ETZ 250 - 251 diffrences

[Puffs]

I see, your 'we' was you & me in May, ta for clarifying.

Well, for me it was quite an eye-opener to see how crudely they apparently adapted the barrel, going from '89 EM250-->'89 EM251. And it's only recently that I fully realised that, with the barrel in hand. That's why I did not recognise myself in the 'we'.

Increasing the intake opening duration (done by lowering the intake port) is quite a common approach in 2T tuning, causing more high rev focus & more high rev power, but at the cost of low rev torque + an increase of low rev spit-back (this is why reeds or rotating intake became preferred). But then commonly it is combined with also extending the opening times of the overflow ports, and particularly the exhaust port: as there is less time at high revs, it needs to open earlier. That is done by taking the level of those ports upwards. If you chop 2 mm from the base of the barrel & leave the porting as per the mould, those ports go downward, by 2mm, just as the intake does. Nevertheless, look like that's what they did - continue using the old moulds.

I still have not done the sums (convert ° opening time to mm below TDC), so let me check: over '89 EM250 --> '89 EM251, the exhaust goes from 181° open --> 175° open, so from 89.5°bTDC --> 92.5°bTDC. I make that from 36.3mm below TDC --> 38.0mm below TDC. Overflow: 124° --> 115° comes to 50.9mm below TDC --> 52.8mm below TDC. So indeed, 2mm off, give or take a bit. Same moulds.

[Puffs]

I put the bike back together again yesterday evening. While previously I used a thin base gasket, about 0.25mm, in combination with a 0.5mm head gasket, I now put 3 base gaskets, in total 1mm. How much of that remains under pressure I don't know, but it will raise the barrel a bit, say 0.7mm. On top I used the thinnest head gasket I have, 0.2mm, so that the squish size should be increased by some 0.4mm (not measured). I had taken out the oil pump & now use premix, as described elsewhere, but other than that nothing was changed.

After putting it all together, had a quick spin on the bike. Maybe it's imagination, but I really believe the bit the barrel has come up makes a noticeable difference. It is rounder & pulls better in the lows. Seems like it runs better, though I now do have to re-adjust the idle. The blow-back should have reduced, a little.

Edit: now adjusted idle (to standard settings (pilot #50, 2½ out, bypass 4 out) & rode some more. Also changed the main jet to a #133. It does seem to pull a bit more willingly, though top speed might have reduced a bit (as you should expect, if you reduce the compression ratio). The power curve appears to be smoothed-out a bit.

[dave47]

The correct squish for the ETZ250 is 0.9-1.2mm, and I daresay it is similar for the 251.
My own (251) squish is 1.1mm with 0.6 mm of head gasket, so if I used a 0.2mm gasket it would be safe to add an extra 0.5mm at the base, ie same as you have done. I will certainly try this out next time I have the barrel off. Unless of course you report in the meantime that it made no difference!

[Puffs]

I see Haynes say 0.5mm isn't available... just re-measured it with a micrometer, the one I used before was 0.43mm. So my squish may have increased even more. If I find suitable solder & time, I'll measure tomorrow.

I think it does make a difference, with this size A =13mm barrel raised some 0.7mm, I like it better. Stronger in the lows, and actually less pronounced surging too. The peak it previously had in the highs has come down a bit, and top power / top speed may have reduced a bit. But by what I've done, I have reduced the compression, and probably have too high a squish size - that will reduce top power. I think it's a smoother bike. But all differences are small, and I find it difficult to judge differences when there is a bit of time between them.

Edit: the squish was measured, it is now about 1.3mm.

[dave47]

I will definitely do that then, it is worth it even if it increases low end a little bit.

Regarding your 1.3mm squish:
It seems to me that because you have had a rebore, even if it is just 1st o/s, it will increase the cylinder volume by enough to keep the compression ratio right, even though the squish is 0.1mm above tolerance. I am no technical expert, however!

[parrbd]

Too large a squish clearance can encourage detonation. Best to keep within specification.

[Puffs]

Ta for the advice & making me think a bit more about this.

While I believe it's too little squish clearance that can encourage detonation (higher average pressure & more difficult for the mixture to escape from the squish band --> in that band, locally even higher P & T), I'm not too worried about this size of the squish, for my 71mm bore. Nevertheless, I crunched some tentative numbers:

250cc ETZ, bore, squish, compression.jpg (18.31 KiB) Viewed 876 times

Which suggest that compression ratio-wise I'm pretty much on spec.

I do believe that my base gaskets, particularly the klingerit one, have compressed more than I anticipated, otherwise I should have had even more squish. So the barrel has not come up by 0.7mm, but less.

[parrbd]

Too small a squish clearance may cause clearance problems but it will reduce detonation. Too large a clearance will make detonation more likely. Refer to page 37 in the Jennings bible. http://www.amrca.com/tech/tuners.pdf

[Puffs]

Thanks for that link! Once again, I learn something from these discussions.

P37.JPG (29.61 KiB) Viewed 876 times

But what I don't find there, is that if you increase the squish size, you increase the risk of detonation.

If there is sufficient adiabatic compression of the mixture, its temperature can increase to levels above the self-ignition temperature (Mr. Jennings calls that 'compression heating'), and detonate, i.e. self ignite, before the spark in the plug causes it to. The squish band can help reduce this problem by locally enhancing contact with the (relatively) cool piston & head. More importantly, it focusses the mixture in an area close around the plug, as close as possible to a sphere, which makes the combustion more effective. Also, the gas flows as indicated by the arrows, further help adiabatic compression of the mixture in that central area, again aiding combustion.

