Engine

Overall analysis

To recap:

1) I knew that GHN racing in Germany had already done this, and got 155 HP. So...

2) A TZ250 makes at least as much hp/cc as an RG500 all through the rev range, and at high RPMs, it makes much, much more. 75 hp is not uncommon for a stock 1994 TZ250. Scale that up by two, and you have a tractable 500cc engine that still makes 150 net Hp on top. Why not swap on a quartet of TZ250 barrels and have the RG500 from hell? Could we make the engine behave as though it were a pair of TZ250s?

Darcy has a complete grasp of what’s happening inside a 2 stroke, and was able to visualize all the potential hurdles before they happened, which is in stark contrast to my usual approach, which involves staring helplessly at two parts that will never fit together, as the project reaches the point where those two parts really need to be installed.

This swap was Darcy’s baby. What’s impressive was, he did all the work remotely, (in Canada) and yet when I assembled it all, it ran, and ran reliably, right off the bat.

Cylinders

I use 1994 “4DP” Yamaha TZ250 barrels.

The TZ barrels are shorter than RG barrels, and are aluminum (of course) with plated bores. TZs also use O-ringed heads (Yahoo!!) and attach to the case via 4 studs at the cylinder base. This is in pleasant contrast with the OEM RG setup, which attaches the cylinders via long allen bolts submerged in coolant, inside the coolant passages.

TZ has larger, higher (damn) transfer ports than the RG, as well as cavernous transfer ducts and a large, triple exhaust configuration with a main port flanked by two subexhausts. The main exhaust port is affected by a guillotine-type powervalve, which lowers the height of the main port by approximately 10mm when in the lower RPM position. The two small subexhausts are not affected by the powervalve. The TZ is a case-reed motor, so the cylinders would work OK with the RG’s disc-inlet system. The TZ has enough exhaust area to support 160 net HP in a 4 cylinder setup. That ought to do the job.

Porting, port timings

From what I have measured, here are the TZ250 port timings, and time areas at 11000 rpm, compared with my ported RG500 barrels.

TZ: Exhaust opens: 24.5mm ATDC with valve up, 32.4mm ATDC with valve down

Subexhausts open 26.5mm ATDC

Transfers open: 38.6mm ATDC

Boost ports: 38.5mm ATDC, all transfer/ boost are level

RG: Exhaust opens: 25.8mm ATDC

Transfers open : Tmain.. 39.7mm ATDC, Tmid... 40.2mm ATDC, Tsmall.. 41.6mm ATDC

Boost port opens at 40mm ATDC

Here’s a tracing made from inside my RG barrels: RG_ports.jpg

Here’s a port timing diagram for the TZ barrels: delta port timings.jpg

Fitting the barrels onto my bike

Problems to overcome

a) The TZ’s stud-mounting system required that an adapter plate be used.

b) The use of an adapter plate meant we would need longer-than-stock rods. RG has 100 mm rods. TZ uses 110mm (depending on year) . But, TZ rods have a too-large main journal diameter to go onto the RG500 crank. We needed a 110mm rod that fit an RG bottom end, and a TZ top end.

c) TZ barrels are too wide to fit on the gamma as-is, not to mention the coolant comes up on the opposite side on the RG, as opposed to TZ.

d) A TZ has coolant that comes up under the exhaust port, the RG’s coolant comes up next to the boost port.

Solutions:

a) Darcy devised adapter plates that would attach to the RG case using the RG fasteners and gaskets, and connect to the TZ barrels on top. I had to cut custom gaskets by hand for the TZ cylinder base side.

b) Switching to an RZ350 rod was just the ticket. It was heavy, but hopefully, sturdy enough. It fit the RG crankpin on the bottom, and 94 TZ pistons on top. Perfect. There ARE TZ rods which will do the job, but they are off of some old model from way back, cost a small fortune, and are nearly unobtainable. Hardly the route I wanted to take, at least as far as parts supply was concerned.

c1) Darcy shaved the facing sides of the barrels, to move them as close together as possible. The transfer port outer wall is fairly thin, but not dangerously so. When mounted, the thin walls butt up against each other securely and cannot be damaged by outside objects.

c2) Darcy rebuilt the crankshafts and converted them to a TZ-style, with locating thrust washers on the big end. He offset the rods slightly from their OEM positions to buy us a few more mm of room for the barrels.

d) Darcy x-rayed the barrels and drilled small passages to allow some coolant to flow directly up from the case, to the transfer side. The rest of the coolant was handled by cross-drilling the adapter plates, to route coolant around the perimeter, and through the center divider, over to the exhaust port side. That way, the TZ barrels would receive coolant in the pattern (mostly) that yamaha had intended.

The work mentioned thus far would allow the barrels to be bolted onto RG cases, using TZ pistons.

Powervalves

It was a bit of a chore connecting all the powervalves. The RG uses actuators on 2 cylinders, and they simply connect to the adjacent cylinder like a driveshaft. The TZ has individual powervalves with no provision for siamesing them. I determined that the RG motor turns far enough to actuate TZ valves. At first I considered a 4 way cable splitter but it was too hideous to contemplate. I tried a few setups, but eventually, I made little driveshafts that connect the powervalve shafts, like the RG’s OEM system. The shafts mate like a flat screwdriver blade and a screw, it’s a bit sloppy and requires careful initial setup, but has worked reliably so far. I’d like to switch to a hex-ball drive, like on an allen wrench. That would allow for misalignment.

A BDK valve controller allowed me to relocate the actuating RPM to something useful, which is now around 9000 RPM, I believe.

Adapters

Adapter plates are aluminum slabs, about 15 mm thick, that mount to the RG case below and TZ barrels above. Darcy machined the first one by hand, after that the plan was to get that CMM’d , and then CNC cut two more. Steve Gyimesi offered to help with the CNC machining, and when all was said and done, I had two complete sets of Adapters, plus two early prototypes that needed a bit of fixing. The coolant passages are cross-drilled and the holes are capped with brass plugs. It’s really a pretty complex piece of work. I mounted the adapters to my cases and had to finish the machining by hand, with a grinder, but they are now a perfect match to my engine. Piston deck was set at zero.

One intended side effect of the adapter plates was to increase the volume of the crankcase. An RG has a case compression ratio of about 1.33:1. The Delta has a case CR of 1.288:1, which is right in line with a 1994 TZ250. This should allow the TZ-style pipes (very fat) to draw mixture from the large case easily.

One unintended side effect of this increase in case volume was a noticeable reduction in metering signal strength at the carb. Below 3500 rpm, my bike had such a poor signal that I could not even blip the gas, much less pull out from a stop sign. it would idle, but I had to thumb the choke just to nurse it above 3500 rpm. My old RG setup used a 6.0 slide and ran just fine, now I was backpedalling to a 3.0 (much richer cutaway) to get any kind of response off idle. I finally got to where I could blip the gas and get sharp response, with the 3.0 cutaway. It’s still pretty fluffy pulling out, but once over 3500 suddenly the carburetion picks up and the bike runs well. Unfortunately, the super-rich carburetion that works well below 3500 RPM is way too rich for the engine when it’s running at 4500 rpm. This has presented a challenge that I have had problems beating, with the adjustments available on my TMX carbs. This year, I’m installing adjustable main air bleeds and possibly a few other tricks, and hope to get the bike more driveable. Even I don’t want to struggle with a bike that’s crappy away from every stop light. On the other hand, since I change pipes and heads as often as most guys change spark plugs, it makes it difficult to pin down a precise carburetion soultion. Comes with the territory, I guess.

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