Yamaha YZ125 rear wheel offset

[ChrisACT]

M/F 100cc

[Lozza]

what's the record in that class?

[ChrisACT]

In Australia it's open.  Internationally, the fastest record I've found for this class is about 102mph at Bonneville.  There's no "world record" as such.

A record would be nice and is obviously a goal but ultimately I just want to see how fast we can go with it.

The M/F 100cc class is for naked bikes, so no fairings on this one.  Can't even have aero aids on my leathers or helmet.  I reckon at least 30HP is required to hit 100mph.

What could possibly go wrong? 

[OverTheHill]

Hell, now i'm sitting up & taking notice. Good luck with the project in taking the speed record.

[Lozza]

Are you allowed water cooling? Aero will play a big part in top speed even down to how you have your feet on the pegs. What engine?

[ChrisACT]

The engine started life as a DT100D engine.  It has had a fair bit of work done to the barrel (it was my old Superlight race engine) but I'm going to do a lot more to it.  Planning to spin it to 12,000RPM which it'll do all day.

MOTA reckons that 30HP on methanol at 11,250PM is easy peasy.

If I can't get the required HP out of it on straight methanol, we'll start playing with nitromethane.  A 10% nitro, 90% methanol mix should produce 40HP at 12,000RPM with the same pipe.  The question is, will it become a 100mph grenade?

I can water cool it if I want to but that adds another degree of complexity and stuffing around.  If we get the record, the next thing to do will be to streamline it and go with a cooling tank (no radiator to slow me down) and then run it in the special build class.

The record for that class is significantly higher (off the top of my head I think it's about 170mph),   

[Lozza]

Hmm give me a ring one day or PM your number. Nitro will mean wrecked pistons as you will be WOT for 2-3 miles

[ChrisACT]

Awwww, but I only wanna use a widdle bit. :'(

[Lozza]

That's what every HP junkie tells me in the beginning 

[ChrisACT]

The plan is to do many runs on straight methanol before we start playing with nitro ... and even then, only if we can't get it to 100mph.

We should have some good data by then of EGT, CHT etc which will hopefully give us some idea of whether or not it'll take any more heat for that duration.

The track we'll do most of our work on is only 3 miles long so the most it'll be WOT for is 2 - 2.5 miles including the run up.  Probably only on the redline for less than half a mile.  There's no timed area on the short track so it's GPS speed only.  Once we get it singing along, we move to the 9 mile track and run it there for a timed run.  The timed area is only 400 metres long.  So the trick is to peak at the start of the timed section and run it flat out for that 400 metres then shut it down.  The data we get on the short track should give us a good idea of when to coast and when to run it hard so I'm not just holding it wide open for miles.

Rear drum brake only plus the surface of the salt itself means stopping quickly isn't an option which is only an issue on the short track.

EDIT: After reading the 2013 rules, it seems I won't be on the long track anyway.  I'll be doing record runs on the short track where I'll be timed over 400m leading up to the mile 3 marker.  Then I have to turn off before the mile 4 marker.

[ChrisACT]

After a bit more simulation work, I agree with you Lozza.  There won't be any need for nitro.  I can get 35HP @11,000RPM on methanol without too much trouble.  Will run cooler and there's less chance of making mistakes with the mix.  No need to muck about with Hygrometers at the salt then either.

Another mod I'm looking into is a V-Force reed block.  Apparently the '93 - '01 YZ85 reed block uses the same bolt mounting pattern as the DT100.  The V-Force item for that model should almost bolt straight in (might need a spacer and maybe a little die grinding - a little more crankcase volume might be a good thing anyway).  Simulation says another 1HP across most of the curve.  Every little bit helps.

[ChrisACT]

Lozza, what EGT do you reckon I should be shooting for on this baby?

[Lozza]

I don't think you will see 35hp from a 100 you would need to spin to 14-15000 rpm (which is where I would be tuning it for). Would use a CHT and EGT, if the CHT stayed stable at say around 100deg C might be able to get away with 1150-1200F.

[ChrisACT]

I know guys who were allegedly getting 32HP on e85 in Superlights (Honda H100 engine).  I'd believe it too.  They were blowing everyone else into the weeds.  They were spinning it to 14,000 RPM though and regularly eating small end bearings.  The H100 has a longer stroke and smaller bore than the DT100.

I have EGT, CHT and EGO for it.  Running a Mychron 4 2T data logger and dash with all the fruit.

I'm also looking into whether or not I can graft a Yamaya KT100J head onto it.  The bolt pattern is almost identical (haven't measured one yet) to the DT100 head and it has more cooling fins.  And being a dedicated race head, presumably it's up for a bit more torture than the RT100 head I have on it.

[OverTheHill]

would a KT100 cylinder work too [or be allowed], should be same 52x46.[46.5] used a piston in my bucket once when de-stroking & over boring my TF100 [another cock up of mine but did work ok for a bit, used an Rd200 crank [one side of]. KT100 is piston port of course but [have 'kart sport manawatu' down our road] the modified ones fair scream. Was a bit slow off the mark other day & missed what it was, but something down there sounded "off the planet" for revs & bulk power--possibly a rotary valve 125 Italian motor i had to chase the noise of once. Sorry, that was a lot about nothing wasn't it. ps, KT100 pistons go in very small increments ie 52.25--52.30 etc etc. long ago but think that's right, plus very thin single ring.