4T Camshaft: Ongoing Evaluation

[az_slynch]

While the aftermarket has provided we 4T fans with options for upgrading the intake, exhaust and cylinder, it's bugged me that a critical component of the formula has been overlooked: the camshaft. A while back, I suggested that maybe we could somehow fit a 4V head on. While it sounded great on paper, another forum member PM'ed me a thoughtful evaluation of some the challenges of dealing with the nuances of the 4T top end. At the time, I didn't have a 4T or a good answer for them.

Now, the former has been addressed and the latter needs to be looked at.

To begin this project, I bought a new 4T cam from Scooterworks. I do have one in my motor, but I wanted to have a base cam to modify without having to keep the bike off the road longer than necessary. I have taken some preliminary measurements with a caliper, now I need to get a degree wheel and a dial indicator to get the rest of the important numbers. From my initial observations, the cam on this engine is quite mild:

The base circle of the cam lobes measured .984", or 25mm.
Lift on the intake and exhaust lobes measured .176", or about 4.5mm.
Lobe separation angle on this cam is on the tight side; I'd estimate 106° or 107°.

We're still missing critical information, like duration for each lobe. So what can we say about the cam from the information we have?

Starting from the last piece of information, the lobe separation angle, we can infer that the cam works best for making low end power. Narrow separation angles generally work best at lower engine speeds and wider separation angles (> 110°) work better at higher engine speeds. Considering that these engines redline around 6000rpm and would likely fly apart at higher RPM, the narrower separation angle is quite suitable for this application.

Looking at the lift of the cam lobes, they seemed a bit pedestrian. In looking for comparable camshaft profiles, i found a lot of similarity in the small horizontal Honda (e.g. CT, CRF) camshafts. The closest analog I've found so far was for a CRF110, which has less lift (.170" intake, .152" exhaust), but is otherwise strikingly similar in design. The CRF110 engine has a 55.6mm stroke length which isn't too far off from the Stella's 57.8mm stroke length, so the timing of the cam may even be in the ballpark. Looking at performance cams for that motor, I noted lifts of .188" (or 4.8mm) and durations increased by about 15° to 230°. Not radical changes in terms of dimensions, but when pumping fuel and air at higher engine speeds, it adds up.

I ordered a "race" cam for a CRF110 this evening so I can get more measurements and make a better apples-to-apples comparison. I don't believe the CRF110 "race" cam will be a bolt-in modification. I do believe it may provide a good base model for regrinding a Stella cam for better performance, but I'll have to wait a week or so to see if I'm on the right track.

Sorry about not unveiling a 4V solution yet, but I figure we need to learn how to jog before we learn how to run.

P.S. Before anyone asks about valve springs, Scooterworks doesn't sell springs separately. You have to buy a whole head. I intend to pull and measure mine, as soon as I get ahold of a spring tester.

[BigDaddy SnakeOiler]

Thanks for these write-ups. Especially here in America, the 4T motor is quite the red headed stepchild.

[az_slynch]

Received my CRF110 "race" cam. As expected, it's nowhere near a drop-in. However, the base circle on the cam lobes is 25.5mm, so the lobe profiles are close to working in the 4T Stella (25mm base circle). It's also designed for a roller rocker motor like the Stella, so I believe it can still be a decent model to base a Stella cam on. On the downside, the lobe separation angle is wider and probably suitable for a higher-revving engine.

I'm thinking I need to look at the stock cam and head again to determine clearances at full valve lift. I really also need to figure out the spring height at full compression to determine if there's a potential for coil bind with a higher-lift cam.

EDIT: I forgot that the CRF cam intake and exhaust lobes list as having 230° duration, so the base circle measurement is skewed by some lift. Can't measure 180° of base circle when there's only 130° of cam rotation where some lift isn't happening, so a caliper isn't going to give a good measurement. D'oh!

[slotrod65]

Thanks again, I look forward to hearing more.

[az_slynch]

Thanks for the props guys. I really do like the 4T and I agree that it is under-appreciated. My long goal is to work up a build spec so they can tour with their 2T bretheren without being left in the dust.

I've attached two images, so please log in to view them.

The first image is a top-down comparison of a stock 4T cam and a TBoltUSA "race" cam for a CRF110. The CRF cam is shorter and the shaft diameter is reduced at the far end, likely to reduce rotational mass. The Stella cam comes with factory patina. Lobe width and spacing between the lobes looks pretty darn comparable. I've also learned that the MSX125 "Grom" has a similar cam to the CRF110 and might deserve a look, as it runs roller rockers too.

The second image shows the lobe profiles of each cam. The CRF cam has a much beefier lobe profile, but also has a wider lobe separation angle and a lot more overlap. Bit too much sauce for the Stella, with our heavy clutches and flywheels...probably the crank bearings too!

Another item of note is that the lobe profiles of the two cams appear to "mirror" one another. This is because the cam on the CRF rotates in the same direction as the crankshaft, while the Stella cam rotates in the opposite direction of the crankshaft. The reverse cam rotation is specific to the Stella, as the cam is not driven directly by the crank, but driven by a gear fitted about where the autolube pump gear would go on a 2T. I believe this is specific to the 125/150cc engines, and that the 200cc engines drive the cam in the same direction as the crank. This design element is a big barrier to simple dropping a cam in; I believe we'll have to develop our own cam grind to overcome it.

