Hi there,

So I have a question.

I fitted gsxr600 throttle bodies on my engine (GSXR1100). It runs nicely and I have read on here that these TB's can flow 200bhp, which is more than I will ever manage.

The 2001-2003 gsxr600 TB's (engine rated around 100bhp) have an ID (at the airbox end) of maybe 42mm.


Now, I have been sprucing up a spare engine (big valve, ported, all that stuff), and thought I would put some 2001-2003 gsxr1000 TB's (engine rated at 154bhp) onto it as "more area = more air = more power". I have preped these in the normal way, the ID at the airbox end is 49mm i.d. (tapering down to a somewhat smaller throttle plate).


Finally, I purchased an airbox off a recent CBR1000rr, this is a fireblade (rated at 170bhp), as I wanted to use the secondary injectors and thought I might be able to re-use the airbox. This arrived complete with trumpets, and on measuring these the ID of the trumpet is 42mm, and the o.d. is about 47.5mm.


So, what is going on? Did the manufacturers go overboard with their sizing of earlier TB's and now realise this? I appreciate that at partial throttle smaller TB's will give higher airflow and improved running, which though less significant on TB's than on carbs may still be of value.

The Megasquirt manual suggests that all these TB's are big enough for my engine, but recent Dave Walker comments (in PPC) support the use of super massive TB's.

Also I appreciate that flow reversion is a bad thing when running secondary/staged injection (as the fuel can get somewhere other than the inlet), and that might be why Honda have gone this route.


So, a question. Why have Honda gone smaller, and why do they get more power?


Sorry if this is a long question, but I am struggling to understand.


Matt

These are indeed all big enough to flow the required air. After all, a 36mm choke in a set of Weber's will flow sufficent for close on 200bhp, and there's a huge difference in area when comparing 36mm bore and 42mm bore (36% more or something like that).

Its probably more to do with emissions, rideability, engine design why Honda have gone smaller

Basically the larger a throttle body is, the more cubic air will flow at maximum, but it will also tend to have a lower velocity.
A low velocity can create all kinds of problems, depending on your set up, and even cost you power.

I suppose my question is:

"Given that these companies are damn good at what they do, then why the variation in TB dimension?"

I'm guessing it was "fashion" in the same way that the really big bikes had 40mm carbs (which didn't work optimally except at very high RPM). Were huge throttle bodies a macho status thing, or did they just get it wrong in the early years?

I am now tempted to stay with the gsxr600 TB's on my rebuilt engine. I guess I can do the experiment (optimise fuelling with GSXR1000 and with GSXR600 for fixed target AFR, and see which needs more fuel, and hence is getting more air).


Thanks,

Matt

You should be looking at the head end ID as they'll taper somewhat, the amount of tapering will vary.

The problem with thinking about a bike engine is it's different to a car, not only in terms of engine speed and intake volume for different power figures, but also it's use. Fast/high performance cars are much more about WOT when going fast so speccing up the ITBs is more about peak flow. On a bike it's more about part throttle and driveability, I.E fast, high performance riding without falling off.
As part throttle is so much more varied than WOT, e.g 40% throtttle on one engine will give the same power as 50% on another, there is alot more oppertunity for variance.

Using the bike engine in a car doesn't make the part throttle as 'important' as on a bike, so just fit ITBs that are for a similar power/size/rpm spec and hope the mappers abiltiy will make the most of them.

Edit, part throttle doesn't nesesarily mean low revs, so they'd still need to flow high volumes of air.

The tapering smooths the airflow through the ITBs, helping to minimise pressure drop/losses.

[Edited on 28/1/10 by brianthemagical]

Have a look at the FAQs on the Jenvey site, much information:
http://www.jenvey.co.uk/

Also, if you read Dave walkers comments again, you will notice that the super massive TB's worked well on a high reving highly tuned Duratec.....His conclusion that when you get to a certain state of tune on this engine, the bigger seems better, it was based purley on peak hp, It would have been more use to most of us to see the curve. But this is by no means a written law that bigger is better....

You also start getting into different flow characteristic requirements when looking at very high revving engines (i.e. bike), and hence can require bigger TB's.

As mentioned, you should be looking at the smallest diameter for your ID size, this is the restriction.......

I must admit there does not seem to be too much like for like comparisson plots available, it would be interesting to see a more normal engine plotted with different size TB's.....

As an example, I'm one of those who have managed to find over 200bhp through the gsxr600 TB's - BUT, you can see from my print out my engine is not very high reving (peak hp is around 6100rpm), it's a 2ltr 8v engine....for me I think the smaller TB's work very well and are matched to the characteristics of the engine - If my peak hp was around 7500rpm, I suspect I may require larger TB's...

Thanks for all the help here.

Why does the RPM matter again? Clearly you guys have thought about this, but I would have thought 1litre engine at 12k rpm, would be the same as 2litre engine at 6k rpm. That is the airflow (litres/min) would be the same.

Are getting into a tuned length type discussion?


My gut feeling from investigating this is that the GSXR1000 TB's are bigger than makes good sense, and that the GSXR600 and newer Fireblade ones should both work better. But then I measured some 2004 ZX10r ones and these are super massive (bigger than the gsxr1000 ones).

FWIW on my MAP sensor I see only minor (1-2kPa at most) drop in pressure at WOT and max RPM at the moment. I assume this is as good an indicator as any as to whether these things are flowing enough.

Maybe I will investigate where these different bike engines are getting max torque.

Thanks,

Matt