Dual Throttle setup

[Godless Commie]

I just finished my dual throttle project today..

My problem was matching the wire travel between the two TBs.
I have a BBK BTB (big throttle body) on the S/C.

I measured wire travel on both TBs. Stock requires 3.5 cm from closed to fully open, and the big one takes 2.75 cm.
I found out that the stock pulley rotates 75 degrees. (One would assume the full motion would be 90 degrees..)

3.5 cm wire travel at 75 degrees needs a pulley which is exactly 42 mm in diameter (where the cable rests) mounted next to it to provide 2.75 cm of travel.

So I made some drawings based on my calculations, and had a machine shop turn a pulley for me with the correct diameter and offset.
I mounted this pulley on the stock TB, and had a "wire shop" build me a custom throttle cable.
The cable sheath is 95 cm. Has adjusters on both ends, and the cable itself is about 135 cm or so. Cost me 20 bucks to have it made.

The next step was to modify (extend) the existing cable bracket on the IM to allow for the offset on the new pulley.

I did not use a BOV or a recirculating valve.
The trick is to crack the SC butterfly open a bit. Imagine setting your idle at 2500 rpm. That much. (Stock TB has authority on idle, anyway.)

The big butterfly has a very aggressive opening/closing rate, which is a definite advantage.

Adjusted both cables so there is no slack, and made damn sure WOT on the stock TB is actually WOT on the other side, too.

Turned the key, the car started and immediately settled into factory idle. It took a couple of very minor adjustments on the SC butterfly to smooth things up.

Revs instantly, has great pick up, punching the throttle idling down the road in second gear smokes both tires forever.
I have a 130/65 pulley combination, BTW.

I took it out on a nice, long and SLOW test drive after dinner. Very civilized behavior overall, drops down to idle perfectly from all RPM levels, and has perfect throttle response. I got on the gas a few times, and no problems whatsoever.

I am very happy with the results. No idle related problems any more.
Should have done this a looong time ago.

The only concern I have is the stiffer throttle pedal.
I'll try it with one coilspring removed from the TB. See what happens.

I will also post pics when I have a chance to take pics during the day.

[Tom @ Fast Forward]

Great job comrade. Can't wait to see the pictures.

[FormerDatsun510Man]

Sounds great! I'm glad this worked out for you. The stock TB has some pretty hefty springs on it. Ironically, we had the problem here with the Power TB that the spring was too light. Sounds like you probably need a pair of lighter springs to put on both TBs.

[Godless Commie]

Thank you Tom.. er, comrade!

Bill, do you think I'll be opening a can of worms if I were to remove one of the two springs on the stock TB?

Would it have any trouble overcoming the internal resistance of the throttle cable and returning to idle?

[FormerDatsun510Man]

With half the spring tension, the throttle body should still close all the way to idle. I did a lot of spring testing with our big TBs and found that springs even less than half the stock torque would still close the throttle body. The key is that the spring is wound so that it is still exerting force all the way through the motion of the throttle. I would check how it feels when you remove the second spring.

[Godless Commie]

Bill, I just removed 1 coil off the spring on the stock TB.

I didn't remove it all the way; all I did was to "free up" both ends, so it is still there, but useless in terms of exerting anything.

Pedal feels great now.

Thank you for your valuable input.