250 GN, the 100 mph project

[renault_megane_dci]

Due to the taller new rear tire, the top speed is now down from 125 km/h to 120 km/h without any aero mods.

I am now working on the much smaller indicators. Needs to cut some brackets.
End weight of the indicator is gonna be a third of the original equipement ...

[renault_megane_dci]

Last week I found the bike dropped on the side.
Luckily only issue is a broken mirror and an unknown amount of gas spilled !

Also around half tank, I changed the rear tire for a taller one (120 90 R16).

Only trouble is I didn't foresee the change would be this big (it is standard size for Germany) so I didn't make any notes of the size of the older tire (3.70 x 16, no idea what the wall size is).

I seem to remember I dropped 200 rpm @ 90 km/h

[renault_megane_dci]

The bike has been on hold for 2 weeks due to funky engine noises when hot and erratic power drops.
I am guessing either :
- head gasket (there is an oil leak) + chain tensionner guide
- shot clutch (was reportedly worn but stopped slipping as soon as I changed the oil) and crap in carb and tank

My only issue is I have no room to open up the engine.

Anyway, there is opportunities even in drawbacks so the plan is to ugrade the engine as far as FE is concerned

I am thinking :
polished combustion chamber
polished exhaust ports
higher CR (from 8,9 to 10 maybe)
flowed carb (if needed)

Keep in mind the GN is air cooled with no oil cooler.

Any advice on the spec ?

[beatr911]

The rage at the Honda marketing department seems to be friction reduction. This is likely a contributor to the reduced fuel useage seen of late. If you are increasing compression with an aftermarket piston, ask about a low friction coating. If you are milling the head, consider that it will retard cam timing making the engine run a little stronger at higher RPM, at the expense of lower RPM power.

I understand that friction reduction coatings can be used on other parts of the engine as well. Make experience with friction reducers a consideration when choosing engine builder.

The compression ratio chosen depends on the grade of fuel you plan to use. The 200cc Chinese derivative of this same motor that I had probably wouldn't like higher than 9.0:1 on our regular gasoline. I'm guessing your fuel is a higher grade. Consult the TTR 230 forums as this engine family is widespread and long lived in Yamahas production runs. Again your engine builder should be able to answer this question.

As for polishing, it can help with pre-detonation if there are some burrs or protrusions in the combustion chamber, as you know. Also, I've had success in smoothing any sharp transitions in the intake, like around valve seats.

A co-worker of mine has been doing the motoman.com high velocity intake service for racing quads for several years now with success. He says that the dyno has shown a good bump in bottom end and mid-range with no loss, or minimal loss of top end. I didn't get it done on my 200 before selling but do know that they have very large ports that could very well benefit from it. No idea how it affects fuel mileage though.

[renault_megane_dci]

CR : I was thinking about having the head milled (10,5 is 1,5 mm if my calculations are correct)

So far I hope to just DIY a basic overhaul with the only outsourced operation being the head milling (I'll first investigate a thinner base gasket)

Polishing is also about running cooler, heat being reflected more efficiently through less surface exposed to the flame front.
The engine does not have an oil cooler so it is a predominant factor.

Friction work seems interesting for the bottom end wich I hope I won't need to work on.

[beatr911]

Internal heat managment may also be partly managed with ceramic coatings. Porsche started using this with the old air cooled 964 motors in the early '80s. They only lined the exhaust port with it.

Sounds like you've already done some research, but here is a link to an article that hits the highlights in case others are interested.

A Look at High-Tech Engine Coatings and What They are Worth - Hot Rod Magazine From the results of the naturally aspirated engine it appears there is no difference in power production. The change is so little to be of significance. They didn't state temperature differences though.

So I guess I don't know what I'm talking about. Does anyone have some details on how Honda (or others) build motors to get maximum efficiency that might be used here?

[renault_megane_dci]

Originally Posted by www.hotrod.com

about a 1.1 percent gain in torque and a 1.5 percent power gain. Overall average output was up as well.

Originally Posted by www.hotrod.com

there is a trend of increasing gains for both the full-coated regimen and the oil shedder/antifriction regimen. This indicates that coatings become more effective as rpm increases, so a 7,500- or 8,500-rpm combo might show greater improvement

Basically, it is free and can do a little good so I am going for it (polishing, not thermal barrier)

[renault_megane_dci]

Bike has been stored in the basement,
Head has been removed,
Cam chain tensioner guide has been checked, it's likely not his fault.
I'll renew the cam chain as it is opened and a good time to do so (bike has 40K miles on the speedo), also the cam tensionner is far out.

So the head has been cleaned, the exhaust ports (one for each valve) have a funky shape and they are very much shrouded on the begining.
I did some porting on one of the ports and I will crudely check its flow and comparing it to standard clean port shape flow.

Also I did some calculations and the end pipe diameter is much smaller than the equivalent surface of the exhaust valves minus the stems.
Spells back pressure to me.
Might as well hurt 3/4 throttle P&G

Inlets are 26 mm times 2, equivalent single diameter is 36mm, carb is 34mm
Exhaust are 22 mm, pipes are 22 mm inside, tail pipe is a SINGLE 18 MMM !!!

Valves were very whittish in color indicating a rather poor mixture (I used to run on E10 and the bike was probably set up weak for regular gas to start with).
Also there are some distinct knock scars in the quench area between the exhaust valves meaning the engine was runned too hot.

I have some ideas for improving engine cooling in this area (drilling holes in the fins as close to the quench area as possible).

I'll add some pics tomorrow.

(money is on the clutch now for the funky noises)

[renault_megane_dci]

Here is my 20 minutes homemade flowbench :


The idea is to time the exit of the water through one of the exhaust valves (they have their dedicated port) and compare the result between the modded port and the stock port.
So far I have to improve the leaks between the "cylinder" and the head and to make sure the gasket (destinated to seal between a WC and its tank !) has no adverse influence on flow or at least evenly.

To make sure the result is mesurable, I chose to lift the valve only a small amount (I assume half the lift)

[renault_megane_dci]

Nobody seems to be interested but the modded exhaust port spits out its liter in an average if 4 seconds when the unmodded and not de coked standard port is closer to 5 seconds.

It is encouraging and the modding is not fully done too, the port are very much shrouded in the first bend in spite of the valve stem already blocking off the area.