2 and 4 STROKE DESIGNS

[Jonathan W]

From full size automotive engine design, the advantage of the wedge shape chamber as per the Laser is that it straightens the port shape, so on the Laser you have a very straight flow path from carb direct to the valve throat area. Also quite a straight flow out from the exhaust valve in this case, since the exhaust is going back the same direction the inlet came in. That's probably less good for keeping the inlet port cool though. It has generous squish area so the combustion chamber is quite compact and the flame propagation path therefore short.

The Enya head is kind of a cross between a "bathtub" and wedge. Does not look like it has such good squish as the Laser, but still pretty good.

With the FS56A, OS have gone to the trouble/expense of making the inlet valve slightly larger than the exhaust. This is beneficial to inlet flow. The detriment to exhaust flow is not so important, since the exhaust is expelling under its own high pressure anyway. It's still a basic bathtub chamber, in theory less good inlet flow and less efficient combustion than the wedge.

The disadvantage with both bathtub and wedge is that the valve sizes are constrained by what will fit within the bore diameter.

The hemi head has the theoretical advantage that you can make the valves somewhat larger than constraints of the bore size and the potential for nice straight ports with good flow capability. The disadvantage is a larger combustion chamber with longer flame propagation and it's difficult to achieve a high compression ratio. That's not such a problem for a glow engine, but would be for diesel or high performance petrol. However, the Saito implementation of the hemi head is somewhat compromised, due to the orientation of the valve ports relative to the valve inclination. The inlet port is 90° out from best alignment to the back of the valve, so it's very far from a nice straight flow path for the inlet. Therefore any potential flow advantage from the larger valve is compromised. The exhaust is not so bad, but that is less critical anyway. Also the mono-block construction of cylinder and head makes it awkward to machine the combustion chamber or valve throats to the best design. The mono-block does help with cooling though.

In the end, model 4 strokes are quite low performance relative to full size automotive or motorcycle engines. They are low compression ratio and also very low revving relative to their size. Therefore going for maximum valve sizes, best possible port design etc is probably a moot point. In a model engine you want reliable running and good throttling. Also you want simplicity for less moving parts, compact and low weight design. So even on the bigger model 4 strokes, we are still using basic pushrod valve actuation and 2 valve heads.

Lets say a 10cc model 4 stroke is giving 1 bhp. That's only 100bhp/litre. Generally you should get higher bhp/litre from smaller engines, but it's quite normal these days to be achieving 200bhp/litre with 1 litre motorcycle engines.

Anyway, of these 4 heads, going on the photos only, my favourite is the Laser, assuming the ports are nicely shaped. The Laser would be even more top dog if it increased the inlet valve size, but that is maybe down to economics. I guess that Laser only makes 2 different valves. One valve for all the smaller range engines and another for the larger range. The extra cost of optimising inlet and exhaust valve sizes for each engine is probably not worth the expense or effort.

[TartanMac]

I pretty much agree with you Jonathan W. The Laser head I put up is a very old 1985-1987 as far as I can tell. The heads are symmetrical which I think is genius as they could make the 75 single and both heads for the 1.50 V all the same and all the valves the same. I can take the heads off and change them over and it is the same. The newer heads have larger intake valves but as far as I know the same wedge shape but the plug was relocated to a vertically mounted one.

The Laser wedge is also my favourite however the Enya and the O.S head I am still interested in as they make very good power at high rpm and that is pretty interesting. I'm not sure and hope someone can tell me more about that design. I think maybe the vertical valve or near vertical valve gives good delivery of fuel from all the way round the valve and possible good port velocity from low valve lift and with turbulence it gives a better charge at high rpm. This is all an assumption and ideas though.

Jonathan that is a good post thanks you put the head shape and the port relation together very well. I am very keen on learning enough to get a design of a head for my 41 4C together I love that engine. I put a 4C Yusa Tuned pipe on it over the original muffler when I was testing it as it is really loud in the back garden. It does not run right because of low compression but I can get it to idle at 3,600 rpm and throttle up to 10,500 rpm and it sounds like a mini rally car engine really cool engine. I would love to have a rever of a 4C engine, you could get more power on the valve shape also as they are very simple it weighs around 370g with the std muffler on it. So 11x6 @11,000 rpm would give 0.85 hp roughly. That is a lot for its weight and its capacity.

[Rich too]

Perhaps I am out of touch, but although possible, I don’t think 200hp per litre is the norm? Very impressive figure, but not that common.

[TartanMac]

Posted by Rich too on 04/11/2017 07:43:31:

Perhaps I am out of touch, but although possible, I don’t think 200hp per litre is the norm? Very impressive figure, but not that common.

I don't follow motor bike stuff. I know my dads Triumph Daytona 955I is 160 hp with a muffler and remap. I think 200 hp is a bit high most super cars are around 125 hp/ltr. To be honest though I think there is no point in comparing full size engines to model engines there designs are based on the support of electronics and spark ignition. I don't think there is anything to be had looking at those engines to build a model engine.

[TartanMac]

I also remember my old 1.3 MKII Astra head having a similar design I'm sure. The ports were at a very steep angle upward from the valve backwards up to the manifold. Probably to make better port flow.

