TY175 - DT175 port timing and head differences

[David Lahey]

Theres been quite a bit of discussion over the years about how to make a TY175 motor better for trials and a few people have discussed using DT175 components to achieve an improvement.
I started to get interested in this topic when I bought a very derelict TY175 in which someone had fitted a late model DT175 motor ie about 1981, with the radial fin head, and most importantly, with the barrel type that has the bolt pattern that does not match the TY175 crankcase bolt pattern. I think this bolt pattern came out on 1977 IT175 and YZ125 (D models) and in 1978 on the first monoshock DT175 and DT125 (E model).
Modifications to the DT175 motor in the derelict TY175 included the ignition and flywheel from a TY175. It still has the (26mm?) standard DT175 carby. The exhaust is fully TY175 as is the airbox. It still ran so and it was not much trouble to get the bike ridable. The motor responded very well for trials use at low and medium RPM (just like a standard TY175), but when the RPM went up above mid range, there was quite a surge of performance that was definitely different to a standard TY175. I'm not sure it would be such a good thing for competition trials use for the motor to respond like that, but it would be nice if the bike was used for trail riding or on the road.
Anyway, this experience aroused my interest in what was different inside between the TY175 motor and the DT175 motor ie the port timing, port size and compression ratio.
Did Yamaha simply fit additional flywheel mass and a smaller carby to a DT motor back in 1975? We know that the TY175 was developed on the quiet by Mick Andrews and his Japanese bike development team without senior management approval, and when management were presented with the prototype, they were so impressed, the concept went ahead and it became the highest selling trials bike ever made.
The next stage in this little quest has just started because a DT175 motor of the same vintage as the TY175 has arrived at my place and will soon be getting pulled apart for examination and comparison with a TY175 motor.
I'll post again on this when I've done some comparisons.

[Tee-Why]

Hi Dave, this is basically a line of thought that I never actioned myself, as I started to look at the DT175C (1976 Twinshock model) that I also have.
I havent got round to pulling the DT down to compare, but this I have observed, the reed block opening and reed cage is bigger, and excepts the RD350 rubber boot (complete with equalizer tube insert that would suit boost bottle. This manifold sits a little towards the right further and gets the carby very close the the TY exhaust.

I too am interested in the port differences and was going to port map the TY and DT and try and back engineer the best fit for trials, basically a better mid range response, and not sacrifice the bottom end too much more.

But I did experiment with the DT carb to great effect on the TY, the 24mm compared to the 22mm had a huge difference to the response and overall power, and when revved right out in the open it actually breaks into a small power band in the upper revs! The low end jetting needs tidying to tune it better from idle to loading with a smaller idle jet.

This carb and slightly lightened flywheel had this motor more aggressive when required which is way better than stock.
My only fear is the DT barrel (that will just bolt on as its 4 stud) will push the usable power too far up the rev range. Now the Vintage MX boys dont rate the early DT for a converted MX'er much, but might just fit better for trials if tamed.
Compare this to the 200cc kits for the TY, and read that the power is something like this anyway, the DT may just be closer in the ballpark than we think.

I think this has possibilities Dave and should be followed up, but one thing I did do was measure the overall length of the TY and DT exhausts, as this affects the port timing and found the DT was heaps longer and means the rev range is higher. So a de-tuned DT-rised engine might still be possible. All I need then is a Mini Maj frame for the right price to put it to full effect!

But to put perspective in all this, for what gain is achieved, if I rebore a DT barrel and is no good for trials, I end up with a start of a DT restoration that I am not really ready for as it was way down the list. And basically if I get a sweet trials bike out of this, I might actually get to ride against 3 more twinshocks in a trial if I am lucky on a good day, the interest and support in WA is severely lacking and so has mine at this point in time.

Cheers,
Roger

[TYYamFan]

Hey Roger,

My name is Mark and I live in Perth. Some years ago I bought a TY175 from Steve Scraggs and a wreck from Steve Cox. It's sad to hear that there is poor support for twinshock in WA.The first of my 2 projects is nearing completion and my entheusiasm levels are definetly high, I'll go mad if I don't get to ride again soon. Back to the point, the first bike will be as standard as I can make it, the second I'd love to trick up a bit within reason, so I'm interested in any suggestions. The Mini Majesty frame is just too expensive for my liking and I'm told that the shorter wheelbase makes it a bit of a liability for anyone over 5'6"! I'm thinking flywheel lightening, electronic ignition and porting of some kind. Does anyone know what Shirty used to do to them? Incidentally, David has been an invaluable help over the years and is a walking TY database, Thanks David!

Regards Mark

[Tee-Why]

Hi Mark, ah good to hear someone has some passion!
I sort of know of the bikes you have, there are a few more about too that dont get brought out, pity the riders will choose a modern over a Twinshock in a twinshock handicapped event .???

