Wiseco high comp piston questions. 1981 GS850G.

[Chuck78]

You will get a higher compression ratio and have deep valve reliefs that will accept large lift high performance camshafts. The added compression ratio alone will give you a bit of a boost in power, but nothing astronomical, but a good step in the right direction. The ability to run the more performance-oriented aftermarket web or megacycle camshafts using stiffer valve springs will give you a huge boost. Having the cylinder head ported and having a Serdi valve job done to it will also give you some massive gains. Rapid ray on the forums is a pro at this work, check with him.

[Chuck78]

Also, getting some custom pistons made is not that difficult. Wiseco would probably design one for you based on the 69 mm K844 piston, but you would have to buy 12 minimum order, and the price would go down from there per piston if you got several other buyers. I have 1-3/4 sets of 920cc GS850 pistons, but they are not for sale. They are from the 1980s. That was a killer street engine (chain drive gs750 with 850 cylinders), a bit unruly with me at the helm. Unfortunately I blew up 1 cylinder. Ray bored out another cylinder for another set of pistons, and I need to get my ported cylinder head back from him also! And my crankshaft back from Pearson racing. This will be the best 750 of all, Yoshimura cams, ported head, 850 cylinders, 920cc pstons.

[Drybear]

Thanks for the information.
The K844 pistons and webcams grind #110 are on the way, and I have also got a mild porting of the head.
Open exhaust and inlet system as well..
Anyone who has a similar setup and can estimate the Power gain?

[Chuck78]

Big power gains by the seat of the pants feel of it!

I'm not sure exactly how the Wiseco Pistons will fit into a used stock 850 cylinder, however. You need to check the piston to wall clearance and make sure it is within spec especially according to Wiseco, because that piston will expand and contract differently than the stock cast piston. Also, setting your ring end gaps properly according to Wiseco specification is extremely critical. This can make-or-break your engine literally, and also can be responsible for a lot of oil consumption if not done properly. Or breaking the rings if too tight!

[Drybear]

Big power gains by the seat of the pants feel of it!

I'm not sure exactly how the Wiseco Pistons will fit into a used stock 850 cylinder, however. You need to check the piston to wall clearance and make sure it is within spec especially according to Wiseco, because that piston will expand and contract differently than the stock cast piston. Also, setting your ring end gaps properly according to Wiseco specification is extremely critical. This can make-or-break your engine literally, and also can be responsible for a lot of oil consumption if not done properly. Or breaking the rings if too tight!

Thanks for your advice.
The cylinderblock is in good condition but I will take it to a workshop for a proper hone job anyway as soon as the pistons arrive.
The bore is 69.02mm and I hope that will be acceptable?

[exdirtbiker]

Double check the valve lash spec with those Web 118s.
I think I just realized why my Beast never ran as good as it should... I found on Web cams own web page that #118 cams should have .006" clearance.

Yes, .006". I thought "gee, that's a lot, but they're the mfr"... In a FREAKIN VOLKSWAGEN !!! ?? F'n A !!! They make 2 cams with same name, and I found the spec sheet for the wrong one !!!

Now I gotta go call the guy that bought my bike... I still don't know what the gap should be. I just know I got it wrong.

[Drybear]

Double check the valve lash spec with those Web 118s.
I think I just realized why my Beast never ran as good as it should... I found on Web cams own web page that #118 cams should have .006" clearance.

Yes, .006". I thought "gee, that's a lot, but they're the mfr"... In a FREAKIN VOLKSWAGEN !!! ?? F'n A !!! They make 2 cams with same name, and I found the spec sheet for the wrong one !!!

Now I gotta go call the guy that bought my bike... I still don't know what the gap should be. I just know I got it wrong.

Thats interesting.. My cams are the #110 grind and the valve lash spec. is 0.006 and 0.008" Its a lot compared to OEM for these engines, anyone who can explain why?

[exdirtbiker]

I could not find where they specify lash anywhere on their website, except for the above mentioned VW cams.
Did they send documentation that says .006" ?? I'm even more confused now!!
I would call them ... web site says they will not answer tech questions via email, you must call.
951.369.5144 Phone (8 am to 4:30 pm California time)

[Drybear]

I could not find where they specify lash anywhere on their website, except for the above mentioned VW cams.
Did they send documentation that says .006" ?? I'm even more confused now!!
I would call them ... web site says they will not answer tech questions via email, you must call.
951.369.5144 Phone (8 am to 4:30 pm California time)

If you click on the "grind" number on their website you will find a timing card with the valve lash.

[exdirtbiker]

Thank you ! I actually called them today, I was told .006" intake. .008" exhaust.


web cam 118.jpg

[exdirtbiker]

Thats interesting.. My cams are the #110 grind and the valve lash spec. is 0.006 and 0.008" Its a lot compared to OEM for these engines, anyone who can explain why?

it appears your numbers are 100% correct. And I feel even dumberer than before.

[Drybear]

Hi,
I am almost finished with my GS750-79 engine rebuild, It hasbeen completely renovated and has got some performace modifications as well.

The cylinderblock has been changed to a 850 with Wiseco high compression pistons. The head has got a mild porting and has been milled 0,3mm to a squish of 1,5mm.
New camshafts (webcams grind 110) set to LSA 106,9^o.

So my question is if anyone have any idea how these modifications will effect the carb jetting?
I guess the increased cam overlap will reduce the idle vacuum a bit but at the same time the 100 cc extra cylinder volume will give a higher intake velocity?

The carbs and exhaust system will remain the same (almost completely open) as before modifications, WM26 with 120/15 and 1 step up with needle.

All feedback are welcome, shall I go up or down with jetting as a start point?