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slower than a turtle
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Discussion Starter · #1 ·
i have serched on this forum and google and havent not found a concise answer to this question

what benefits powerband wise do you see with a short runner ffim vs a long runner ffim.

just from what i kind of know shortening the runners changes the torque curve resulting in a smoother powerband but other than that i am clueless. also how do each of these manifolds respond to a big throttle body, in respect to each other.

the reason i ask is im trying to decide which route to go for my autocross car, trying to keep up with 300hp camaros isnt to easy
 

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supraguru05 said:
i have serched on this forum and google and havent not found a concise answer to this question

what benefits powerband wise do you see with a short runner ffim vs a long runner ffim.

just from what i kind of know shortening the runners changes the torque curve resulting in a smoother powerband but other than that i am clueless. also how do each of these manifolds respond to a big throttle body, in respect to each other.

the reason i ask is im trying to decide which route to go for my autocross car, trying to keep up with 300hp camaros isnt to easy

good question! I'm going to get a short runner ffim just cuz i like the wya it sits in the enginebay, but i'd love to know the benifit/downfall to it as well
 

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Hardcore Night Warrior
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From what I understand myself, the long runners benefits are more tq, but less hp. Short runners are more hp, but less tq.
 

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chevydude said:
From what I understand myself, the long runners benefits are more tq, but less hp. Short runners are more hp, but less tq.
I dont know how it works, but that is how it is. Thats how automakers know it too.
 

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Plus short runner increase hp on higher rpm but loosing some at lower rpm and vice versa on long runner.
 

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boost freak
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chevydude said:
From what I understand myself, the long runners benefits are more tq, but less hp. Short runners are more hp, but less tq.
correct.

then again, in forced induction, it really doesn't matter that much....certainly not something to get too stressed about since no one here is at the level at which it would matter....
 

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slower than a turtle
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Discussion Starter · #9 ·
ok so it was what i suspected i had heard that the main effect was onj the torque curve and shifting it around the rpm band. well then i guess i dont really have a defenitive answer on which owuld be better for autocross. i supose that the short runner woudl be better overall because ill be in the high rpm range all the time
 

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The way that I understand it is that in a tuned intake system A pulse or wave of compressed air is created when the intake valve closes, which bounces off of the closed valve and travels back up the intake runner. This wave then reflects back off the inside of the plenum and travels back down the intake runner. The idea is to tune the manifold so that this reflected pulse arrives at the intake valve just as it is opening thus improving cylinder filling.

The wavelength of the reflected pulse is determined by the runner length among other things, with longer intake runners yielding a longer wavelength. This results in the wave hitting the valve at a lower RPM thus improving low end torque. While a short runner design decreases the pulse duration making it coincedent with the valve opening at a higher RPM thus moving the torque further up into the power band.

This is how I understand it and it may not be completly accurate, so feel free to correct me. I am designing a FFIM right now and my philosophy is to use the bottom part of the stock manifold, since the runner length was designed to be length it is based on dyno testing by Toyota. I am also looking to keep the same plenum volume as stock, although I don't think this is as big of an issue. Basically I plan to keep the fairly long runners because the 7M doesnt operate at very high RPMs anyway, and it seems best to have it tuned for a RPM range that is in the usefull powerband.

sorry for the long post.
 

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Sir Digby Chicken Ceasar
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Nice explanation, I would have to agree with the usefulness of having more power at low rpm considering our motor.
 

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Moderator, l337 M0d3r4t0r
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X-MAN posted results form stock mani to a FFI and showed that he was getting more torque and better mid range but the FFI made more HP and power top end.
 

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Shorter runners also contribute to better throttle response ;)
 

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IJM Mafia!
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ZaZZn said:
X-MAN posted results form stock mani to a FFI and showed that he was getting more torque and better mid range but the FFI made more HP and power top end.
i'm confused

stock manifold made more torque and better midrange

FFIM made more top end HP

correct?
 

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slower than a turtle
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Discussion Starter · #17 ·
well the ac is already gone for weight issues and the ps will be gone shortly. my ultimate mission is to create a nationally competitive mk3 supra in esp. and then eventually i mean way down the road make a autocross nationally competitive SM 1.5jz supra
 

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Just some guy
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supraguru05 said:
i supose that the short runner woudl be better overall because ill be in the high rpm range all the time
have you ever competed in an autocross before? You're usually in the low/mid rpm range.
 

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Here is a formula to pick your runner length. 97000 / peak rpm (goal HP @? rpm). IE: 300hp @ 5900 rpm--- 97000/5900= a runner length of 16.4 inches. Good luck which ever way you go
 

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slowest restoration ever
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Lzalusky had a good explination.
Allan has a good tip too, but that formula ignores runner diameter so I don't think it is universally accurate.
Manifold design is actually very complicated.
The resonance of the runner needs to match the frequency of the intake valve opening, so that by the time the valve opens again, a positive pressure wave is just getting there.
Resonance of a tube, like with ports on a speaker enclosure, is a function of diameter/length.
You can have two different ports of different lengths and diameters but with the same proportions and they would have the same resonance... With intake runners, there are a whole bunch of other variables which affect how much they flow at a given rpm, like manifold pressure, cylinder volume, head flow, etc.
I.E. a bigger cylinder with more head flow would need a runner of the same proportions but wider and longer to be tuned for the same rpm... I think?
I don't really understand the relationships, honestly... but I know there's a lot involved.
I have barely taken any physics, but I love it, so I try to figure it out.
 
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