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Old 05-12-2020, 03:25 AM   #1 (permalink)
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Insight Gen 1: variable length intake manifold

I've always liked the idea of a variable length intake manifold on my Gen 1 Insight to improve torque off boost. (It's turbo'd and doesn't use IMA.)

The Australian Ford Falcon six cylinder has featured over the last few decades a really nice variable length intake, with the changeover effected by butterflies inside the manifold.

Generic pic of Falcon engine bay (inlet is the snail-like casting):



Lots of Falcons were sold, so the cost of the inlet is basically nothing - eg US$15 from a wrecker. Here's what it looks like inside. Note the butterflies that cause the air to pass through the longer runners (the snail). When open, the air takes a shortcut.



I sliced it up and bent some heavy wall aluminum tube that will connect to a manifold plate. Shown here with standard intake manifold in background. The bent tube is much longer than it will finally be. I have a TIG welder but I don't know if my skills will be up this one - might need someone else to do the welding.



My measurements indicate these runner lengths (approx):
  • Std Honda - 330mm
  • New intake manifold short - 350mm
  • New intake manifold long - 610mm

It's not a quick project, so don't expect results next week. Will be interesting to see if it makes an improvement - lots of unknowns like the impact of larger runner diameter, different plenum volume, etc, etc.

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Last edited by JulianEdgar; 05-12-2020 at 04:14 AM.. Reason: typo
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Old 05-12-2020, 10:48 AM   #2 (permalink)
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Old 05-12-2020, 01:44 PM   #3 (permalink)
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Very interesting project. The much larger diameter would be a definite concern and might kill any gains you get from longer runners. However, I look forward to what actually happens!
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Old 05-12-2020, 03:35 PM   #4 (permalink)
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I failed to find the article I read, but I recall seeing a dyno chart for a K20A3 engine with the variable intake runners enabled vs disabled, and the result was (if I recall) something like 11 ft-lbs of torque (~8%) over a pretty wide RPM band. I'll post it here if I find it.
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Old 05-12-2020, 04:58 PM   #5 (permalink)
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Quote:
Originally Posted by Daox View Post
The much larger diameter would be a definite concern and might kill any gains you get from longer runners.
Reminds me of those French Ford flatheads which had thinner manifolds to improve the torque when applied to those SUMB military trucks.
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Old 05-14-2020, 02:07 AM   #6 (permalink)
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Slow progress, but I think it will fit. (Here it's being tested on a spare parts car.)



One inlet tube in about the right place. The vacuum canister that operates the manifold changeover valves will need to be moved in position (as being held).

The throttle body will need to be placed at this end of the manifold, so requiring a new entrance.

I am wondering what throttle body size to use. Standard Insight is just 38mm, and with my power increase, I calculate maximum airspeeds through it of about 300 km/h. (Standard max would be about 170 km/h.) A 50mm throttle will reduce speed back to standard.

But I really don't know - there doesn't seem to be any tech literature around about matching throttle body size to - well, anything much.
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Old 05-15-2020, 05:42 AM   #7 (permalink)
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Made a wooden jig to locate tubes.



Also cut new hole in one end to create new inlet for throttle butterfly. Amazing how space tightens up as you try to fit things in! Am going for 50mm throttle body.
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Old 05-16-2020, 05:18 PM   #8 (permalink)
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I've decided to put this project on the shelf. The amount of work to make it is too high I think in the context of the risk it won't be successful.

I think it might be better to roughly make two inlet manifolds from steel tube, each replicating the runner lengths/diameters, plenum chamber volume and throttle body size of the ex-Falcon changeover manifold, and then test them both on my dyno. Obviously they can hang out of the engine bay during that testing. I could MIG them together much faster than fitting the aluminium changeover manifold. But even a pair of rough steel ones would be a lot of work to make.
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Old 05-16-2020, 06:46 PM   #9 (permalink)
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What about spacers between the head/manifold and manifold/throttle body? Would small changes like that make a difference?
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Old 05-16-2020, 06:49 PM   #10 (permalink)
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Quote:
Originally Posted by Gasoline Fumes View Post
What about spacers between the head/manifold and manifold/throttle body? Would small changes like that make a difference?
I would think yes (to head-manifold) but one would need to be very careful to place the injectors at exactly the same spot.

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