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Intake lenght and Throttle body sizes

Hey guys, I have been doing some reading "Four stroke performance tuning" By A Graham Bell, Its a good book if you like technical stuff.

Anyway I have been reading about mostly intake stuff. On one of the pages it list HP per cylinder and recommened throttle body sizes. So I go out and measure my throttle body on my 87 924S and it came out to be approx 3 inches or 76.2mm, so I check the chart and that sizes isnt even listed! It stops at 62mm which the book says is good for 80-98HP per cyclinder. So doing a little math, if you take 80hp per cyclinder, lets say a 4 banger, you get a total of 320hp, So now here I am thinking that my throttle body is way to big for the HP out put of our engines. Am I doing something wrong?? Could the book be wrong??

ANother cool thing Im learning about is changing the intake runners will effect HP gains and where your powerband is. The book has dyno numbers of a 4 banger engine they where messing with, They took away 15mm of intake runner and LOST 4hp threw the entire power band. That seems wild to me 15mm is like 0.6 inches, I think thats crazy how that such a little amount would effect something so much.

Well just thought I would post some of my thoughts. Sorry for the long post.

1986 951, Stock for now.
]87 924S Gaurds red- SOLD after 11 years of ownership
Old 01-12-2005, 01:58 PM
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Old 01-12-2005, 04:05 PM
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I believe I remember reading that the intake runner size is based on the wave pressure harmonics that are induced at a particular RPM (might be peak power, might be redline, don't remember). Basically what happens is the air moving into the engine attains a certain velocity and if the intake is too short, the molecules bang into the closed intake valve, inducing a "pressure wave" that propigates all the way back to the intake point. If it's too long, there is too much friction against the walls of the intake - it's like sucking soda up through a ten-foot straw. Doesn't work too well. Ideally you want the air to proceed at a constant velocity all the way through the intake, into the cylinder and THEN stop.

I'll try to find where I read that for you, but it makes sense. . .
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Old 01-14-2005, 05:55 PM
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Intake runner length and size has alot to do with the power band of a particular engine.

Many late model engines are using dual intake runners controlled by the pcm and an actuator. They use a long runner for bottom end torque, then open up a short runner for top end hp.

Aftermarket throttle body spacers are trying to tap into this theory of harmonics. However not every engine will benefit from increasing runner length and plenum volume.
75 914 - Undecided.
80 931 - Gone, but not forgotten.
72 914 - old toy- sold.
And a whole bunch of German scrap metal shaped like 924's.

Old enough to know better, and stupid enough to do it anyway!
Old 01-14-2005, 08:37 PM
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The measurement you probably need for the throttle body is the butterfly, which is about 2" or 50 to 52 mm. A Mustang GT has a 65 I think.

I wonder if the book isn't talking about a situation of 1 throttle body per cylinder, like the 911 IDA Webers or the IDFs that some folks put on 914s. In this each carburator throat is feeding a cylinder.



wife has 924 turbo
Old 01-15-2005, 04:51 PM
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