stevec33
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| posted on 24/4/16 at 04:43 PM |
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Bike throttle bodies
Im looking for a bit of info on if there are many people using bike throttle bodies on highly modified engines, especially vauxhall xe engines, how
they get on with them, which ones they are using and what size ect.
The reason is ive changed my engine spec this year from basically standard with mild cams to a full build with rods/pistions/cams/headwork ect and im
wondering if my current gsxr 600 tb's will be suitable for the new spec, they taper from around 50 mm to 38mm at the butterfly which ive been
told may be a bit small and could limit power ect.
I need to get a new tps as mine has broken, and it may not be possible to get one seperate so i may have to get a new set of tb's anyway, the
bigger cc bikes have larger tb's and are cheap and easily available so im wondering if this may be a sensible thing to change at this stage.
Cheers
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rusty nuts
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| posted on 24/4/16 at 05:07 PM |
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When I was looking into fitting bike TBs to my Xflow engine I found a guide to tuning Weber carbs which gave a guide to choke sizes /BHP
Worked out the 36mm would be in the ball park, rolling road results seem to agree. Can't remember where I found the guide . The only gripe I
have is at low speed throttle response is very quick . looking at making a progressive linkage
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SPYDER
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| posted on 24/4/16 at 05:55 PM |
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I have GSXR750 throttle bodies on my Toyota 3SGE engined car. This is a similar engine to the XE. 178hp standard.
Mine is standard other than the throttle bodies and a larger bore exhaust manifold with a proper merge collector. It does have a standard R1 can which
many people say is restrictive. The engine is making 194hp on the rollers, although I appreciate this may not be 100% accurate.
GSXR750 bodies are the same size as GSXR1000, ie. 42mm at the butterfly. This size should be good for 220 hp on a four cylinder, according to
Jenvey.
Using datalogs I drew up a graph of throttle position versus manifold depression, using only hits where the MAP reading is just below atmospheric. You
end up with a diagonal line that will intersect the top edge (full throttle) of the graph at the point where your inlet tract starts to restrict the
airflow. On my engine this was at around 6500- 7000, so the 42mm bodies seem to be only just big enough.
I should say that the air filters were removed whilst datalogging so its not the filters restricting.
If you are aiming above 200hp you might find that 42mm bodies start to restrict things. They won't limit the power to 200hp though.
The datalog idea came from this site...
CLICK HERE
Have a read. It explains the importance of not over-sizing too.
Like rusty nuts says, you'll need a progressive throttle linkage. I've got pics of mine if you would like to see them.
[Edited on 24/4/16 by SPYDER]
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stevec33
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| posted on 24/4/16 at 06:29 PM |
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Thanks for the info. The engine is quite high spec, steel rods, forged hi comp pistons, ported head, big cams with solid lifters/double springs,
simson exhaust manifold. Im hoping to see 230-240 bhp out of it.
I didnt really consider that the tb's may be restrictive when i decided to go for a higher spec engine, the only reason im looking into it is
cause ive been looking high and low for a new throttle position sensor and have had a couple of tuners/suppliers suggest that mine may be quite small
for this spec engine.
I was thinking if it does need some bigger ones now would be the ideal time to do it before i get it mapped and i may even need to buy a whole new
set of tb's just for the throttle position sensor as they are very hard to find seperatley.
It would be great to see your linkage setup thanks, another point mentioned to me by a tuner was that bike tb's ideally need to be converted to
run so they open from/on the screws, not from the springs as this can make them not good low down in the rev range, does this make any sense to you as
i had no idea what he was on about!?
I think most people from what ive found tend to run bike tb's on fairly standard engines as they are obviously much cheaper than car tb's
like jenveys for example and offer similar gains for a lot less money. But perhaps most people who are running higher spec cars take a more no
compromise view of their setup and not so bothered about what it costs. I don't mind spending a few bit on some new bike tb's if its going
to help it make quite a bit more but certainly wont be spending a couple of grand on fancy car type tb's
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SPYDER
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| posted on 24/4/16 at 07:15 PM |
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Here's a few pics of the throttle quadrant. I firstly worked out a suitable diameter using the cable travel and degrees rotation.
I machined the disc up and cut a "D" shape from it. It is mounted offset onto the existing tiny quadrant. The centre of rotation is in the
middle of the four little mounting bolts. So the radius decreases as the throttle is pushed. The change in angle of the cable is catered for by the
little spherical bearing.
I understand what the guy means with the "springs-screws" business. I wouldn't worry about it.
Hayabusa throttle bodies are bigger. 46mm I think.
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kingster996
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| posted on 24/4/16 at 09:40 PM |
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Funny enough, just fixed the spring issue on my gsxr1000 itbs today.
It was giving me grief by opening the first butterfly but then the spring would compress just enough so that the tps didn't move, so it goes
lean and splutters. It only happens when engine runs and adds pressure to butterflies, so was hard to diagnose when bodies were off engine and
accessible!
I just put a bit of a rivet head under the spring to compress it more and to act as a stop.
I used to be indecisive, but now I'm not so sure
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stevec33
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| posted on 25/4/16 at 05:12 AM |
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Yes the guy who i was talking to said that the spring can can cause them to open one after the other like a concertina effect. Is it a spring on the
end he's talking about as im not really sure what he's on about.
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SPYDER
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| posted on 25/4/16 at 07:23 AM |
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He's talking about the little balance mechanisms between the individual bodies. I've grabbed a pic of the net. The turning motion is
transferred from body to body via a small spring.
Initially I sort of had the opposite problem to kingster. I balanced the butterflies with a synchrometer so that the flows were equal. After a few
throttle blips the end pair weren't quite shutting properly so I fitted an extra return spring between the pairs. That ought to have made the
"spring" problem worse. But I don't think it has. I'll check the balance today and report back.
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rusty nuts
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| posted on 25/4/16 at 05:37 PM |
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Neat solution to the throttle quadrant SPYDER , must find the time to sort mine out.
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