Mr. Jennings explains, in the pages preceding P37, the background to the squish band: poor fuels from about 100 years ago, in side valve engines suffering from detonation. But detonation is not much of a risk in my case: while all the design specs for this engine are based on DDR fuels with rather modest quality (to quote the original ETZ250 manual, section 2.1. Kraftstoff: "Entsprechend der konstruktiven Auslegung des Motors ist ein Vergaserkraftstoff mit einer Oktanzahl ROZ von mindestens 88 zu verwenden (in der DDR Kurzbezeichnung "VK 88"). Außerhalb der DDR wird ein Kraftstoff mit ähnlicher Oktanzahl empfohlen.", the worst I can buy is RON95. More on octane rating & detonation in https://en.wikipedia.org/wiki/Octane_rating . Anyway, the engine runs fine & if you don't have detonation, you will not increase the risk of detonation by adding more gaskets & thus lowering the adiabatic compression. For more, see 'Adiabatic heating' in https://en.wikipedia.org/wiki/Adiabatic_process .

But rather than compression ratio's, the most important thing is, and Mr. Jennings says that too somewhere, the actual P & T of the mixture. In my numbers/spreadsheet I used the quoted compression ratio as an indication of how sound it all is, and conclude that 'compression ratio-wise I'm pretty much on spec', but of course the mixture knows nothing about any compression ratio - if P & T get too high, it just ignites/detonates. And if P & T are too low, combustion, as caused by the spark, is slow & inefficient. But I don't know the P & T in my bike, and particularly in a 2T they are very much dependent on the porting & port timing (all standard in mine). So it's a lot easier to compare in terms of compression ratio...

[parrbd]

I think my reading of this research is that the squish area stops ignition of the enclosed mixture because too much heat is lost to the surrounding metal. This only works within a small range and the ideal was tested as .015". What I am inferring is that as the clearance increases it stops acting as a squish area and ignition can occur at the outer reaches of the combustion chamber, so bringing back the possibility of detonation. BTW, from experience I have had results that diverged from Jennings advice, particularly with inlet timing. (On a TZ350A I got a much more tractable engine after taking 5mm off the rear skirt and a better top end of 154mph at Bathurst, back in 1975)

[Puffs]

Yes, we agree on the way it works - contact with relatively cool metal causes the mixture temperature to stay a bit lower, thus prevent knocking/detonation. I agree with you that it follows that if the pressures & compression were to stay the same, while the squish size would increase, yes, then the chance on detonation would increase. BUT: I don't think he wrote that, and that's also not what happens here. Here, if you increase the squish size, you reduce the compression simultaneously, because the volume of the combustion chamber increases. To increase the squish size while keeping compression P identical would require modifying the shape of the combustion chamber in the head, going back to an older design (as he showed on earlier pages). Anyway, because of better, higher RON fuels I think I'm safe.

Chapeau - 154MPH = 248km/h with a 350 is quite an achievement. On the 1.916 km Conrod Straight, I presume. Seems like an interesting circuit, Bathurst, unfortunately a bit out of my way.

I haven't read what he says on inlet timing, but a longer inlet timing for more high end is a common approach. And maybe what MZ tried, going to the EM251 size A = 13mm. But - anyway, said all that earlier.

[Puffs]

Actually, doing a bit more reading & thinking on this, I've revised my opinion: I think the cooling effect in the squish band is less important, and I disagree with what Mr. Jennings says on that. You can most certainly have detonation right in that squish band, photo's prove that:
Image
Image
The reason is that the relative volume reduction (compression) in the squish band volume is far bigger than in the combustion volume. Consequently the pressure & temperature rise quicker there, and as long as that compression it is adiabatic (so quick, high revs) it is exactly there that detonation will first occur.

Of course it is that relatively quicker pressure rise in the squish band volume that is the working principle: because of that, the inward flows occur, which cause turbulence and thus result a more effective combustion. The more effective combustion associated with a squish band combustion design is proven by all modern 2T MX & race bikes having it, with specific power ratings in excess of 200hp/L.

If you increase the squish band height (so by adding more gaskets), I think the prime risk is that you reduce the turbulence in the combustion volume, and with that the efficiency of the combustion. Of course also the compression ratio will reduce; more info in table above (download/file.php?id=5329&mode=view).

[Puffs]

The next episode focussed on actually measuring the size of the combustion chamber (using a syringe & water). The earlier table above inferred that size from the specified compression ratio of 10.5 (but slightly wrong), which gives small differences compared to the volume measured on my head:

Squish & CR.jpg (68.9 KiB) Viewed 876 times

Now of course the actual pressures very much depend on the degree of filling, particularly on a 2T, but when using RON95 fuel, at these CR's the chance on detonation is pretty dim.

As I also wanted to perceive the impact of a Maß A = 14mm barrel, I made a 1mm thick spacer:

Cylinder base spacer, th = 1.0mm.jpg (42.64 KiB) Viewed 876 times

In view of anticipated squish heights, the barrel was assembled with just silicone sealant, in addition to this spacer. So no gaskets, also not below the head; for now also there the silicone seems to provide a sufficient seal on an accurately torqued head. The squish height was measured at 1.18mm.

The bike runs fine, compared to the original Maß A = 13mm, more torque low down, and it doesn't bog down so much on low rev + full throttle. But the top end has reduced a bit, and it doesn't go as willingly into the highs, and actually also top speed has come down a bit. Nevertheless, for the bike that it is, I like it better like this.

[dave47]

Thats a very professional looking job.

[JawasandMZs]

I remember when the 251 was introduced and the bike press evaluation was that the 250 etz had hotted up the porting from the Supa 5 making it a (relatively) peaky, fast stroker at the expense of sub 50mpg economy. The 251 reversed this design by making the motor much more flexible sub 3,500 rpm, the point below which the etz 250 is pretty hopeless in 5th gear, resulting in better economy but probably less fun to ride.