piston weights and their effect

[Michael Moore-3]

I've been weighing different pistons for a Motobi project (both 74mm
250cc and 66mm 200cc).

A race kit 250 Motobi cast Mondial piston assy is 363 gr.  A 74mm
GS500 Suzuki piston assy by ART for use in a 250 Motobi is 297 gr.  A
Wiseco Suzuki LT230 quad piston assy for a 200 at 66mm bore is 223
gr.

Ken A told me he had a CR250 piston  (77mm) with two rings
and one circlip that was 143.6g.  Wiseco lists an S686 pin (16mm x
41.5mm x 10.5ID) for their CRF piston at 37 gr so that's right at 180 gr
total.   Until I found that out yesterday I wasn't aware the modern 4T
pistons were so light.

FWIW the S259 pin with the 66mm Wiseco piston is 16x 58.4 x 10 ID
at 50 grams so Wiseco pulled 13 gr out of the pin for the 77 mm piston.

I did a quick spreadsheet using Phil Irvings formula for the inertia loads
of reciprocating parts to see what differences the piston weights and
con rod lengths might have.  I used 93.5 rod length for the modern MXer,
105mm is the OEM Motobi and 111.5mm is the length for a rod for the
Suzuki 230 quad.  I used the stock 57mm Motobi 200/250 stroke for the
Motobi and Suzuki rod lengths.  53.6mm is a common stroke for the
250 MX thumpers.

It appears that with the modern 250MX dimensions, light piston and
13500 RPM redline the upwards piston load at TDC is actually slightly
less than the long LT230 rod and heavier 200cc piston at 12000 RPM
with the Motobi stroke and quite a bit less (2788 lbf vs 3585 lbf) at
13500.  That piston weight really adds up!

cheers,
Michael

[Ian D-2]

>Ken A told me he had a CR250 piston (77mm) with two rings
>and one circlip that was 143.6g. Wiseco lists an S686 pin (16mm x
>41.5mm x 10.5ID) for their CRF piston at 37 gr so that's right at 180 gr
>total.


Just weighed the 2 ring CRF250 slug I have here, 173g with
pin & no rings / clips, 137g ( !! ) bare.  The 2010 model has
a smaller bore and reputedly even lighter piston !!

They're here for a good time, not a long time.



Cheers     IAN



See www.drysdalev8.com for :
- Drysdale 750-V8 Sports & 1000-V8 Cruiser
- DRYVTECH 2x2x2 Experimental
- Carberry Enfield 1000cc V-Twin
- Drysdale Hillclimb Open Wheeler

[Ken Augustine]

FWIW, those pistons are 78.0mm.  

Ken A




________________________________
From: Ian <iwd@...>
To: mc-engine@...
Sent: Wed, October 28, 2009 4:27:45 PM
Subject: Re: piston weights and their effect

 

>Ken A told me he had a CR250 piston (77mm) with two rings
>and one circlip that was 143.6g. Wiseco lists an S686 pin (16mm x
>41.5mm x 10.5ID) for their CRF piston at 37 gr so that's right at 180 gr
>total.

Just weighed the 2 ring CRF250 slug I have here, 173g with
pin & no rings / clips, 137g ( !! ) bare.  The 2010 model has
a smaller bore and reputedly even lighter piston !!

They're here for a good time, not a long time.

Cheers     IAN

See www.drysdalev8. com for :
- Drysdale 750-V8 Sports & 1000-V8 Cruiser
- DRYVTECH 2x2x2 Experimental
- Carberry Enfield 1000cc V-Twin
- Drysdale Hillclimb Open Wheeler