Adjusting squish, etc. Some questions.

[jdosher]

I'm doing the top end in my 300 (its first one), and I've taken some numbers down in order to decide how I want to adjust the squish. I'm planning on putting in a shorter base gasket stack, but I want to get some advice from you guys to make sure I don't get the compression too high or throw the port timing off too much. Here's the measurements I've gotten from the stock setup:

squish, about 2.3 mm
trapped volume, about 26.3mm
computed UCCR, 12.1:1
deck height, -4.45mm (I used digital calipers as a depth gauge, so this is not very accurate)
squish band offset from head surface, 2.40mm (just for reference)

I have 2x green base gaskets, and one grey. I've measured the thickness at 1.3mm.
Q: Are the numbers on the "measuring the squish" page from JB correct? Green is .3mm and grey is .5mm? That would give me a 1.10mm stack, which doesn't measure out. But if green is .3 and grey is .5, then that matches what I've measured.

At BDC the piston is even with the bottom of the exhaust port.

On reassembly I'm thinking of putting in a single .50mm base gasket. That'll give a squish of about 1.5 - 1.6mm, which sounds like a good target. However, it will lower the ports so that the piston will be 0.8mm above the exhaust port at BDC. That should give some improvements to the bottom end, but I'm wondering if .8mm is too much? I'm fine with sacrificing some top end, but I don't want to go overboard.

I currently have a .3, .38 and .5mm cometic base gaskets on the bench.

I have a lathe, and could cut the cylinder head, but 1) I'm not a professional and 2) it's a mini lathe and not all that rigid so I'd be concerned about the interrupted cuts over the cooling ports.

As to the change to the compression ratio: going from a 1.3mm stack to a .5mm stack = .8mm difference, which should result in a change to the trapped volume by about 3.3cc. So 26.3 - 3.3 = 23cc, which should put my UCCR at about 13.7:1. I'm running 98octane pump gas (or whatever is the highest available octane when getting fuel). Is 13.7:1 a tad high for pump gas, or should I be okay? I'm generally riding near sea level, low 40F in the winter to low 70's in the summer.

I know I can send the head off, but frankly I'm happy enough with the bike and I'm just looking to make improvements with what I have available.

Thanks in advance for your advice!

-Jesse

 

[Jakobi]

Jesse,

Looks like you've crunched all the numbers. I'm assuming they are correct.

If you want to change it out for a single 0.5mm gasket with piston 0.8mm above at BDC you'll boost the bottom. You may experience some loss of top end but that might suit you.Its not much to try. 1.5-1.6mm is still excessive but will be a vast improvement over 2.3mm though. Ideally you'd send the head to have it machined. Re 98RON fuels, I've been running my 250 with 1.25mm squish at 14:1 UCCR on 98 just fine. Yours at 13.7:1 will be at the higher end of the spectrum but should be fine for your purpose. Worst case is that you don't like it. Pull the cyl off, slap an extra 0.3mm of gasket (0.8mm total) and try again with 1.8mm squish. Everything will be a compromise until you have the head set up professionally, and then it'll be a constant setting for each and every rebuild.

Re the gaskets why not measure a green one individually? I'm sure green was 0.3mm and noticably thinner/flimsier than the 0.5mm grey ones. Are yours OEM?

Cheers
Jake

 

[jdosher]

Jake,
thanks so much! That's just the info I was looking for. I'm going to just put in a single .5mm base gasket and see how I like it. I'll re-measure the trapped volume with the new setup to double-check my calculations vs the real world. I'll quite likely have RB do the head sometime down the road, but for now I wanted to just tackle one thing at a time, as well as get back into the woods next weekend.

You've done a fair amount of research into this- what do you think is the idea squish value and piston-to-port distance for low-end performance? I ride mostly tight single track, twisty double track, and once a year get out to the desert (where I'm completely out of my element, lol). I can use "lugability" and a clean(er) combustion more than top-end peak performance. From what I've read I'd guess around 1mm squish and the port 5.-.7mm lower than the piston at BDC?

Thanks again.
-Jesse

 

[Jakobi]

There is no 100% correct setup. Just the setup that works for you. From what I have read once you start moving the port timing too far (much past 0.7mm above piston at bdc) you noticably start to compromise top end. Its only 1 of many variables all working together though so no hard and fast rules.

Minimal squish is ideal, up until the piston starts skimming the head, and then once its full contacting its not to good. 1-1.3mm is generally a pretty acceptable figure for a 250/300 of this nature. Its not like they see insane rpm. Squish angles, squish band width etc is all well out of the scope of my knowledge. The UCCR is best used to determine fuel requires (some prefer cranking compression for this). Obviously compression ratio also moves the power around a bit too. Lower comp = freeer revving vs higher = more snap off idle.

I'm actually going to discuss things with Dave who did my head and see if I can get him to run some simulations with smaller exhaust ports off the bottom. Rotating the pv through its motions there is still room for the valve to drop down lower. It may be possible to modify the actuator plate in a manner that allows it to rotate around a bit more and pick up some more bottom end grunt without sacrificing anything up top.

 

[jdosher]

I'll be interested to hear what you find out about the PV actuator plate/range of motion.