I was messing around on my flow bench with a zx14 head and noticed the throttlebodies looked to small for the port.I machined the throttlebody out to 46 mm from 44mm and the head really liked it.the fit to the rubber manifold was perfect with no cutting. picked up about 6 cfm. when the upper butterfly was removed nothing changed on the bench but when the upper shaft was removed it picked up. trying to get it on a dyno to have real numbers. looks promising.

got some testing done and we picked up about 4hp and 1.5 torque.turbo todd said the one test he got about 6 hp.and 2 torque. going to try another idea and see what happens.

making a jig for boring out the throttle bodies. might have a couple of sets for sale. any interest?

If I may add to this thread before SALES are made....

Just because the flow bench and dyno liked it... doesn't mean it's going to help.

There are many instances where the flow bench and dyno liked certain things, yet real world testing concluded the exact opposite.


A common example of this is with hogged out heads. Sure, they flow great on the bench and make big power on the dyno...but run like shit in the real world. You may be hogging out the TB's and coming up with the same positive results on the dyno/bench as those with hogged out heads.

I'm not saying going to 46mm TB's is a bad thing, I'm just saying you won't REALLY know anything until you do back to back test runs and datalog with both the 44 and 46mm TB's in the real world.

Even if this mod does show gains at peak power, if it comes at the cost of area under the power curve (particularly @ recovery rpm's) you could actually go SLOWER with more peak power due to having less AVERAGE power throughout the powerband.

Anyway, good luck with this mod. Even if it doesn't work, it may still benefit big bore/stroker and/or turbo applications!


Either way, my hat goes of to you for having the courage to attempt it and spending the time/money doing it.



P.S.- I wasn't trying to piss on your thread or pop your joy bubble, just suggesting that there be some actual real world testing before you go ahead and start opening up a bunch of TB's.

you are correct in what you are saying. we are in the process of doing some real world tests. I have a set on my bike and mph went from a best of 149 to 152. et sucked (9.42) because of my lack of experience in launching. once I get the tbs done another set will be going on a friends bike. he has a lot more seat time on the 14. you are only opening up the hole by .080 inch but if you look in the bore the transition from the casting to the machined hole is not the best. I am also machining the upper shaft down which helps. just taking tthe bumps out of a bad port is whats happening.

this may be of some relevance!!!

and possibly well known!!!

with SU carbs on 'A Series' engined Austin Mini's
the throttle butterfly could be modified to a knife edge ( leading/ trailing edge),
and half of the throttle shaft diameter removed (slotted area which butterfly slots through) to allow more CFM through the carb.

Thanx Dave.

Very cool!

went to the track today with the 46 tbs. stock motor brock, shorty, pc and bmc. 5 over with a lot of lightning up. I weigh 220 stripped. went 9.42 149mph with the 44tbs earlier this year. to day I ran 9.27 153mph with a 1.62 short. still sorting out the sb6 just ran out of time.

What was the change in track conditions, density altitude on each day? Other changes?

cant tell you what the air was. all I can say is best mph with the 44tbs was 149 with colder air then the 46tbs going 152.99 with no wind. it went 123.88 with a 1.62 short with shell 91oct, not race fuel.

sorry about taking so long but I had to make up some fixtures to do the job properly.reassembly is really important. to balance the plates adjust the 2 tps sensors as well as even bolting the 2 halves together I had to make a jig. I will ship them to you with no injectors and you send yours back. its $500 plus shipping. if interested email me and I will send you some pictures. hmanovich@sympatico.ca