Best engine size?

[kowached]

Originally Posted by Biggles

Going back a few years when I was in to motorbikes all my mates used to get their cylinder bores skimmed and put oversize piston rings on. This used to give them more HP. Doesn't anyone do that in the boat world ?

Yup, I've noticed that the "rebuild kits" for my Nissan 40hp are available with 0.010", 0.020", and 0.030" oversized pistons. I guess it would be wise if I compared the overbored 40hp to the stock 50hp, it might be considerably less expensive for comparable performance...

[Biggles]

Thats the sort of lines I was thinking along.

If the 40 and 50 have the same size block etc the difference must be in the bore. And if you have the smaller of the two it must be pretty safe overboring to the larger size.
Put simply I know but it makes sense.
Well it does to me anyway.

[ian parkes]

Originally Posted by Biggles

Going back a few years when I was in to motorbikes all my mates used to get their cylinder bores skimmed and put oversize piston rings on. This used to give them more HP. Doesn't anyone do that in the boat world ?

That is going back a few years .I doub't you could have measured the increase a re bore gave but owners always claimed it had more power , main reason was the engine had been restored to new condition and they were used to a knackered engine .
It doesn't happen so much these days as most decent engines have plated bores so a new piston and rings is the normal way or a new or barrel if the its been damaged

[ian parkes]

Originally Posted by Biggles

Thats the sort of lines I was thinking along.

If the 40 and 50 have the same size block etc the difference must be in the bore. And if you have the smaller of the two it must be pretty safe overboring to the larger size.
Put simply I know but it makes sense.
Well it does to me anyway.

Ill bet you they are the same bore and capacity , the 40 will simply be a restricted 50 . the retriction will iether be on the fuel side or the ignition , could even be as simple as a flow restrictor in the inlet .
Worth some research I expect .

[Nos4r2]

Yep, Ian's right.

A 30 thou overbore is bugger all. Maybe 5-10cc extra capacity that's all.


We've just paid for a serious bigbore (190cc overstock on an xs650) and for the extra 15ish HP it'll give the pistons are soddin enormous.