cylinder swap

[yamahansolo]

hello

i was just thinking and i am searching but i am not getting anywhere...

we just brought the new baby home from the hospital yesterday. this is number 4.
i now have 2 boys and 2 girls... any who.

can i use my xj cylinders on the xs block and xs cylinder head so only the section that the pistons are in would have the xj part... i thought about getting it bored and everything while i am still riding my xs and then swapping part of the top end... i know i need to keep the 79 cams and head for the power...
any thoughts

[DiverRay]

The engine parts can be swapped back and forth almost at will. The only thing you need to watch is the YICS ports on the head/cylinder of the XJ, and the LACK of them on the early engine.
The XJ has the bigger valves, but it will be a tight fit with the XS head on the XJ. If you do bore it, the Wiesco pistons will negate the problems, and it should run just fine.

[yamahansolo]

cool... also i thought about tapping the yics and putting fuel injectors or something in there??? i don't know? i think an obd1 aldl type computer from a gm might work with the right injectors. i had an older camamro that you could burn your own chips for. pretty straight forward. they also used a manifold absolute pressure sensor instead of a mass airflow meter... and i happen to have the map sensor from my xj..... maybe a tpi 4 cylinder gm car or truck ecu would work?
also are you saying the pistons are bigger in diameter??? or are they a little taller in the cc.
and if i do this i plan on having the head ported and ploished anyway. maybe i'll get a good improvement.

also i thought about doing a vmax vboost type mod. i know that the carbs flow plenty but if you had cylinder 1 and 2 hooked up to the same carb it will draw more air, it would pull a little from the cylinder that is not firing at the moment. i had a performer rpm manifold for my camaro that allowed this to happen. i think it might even out the flow a little. similar to the yics.
vtwins use two carbs sometimes.
they have some cool carbs out now days. there is one that has a ribbed venturi on the top part of it and it uses 3 adjustable screws for tuning it. no jet changes needed. you could fab a manifold up for two of them and maybe get some gains?? i don't know??? it be different i guess.

[trbig]

Do your homework on the port and polish. If you polish the ports to smooth, you will LOSE H/P! You can smooth up the valve relief area of the head and do some grinding of the intake to make smoother transitions into the valve, but the roughness helps atomize the fuel better. I wouldn't touch the exhaust port.. but that's just a suggestion.

The earlier XS (78-79) has a lower domed piston than the later XS/XJ models.. but a smaller valve relief area.. making the compression higher. The Wiseco pistons have a dome that is shorter than the later XS/XJ, but taller than the early XS's... so it's in the middle.

I don't know the answer as to which head would give better performance. If you use the early model head, you will have increased compression, but the smaller valves. I don't know if you used the later head if the larger valves could help you make more HP. You would probably need to change the cams to an aftermarket to let that become a factor. Maybe Dan Hodges will chime in here.


Tod

[skids]

CONGRATS man! Where's my cigar?

hello

we just brought the new baby home from the hospital yesterday. this is number 4.
i now have 2 boys and 2 girls... any who.

[yamahansolo]

ok i don't get it about the exhaust... on chevy's people usually polish the exhuast side until it's very shiny so the carbon doesn't stick and it has a smooth airflow. and the intake side shouldn't be smooth because of the fuel atomizing. i can't remember exactly though it's been a few years ...

[trbig]

I don't know exactly why the exhaust thing is how it is. Something to do with needing back pressure and scavanging. On a Chevy.. or anything other car, you can usually get the best performance from running open headers. On these bikes, a 4 into 1 is usually the best for performance. ??? Don't ask me.. it seems backwards. To me, it seems like you should be able to cut off the collectors and run straight headers for best performance... but it just ain't so.

Hopefully someone that knows will chime in. I do remember reading a long post one time that warned against messing with the exhaust ports. But.. I have also read long posts about not jump starting a battery from a running car.. which proved to be wrong?

Tod

[Ivan]

I wonder if the 4 int 1 headers work somewhat like a tuned pipe (expansion chamber) on a two stroke engine.

One charge exiting the header causes a venturi effect on the other headers, in effect, pulling the next exhaust charge out.

Just a thought.