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Morris

Morris

Yoda
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Soooo... since my car is temporarily down again, I am thinking it's project time. Inspired by a box of motorcycle parts my brother left in my garage, I am thinking a set of throttle bodies from a Kawasaki zx6r might make a nice addition to my Megasquirt set up. Fabbing an adapter for the zx6r bodies will be much easier, cheaper, faster than designing and building a custom MPI manifold.

The individual throttle bodies have a 38mm bore, which seems like a lot for a 1492cc engine, but they work pretty good on 636cc engine, right?

Can you guys think of compelling reasons not to attempt this crazy donkeyed project?
 
Need for long runners?
 
These guys use very short runners, but the throttle bores they are working with are much smaller.

Can you explain why I will need long runners?
 
I think once again you are my hero!

keep us posted!
:cheers:
 
My friend put quad carbs like that on his Spitfire.
Man that car sounds good!!
 
go for it. they do it in the UK on all types of engines. They even use the earlier yamaha r1 carbs and just rejet them for car engines. i think it would sound great and you def would not have any restriction on the intake side of the engine!
 
I was going to fit motorcycle carbs to my Datsun A15 engine swap but found that there is such a demand that other options looked far better. The nicest thing about the MC carbs is how simple they are to plumb-just short pieces of hose to connect to a home made manifold.

Kurt.
 
Morris said:
These guys use very short runners, but the throttle bores they are working with are much smaller.

Can you explain why I will need long runners?
Click to expand...


Sure, the longer the runner, the lower in the RPM range the power comes in. The shorter the runner, the higher the power comes in.

Short runners are for high RPM engines that make power way up top. ( Or are used on an engine where you want it to make power up top.)

You don't want or need little stubby runners on a small street engine that'll only see 6k down hill with a tail wind. A 1500 would frag to pieces before it ever even came close to using short runners to their full potential.

IIRC 11 FEET is the optimal runner lenth for a street engine ( depending on charateristics, but that's pretty close for everything street wise.)

Myself I would look into making something grunty from idle to 5K. You can use ITB's but I would make the runners as long as I possibly could.

Ever notice how intakes wrap up and around on new street stuff?

Ever notice how you only see stubby intakes on high power/ high RPM stuff like motorcycles and Ferraris? BMW does it too but they scream.

You can do it but you won't have the bottom end you could have with longer runners.
 
The idea is inertia. Just like water traveling down a long tube the air is a fluid also that has inertia. You want the air's inertia to push the air in front of it in. You can't do that with a short tube.

There is a point where air speed is so great a long tube become a restriction, thus the desire for a shorter tube.

You want idea air speed for whatever your "usefull" RPM range will be.

Pick the RPM range where you want power, ~then~ build an intake to facilitate that.
 
Okay... I understand that aspect of manifold design. According to my calculations, to make a real difference in the performance of the car, the runners I would need would be 24" long IIRC. I have mocked up a manifold in a 3D software, and really there is no easy place to put runners that long under the hood. As my the runners on my current stock manifold are about 3" long, I don't see how I would be loosing anything by having short runners.

My main concerns with the 4 x 38mm ITB set up I described are:
1) Will I be able to make the engine idle speed low enough with that much collective throttle bore.
2) Will I be swamping the engine every time I put the petal down... or worse...
3) Will dropping the pedal bring power on too quickly causing dammage to my engine or drive line... or to a lesser degree, will power come on too quickly to be able to ease the car into gear from a stop.
 
Here is a free tip: if you are going to build your own intake manifold and you need tapered alloy tubes, try using aluminum baseball bats....... Cheap, readily available and seamless.

I was going to do this to my A-series, but the quirks of the Siamese ports were just too much....... However I gave a lot of thought to how I would go about it.
 
I heard a experienced machine shop guy say the material bats are made out of doesn't weld good. That being said, I ran right out and bought 6 on sale. You need to get the kiddy bats. I never did try it and donated them.

Don't worry Morris, with that short of a runner and that much throttle bore you'll kill enough torque that breaking drivetrain won't be an issue. :jester:


IMHO, you'll need more than three inches.

Look into a DGV manifold. A TB from an Old Jetta will bolt right up and they are cheap. It shoudn't be a problem putting injectors in that.


Hey, no need to put 24" runners under the hood. Cut the fender and bring 'em out into the sunshine. :hammer: :laugh:
 
Thank you Westfield 11! I had heard that tip before but forgotten it! I have been really torquing by brain to try to figure out the taper situation!
 
They're not cheap though. I'd thrift store one and see if it'll weld good. If that doesn't work, split one side of an aluminum tube halfway up lenthwise, squeeze the cut end in a vice then weld.
 
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