| Author | Message | ||
     Martin |
I am constructing a twin-inlet system for my XB9 but am uncertain as to the best diameter of throttle-body to use. I have available anything from 40-50mm. My early XB has the 45mm standard single at present but most of the books I read suggest that that is a bit 'big' for the hp. I know that the race-bikes use 51mm but they are considerably larger and rev higher. My gut feeling is 48mm but am I guilty of falling for the 'more is better' trap? The close firing intervals of a 45degree twin might cause charge-robbing with a single t/b which won't happen with the twin. What is the consensus? | ||
| Slaughter |
Kinda depends... It's like carbs - if your motor isn't pumping enough air, it never makes sense to mount a huge carb. I run a single 51mm on the racebike but I can spin it to 8000 rpm (well... for a little while) - and I have BIG valves. I don't think there's much to be gained in a bigger throttle body until/unless you're doing something else to get lots more flow - bigger valves (stage 3 headwork) - even "bigger" cams don't really call for bigger induction. I don't see "charge robbing" as an issue because of how the 2 separate injectors are controlled. This aint a carb system. Your ECU calls for a specific timed fuel pulse for each cylinder's injectordepending on RPM, Throttle Position, and other factors. Just my $0.02, devalued for inflation. (of course, this ignores the "cool factor" of being the only kid on the block with dual induction) (Message edited by slaughter on April 04, 2005) | ||
| Wyckedflesh |
I would look into 40mm-42mm throttle bodies. Or look at the other V-twin engines that run a dual intake and compare that way. I know the older TL1000 zuki used twin 35mm for 101hp or so. | ||
| Fullpower |
i think the ducati 620 uses a 52mm throttle body for EACH cylinder. they run real sweet, no flat spots, just smooth even torque curve from 2500 till it tapers off over 9000. | ||
| Tank_bueller |
I would say dual 45mm should be waaay sufficient, but I'm un-educated and un-informed. Like said above, the head and valve design should have more effect than a slightly larger/smaller TB. Definitely cool though  ($.02) | ||
| M1combat |
Hmmm.... I think I would add the header pipe diameters together and divide the stock TB diameter by that (giving you about .7-.85 I would guess)... Then multiply a single header pipe diameter by that number. I could be way off base though because the stock TB deals w/ two cylinders... | ||
| Martin |
'I could be way off base though because the stock TB deals w/ two cylinders...' But not at the same time. If I built a tunnel ram then the cylinder can draw air from 2 t/bs but so long as I stick with the existing injection only one injector will supply fuel. I have 2 objectives with the twin system: to speed up gas flow at slow speeds to give more torque, without sacrificing top-end, but also to be uniquely cool!By my definition, cool is only cool if it works. I think that smaller is most definitely better to achieve higher gas speed. This is, after all, a road bike. | ||
| Martin |
dia area twins 45 3.141 4995 49 5922 40 3946 7893 51 6415 12831 From the above calcs, a pair of 40s will give more flow/area than a single 49 and still falls way short of the race bikes humungous area | ||
| M1combat |
Keep in mind that the existing system is sequentially injected, so only having one injector supply fuel in your case is exactly what you want. | ||
| Kowpow225 |
I'd want the best balance between low and high rpm cylinder fill without sacrificing much at the upper rpm range, meaning if you go too small with the diameter in order to gain air velocity, some upper end power will be lost because it will 'choke' the engine of the required amount of air. Equally important when considering intake changes is the valve timing/overlap etc. and slightly less important, the velocity stack. ALL of these elements work together to give you the end result. If it's diameter only you're after, I'm sure there's a method of figuring out the sweet spot in velocity and volume. | ||
| Imonabuss |
Martin, you are right if he is feeding each cylinder with both throttle bodies. If he is constructing individual runners, then he needs at least the twin 45's. |
