The 90 engine topic.

[TimD]

Starting this thread to stop side tracking Scott's thread B) this is the thread for those that want to read it so far; http://www.intlwaters.com/index.php?showtopic=3768

The discussion was at the point of diccussing nitro % and it's relevance to K90's and A90hp's and port numbers etc.....

 

[TimD]

The intake window in the housing and the disk are small on the standard A90HP but Ian's motors have seen a lot of dremel work in this area and he does open them up a lot!

I have always been of the opinion that the case volume has always been an over rated aspect of a piped engine, but what Terry has mentioned sounds like it could be feasible....opinions please?

Andy,

Do you approach your timing numbers differently for the MAC84 to the MAC67 based on the bore/stroke ratio scenerio?

Tim.

 

[Scott Schneider]

:lol: :lol: :lol: See guys are allready checking this out. :lol: :lol: :lol: Scott

 

[Ian Inverarity]

Okay, posting over here now!

I made a mistake saying I have a 40 thou shim in the K-90, actually it 0.82mm which is pretty much 30 thou. The timings I stated in the other thread are correct tho.

Terry,

Yes, I was thinking along similiar lines about the crank-case volume effects as the K-90 drum is very large volume indeed, maybe at engine speed below which the pipe scavenging is efficient (on the starting stand) the higher effective case volume reduces the ability to fill the cylinder.

That other drum version that Anders has a picture of in the Russian motor thread might be the key here....

Andy,

In reference to the small transfer ports on the A-90 as compares to the K-90, I guess you are reffering to the ports in the case? I have measured these and the cross sectional areas are quite comparable actually...

As Tim suggests I do open up the inlet port and the passage between the case and carb. Craig (EMS) should be able to verify this by inpecting his conrods thru the open carby's of the motors I modified for him... B)

Nitrocrazed racing: Data overflow...

 

[Ian Inverarity]

Andy,

On the topic of disc diameter I probably wasnt clear in what I said. A larger disc will be fully open (and not partially obscured by the disc) longer than a smaller disc with the same port area and same timings. Which is probably why over the years that both CMB Evo's and the A-90's have had larger discs fitted. A drum is in area terms the equivalent of a small disc, thus for the same port areas and timings the port is fully open for less duration than a Zimmerman disc...

Ian.

 

[Ian Inverarity]

Anders,

You have been playing with A-90LS motors, these are very similar to what I use (A-90HP's) but they have the same bore and stroke and rod length as a K-90. Why did you chose LS motors over the HP's? Have they been working well?

Ian.

 

[DaveMarles]

On the topic of disc diameter I probably wasnt clear in what I said. A larger disc will be fully open (and not partially obscured by the disc) longer than a smaller disc with the same port area and same timings. Which is probably why over the years that both CMB Evo's and the A-90's have had larger discs fitted. A drum is in area terms the equivalent of a small disc, thus for the same port areas and timings the port is fully open for less duration than a Zimmerman disc...

Ian, Not wishing to be pedantic (as if) but what you are saying is not actually correct and in this case its probably important. A larger disc will be fully open longer than a smaller disc with the same port WIDTH not port area. As I said, its a while since I looked at an A90 but I thought this engine had a wide but not very high induction port and I'm sure that you can carve some metal away but I don't think you can get anywhere near a decent shaped induction port on this engine.

The crankcase volume will have some effect in comparing the 2 motors but in which way? Many schools of thought on this and most modern two stroke bikes have a low crankcase CR and more power than the engines with the old ideas of small crankcase volume high crankcase CR which is put down to improved pipe designs.. Personally I havn't seen a lot of difference on my engines when I've tried stuffing the cases.

Dave

 

[Anders_Martinelle]

Ian, the A90 i have is the newer A90LS MkII.

It has the dia/stroke as a LS but have the needle rod as a HP, so it`s a combo of both.

So far it has been good but the K90 still has the advantage...

Hard to beat the K90......

Anders

 

[TimD]

I pulled the liner shim out of Ian's motor last night and it is 0.030" (.77mm).

A quick visual inspection of the motor did not reveal anything that could be wrong. Guess I'll just have to put it in the boat and run it on 50% and see whether it behaves for me :unsure:

Tim.