I marked the intake and exhaust lobes on the Stella cam. I'll need to look at a CRF manual to determine if lobe positions are swapped as well.

Made some V-blocks for the cams and ordered a dial indicator set. I might need to get cam sprockets so I can figure out the TDC positions for each cam too. Didn't get to the shop today, but I'll try to get by this week and figure out the valve and spring measurements, as well as the rocker lift ratio.

[Neurotic-Hapi-Snak]

I wish I had some of my tools and measuring devices with me, I'm currently living in Japan. I have a Super Cub 110 and I just installed a 125cc piston/cylinder and a 'sport' cam. I wish I could take some measurements for you of the stock cam, stock at 109cc it put out the same HP as the Stella 4t and almost as much torque (at 7500 and 5500 rpm, respectively), and got about 50% better mileage, 63.5km/L, about 150mpg. Could help that it's FI, though, oh and a Honda. The head was pretty darn close to the Stella head, but then again, in the small motorcycle game, everyone copies Honda.

[Tipper]

This is a very interesting post,any updates ?

[az_slynch]

This is a very interesting post, any updates ?

Tipper, the head is on my workbench. I picked it up from the shop to get measurements and a bearing spec for the far end of the camshaft. I did get a valve spring compressor set so I can dismantle them, but time hasn't been on my side to actually do the work.

I intend to measure the assembled valves in situ and then the disassembled parts. Looking for someone with a local valve spring compressor so I can ascertain how far we can actuate the valve without going onto coil bind or hammering the valve.

I did note that the 4T does have ratio rocker arms. Without measurement, I can't speak to the ratio yet. This will play a part in the lift calculations, that much I know. The rocker rollers appear to be copped from small bore 2T top-ends; I'm certain that with measurement, a better bearing can be sourced.

I hope to have time to address this soon, as the annual rally comes around in October-November and my engine's still in pieces!

[az_slynch]

Took some time to tackle the cam removal tonight. Used a grade 10.9 M6x50 bolt, a thick M6 fender washer, my Tusk crankshaft installer and some steel to support the puller against the cam cover sealing face. Took it nice and easy and the cam came out with minimal fuss. I used a 15mm mandrel on the blind hole puller to extricate the far-end cam bearing; that one was a bit fiddly to push the mandrel through and required a bit of compression with a long thin flathead screwdriver to get it guided into the bearing. Once fitted, the bearing came right out.

For those not enamored with quality and tolerances of Indian bearings, the driven end of the cam is fitted with a 6202 bearing and the far end bearing is a 6002. Though the cam has never been out and the puller used to extract it was operated in a deliberately delicate manner, the 6202 bearing was quite "lumpy" and sloppy. This was a bit surprising, as the bearing was nearly a sliding fit into the head; the snap ring and the snug fit that the cam has into the 6002 bearing are what kept it in place. I'm wondering if this bum bearing was the source of my "ticking valve" noise...

I did measure the rocker bearings. Their dimensions are 10mm x 13mm x14mm, which is commonly found in Piaggio Bravo and Ciao mopeds. This likely means that better needle bearings are available.

I also measured the rocker arms. The dimension from the rocker shaft center to the roller center is 18mm and the dimension of the rocker shaft center to the center of the adjuster is 25mm. I expect that the rocker ratio would be described as 1.4:1 for this assembly. This means that our stock 4.5mm of cam lobe lift could yield 6.3mm of valve actuation.

If time permits tomorrow, I'll get valve assembly measurements.

[Neurotic-Hapi-Snak]

Az_slynch, have you messed with the ports yet? I really think before you mess with more aggressive cam profiles and larger valves, the ports need to be addressed, otherwise you won't see a drastic gain, in my opinion. In my limited knowledge, just what I could learn from the internet, the ports on the Stella 4T are atrocious. There are diagrams online of what you want your ports to look like and what you don't want them to look like, and the Stella's almost matched what you don't want them to look like. I tried to improve them as much as I could with what little material was available to work with, I'm thinking of buying a second head to experiment with building up material, most likely epoxy, to try and go farther. With some knowledge gained online and a few rules of thumb, it isn't hard to do a good, mild port job, anything more drastic would require more knowledge, experience, and tools, like a flow bench, than I have access to.

[az_slynch]

Az_slynch, have you messed with the ports yet?

Yep. I went and saw an OG mechinist who specializes in motorcycles and ATVs. He's semi-retired and has probably forgotten more than we will ever know. The ports have been reworked and the intake's been matched to the Polini manifold.

The cam profile isn't that agressive. I'm just looking to compliment the 22mm carb and the GPR pipe.

Really should finish that engine soon. Been fiddling with my K-Pipe and rebuilding a CL125S recently, working on a friend's Zuma and another friend's Helix this weekend.

[az_slynch]

So yeah, the valves on the 4T are comparably sized to a Honda CRF110. Can't use the CRF "big valve" kits, since the stems are roughly 10mm too long. Still working on this project and it's moving slowly, hopefully the end result will validate the effort.