I wanted to go the same way, a stocker and a modified, and I have a theory that would require mods to swingarm and mounts, but requires some welding and an alloy swingarm.
In fact my mods are all Yamaha sourced era parts too! (look outside the box)

I have the John Shirt mods listed somewhere, they dont seem to be too radical, but heaps for a trials bike, might make it a bit peakier. The electronic ignition, there is a fair bit discussed in Trials Central about this. Alternative is the BJ racing one (USA) if the exchange rate is more favourable. Its UK sourced anyway, not sure who's it is?

BTW, the compression is higher in the DT as well, havent had a chance to compare pistons, but believe its more to do with the head anyway.

Keep your eyes peeled for a dedicated Twinshock event (and non stop rules too I think) in August run by the Vintage Motorcross Club of WA, I hope to be at this, might see you there.

Cheers,
Roger

[David Lahey]

Well I finally had a look today with a TY175 barrel and head and a DT175 of the same era barrel and head side by side.

Sad to report that the port timing and shape on both is exactly the same
Also sad to report that the inside shape and volume of the head is also exactly the same

There are differences of course. Both the head and barrell are made from different castings to the TY, with different finning and there is an oil injection port on the DT barrell. The DT head has an extra sparkplug hole and the head finning is cut away to fit the DT exhaust in a different pattern to the TY finning cut-away. The DT head also has lots of square holes through the horizontal fin much like a TY250 head. The DT barrell finning is the same shape all the way to the bottom.

The only performance differences I could see are that:
The DT motor ignition has a timing advance mechanism that rotates the points cam relative to the flywheel.
The DT flywheel does not have the additional steel disc fitted to the flywheel face like the TY.
The DT has a thinner head gasket than a standard TY gasket.

David

[Tee-Why]

The only performance differences I could see are that:
The DT motor ignition has a timing advance mechanism that rotates the points cam relative to the flywheel.
The DT flywheel does not have the additional steel disc fitted to the flywheel face like the TY.
The DT has a thinner head gasket than a standard TY gasket.

David

Hi David,
Very interesting your notes, and I am also disappointed. I was hoping for a bit more internal differences.
I forgot about the advance cam bit, but I know the size of DT flywheel wont go on a TY as the stator size and shaft taper are different. So a crank swap would be needed, but the DT flywheel is so small without the mentioned external weighted ring. You would get a better ignition advance from an electronic unit probably fitted to a TY.

So the DT power must be worked from the pipe design and longer pipe, and it has slightly higher compression, must be the piston shape? Surely the thinner head gasket wouldnt be that effective. Also the bigger carb and bigger reed block contribute.

Cheers,
Roger

[David Lahey]

Roger subanator
I've posted some photos on the OZVMX forum of the TY175 with the late model DT motor in case you are interested. Someone on that forum was interested in making their TY175 better for road use.

http://ozvmx.com/community/index.php?to ... n#msg60597

Regards
David

[Tee-Why]

The DT has a thinner head gasket than a standard TY gasket.
David

So the DT power must be worked from the pipe design and longer pipe, and it has slightly higher compression, must be the piston shape? Surely the thinner head gasket wouldnt be that effective.
Cheers,
Roger

To self answer to the above, I have found out that both the TY and DT share the same part No. for the piston, so ruling that out, it also could be the head shape.

Thanks David, very interesting for the links and pics. I note the radial fin head is that of a non twinshock model barrel and head - the 5 stud one. The relevant year model DT175 to the TY175 had the same 4 stud head/barrel but a more heavier finned version, including a cutaway in the head finning for the exhaust pipe. (I can post a pic if anyone interested)
So how is the gearing with the DT motor?

Cheers,
Roger

[David Lahey]

Yes Roger. That DT motor is a late model and I think is a DT175F. The motor I was referring to in earlier posts where I had compared port timing with the TY175 was out of a DT175A.
I was surprised that the gearing was fine with the DT175F motor in that both 1st and 2nd could be used just like on a TY175. I imagine that the DT would be slower in top gear but I only rode it for one lap of a trial and didn't do any high speed stuff. It certainly shifts gears more nicely than a TY175. The late model DT175 shifting system doesn't have that direction-reversing linkage like the Ty175 so that may be a reason for the nice shifting.
Apart from the power surge at highish RPM, the late model DT175 motor would be a suitable replacement if a TY175 motor had been damaged terminally, and there are probably plenty of those late model DT175 motors around. I wouldn't like to use one in trials without the TY175 igniton system though.

[hot101rod]

You say the DT175 has a longer pipe over the TY175, does the shorter head pipe create more toque than a longer head pipe? You can now buy both "standard stainless" and a "longer" pipe? I'm confused? Normal state of mind.
Rod