Hi Guys,
I'm trying to improve the performance of my motor. I am using a lift plate to enhance the porting and machined the barrels accordingly, (3mm).
I have plus 0.8 piston deck height and also skimmed 2.5mm from the head but machined a squish profile to bring the dome back to 70mm dia
I have 120psi on the centre cylinder but the motor max's out at 145km/h.
I am about to fit 3 into 3 expansion chambers which I will have to sleightly modify to fit around my suspension and swinging arm.
What is you CR and or PSI? I'm looking for advice to squeeze a little more performance from the engine
Phil
Compression ration and PSI
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PhilGT750
- Posts: 11
- Joined: Thu Apr 11, 2019 1:43 pm
Compression ration and PSI
[img]file:///C:/Users/Samm/Desktop/PXL_20220319_060304440.jpg[/img]
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teazer
- Posts: 1035
- Joined: Mon Mar 07, 2011 8:10 pm
- Location: Chicago
Re: Compression ration and PSI
120psi is low even on a stock motor. I would have expected to see a much higher number unless the exhaust port is crazy high.
Normally the pistons are close to 0.5mm below the deck, so with a 3mm lift plate plus an extra gasket, I have 4.0mm machined off the top of the block.
I have never taken that much off a head though. Usually 1.0 - 1.5 is enough to give around 165psi depending on the porting. How tall is the exhaust port now and did you crank the motor over enough times to let the pressure stabilize?
BTW, what make of gauge are you using? Some of the cheap sets read low. See if you can borrow a good one form someone to test back to back.
My last bike hit 110mph (175KPH) in the quarter mile with street chambers and mild porting and stock BS40 carbs. Yours should do close to that even with stock pipes
What is the secondary gearing? https://pinkpossum.com/GT750/GT750%20gearing.pdf If it's way too tall or short, that might explain it.
What ignition system and what is it set to ? If timing is really far off, that could hurt your speed.
But start with the compression. 120 is low so let's start with the gauge and then if it is accurate, determine why it's so low.
Normally the pistons are close to 0.5mm below the deck, so with a 3mm lift plate plus an extra gasket, I have 4.0mm machined off the top of the block.
I have never taken that much off a head though. Usually 1.0 - 1.5 is enough to give around 165psi depending on the porting. How tall is the exhaust port now and did you crank the motor over enough times to let the pressure stabilize?
BTW, what make of gauge are you using? Some of the cheap sets read low. See if you can borrow a good one form someone to test back to back.
My last bike hit 110mph (175KPH) in the quarter mile with street chambers and mild porting and stock BS40 carbs. Yours should do close to that even with stock pipes
What is the secondary gearing? https://pinkpossum.com/GT750/GT750%20gearing.pdf If it's way too tall or short, that might explain it.
What ignition system and what is it set to ? If timing is really far off, that could hurt your speed.
But start with the compression. 120 is low so let's start with the gauge and then if it is accurate, determine why it's so low.