View Full Version : Port maps for Honda-RS250
08-15-2004, 03:55 AM
For those thinking about tuning - here are some
specs from Honda's 70+ Hp RS250 racebike... Unfortunately dimensions have been obscured - but as the Honda's bore/stroke are different the exact port heights are meaningless for an Aprilia anyway. The important points are the relative port positions and shapes (angles would have been nice!). Also head shape and squishband are clearly visible in the head image.
08-15-2004, 03:56 AM
Honda RS250 head.
08-15-2004, 03:57 AM
Honda RS250 pipe (not very useful without dimensions, but included for completeness)
08-15-2004, 04:36 AM
Yep these are very similar to the 89 and 90 model RS.The exhaust ports tops are parallel to the cylinder top and means short piston ring life and this is the way to go on the NSR MC21 for more power as they share similar porting patterns to the 89/90 RS.A while back you could get F3 spec single ring pistons for the NSR but alas no more maybe they would have a five hour life as well?.
08-15-2004, 04:52 AM
This is going to get confusing... but it is also possible to modify the RS ports above to full
NSR (race) spec, Why did Honda decide to call the NSR-MC?? the same thing as the NSR GP bike!
The interesting points as far as I am concerned:
- All tranfer ports same open timing (not staggered)
- 5 transfers, 2 mains (away from exaust), 2 secondary (towards exaust), 1 boost.
- exhaust port shape
- Flat squishband (with slight radius to edge)
- Flattened dome (not hemispherical) combustion chamber
Interestingly the port configuration and head are not too different to the Aprilias... Honda tuners say that the port specs above can be modified to NSR (GP) port timings. The conclusion has to be that the Aprilia head and ports can be modified to GP spec... Obviously bore/stroke will be different... but it is always possible to offset the crank pins and overbore to a squarer configuration - It just depends how far you want to go.
The 70Hp post showed how to relatively easily get 70HP (without altering ports) ... With port and crankcase mods Stan Stephens can get 75Hp reliably (Stan's race tune flows the crankcase, raises and widens and re-angles transfers, widens the exhaust port, modifies the head, adjusts CR to fuel, corrects the squish and converts to an o-ring).
I guess the rest of the power would come from altering exhaust port timing and the pipe, as well as raising the CR and using race fuel.
Recent TZs use ram-air as well ... rather than trying to pressurise the fuel tank (doesn't work) they add a small fuel pump. Ram air seems to work quite nicely if you put the carbs inside the airbox, and use a fuel pump.
At that point (probably slightly over 80HP) I think you are going start struggling to find extra HP ... of course some dyno's read high, and you can talk about crank-HP rather than rear wheel.
It would be nice to see some accurate specs from the 250GP machines. I guess that information is going to remain elusive until 250GP racers are retired in 2009...
On Ed's map , exhaust port area seems small and is confirmed by Time-Area calc :
147.5 (*10^-4) @ 11000 rpm that could allow 10-10.5 bar BMEP
107.4 for transfer (over 13 bar !)
I also think that exhaust porting could help without increasing actual width.
Some link to portmaps or timing ... :
Interesting to check transfer staggering depending on years...
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