What to expect from these port specs?

2smokebob · Aug 8, 2015

I put a degree wheel on my PK80, what's it gonna run like??

Transfer 120' ATDC
120' Duration
Exhaust 110' ATDC
140' Duration
Intake 124' Duration

Measurements are with no base gasket. Crank trued with dial indicator. Crank case matched to transfer ports. Cylinder ports cleaned up, but not changed. Piston skirt trimmed to clear intake port. Intake port matched to Manic manifold. SHA clone carb. Piston comes to top of cylinder, so squish will be head casket. Expansion chamber not made yet. Wondering if anything looks wrong here. What RPM/power characteristics to expect so I can make the pipe right for the engine.

Thanks!
Bob

Deleted member 12676 · Aug 9, 2015

why in the world would you go to the trouble to get incorrect readings due to lack of base gasket?
my god , man

crassius · Aug 9, 2015

my thought is to run it & see

2smokebob · Aug 9, 2015

I found one reference that 120' transfer duration would peak out at 6000 rpm. Another said "120 I find good". So I found it interesting that I hit those numbers with no base gasket. I have a lathe (and I'm not afraid to use it!), so I can trim gasket clearance off the bottom of the cylinder, or just try RTV sealant. The base gasket doesn't concern me at the moment. I'm just wondering what number combinations are healthy.....proper...? I'm sure it has all been figured out by now. I'm not looking for a secret 9HP blueprint, just what numbers work well on a hand built engine.

Bob

crassius · Aug 9, 2015

I've got many used cyls here - all different by a few mm in port height - the H2s have one trans port 4mm higher than the other

all ran acceptably when they were used

butre · Aug 11, 2015

my bike has no base gasket, I put a thick layer of paint on the bottom of the cylinder, waited for it to get tacky, and then put it on the bottom end. it seals perfect, boosts compression, and improves intake timing.

Deleted member 12676 · Aug 11, 2015

I have considerable experience porting these engines and so would recommend you keep the transfers and the intake the same and then raise the exhaust port little by little till you get the top RPM you want. A duration of 156 worked good on my engines for how I wanted them to be.
But before going to a higher RPM you need to balance the crank or you will have even more disastrous vibration.

butre · Aug 15, 2015

jaguar said:
I have considerable experience porting these engines and so would recommend you keep the transfers and the intake the same and then raise the exhaust port little by little till you get the top RPM you want. A duration of 156 worked good on my engines for how I wanted them to be.
But before going to a higher RPM you need to balance the crank or you will have even more disastrous vibration.

I have to disagree with that. the intake on a stock engine badly needs to be lowered, and the transfers need to be widened and cleaned. the exhaust port should just be widened in my opinion.

Deleted member 12676 · Aug 15, 2015

the intake port duration he gave was longer than stock, being now at 120 which from my research is ideal.
the transfers can be widened toward the rear a bit but due to the natural limitations it can't be done very effectively. I wouldn't discourage it though.
the exhaust height is only ok if he is ok with the top speed. But consider this, the blow down from exhaust opening to transfer opening is only 10 degrees which is not enough. To correct that he needs to raise the exhaust port.
ps- all this is assuming he really knows how to correctly determine the durations of the ports.

butre · Aug 16, 2015

the problem with changing exhaust height is that you run at a higher RPM when you do. since it just shifts the power band, and doesn't really improve peak power all that much, I find that you're much better off changing the gearing to improve top speed than you are changing port timing.

of course, changing port timing is free, whereas a good 32 tooth sprocket starts out at about $25

What to expect from these port specs?

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