squish

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tea5ive
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I've just swapped heads on an imola top end. Squish is coming in at 0.80mm. Is this too close?. Using packers top and bottom and don't want to add a gasket aswell as the top packer. Thought it might get away without buying another packer. Cheers Simon
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Simon, that's tight. Why did you change heads? What does the CR work out at (corrected) with that head? I would change one of the packers (no gaskets), depending on your choice of port timing/ exhaust you're planning on using.
Vague perhaps but does depend on port timing, the reduction of components to fail, and reducing CR that becomes a little more ethanol friendly. S
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tea5ive
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B-Race Tuning wrote:Simon, that's tight. Why did you change heads? What does the CR work out at (corrected) with that head? I would change one of the packers (no gaskets), depending on your choice of port timing/ exhaust you're planning on using.
Vague perhaps but does depend on port timing, the reduction of components to fail, and reducing CR that becomes a little more ethanol friendly. S

the reason for changing heads was i used it on another motor as i did not have another at the time. the new head is a recessed type that drops into the cylinder. i am also using a 62/116 crank hence the packers top and bottom. with the old head i had the packers made to suit that with squish coming in at around 1.36 i think it was. i dont want to add a gasket as its increasing the chance of leakage. so i think the safe option is another top packer . as for exhaust i was going to use a mbd rev-tour as thats what i have spare, and i have no idea how to work out the CR ??. should also add the old head was a standard 150cc machined to suit the imola top end . the new head is an imola head with the central combustion area and the extra fixing holes and recessed to drop into cylinder.cheers simon
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I wouldn't have thought putting thicker on the bottom would be good for the port timings on an Imola, but difficult to say without physically checking.
Running an Imola with it's standard design head and a 62 stroke? You REALLY need to measure your CR and correct it, but in doing so, leave your packers as they are and take 0.2-0.5 off the squish area giving you a 'minimum' of 1mm, prefer 1.2 on a road motor. You won't lose the spigot (head into barrel to centralise) Dowel your top packer though. Simon.
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tea5ive
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B-Race Tuning wrote:I wouldn't have thought putting thicker on the bottom would be good for the port timings on an Imola, but difficult to say without physically checking.
Running an Imola with it's standard design head and a 62 stroke? You REALLY need to measure your CR and correct it, but in doing so, leave your packers as they are and take 0.2-0.5 off the squish area giving you a 'minimum' of 1mm, prefer 1.2 on a road motor. You won't lose the spigot (head into barrel to centralise) Dowel your top packer though. Simon.
yes, thought of skimming a bit off the squish area. how do you measure the cr ??
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Accurately measure from the top of the cylinder (including any packing plate) to the uppermost edge of the exhaust port. Your squish is 0.8 but you'd like 1.2 so measure the spigot depth part of your head, deduct 0.5mm from that figure then deduct the resulting figure from the exhaust port height measurement, and record. With the head correctly fitted, set the piston at TDC. With the spark plug hole uppermost, thats the surface the plug seals against totally horizontal. Use a syringe and accurately measure the volume of the cylinder head using a medium viscosity fluid. (And before everyone shouts, if you use a 50/50 petroil mixture or oil alone etc. your syringe will last a week. I use cooking oil). 50ml syringe from a vets / boots will be very cheap. Measure volume to the bottom of the threads of the plug hole. Repeat. If not the same result, repeat.
You can now work out the volume of the cylinder above the exhaust port, without head volume. (area of piston, pi r [squared]) X height above exhaust port.
You'll have to get your own advice on the CR that you want to be running based on use/ fuel/ timing. I am now building in as much safety margin as I dare regarding the fuel issues being experienced v performance and setting mine at 5.8 to 6:1. Take advice regarding the kit/ manufacturers recommendations. If choosing 6:1 (makes the maths easier to explain), divide your volume of the cylinder above the exhaust port (your measurement doesn't include the head) by 5 so you've now got a volume that's a fifth. when you add that 1/5 volume to the whole you've got 6/5. That 1/5 volume of the cylinder is what you want your oil volume to be. 6:1. 6 parts of volume, 5 in the cylinder 1 in the head, compress those six parts into the space of a 6th, 6:1. When you get your head done, you'll find that It'll probably be on/off two/three/ as required times to get it spot on. It IS worth the effort. Set squish first to profile of piston, if possible the minutest of taper from the cylinder wall, and keep width of squish band/ ring as is, or as close to but under 50% area of bore. put small .75/1mm radius at inner edge of squish ring to combustion chamber. adjust volume of bowl to suit. Fit, measure, adjust, refit, measure, etc.etc,. Job done. This is only how I do it, others will have other/ maybe simpler ways. Doing it this way I'm doing what I feel is best effort to prevent ethanol laden fuel detonating. I welcome all comments on above/ what I've missed (didn't sleep last night) or better ways to do it. Don't expect many tuning businesses to comment though.
Noting your location and as your engine is out, you've got many tuners who will give you their recommendations on CR and do the machining for you and a reasonable cost, the obvious of course is Scooterotica surely.
Did you really think mating 62/116 to an Imola was going to be "bolt on" :D
Simon
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one thing to note is that the corrected CR is only applicable before the pipe comes on if using an expansion. once the pipe starts stuffing the exhaust port the CR will be higher!
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^^^^100% agree, (But quantify?). My 6:1 incorporates allowance of an expansion that works well, but some WILL work better, or worse. It's a pretty much un-defineable figure, so purely based on my own experiences over the last 30+ years. (not the ethanol issue, the expansion chamber variable). Simon.
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.quantify?).
if it pings when the pipe comes in its too high :lol:
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:lol: :lol: :lol:
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