discussion of transfer, boost,exh ports,int/exh rotor timing...

[Jim Allen]

"Well shaped transfer tunnels aim the stream much better." This statement turns out to be one of the most important things when building very high performance two cycle engines. Take a very close look at the 4 main transfers & the single boost port of the MB-40 FAI speed engine, that developes it's peak, flying HP, in the 34,000 to 36,000 RPM range. Notice that the 4 main transfers open before the single boost port opens. Also notice that all of the tunnels are in the shape of a radius, inside & outside. Another unique thing is the very wide exhaust (110% of the bore) & the fact that the upper part of the exhaust window is actually over top of the front two "A" main transfers. Failure to aim these two "A" transfers correctly would result in a short circuit out of the exhaust window. Also notice the hole drilled in the top of the crank pin which comes out into the induction track. The hole provides ample lubrication for the bottom end. Notice the ramp used in the induction tract which helps the engine pipe up & the domed piston crown which improves main transfer flow.

Why does this engine not use some type of rear induction? An induction that would open at BDC & close at 65* after TDC (245* total duration), instead of the mild timing of the front intake. Why has the builder of this engine ( Rob Metkemeijer) decided not to use any type of rear induction, except maybe the bell valve which we discussed at length, during the recent Pylon Championship Race held at Liberty, NC in October?

Jim Allen

 

[dwilfong]

A eng designed to run moderate load and un load to reach top RPM on 0% nitro is a totally different then a fully loaded 60% nitro burning beast.

Many have found the a eng designed for aero use just dose not perform the same in a high nito burning boat.

Wonder why?

any thoughts?

How dose all that nitro change the flow characteristics in the eng?

"case" in point CMB .91/ 1.01 RS eng burning up?

Time to ask and answer the real problems with boat eng.

Then again we can just play hid and seek if you want.

 

[LohringMiller]

This is the full story of what we did when working on the CMB 35. Again, the engine had poorly designed transfer tunnels. The single, side transfer ports weren't the best either. A better design should produce a lot more power . The engine in Jim's pictures shows modern thinking.

Lohring Miller

 

[dwilfong]

That was a great article read that over and over a few times when I first started with the RC boating.

 

[Tom Foley]

Not so sure it worked as well as you think !! BTW the old CMB 101 is a piece of junk in stock form and an embarrassment to release to the public .

 

[brad nichols]

Definitely a melt down!!!

Brad

 

[dwilfong]

Not so sure it worked as well as you think !! BTW the old CMB 101 is a piece of junk in stock form and an embarrassment to release to the public .

one out of 6 ain't bad..........O well I missed on the tune at 9000+ RPM.......LOL

 

[Jim Allen]

Lohring,

I sent you a massage to your E-mail address.

JA

 

[Don Ferrette]

Upload of test pic, not part of this discussion

 

[Steve Anderson]

Gordon Jennings wrote about that style of transfer passages in his book Two-stroke Tuner’s Handbook that was copyrighted in 1973. Gordon Blair wrote about them in his book in the early 1990’s. It has been the practice for two-stroke racing engines for well over four decades. The MB.40 is over 15 years old.

My question to Lohring and Jim is… why do you believe no other RC racing engine manufacturers have developed cases or barrels with transfer passages like that? With the long known promise of horsepower gain, surely a company like Novarossi would take advantage of any power increase available from such an easy design change?

 

[LohringMiller]

Quickdraw did and the Pioneer is close to as good as it gets in model engines. I think the main reason is there are very few people designing model engines and they aren't in touch with the larger two stroke world. Also, there's no money in model engines except in a very few mass produced cases. There production issues dictate design. The most bang for the buck is in the industrial gas engines and mass production is the only consideration there. It's a lot cheaper to increase the displacement for more power than to improve the design. The IC engine is on the way out and not much effort is going into new design. After all the 55 small block GM engine is still around with some modern electronics.

Lohring Miller

 

[Jim Allen]

Even though the MB-40 has been around for many years, the technology used in the main transfers & the boost transfer is relatively new. It is also much easier to change the transfers & cylinder ports when they are seperate pieces compared to a one piece upper end that has no cylinder. These transfers with a radius inside & outside contradict Gordon Blairs idea of the tea cup handle shapped transfer.

Manufactured engines always look for the most cost effective method to make the engine. Maybe this is why a very few manufacturers have taken the time to learn how to hard chrome plate aluminum.

Jim Allen

 

[Jim Allen]

Another interesting feature of the MB-40 engine is it's connecting rod which has no bushing in the top end. It is machined out of a RSP aluminum alloy & it is bead blasted to remove stress points. Notice the shape of the bottom & top end as well as the tappered oil hole at the big end. The piston has been lightened to the maximum, but the domed piston crown can still carry most of the combustions heat down to the sides of the piston.

Jim Allen

 

[Mike Rappold]

Jim,

On the MB engine, did the designer chose to not use a rear induction but rather a crank type due to less frictional drag or rotating mass?

Mike

 

[Jim Allen]

Mike,

On the engines we tested, removing the fricitional drag of a seperate induction valve, increased the engines HP. I also saw a HP increase by switching to the the bell valve, which also has no fricitional drag. A front intake engine's crankshaft is mounted in ball bearings, therefore it also must have very low fricitional drag. Notice how the inner race of the rear main bearing is ground into the MB-40's crankshaft. This gives the necessary rigidity for very high RPM's. This system is as good as a crankshaft that is mechanically fastened to the inner race of both front bearings (sleeve over the crankshaft in the photo) that are mounted in a hardened steel front end. The latter system was used in the FMV engine that Rob Merkemeijer work on & is used in my .90 boat engine.

There is no subitute for a valid mechanical design. It doesn't matter how much nitro you put in the engine, if it fails mechanically!

Jim Allen

 

[brad nichols]

Why no bushing in the top end??? How well does it hold up before it needs to be changed?

Brad

 

[Jim Allen]

Why no bushing in the top end??? How well does it hold up before it needs to be changed?

Brad

Brad,

There is no bushing in the top end because the rotational speed is very low is this area. Selection of the correct metallurgy (RSP alloy- proprietary) makes this possible. There is no bushing used in the steel connecting rods pictured. Again the selection of the correct metallurgy makes this possible. Both types of connecting rods have very good lives even when WOT RPM's exceed 32,000. In both cases there is a reciprocating weight savings because the brass bushing or roller assembly (steel rod) has been eliminated.

Jim Allen

 

[dwilfong]

OK its show and tell time???????

What is this??????

O it is a boat eng........

 

[Jim Allen]

Upload of test pic, not part of this discussion

Don,

The photo you posted reminded me of something. I still use it on both the gas & nitro engines with great success.

Jim

 

[Jim Allen]

delete