lengthening bottom of throttle bodies

[cyclecamper]

OK I have slightly bored-out throttle bodies that are also cut shorter at the top. Worked well on the seller's race bike they came off of, but everyone who has used shorter TBs has reported a loss in low-RPM torque and reagarded it as a race-only mod.

I'm considering lengthening the TBs, but at the other end. For one thing, I can probably then use my stock airbox again. The tops were cut off, now I'm thinking of adding to the bottoms. This will effectively move the injectors farther up-stream, and move the butterflies up-stream. The effect could be significant. I'm educated in acoustics and computer modelling of acoustical structures, so I'm not looking for a wrong lecture on the effect of making it shorter or longer...but I'm really still interested in all opinions, especially about whether there will be an noticeable throttle lag from moving the butterflies up-stream, and whether there will be any improvement in atomization from moving the injectors up-stream or will it have worse hesitation when I roll-on with more fuel condensing and raining out below the butterflies, etc.? I know more or less what longer runners do on a car engine; I know Ducati and the Yosh LTR racebikes were successful with secondary overhead shower injectors.

This is for my "future engine" I've been accumulating parts for but never even opened up yet...essentially: J&E high-compression ceramic-coated pistons w/ anti-friction oil-retention coated skirts with stock bore, Vee-Two cams w/ adjustable gears, slightly lightened alternator rotor, balanced crank nearly stock weight, Yosh/Carillo rods, modified ECU w/ 900 more RPM with soft & hard limiters intact and more ignition advance near redline and software TRE, Ferrea ceramic-coated stainless valves with anti-friction oil-retaining coating on the stems, cryo-treated stock valve springs. Bike has bigger aftermarket ram tubes, Yosh full system.

[TLJimmy]

That's gonna be a beast. I can get you some cheap lead if you want to make a BRICK to keep the front wheel somwhere below the vertical

[The Ring-In]

no worries greg.

In practical terms I can see no reason not to lengthen either end of the bodies . In practical terms everythings fits in a std configuration. so there is one reason to lengthen the top. simplicity. Raising the bottom would change the geometry of the linkages between the two bodies. the bottoms also have cavities for the injectors which would make the extension a little more complex.

to incorporate the acoustical knowledge you have there are fields of thought as to the location of the butterfly and injector nozzles in relation to the total length of the inlet tract.(valve seat to top of bell mouth) and that being somewhere midway. others here may have more up to date knowledge here. so taking into account the extended length of the bellmouths and the suggested increase in bottom body length I'd check the location and make my decision from there.

[UKbiker]

First off : I dont know very much about this subject ... But ... I read about KTM and the RC8 twin. Said that KTM had spent millions on computer mapping the complete intake system on the bike. So if you say took the KTM specs for the throttles and made yours the same would that not work ?

Seems most manufacturers are looking down the variable length road anyway.

TLR's

[97TLS]

From what I've gathered, the further up the intake tract the injector is the better for higher rpm power. Like you mentioned, more time to atomize. Here is a link you may have already found.
http://www.dtafast.co.uk/Throttle%20....htm#positions

Ducati uses this set-up but none of the ultra-high revvin' Japanese sportbikes do so who knows.

[cyclecamper]

That link has some things to think about; thanks!