Even the TS400 suffered later on. The later bikes with the center exhaust port were hobbled just like the DT.

The port maps above are very interesting. I do find the DT360 specs odd. All of mine measure about 21mm on the secondary transfer port. Same as the MX360. 11mm would be so small as to be completely useless. That must be a typo.

This is a stock DT360:


This is the MX360:

I do agree though that transfer port area is part of the issue. That is the inherent flaw with large displacement singles. You can never fit enough transfer port area in to keep up with the displacement at high RPM. An RD350 with two 175cc cylinders has much more cylinder wall available for transfer and exhaust ports.

At low revs though, the advantage is diminished. The two smaller cylinders are easier to scavenge and more efficient but the big single still has enough port area to get the job done up to a certain RPM. I'll have to post the RT2-MX dyno I have on my phone, that big single put down over 30HP at the sprocket from 6500-8500 RPM. Just about 20ft-lbs of torque from 5500-8500. That will never keep up with a tuned RD350 running 12000+ RPM but it is certainly respectable.

Just checked mine since it's sitting right here beside me. same thing, about 21 mm

I have a 1973 (side port) and 1974 (center port) TS400. Both engines are easy starting, fantastically smooth, torque monsters without any pinging or surging during cruise or anywhere else. Odd because they are lowly "piston ports". The exhaust port outlet was moved to the center for the 1974 frame changes but I don't think power changed at all. Certainly it wasn't "hobbled" compared to the earlier side outlet port model. It's possible that the last TS400 models (77?) were crippled due to USA emissions requirements that killed off the two strokes starting with the big bores.

What?!? A typo in Yamaha specs?!?!?


(For those that don't get the sarcasm, unfortunately there are quite a few typos in Yamaha specs)

One of the trails I've been down was thinking the pinging was an octane/ethanol blend issue Akara1. My DT360 has a modified combustion chamber with a narrower and tighter (to the piston) squish band to work better with 91 octane.

In general, the engine idles better and is a bit smoother but didn't correct the low speed pinging issue as I'd hoped. Even with high octane fuel.

Pics of the stock and modified chambers are attached.

So the feedback from MarkT about his TS400s indicates that there is hope for the big bore two strokes.

I wanted to clarify one thing... on the TS400 the early exhaust outlet angled off to the side to clear the single frame downtube but the exhaust port itself was centered. I'm pretty sure the center outlet models on the twin-tube frame had the same exhaust port timing.

I also dug out my TS400 Owner's manuals. Found something interesting that somewhat supports a theory I've had and I know some share...

Believe it or not, vehicle manufacturers were known for... ummm... "exaggerating" performance specs in their published literature. Fuel economy and horsepower numbers were often stretched to the limits of believe-ability. That's why for cars you'll see a massive drop in HP ratings around 1970-71 because manufacturers had to use a specific SAE net HP test procedure instead of their own "gross" HP rating test procedures that varied from manufacturer to manufacturer.

Some manufacturers were known to "cheat" more than others but the worst part was a consumer could not reliably compare the specs between brands or sometimes even models.

Similar laws were enacted for motorcycles but I have never found the exact time SAE net HP ratings became required.

Interestingly, the 1972 and 1973 TS400 (side outlet) owner's manuals rate the engine at 34 HP @ 6,000 RPM.

The 1974 (center outlet) owner's manual rates the engine at 33 HP @ 6,000 RPM (SAE net). One HP less but the important part is the "SAE net" which indicates a specific, regulated, and most importantly, repeatable test method was used.

Yamaha seemed to avoid rating the HP in their later manuals and I can't recall seeing a Yamaha "SAE net" HP rating for any of the Enduro models?

Anyway, I think this is a big reason for early bikes possibly having a lot higher "advertised" HP than later ones.

I agree that must be a typo!!!

I find the comparison with the ts400 to be particularly interesting. my first impressions when I compare the two engines are the TS has a much bigger intake port and a smaller crankcase volume.

I took a port rubbing of my late center port ts400
exhaust Port is a similar width as well as the main and secondary transfers, Port timing is similar to dt360 at around 170° exhaust 115° transfers (ts400is 75mm stroke) interesting with such similar Port size and timing how different it runs.

also an interesting combustion chamber shape on my Suzuki, in a past life someone had welded it up into this shape...it seems to perform well.

I work with a guy that bought a 71 TS400 brand new. He says it did 0-60-0 in a block. I don't believe it either but who knows. Suzuki made a few changes over the production run to settle the 400's down. Later bikes got a different crank with more inertia and revised porting. I haven't measured the differences but they did use a bridged intake port on the later TS. Crank inertia won't show up on the dyno but it does have a significant effect on acceleration.

This is from a 1994 issue of Dirt Bike magazine:



All of 31 HP at the wheel from the "widow maker" 1975 TM400.

Piston port isn't too bad. Fixed, symmetrical intake timing isn't as flexible as a reed valve but it does eliminate any restriction and pressure drop in the intake path. Reeds also add volume in the intake tract and crankcase, which is part of the problem with the DT360. Yamaha seems to have left the reeds in the same spot in the casting relative to the bore center that they did on the SC500 despite the 360 having a much smaller bore. This leaves a lot of dead space in the intake port to further weaken the intake signal. I would rather have a piston port than a poorly designed reed valve intake.

When I was looking in my Suzuki Owner's Manuals I noticed something I never really paid attention to...

In about 1973 it appears they needed to include stopping and acceleration charts in the owner's manual.

It also appears that the standards were the same and I assume they also had a standardized government test method. This should be a very "apples to apples" way to compare performance from model to model.

Anyway, RT2 and RT3 should be the same but requirement didn't start until 1973.



1974 DT360



1975 DT400b and 1976 DT400c should be pretty much the same

You can read a detail of the test method in any of the manuals... the high speed pass has a limit of 80 mph and is going to be the best indicator of real-life performance.

Results show that the 74 was a great performer and the 75 and 76 even slightly better. This agrees with my experience with these bikes... they didn't lose a ton of power in 74. Performance was very equal through the years.