Porting the GT top case

advant63 · Post by **advant63** » Sun Aug 30, 2009 6:45 pm

a question from the unlearned.
will the increased crankcase volume (i know it's not much) have any effect on the overall efficiency? will the gains from radiusing everything be lost in lower crankcase pressure?

Suzukidave · Post by **Suzukidave** » Sun Aug 30, 2009 8:43 pm

According to those who know this answer the newer thinking on crank case volume is that a tighter volume isnt as much a issue with todays design pipes that actually draw the mixture up the transfers along with the down strokeing piston pushing the mixture out and then this extra mixture is super charged back into the cylinder dureing the reversion pluse . If i didnt quite explain this correctly i am sure one of the "all Knowing" will correct me

tz375 · Post by **tz375** » Sun Aug 30, 2009 9:03 pm

Dave,
That looks fine. As you can see, the flow pattern will be very different from one side to teh other. Replacing sharp angles with radii is not always a good move and in intake ports it has been found that a simple sharp change of up to 15 degrees has the gas changing direction nicely. With a radius, the same flow often separates from teh surface and results in less flow. Strange but true.

In the case of an asymetrical flow like the GT, what you did should work fine. If you really want some work, try to make the floor concave so that it encourages the flow to change from almost horizontal to vertical.

tz375 · Post by **tz375** » Sun Aug 30, 2009 9:13 pm

The simple answer to the crankcase compression question is that it has enough already and creates more than enough combustion chamber turbulence. Removing such a small amount of material will not make much difference to the primary compression.

Lets say that stock primary CR is 1.3:1 so the crankcase volume is about 823cc. let's say that Dave removed 5cc of material, the crankcase is now 828cc. The difference in CR is tiny.

Suzukidave · Post by **Suzukidave** » Sun Aug 30, 2009 9:41 pm

Thanx as always for the input Richard , i guess if it really doesnt work out it will just mean replaceing the top case with another . Do you happen to have a picture of the adjustments you made to your case ?

tz375 · Post by **tz375** » Mon Aug 31, 2009 9:00 am

Not this time. Funnily enough I seem to be in teh habit of taking lots of picturs before I start something and I rarely take any after I finish the work. When it's done e bits go back together and then I remember that I didn't take any pictures. Ah well. There's another motor to be done soon.

I'll see if I can remember to take a few next time.

Suzukidave · Post by **Suzukidave** » Mon Aug 31, 2009 9:07 am

Oh Well , but basicly instead of removeing so much aluminum you just rounded the sharp edge ?

tz375 · Post by **tz375** » Tue Sep 01, 2009 11:07 am

Not necessarily

This motor is tame, so I didn't spend much time on the cases. The next one is a whole nother matter - as they say.

Suzukidave · Post by **Suzukidave** » Tue Sep 01, 2009 8:49 pm

Thats not fair .. so i have to wait to see whats up