 

[Ian Inverarity]

Dave,

You are dead right about port width not port area, my mistake!

There is not a lot of room to play with on the A-90 inlet port itself, I raise the top by about 1mm, too much more and there wont be anything to seal the disc in that area. But I modify the area after the port leading into the case to increase the flow area quite a bit.

Any idea what the inlet port area is in a CMB 90?

Personally I dont think high crank case compression is the way to go with modern two strokes, but I am wondering if the low crankcase compression of the K-90 compared to the A-90 is what is causing the poor running at low speeds that I am experiencing. Hence my interest in that alternative drum configuration... B)

Anders,

So far, in my experience at least, I have no trouble beating my K-90 with my A-90HP's...

Are your LS MKII's fairly standard still? I would expect a K-90 to be better than a stock A-90.

Ian.

 

[RIGGERMORTISE]

Ian

Where do you find the time??????

 

[DaveMarles]

Ian,

CMB 90 induction port is close to 15mm wide at mid height and close to 13mm high with a disc diameter of 49.5mm.

Dave

 

[TimD]

Will we see a MAC 90 or larger some day?

 

[Ian Inverarity]

Dave,

The A-90 inlet port is about 20mm wide at the top and only about 9.5mm high, but I increase that by 1mm. I dont have the disc sizes to hand and dont have any motors in bits at the moment... B) But it does sound like the CMB port is a good deal larger.

Riggermortise (Dave)

I have no life....

Nitrocrazed racing: Re-building boats after the nats....

 

[DaveMarles]

Ian, Thats a puny port for a .90 and I think the disc is not as big as the CMB disc either which is not good. The engine is really restricted by the induction and its a pity because its very well made. On the example I looked at, the disc was aluminium and I might be mistaken but it did not seem to touch the backplate at all. <_<

Dave

 

[Ian Inverarity]

Dave,

I am not sure which engine you would have seen, but the earlier ones had a steel rear bearing housing and a smaller diameter aluminium disc. The disc only seals on the rear bearing housing, so is not a zimmerman disc.

Later motors have an aluminium rear housing which is hard anodised yellow, and again this is the sealing face for the aluminium disc, which is larger in diameter

I just pulled apart an A100, the disc is about 49mm diameter, so it is not that much smaller than a CMB. The bigger the disc, the wider the motor, so there are simply practical limitations to how big the discs can be made.

The port has much more area than the carb, and while probably even more area would be better, how much more area over the carb do you need?

Did you see the induction I drew for the twin cylinder motor in the 180 topic? Still a 4 bearing crank, but with integral drum induction. B)

Nitrocrazed racing: How much area do you need to run a 14mm carb....

 

[DaveMarles]

Ian, I think that its the port shape thats important. A narrow deeper port is much preferred. Like I said its a good motor spoiled by the induction.

Does that mean that the disc on the A 90 doesnt seal on the carb backplate?

Dave

 

[Ian Inverarity]

Dave,

You may be right, but I am not sure! Mine go okay! But it could be improved.

The disc does not seal on the backplate at all, so it isn't a zimmerman disc, effectively it is a disc like many Picco's and OPS' had, only larger and with a more zimmerman type porting arangement. I dont know what this would be called.

Ian.

 

[DaveMarles]

Anybody know another engine where the disc doesn't seal against the backplate? How can that work correctly without losing a lot of torque?

Dave

 

[Ian Inverarity]

Dave,

The disc seals against the rear bearing housing. It only needs to seal on one face to work properly??!! :huh: As far as I know anyway....

The discs used by Picco and OPS sealed against the back plate but are open to the case on the other side. The A-90 disc seals on the rear bearing housing and the other side is effectively open to the carb. The case volume is still sealed from the carb by the disc, I dont see why it would need to do this on both sides to work? The Zimmermans do this because of their mechanical arrangement: they are floating so cant be held on one side to seal, thus seal both sides with fine clearance. The A-90 arrangement is bolted to the rear crankshaft so that the disc is held at a controlled clearance to the rear bearing housing.

Ian.