Side Port(vintage) Design

[Don Fry]

I’m happy for you Gonzo, send it back. You have changed your mind. Only cost is the return postage. Your choice.

[Jonathan W]

Gonzo, did you draw any conclusions in the end? Did you keep and run this engine that "might" be a SAM 125, or send it back? Any feedback from the supplier/manufacturer?

[GONZO]

I've still got the engine, unrun, for now. I had some feedback on another forum stating that stepping the front lower edge of the piston skirt had been found to be ineffective on certain engines like the one you mention. This suggests to me there is some other power robbing factor at work like induction or port timing. There is a tendency to use pre existing parts to cut production costs. Maybe, the con rod is from some other design or was made to the wrong dimensions. I say this because I'm beginning to suspect that the con rod is too short. I've yet to draw out a timing diagram, with different con rod lengths, to confirm my suspicions but with the addition of 2mm in length it looks like port openings fall into place. Of course I could be totally wrong.

[Engine Doctor]

Suck it and see . You won't know until you put some fuel in it and try it. If it turns over smoothly it can't do any damage so why wait ? 

[Martin Dance 1]

I'm inclined to agree with ED, if you run the engine and its performance compares with other engines of similar design then its performing as designed and will allow the model its fitted into to also perform as designed. It is irrelevant  that its design and construction doesn't allow the engine to meet the theoretical performance that Gonzo believes is possible. The basic construction of this type of engine limits the maximum performance anyway.

On the other hand it  may be a good idea to carefully wrap the engine up and put it into storage for the factory in the Ukraine was destroyed by a crashing Russian fighter aircraft. Should soon be worth £ Thousands on Ebay  'rare collectable vintage war relic etc.'?

[GONZO]

Do people not read my previous posts stating that the test performance(carried out by another person) of this engine is below that of my 'Indian' Mills 1.3. So, what purpose would be served by me running it? This engine is supposed to be a modern version, 'in the mode of', the Mills 1.3. It's a puzzle why such a well made engine should be below par on performance. My original post was to obtain information for possible reasons for it's lack of performance. Unfortunately I seem to have drawn a blank.

[Jonathan W]

I would not say that you have drawn a blank. The free engineering plans for the Mills 1.3 have been pointed out on Outerzone, where the port timings are specified. You can compare these to the port timings on the new engine. It's also been suggested that you talk to the UK supplier/manufacturer to ascertain whether or not your engine has been supplied in its intended form. The manufacturer could also perhaps enlighten you to the reasons for the design details which you are challenging.

 

Further, it was pointed out that these engines are made in Ukraine and now bombed out, so if you wish to obtain a rectified unit or replacement parts, perhaps best to move on with it.

 

I'm sure that you are aware, but fitting a longer con rod will shorten the exhaust and transfer timings, increase the inlet timing and increase the compression. Then it depends if the existing timings would benefit from that change.

 

After all this, with a number of posters showing interest and trying to help, you continue with the cloak and dagger act about the actual make of engine, when it is quite obvious what engine you are referring to. It's just a model engine, for people to enjoy, including discussing about it openly!

[GONZO]

Spoken to the relevant person and existing stock is as mine, so is as intended unless all remaining are wrong. Pointing out the uncovering of the inlet ports by the piston crown came as  new information to the vendor. Original tester Maris Disler (performance data supplied by vendor) stated that he was expecting more from the engine and has commented elsewhere that this engine does not benefit from modification of the piston skirt. This I find strange as from the diagrams the transfer ports appear to be totally covered by the piston skirt at BDC. Perhaps this suggests incomplete/restricted charge getting into the crankcase. I'm aware of the port timing changes but as it's a diesel the compression is variable. I've obtained copies of the Aero Modeller tests of the SAM 125 and the Redfin 061 and the 061 performance is better on smaller props and only approx 200 rpm worse on 8 inch props(061 gave 0.1bhp @ 12500rpm; 125 gave 0.078bhp @ 9000rpm and flat further up the revs. An original Mills gives 0.105bhp @ 9000rpm). From the Utube vide by fiery 1962 the engine appears to start well and run on an 8 inch prop but no rpm figures are provided.

I've sought to understand, with the assistance of others, why such a well made engine should be such an under performer(the testers figures not mine). Not just for my own enlightenment and satisfaction but for the vendor who could thus possibly rectify any identified shortcomings. The last communication I had with the vendor before he went on holiday was that he was going to contact Maris for more information.

[Jonathan W]

That's good, a bit more meat on the bone now! I have just checked the Disler article again and he gives port timings for the SAM 125 of Ex 160, Tr 110, In 110. If that exhaust timing measurement is correct, it's awfully long and what you would apply to an engine for use with a tuned pipe. And the transfer timing is long too. With such a long transfer period, that will be why modifying the piston skirt does not improve anything. If you had a longer rod to make the inlet timing about 120, at least the exhaust and transfer would become more reasonable, if not ideal. From what I can glean, going beyond 120 inlet timing on this type of engine is counter-productive.

 

Maybe the supplier will feel inclined to have a new batch of rods made, though probably not in Ukraine at present.

[GONZO]

The exhaust timing is so long as indicated by the piston crown going lower than the bottom edge of the exhaust port by 1.5mm to 2mm by eyeball guesstemate (not measured it yet).

I agree with your comments on the inlet timing and you seem to be agreeing with my conclusion that the conrod is short, possibly by 1mm to 2mm. The possible reason for the short rod would be to lower the overall height of the engine. Installing a longer rod would mean part of the contra piston being out of the top of the cylinder at running settings. Not best practice but have seen this on other engines. The knock on issue would be the need to install a spacer ring of say 2mm between the top face of the cylinder clamping flange and the screw on head/cooling jacket to make space for the contra piston. I have discussed these points with the vendor but he favours reducing the height of the crankcase(it would be cheaper) by the available 0.5mm to go some way towards addressing the problem(spacer ring, thinner, still required). The issue with this, as I see it, is that it would cause a misalignment between the cylinder and crankcase exhaust ports, more aesthetic than a performance issue, and may not be enough.

As you say, once the port timings are addressed by what ever process, the piston skirt modification may further enhance the performance of the engine. An additional difference I noticed between the 125 and the Indian and Irvine Mills 1.3 is the intake port in the cylinder wall. Indian and Irvine use a slot, 125 uses two drilled holes. It may be that the two holes could be joined to form a slot with further benefits.

Thanks for all the input. It's been a useful exercise discussing the engine issues which seems to have confirmed my suspicions and not thrown up any viable alternatives.

I'm torn between solving(possibly) an intriguing problem(at my expense on a new engine) or just sending it back.

[Jonathan W]

I hope you can reach a satisfactory conclusion with this engine. On the face of it, the port timings from the Disler article are so far from the norm, a longer rod might well only be a partial fix. I'm interested to hear what happens in the end.