Piston below exhaust port - is it a bad thing?

[dickie]

Over the winter I'm planning to swap the 58/107 crank on my gt186 for a harry Barlow 60/110 I have lying around.

I thought I'd take the opportunity to raise the exhaust a few degrees by using a 4mm packer rather than the 'standard' 3mm.

By my reckoning this will leave the piston 2mm below the exhaust port at BDC.

I could cut the bottom of the port but if I don't like the result I'll be stuck with it.

So is it OK just to leave the piston this far below?

[Dimitrios_231]

If you don't plan to do any porting work,don't do it.
The only benefit that I can think is that you might not need to use a head gasket to get the squish right.

Transfer timing will raise too much,blowdown will become a little smaller,
transfer port bottoms will also be above piston.
The scavenging will be less efficient,if you use an expansion chamber this will be more noticable,
the air/fuel entry to the bore from the transfer ducts will also lose flow.

[Warkton Tornado No.1]

By your reckoning, the modification will take your piston to head @ TDC exactly where it was & your transfers & exhaust will be raised by the 2 mm difference in stroke which Dimitrios ( ) has commented upon. Although his comments are true about blow-down & the advancement, all of these things are relative.

Dependent upon known quantities, like the existing port timings, exhaust in use & power/torque, I would venture to say try it, for how little work is involved.

You can always subsequently recess the head & fit less packing if the engine proves too peaky.....

[Dimitrios_231]

Transfer duration will be higher than 135 (around 136.3) degrees having 22 (around 21.6) degrees blowdown,
I wouldn't like to ride an engine like this.

Exhaust port length is more than 60% (around 62%) of the bore having two rings of 1.2mm and 1.5mm,
port shape is quite squarish so the only way to go is to raise it (already close to 180 degrees),
the motor will be peaky,might be good for racing but not great for road use.

P.S. I work on a GT186 kit this period,I'll also do another one with a 60/110 gran tourismo crankshaft soon
I'm thinking to open a third port (transfer duct) on the cranckcase/barrel above the inlet/reed straight into the reed but this is another story...

[Lambretta Sash]

Piston below exh port is ok as most modern two strokes have exh port this way.
I've had good results in giving more blown down to cylinder which gave similar dyno curve and output as ss200 kit.
t im sure transfer timing will be too high as already pointed out

[Warkton Tornado No.1]

As I said in my original response:

“Dependent upon known quantities, like the existing port timings, exhaust in use & power/torque, I would venture to say try it, for how little work is involved.”

However, apparently the port sizes & timing are a known quantity as Dimitrios has stated, in which case ignore my earlier advice. I didn’t realise such kits were so finite, unlike the Rapidos etc that I have always set up on an individual basis....

[Dimitrios_231]

I could cut the bottom of the port but if I don't like the result I'll be stuck with it.

If you ever decide to cut the exhaust floor,make sure that the piston skirt at TDC will cover the port.
Some modern 2T engines use really short pistons,this could be a reason for having the ex. floor higher than the piston crown.
On the Aprilia RSA 250 (one good 2T engine ),I think that the piston at BDC is 1.5mm higher,

[dickie]

My exhaust is already raised a little so my timings are about 178ex and 124tr.

Maybe I'd be better with 3mm packer so that I don't move the transfer too much?

[Warkton Tornado No.1]

Now that you’ve posted some port timings, then I can agree that you shouldn’t be adding a packer anything like 4 mm.

If were you, I’d think through what exhaust & transfer timings you’d like to aim for & calculate from those goals the packing required.

You can either do it mathematically or draw it on any 2D CAD package such as AutoCAD.

[Dimitrios_231]

In standard form,using a 58/107,no packer/base gasket,(no head gasket) I get:
Ex 174.85
Tr 128.93 ??? Bigger than yours

Using a base gasket or a packer,my squish gap becomes large.Do you use any?
Is your head recessed?

To get 124 Tr,I must machine my cylinder base 1mm,I use a S2 Italian crankcase (height difference ?)
With 1mm out,I get similar results with that quoted in the GT186 manual (EX 170.86 Tr 123.86) so my measurements are not off.

I did some maths,to get 178 ex degrees and 124 tr,
the exhaust port must be raised 1.8mm,(cylinder base still 1mm machined )
With 1.8mm ex. raised,using a 60/110 with 3mm packer

EX 178.74
Tr 126.82

Changing to 4mm packer

EX 182.55
Tr 131.63

Not so bad
the initial calculations were taken using my cylinder's (standard one) data.

[dickie]

Gents, thanks for your advice. I'm away from home at the moment so I can't check, but I know my exhaust is 178. Maybe I'm wrong about my transfer timing.

Also from memory I only have a small Base gasket (maybe 0.5mm) and no head gasket. Best not to guess anymore and check when I get home.

[dickie]

Been crazy busy with work so I've just got the chance to check my port timings.

The exhaust is 178 degrees as I said but the transfers are 126 rather than the 124 that I said initially.

I'm going to speak with Chris sturgess about porting work as I need to take the engine out to replace a knackered silent block. May as well kill two birds with one stone.