The O.S makes 1 hp so 1 devided by 56 then multiplied by 41 makes 0.732 hp

My 41 turns 10500 rpm on an 11x6 and that turns out at 0.732 hp that is the same HP / capacity at 500 rpm lower in a smaller engine that has the same size exhaust as Intake that is pretty impressive and freaky that it is the same number. I just figured that out and thought I'd share it as it is amazing to me that it is the same number.

[Jonathan W]

The point of the 200bhp/litre was merely to show that these model engines are in quite a low state of tune. A current model Suzuki or BMW etc 1 litre sports bike is indeed quoted at around 200bhp at 13-14000rpm. That's in road legal trim, compliant with noise and smog regulations.

Even though these model engines are relatively low tech 2 valve per cylinder pushrod jobs, there are still lessons to be learned from full size engines.

The OS and Saito engines both have fairly torturous inlet passages, when you include the 90° elbow where the carb pipe joins the head. At least that is giving you plenty of turbulence! By contrast, the Enya 4 strokes have a much smoother tubular inlet pipe with nice wide radius curves. Then we have the Laser with a direct straight path from carb to back of valve.

In the end, you could make a new head for the Enya 41 with a full wedge chamber and larger valves, but it also then depends on the cam lift and timing whether that would yield much improvement. You might also get more out of the Enya depending which carb you have. The earlier ones had the "GC" series, with main needle and air bleed adjustment. The later ones had the "TN" carbs - twin needle with slightly larger choke area.

[TartanMac]

Just wondering Jonathan do you think it would make sense that O.S and the Enya head designs are based on having the carb with a 90 degree bend so that the fuel trapped or built up in the bend gets distributed better in the cylinder because of the bath tub turbulence ?

I was thinking about carbs I really like the butterfly carbs I have 2 of them. MVVS and the O.S Hanno. They look perfect for a direct port attachment as the jet is in the butterfly and at idle and full throttle you can have the jet closer to the high flow part of the port. I was also thinking the barrel carb has a large area inside the barrel at idle then the opening is at the side and the opening is not straight from the jet.

[TartanMac]

Not as close to the edge as I remember but still nice straight flow. into the port.

[TartanMac]

I also found that 2024 and 7075 T6 aluminium seem to be popular for model engines and I looked at 510 hard tempered phosphorus bronze and from what I could see it seems to be similar rockwell B and tensile strength to 7075 so I could save myself some work on the valve seats. I have no real clue what I was looking at though. But I was thinking about making a head from billet. I would also like to try a cast head but I think if I was doing that I would want to use a lost wax investment casting method as it gives a better finish no flashing and more aesthetic design abilities and a nice seamless look. But from what I have seen most home castings are made from plaster and it seems hit and miss in terms of success and the finish does not look like what I would be happy with. I am seriously considering just making new heads for my Enya 1.20 and my Enya 41 4C engines. I was considering a V twin out of my 41 but the advantage of that engine is its weight when you up the power the crank and bearing sizes go up then you loose the advantage. This engine design business is a lot more complicated than first meets the eye. Out a V twin around 1.36hp is what I would want and around a 60-70 size 12x6 @ 11,500 rpm = 1.36 hp 8lbs thrust 65 mph sounds like a really nice out put. If I was going to build an engine I think I would build that as you cannot buy it and the saito 60T was around £480 I would not pay that for such a small engine it is too much.

Edited By TartanMac on 02/12/2017 21:11:01

[Denis Watkins]

Lots to consider TM, as you move forward.

Machining from a billet just ensures there is no porosity in the metal, or weaknesses that you might get from casting.

Lost wax is very accurate but the former is melted away each time

High end gold jewellery is lost wax cast as each piece can then be exclusive.

And die casting is repeatable but expensive tooling to make the die, but then you could offer more heads to modellers.

Machining from a billet could be the best way forward

[Simon Chaddock]

It is a bit unfair to compare aero and road going engines as they have different aims. Aero engines seek power to weight rather than power to capacity.

If you have the inclination and the skill you can build a true hemi head model engine although you have to get clever with the rocker and push rod alignment to do it.

I have actually run this 60 year old 5 cc petrol but it was made by my Dad.

The engine on its 'display' stand.

Long before the days of electronic ignition so it has 'points' and a coil.

With a scavenged sump and positive feed to the big end from a separate oil tank it runs on ordinary 'pump' petrol.

From memory it turns a 9x6 at close to 10,000 rpm. Quite a modest performance but it was built for fuel economy over power.

Edited By Simon Chaddock on 03/12/2017 01:34:45

[Denis Watkins]

A 9 x 6 is a good result from your Dads motor Simon

5cc is in the .30 class, and most would struggle to get a 9 x 5 to 10k

There was a time that all our small aero motors were petrol

Edited By Denis Watkins on 03/12/2017 08:38:07

[Jonathan W]

Lovely job by Simon's Dad. A true hemi head, instead of the Saito cosmetic hemi. Was it his own design, Simon?

Back to TartanMac making a new cylinder head, machine from solid seems to make more sense for a one-off. The alloys mentioned are specialist aero alloys. Even if obtainable in the small quantity required, is this really needed and how easy are they to machine? I would have thought some normal HE30 (6082) would be adequate and easy to machine. Perhaps some leaded bronze for the valve seat/guide inserts.