAF calc but it is amazing how large the throttles have to be to keep the air velosity down. I found out about this when the CRX went from four individule air horns to an airbox with a single 3" inlet air filter. The WOT map jumped from 97KPS to 92KPA with the air box. Things were even worse with a 3" dia 30" long "cold air intake" I found the air flow in the CAI of around 90MPH

AW

[efahl]

Yeah, that's why Scott got rid of the MAF on Jim's truck. If I remember right, he measured a 2 inch vacuum across the thing and immediately got rid of it...

[eliotmansfield]

Fuel injected Rover V8' (3.5L & 3.9L and possibly the bigger ones too) came
with a 65mm throttle bodies I believe. (offline, cant verify)

Eliot.



Posted by email.

[whittlebeast]

But the real question is what is the MAP compared to ambiant air pressure with that throttle body and at max RPM. I have seen pressure drops of from nearly 0 kPa to 8 kPa depending on the CAI/filter/MAF/throttle combination.

AW

[Thomas123]

Hi,

I've just done the calculation for my engine. I'm running a 3.5litre V8 (215 cu) and used peak rpm as 6,000. I plan on using a single throttle body when I move over to Injection. According to the calculation I need a single throttle body with 20sq.in, which results in a 5" throttle body!

I was planning on using either a single 72mm or 90mm?

Anybody provide any advice?

Yeah, that sounds way huge!

BTW, is it a 1960s Buick 215 or the Rover one?

I have a 4300 series Buick 215 I'm rebuilding right now and have a selection of throttle bodies to choose from depending on how I choose to design the intake manifold.
Using the calculator here they list at.....

One @ 50mm is rated at 144 HP 5.0L V8 Mustang.
One @ 65mm is rated at 243 HP 4.6L V8 Thunderbird.
Two @ 45mm is rated at 233 HP 3.5L V6 Concord.

I'm thinking of two 180 deg (all 4 intake pulses 180 deg apart) plenums with a 45mm throttle body for each one.

In the Buick 215 V8 would be.
Cylinders 1 4 6 7 fire 180 deg apart in one plenum.
Cylinders 2 3 5 8 fire 180 deg apart in one plenum.

[mrmad]

Here's a good article on throttle body dimensions at Team-Integra.net

http://www.team-integra.net/sections/ar ... icleID=484

[Jakor]

I would like to throw something else out there!

While all this calculation is making edjucated guesses, I would like to add to the edjucated part.

Volumetric efficiency doesn't have to mean 100%. It means more like 102% due to the fact that the air has pulses to it. These pulses can be timed up to give yourself a set powerband. Take a throttlebody spacer on a 1993 sohc vtec del sol. My friend added one and it dropped his powerband around 1,000 rpms.

This along with the fact that noones intake is going to match their rev limiter means that at a perfect volumetric efficiency (102%) with a intake set for 7000rpms and a rev limiter at 8000rpms: You will be using more air at 7000rpms than 8000rpms. Matching to the direct cfm that you need may be ok since we don't live in a perfect world, but a bit larger would be best so you can have some sort of a velocity stack before the motor. We all know that it's air velocity the makes power, not sheer volume of air.


I'm currently working on my 86 mazda rx7. I'm porting the intake ports (effectively changing "cam" timing + size of the port for more airflow) and matching a custom short runner intake. Rev limiter bumped from 7800 to 8000 rpms. I'm calculating for an intake pulse at 7000 rpms which this pulse will work a little on either side, thus leaving me run off to 8000 rpms. I just wanted to drop by and add my $0.02

[Uncle Bob]

^^^ Same question here actually, I've got two turbos bolted to mine. Do you have to consider boost? I would imagine you would, as forced induction increases the air flow. If so a large single throttle body like I mentioned would probably suffice (90mm)?

cheers

very late in the discussion, but hope it helps someone.

If it is a blow-thru setup, you are still dealing with the exact same parameters as the NA folks. IE whatever HP your engine makes NA, the numbers from that will give you the "correct" TB size, because a turbo does not increase CFM, it increases density.

[357supermagnum]

There is a calculator here that is very good:
http://dairally.net/daihard/chas/MiscCa ... rottle.htm