Squish band on a road racer

I'm in the process of turning my ye-olde 1992 Yamaha WR250ZD into a Supermoto/Road racer and while I've got the engine on the bench I thought I'd contemplate my setup options. The motor has a Forward Motion 'more better' ported and bored 91 YZ250 cylinder, making it a 265cc machine. I sent the two ugly head specimens I had in with the cylinder as part of the service is reworking one and I figured I'd let him decide which was the better salvage case. He sent back both cleaned up and cut, one set 'for pump gas' and one tagged as 'high compression'. It looks like both started with the same squish band and combustion dome, on the pump gas head he made a third cut in the middle, narrowing the squish band and increasing the volume of the head. I've run the bike as a trail machine with the pump gas head, no complaints. Now that I'm throwing the bike into an environment where MOAR POWAH is a good thing, I'm curious if I should run the other head in combo with higher octane fuel? One friend's initial reaction is that the squish band is too wide and will lead to detonation, I know nothing on two strokes hence my curiosity.

First pic is the two heads, pump gas on the left. Second is a clearance test with the race gas head, third same test with the pump gas unit. .050" clearance on both at the closest point.

 

Other head with race fuel. You should not have detonation. Sunoco 110 works great in my 2-strokes and is cheap in comparison to VP. It is also very stable and won't corrode anything.

 

Any negative impacts from using the high comp head? Loss of overrev is bouncing around my noggin as something to look out for for some reason...

 

Too much compression can be a bad thing in a roadracer. I know on my RZ race motors I don't want any higher than 148 psi.

 

Since it sounds like you are new to road racing 2 strokes I will mention this. IMO you want to run much more oil than you do dirt biking. I would recommend somewhere between 20-24 to 1 ratio.

 

That much? I've normally run 40:1 despite my slow pace in the woods and H1R suggesting 50:1. I was figuring 32:1 to start on pavement, better to be safe than sorry though I suppose. Other than not melting the top end down, how do you gauge where you ultimately should be ratio wise, or am I wrong in thinking there is a 'too much' option to worry about once jetted to match?

I need to put a compression gauge on the bike as well, given how it kicks over I'd be floored if I'm below 200psi with the pump gas head. It'll support my full weight just past TDC if I don't jump on it. (Ok, so I'm 155lbs, not exactly a massive guy.)

 

If you're racing or riding this bike on the track with extended periods of WOT, yes, run a lower oil:fuel ratio than used for dirt. And use good oil designed for track use.

 

You want the squish to be as tight as possible, but you want the dome to not have too much compression.. On a 54x54 motor I shoot for 25 thousandths without having the compression ratio too high. The squish will combat detonation but you still can't have too much compression or like you said you'll lose your over rev.

it doesn't always work but take the stock motors compression and measure it. Then, close down the squish to as tight as possible given your crank intervals. Then cut the dome back so the compression ratio drops back to when the motor was stock. This gives you right squish without being too high comp. you can often run it with stockish ignition maps without an issue.

I run 100octane . More oil saves your crankshaft.

 

I never went below 30 thousands on squish. I ran 32:1 for 13 years of roadracing. Sunoco 110 with Motul 800. Great oil.

 

I used to run .030" and closer on a way overbuilt CRF70 but that was only dealing with a tiny rod and 41mm of stroke. This machine is a tad oversquare at 70 x 68.8, bit more stroke, good chunk heavier rod. The normal advice from the MX side of the house is shoot for .050", I've seen claims of one person running at .040".

Sounds like I need to talk to a two stroke shop and possibly get a head recut for the new application. RB Designs is popular amongst the MX crowd, RK Tek is popular amongst sled and desert guys, is there a go to for road racing two strokes?

 

Fuel - Seeing as it was mentioned, I've got the following legal fuel choices:

Sunoco Pump 93 (up to 10% ethanol)
Sunoco Supreme (109 M octane, leaded)
Sunoco 260 GT (95 M octane, unleaded, oxygenated and 10% ethanol)
Sunoco 260 GTX (95 M octane, unleaded, no ethanol)
VP C12 (108 M octane, leaded)
VP 100 (96 M octane, unleaded, oxygenated and 10% ethanol)

Personally I'm not a fan of ethanol, other than health effects, thoughts on leaded vs unleaded? (My RS125 friends are running C12 so I'm already breathing lead. Bah.)

 

On our roadrace KX65 and CR85 motors, I ran pump gas. When we went to a RS125, I used C12 or Sunoco Supreme. The Honda RS125 manual said you could run Av fuel, but I never tried it.

I lost a crank on the KX before we started running more oil.

 

FWIW, for the RS125 I've ran mostly C12 with some Sunoco 110, Motul 800 roadrace 2T oil at 20:1, with somewhere around .029" and (I think) a '97 leaded head ('98 and up was unleaded with different shaped dome). With the jetting figured out, everything has looked great with the past several years of teardowns.

Matt

 

I'm kinda leaning towards having 6 to 8 thou cut off my pump gas head, minor bump in compression, should end up just over .040" piston to head clearance so still safe. I need a dyno and a detonation counter so I can play...

 

Another nod to 22:1 oil ratio, I went through two cranks before I committed to the larger jet and more oil.

 

Your best bet would be to call Eric directly. He has done many 85/125/250 kart motors which is basically what you want in a RR application. He might suggest sending him another head that he can cut for the best results. Especially after he might suggest using an extra or thicker base gasket to alter the port timing better suited to RRing as his Mo-Power porting is better suited to MX/Offroad, especially if he epoxied the rear transfers. JMO

 

When I last asked him about this, he said I was going to scatter the transmission across the track... If I start with a modern YZ base, he's got a setup ready to roll that he's used in RR already.

 

FWIW, there was a Yamaha technical paper in the 60's that showed producing more power the richer you went with gasil all the way down to 17:1 or something like that. There was a Cycle or Cycle World test in the 70's that confirmed the results.

 

Feb 78 Cycle magazine, Gordon Jennings: http://www.bridgestonemotorcycle.com/documents/oilpremix6.pdf


Also addressed very well in A. Graham Bell's two stroke tuning book, from (pdf) pg 159: http://iheartstella.com/resources/manuals/tuning/Graham-Bell-Two-Stroke-Performance-Tuning.pdf

 

So, to add complications to this discussion: I'm running an APT SmartCarb, basically an evolution of a Lectron. In theory if I'm fattening up the oil ratio I need to up the fuel flow to match, on a normal carb that'd be a fatter main. On the SmartCarb I can raise the needle which in theory primarily impacts the mix at idle, I dunno if it will supply a proper amount of fuel at higher mix ratios at WFO. Guess I've got some more research to do.