engine tweeks to increase hp

[speedracer831]

huh. well, folks, just tried putting on the head. it fits my gt4 PERFECTLY all it needed to have clearence was another head gasket. it has higher compression for sure, but in the process of torquing down the head bolts i snapped one, so off to sbp i go to get some 8 grade ones. too bad, i was psyched to ride it with high compression.

 

[clay]

busy afternoon

 

[speedracer831]

is that to balance the flywheel.. im not fully understanding. i have a friend with a lathe so ill eventually get my flywheel lightened. have you done this clay? also, are you big into porting cause if so i could give you some useful specs

 

[clay]

if you were to measure your crank the same way, it would read 20mm on the calipers because the your motor has a stroke of 40mm. that means mine will have 50mm stroke. since my bore is 45 that will give me 79cc of displacement, under the limit for the death race.

 

[Egor]

Clay, So you welded up the crank and bored the pin holes out 5mm. Are you going to run the bolt on parts? I saw one fella left them off. There is just one problem with leaving them off, if you look on the inside of them you will see that on the big end side they are hollow. This is actually smart on there part, it keeps the crankcase volume down. The problem I have is that I have spun the wheels with just my porting. I can get the rpm up there easy, just starts tearing everything apart. I took one of the cranks apart and found that it was just mild steel. I was able to put it back together with a common shop vice. I saw one fella welded the pin to keep it from slipping. It would be nice to find a bolt in replacement. One of the reasons I don't go high comp. is that there is nothing in the lower end that can handle it. I have some Honda cranks, and I will have to look and see if it would be possible to narrow one down. I can turn the ends on my lath, but they also have the 12mm gudgeon pins. I am interested to hear how the stroked crank turns out. Have fun, Dave

PS: Pic of my new carb. Off a small outboard.

 

[clay]

in the next day or so i will get out the gram scale and balance the whole assembly. once it's balanced i'll throw a little bead on the pin. i beg to differ on the high compression though, my head gives me 140-145psi and the only issue i've had is with the top rod bearing, (which should be replaced anyway).

 

[bigdan]

it fits my gt4 PERFECTLY all it needed to have clearance was another head gasket.

So then, am I correct in assuming the Puch head is pretty much a bolt on replacement for the 70cc HT motors?

 

[speedracer831]

yep, its a direct bolt on. but i would buy something thats for a 47-48mm bore so the piston actually fits in it and gives a proper squish band. my combustion will be a little screwed up since its for a 45mm.

 

[clay]

i had to clearance the bottom of the skirt because of the extreme increase in stroke.

made the balance pad sections of the flywheels out of aluminum and balanced with brass slugs.

money shot

 

[Egor]

Clay - I am looking forward to hear about the stroker crank you made up. There is an interesting fact about stroking a two stroke engine. Unless you find a way to move the piston in the center of the swept area you only gain the leverage the new crank will have to motion. It is true that the time that the piston covers the exhaust port to the top will be the same as it was with the old stroke. But If you were to add a few head gaskets (or a spacer) instead of moving the cylinder up to keep the piston from hitting the head you would gain more swept compression volume. There should be enough area from the top of the top ring to extend the piston out of the cylinder without letting the ring out also. The distance in relation to the head would stay the same IE (compression ratio) or squish area. Let us know if the new cylinder has a longer area from the top of the exhaust port to the top surface. Also you have to be careful not to let the piston go so low that it exposes the intake runner. You will have the opportunity to lower the transfer ports and gain transfer time. Ingenious way to make up the sides of the wheels. What formula are you using to balance the crank? Thanks. Have fun, Dave

PS: I Need to get a turntable for my mill!, So I can do some work like this.