Squish clearance question

[dirtyhandslopez]

Stupid question no. 873.

I understand the need for a correct squish, but no standard set up scoot I have ever worked on ever had a squish that could be measured, but they still run fine, didn't overheat etc. The sqush definately did not seem to be of importance at the factory.
So why are we all hot and bothered about the squish? Just cause were tuning them a little?

[J1MS]

Stupid question no. 873.

I understand the need for a correct squish, but no standard set up scoot I have ever worked on ever had a squish that could be measured, but they still run fine, didn't overheat etc. The sqush definately did not seem to be of importance at the factory.
So why are we all hot and bothered about the squish? Just cause were tuning them a little?

Apart from fuel being unleaded and lower octane than when Lambrettas were designed, the squish type head that is fitted to a lammy has a cooling effect and did even at 2+mm as most left the factory with, but not as effective as a tighter squish.
If you are one of the tuning Guru's that makes expansion chambers squish is an important part of the design formula (MSV) the speed of the flame front is influenced by squish band area position and clearance plus combustion Chamber size has an effect, both depth & shape.
For some it's a piece of solder down the plug to check clearance for others it's an essential part of obtaining more power while keeping reliability (offseting detonation) and cooling the engine between power strokes.
Me I just sick a bit of solder down the plug and hope it's around 1.2-1.5mm.

[dirtyhandslopez]

So even a 2mm+ clearance helps with the cooling?
I've read, read and re-read and then read agian A.G. Bells tuning book, so I understand the theory behind the squish, But I have in the past built motors without checking squish and they have and do run fine. Just lucky or is there still a certain amount of lee way in that respect in the Lammy motor?

[dirtyhandslopez]

Sorry for hijacking your thread TS1
I just read you squishy post in the tuning section Jims, and the last part didn't sit well with me.

"5. Summary
So what have we learned:
• As far as squish is concerned whether it helps or not depends on the
residual turbulence at the time of combustion.
• As for squish quenching the flame at the edge of the squish band to
stop detonation is not looking very likely
"




So is it all worth it? Has anyone done actual heat testing after tuning a Lammy using the squish as an agent to stop detonation? These would be stupid questions 874 and 875

[J1MS]

The post I put up "squishing stuff" was in response to a question I was asked.
It wasn't my thinking in all points but it was relevent to a question a mate asked & that I couldn't explain to any satisfaction. This post covered the topic of squish velocity as this was the point I was trying to get over. Two cylinder heads of identical compression ratios can have different characteristics and suit different exhaust systems and port heights. I was trying to explain the compressed gas having different speeds across the piston and how it could slow or increase the burn time effecting power delivery.

The point you make threw me too. But This was a footnote at the end & not fully explained.

[TS1-200]

Wish I never have asked anyway all sorted now, erred on the side of safety and re-profiled the head very slightly,( by hand) and now have a squish 1.2-1.3mm, run it today and its nice. Change of subject though, Langcourts did a brilliant job repairing & replating the barrel, could be mistaken for a new barrel, took 2wks.