acdii

Yep, already did. I can use both sizes, and had considered ordering both, but didn't want to spend that much at once, so now I get to pay twice as much for shipping.

AWW CRAP. I just now remembered I had a 5% off code.

Hobbsy

My Feedback: (102)

The left cam timing is different.




They are very different from each other.

Glowgeek

Wow, that left cam is an eye opener for me!

With those lobes having virtually no overlap how can the left cylinder run with the same efficiency as the right cylinder? I'm stumped, never seen that on a multicylinder engine.

Glowgeek

Got the fa180 all cleaned up and ready for the new bearings. This one, like a few others I've seen, has a collapsed exhaust valve spring. Obviously another case of the engine being stored long term on the exhaust stroke.

If you're new to four stroke model engines I should mention, always rotate the engine over until you feel compression before storing your engine for any length of time. The beginning of the compression stroke is also a great position for for a 2 or 4 stroke engine to be in while mounting your 2 blade prop horizontal. With the prop mounted this way if the engine quits in the air the prop will be horizontal on your dead stick landing and have less chance of striking the ground on a rougher than normal landing.

Hyjinx

I was wondering what the cams would look like in these opposed cylinder designs. Figured the valve timing would be tricky.
Just found a good old thread in this engine. Looks like a lot of help.

Hobbsy

My Feedback: (102)

The action with the engine running is far more fatiguing than sitting compressed. Think of the coil spring as a torsion bar wrapped up for our convenience.

I will check out my 1.30 and see if has overlap on the left.

Glowgeek

I won't argue that running the engine promotes valve spring fatigue. Sure it does, especially if the engine is constantly over revved (harmonics) and/or run very hot (annealling), but so does leaving a valve fully open for years during storage.

You don't leave your guns cocked for the same reason. I've replaced more than a few firing pin springs in semi-autos that were left unloaded and cocked in someones closet for years on end.

Hobbsy

My Feedback: (102)

That's the reason I rarely use a prop with less than 6" of pitch, those popsickle stick props have no place at my house. On my little 30's I alternate between 10 x 5.5 and a 10.5 x 5. depending on the grass. On an LT 25 the 30 only needs about 3' of take run when using the 10.5 x 5.

Hobbsy

My Feedback: (102)

The 120 R-3 sold yesterday and I shipped it yesterday. It went to a great Saito guy.

Hobbsy

My Feedback: (102)

I did a check of the left cylinder at TDC between the intake and exhaust stroke and it has overlap in the same amount as the right cylinder. I pulled the cam and made sure the left cylinder had the wide spread lobes and the hash timing mark. I did this on the 1.30 twin.

Glowgeek

That would seem physically impossible.

Looking at the images of the 90T cams the difference in lobe separation angles is readily apparent and large. Small changes in LSA doesn't always affect the valve overlap period, depends on the lobe profiles but changes that large would certainly show up as a change to the valve overlap period.

That said, the left cam may have been designed by saito to compensate for fueling effeciency problems with the right cylinder, essentially and deliberately reducing the effeciency of the left cylinder to match the right. The left cylinder with the wider LSA would also produce peak torque at a higher rpm than the right cylinder as a consequence.

Hobbsy

My Feedback: (102)

I used a long ago suggestion made by w8ye to check the left cylinder overlap and sure enough at tdc between the end of the exhaust and the beginning of the intake the valves alternately open and close. while doing that I discovered that left cam was one tooth retarded. In other words the alternate opening and closing was happening off center with the crank ahead of the cam. That gave me the opportunity to make certain that the left cam was indeed a left cam, and it is. LSA? It would be very difficult to get the cylinders to turn at different rpm for one to peak higher.

You should check a left cylinder cam also and become a believer.

Glowgeek

Nevertheless, having different lobe separation angles between left and right cams as seen in the 90T is not the norm for multicylinder engines. There's some monkey business going on with that left cam and it's cam timing to compensate for something. Admittedly I don't get it.

Heres the cam for the 90TS

Both cylinders run off the same cam on the 90TS and run fine from the reports I've read.

Of course the two cylinders in a twin can't run at different rpm at the same time Dave but each cylinder can and does make different torque numbers at any given rpm. Nothing is perfectly matched between cylinders.

Hyjinx

Ok guys. I have started a fire again.lol
Being a much less forward thinker than you guys, I simply want to know where what cam timing mark goes where when I am lowering the cam covers to engage the cam gears.
But as usual, I am learning much more than I ever thought when I kindled the embers.


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Glowgeek

https://www.rcuniverse.com/forum/glo...lem-print.html


No fires started here Aaron, Dave and I are just sharing thoughts.

Hobbsy

My Feedback: (102)

It has been stated that since the right cylinder is now the left cylinder that the right intake port is now the exhaust port, and the right exhaust port is now the intake port the timing needs to be different, I have no clue about that, I'm OK with the fact it works and works very well. When flipping there is no difference in the feel between the cylinders. When slowly turning the engine with a tiny ball driver in the glow plug hole the left exhaust valve closes well before bdc and the intake valve closes well after bdc, same as the right side.

Hobbsy

My Feedback: (102)

Some skinny from Clarence Lee, he did not differentiate between left and right cylinders. The plot thickens.

intake valve opens 50 degrees BTDC
Intake closes 60 degrees ABDC
Exhaust opens 65 degrees BBDC
Exhaust closes 35 degrees ATDC
Overlap========85 degrees

Hobbsy

My Feedback: (102)

Still digging, this 1.82 timing.


I hope this readable.

acdii

Well, the cylinders flipped from each other with both intakes on top and both exhaust on bottom so only natural to have two completely different cams. Right cylinder would be normal as in any other saito, intake on the right when facing from the prop, but the other cylinder with the intake on the left would be timed different, the exhaust lobe is the intake lobe, and the intake lobe is now the exhaust lobe. It wouldn't use the same cam since they now open and close at a different relation to the crank.

Glowgeek

Also consider that the cams are spinning opposite directions, so the opening and closing sides of the lobes are reversed as well. I have not seen an actual full camshaft profile yet so there is no guarantee that the opening and closing sides are symmetrical on either lobe.


As far as why the the cams are timed differently between left and right cylinders, even the author states that would be speculation! Well, if it's speculation to him I'll never figure it out!

acdii

Crankshaft splines would do that. What works on one cam, 180* apart may not on the other, but both cams may use the same gear stamp at the factory, so the second notch is added by hand to compensate for the spline difference.

Glowgeek

True, good point acdii.

Hobbsy

My Feedback: (102)

Lonnie, check this crazy geometry with one cylinder fo

rward of the other.

Glowgeek

Yep, two crank throws on that one and it uses a single camshaft; the one I posted a pic of earlier.

Very nice engine Dave. That Jet bandsaw is pretty handy to have around, huh?

Hobbsy

My Feedback: (102)

It looked like this for a few days. Check out the price of these rods: https://www.horizonhobby.com/connect...d%3Ap-sai60t10


The jugs are different construction, as you can see. The left one, on the right is new.