Compression/Squish Clearance

[Seagull 170]

Nice work John
I tried the same on a domed piston on a lathe, with the tool ground to the combustion chamber radius, so the step was less pronounced.
This never worked as well as CNC milling the head which shrank the combustion chamber and allowed the head to match the piston to a thou, before it got hot.
I get the carbon deposit from the transfer port right into the combustion chamber area on the piston crown, maybe I run my pistons hotter than you do.
I was thinking of ceramicing the top of the piston to prevent some of the heat absorbtion but with very little signs of excessive heat on the underside of the piston thought it wasn't super critical, & anything that raised the crankcase compression was beneficial, on a motor with 2 smallish transfer ports & no room for any more.

[quick2008]

Thanks guys, John thanks for the pics
Have a huge library of info now!, but may need some more money yet.

But just for now, till i start going blueprinting, balancing, decking. etc
If i machine my HEAD to be within the 0.8mm squish clearance,
Im guessing that i will see mild power gains from doing so.?
keeping in mind its a 30hp engine.

Thanks!

[john miffco]

the reason i o ringed the cylinders and machined the pistons was to remove
the head gasket to get tighter tolerances
and also to be able to use universal heads for different compressions

this way no special machining was needed to be done to the heads
for that 1 particular motor,,,,,,,
i could use any head and compression ,,or head design for testing to find what the motor liked best,,,,,
simplier method for fine tuning

when you cut the head to gain compression which will see signifigant gain in hp over the squish tolerances
but if use same comp heads with tighter squish then there is more signifigant gain

my theory is to make any motor efficient
this squish area and compression is where combustion effiecieny is achieved
and combustion efficiency is only 1 item of 100
to make a motor stronger than another
wheather it is a spec motor or unlimited
john

[quick2008]

Thanks john,
Ill start to try putting some of this to work!!
Regards.

[gedopa]

Hi you folks, I have done blueprinting of the 30 Yamaha 3 cyl in accordance to the UIM S-550 class homologation. Tunnelboats with maximum 550ccm engine.
There is no doubt about that the 30 Yamaha easily runs with 0,8 mm squish clearance. But if you are trying minimize the clearance you have to think about max rpm also. With the Yamaha 70 C engine we used 0,8 mm clearance, but if we where closing up around 9000rpm the piston began nocking on the head.... 8700-8800 rpm no problem. Some other stuff to take advantage of is, (just to make complete confusion:-) what about doing the combustion chambers different??? If you are meassuring the compression pressure you will find difference between cylinders, even in i a new engine, even if you reduce combustion chamber volum.... Then there should be a difference in combustion volum between the cylinders to make the cylinders run similar...The reason for this is cylinder heigth and cylindehead temperature differences.... Nothing is impossible, it just take some more time or money;-) Good Luck quick2008

PS! The 3 cyl Yamaha`s, the best 3 cyl engine ever built!!!!!

[quick2008]

Thanks Gedopa,

Has been a been something ive tried to be careful of when getting the correct clearence. As being an engine with standard internals, I understand that factory/mass produced rod/piston length can vary by up to 0.4mm!.
Thats where the blueprinting is essential for tight racing tolerances,
Probably wont be looking at revs of 8500+ in my application. so i think i should be ok at this stage.
have you done much with these engines?
as for mod'n the combustion chambers, definately something id like to work on, but as you said, "time and money". We are getting there slowly with both. but enjoying.
Thanks again---,quick

[gedopa]

Hi again, I have done a lot with this engines, they are also a "smaller copy" of the 70 C engine, who I also have worked a lot with. (UIM S-850 class) The 30 hp 3 cyl was not exactly dominating the S-550 class, it was and still is the 40 hp 2 cyl Tohatsu doing best overall. According to the class and rules we`re not allowed to do as much as we want, but with a little patience and creativity you can do much even with tight homologation tolerances. Its about blueprinting afterall, and my opinion is that a well "balanced" engine, (talking about weights, combustion, ignition, carbs, exhaust and all things in accordance to physical and mechanical laws of the combustion engine) is not about doing much of all, but more about doing the rigth things! Ex carb setup, we were running different jets in each carb, why? Check a standard engine, and look at combustion picture at piston and spark plug, never the same picture;-) The same with measuring the port heights and similarity on both placement and heights on each cylinder, never the same, I think in a good engine there should be no difference. The cylinder liners is possible to take out of the cylinder block, there you can grind and adjust the channels behind the cylinder liner to be more effective and to fit better to the ports....as you read, lots of work possible to do, and lots of time running... Good Luck;-)