250 GN, the 100 mph project

[sendler]

Originally Posted by renault_megane_dci

Here is my 20 minutes homemade flowbench :

Nicely done. It's interesting to see what economy improvements you can end up with.

[renault_megane_dci]

Thanks Sendler.

My modding reference for the head is David VIZARD and his yellow bible AKA "Tuning the A-series engine".
There is a chapter about improved FE and what can be done to the engine to enhance it.

He tried various states of head modding and had to conclude the best is to let the inlet standard and mod the exhaust to full race trim.

So far, I saw a lot of potential in the exhaust port whereas the inlet is on the biggish side for me.

The potential in the exhaust has been found to the point of improving by a little under 20% the time required to expel a known amount of water.
The conditions of the test are far from ideal but 20% is more than a trend.

When I will be happy with the exhaust ports, the next step will be :
- a slight skim (there are knock scars on the exhaust side quench area),
- some work performed on the exhaust side fins (drilling some holes where it looks like it will improve cooling area)
- building it back together

[minimac]

The improvement due to your exhaust work is impressive. Even if the flow bench is crude, the results prove to be effective. It shows what can be done using what you have. My hat's off to you. I've found that just a quick blending on the intake side is usually all that's needed, while some heavier work on the exhaust can yield big results. One thing I can"t explain in any follow up testing is that even if measured results are minimal, the "Butt Dyno" can show a big improvement.
I think you'll experience a much bigger gain by dropping your riding position. I'm a proponent of the feet forward position. It's a whole lot more comfortable and more effective than being all hunched over. But one step at a time.....

[renault_megane_dci]

Actually, it is 14% improvement at an unknown lift.

Weird thing to start with on this head is it has a 2 angle valve cut instead of 3 (and that is stated by Suzuki)
15° and 45° but the seat goes from 45° with a big radius that reduces very much the diameter of the port.
I red somewhere that it is not detrimental to flow to have sharp edges up to 15° rather than curves and it gives you more surface for the flow.

Then the exhaust port is very much shrouded and apparently not aero friendly because the long side radius (where most of the flow usually takes places) is inverted towards valve guide (and obviously valve stem too).

Feet forward on this bike would require major surgery to the frame and France (where I live) is really not DIY friendly.
I will convert the seat to barely acceptable foam thickness one of these days but first things first : refurbishing of the head needs to be completed.

[renault_megane_dci]

I had come to the conclusion the chamber is larger than the bore.

the way I see it, it is not good for flow or efficiency so I am tempted to go overbore.

This could also improve torque to capacity ratio with smaller ports capacity wise => lower peak torque rpm.
Obviously, more capacity equals more fuel and I am already geared not as low as I would have liked so I can't see how to offset the increase in capacity ...

That being said, it would also raise CR very easily.

Thoughts ?

[sendler]

Now you are starting to spend money.

[renault_megane_dci]

True (althought I thought the engine needed a freshen up @ 40K since it is not nickasyl plated so this means an OEM piston and set of rings anyway, the extra being the boring)

[renault_megane_dci]

I just used this page for air velocitiy in the pipes :
FLAT4EVER.COM - VW Aircooled Cox Coccinelle Karmann-Ghia Volkswagen Buggy Beetle - Calcul de la Vitesse des gaz
it is made for Beetle engines so I had to convert my valves equivalent surface to a single valve equivalent.
Anyway, going from 72mm bore to 79mm bore (DR 350) meant an increase from 76m/s to 92m/s @ peak power (as well as an extra 50cm3)

[renault_megane_dci]

Crap !

More modding of one exhaust to compare to the (now decoked) standard port and I have to come to the conclusion that I got too far ...
I think I got too wide.
Or maybe the kind of beak before the guide got flow away from this tricky part ...

Ho well, you live and learn ...

On the plus side, the polishing of exhaust and chamber should make the engine run cooler and allow for a slightly increased compression ratio.

[sendler]

Originally Posted by renault_megane_dci

fun

Interesting bike culture you have where you live.