boring out cylinders ??

[fs5]

do you get much of a gain by boring out your cylinders?
would goin from a standard bore size to a .030 or .040 give you much more performance ?
thanks

[zul8tr]

I know "there is no replacement for displacement" but I believe the gain is not that noticeable in a high rev 2 cycle engine when you consider that the larger pistons weigh more and that is a negative. I would only bore from standard if needed to straighten the cylinder and keep it to a minimum overbore. An example is my 2 cylinder 25ss that I raced a lot. Had to straighten the bore after all the use to +015 next size up. I went from the std 3 ring Merc pistons to the 2 ring Merc +015 pistons, pin rings and retainers and they weigh more than the standard piston assembly. The engine runs about the same with the butt dino and my tack and speed info. Compression testing revealed no significant increase over the std. setup.

[fs5]

thanks mate, what about a low rever like mine ? it likes to sit around 5300/5800 tops.
i run longer plugs , with a couple extra washers so the plugs sit further in the dome giving me a bit more comp. this works for me giving us around 3kph better topend.
would oversize boring increase compression ?
thanks

[Powerabout]

thanks mate, what about a low rever like mine ? it likes to sit around 5300/5800 tops.
i run longer plugs , with a couple extra washers so the plugs sit further in the dome giving me a bit more comp. this works for me giving us around 3kph better topend.
would oversize boring increase compression ?
thanks

of course, you wont find anyone running in sst45 with stock bore, they rebuild and start at 030

[zul8tr]

thanks mate, what about a low rever like mine ? it likes to sit around 5300/5800 tops.
i run longer plugs , with a couple extra washers so the plugs sit further in the dome giving me a bit more comp. this works for me giving us around 3kph better topend.
would oversize boring increase compression ?
thanks

That is good that you find the longer plugs work and not a problem since you pull them often before carbon and heat affect the exposed threads. But note that the sharp edges of the exposed threads is a potential hot spot that could promote pre-ignition.

Since you ask about compression ratio only way to get specific is by math, so here is an example.

CR = Compression ratio = (CV + HCV)/ HCV

CV = cylinder volume TDC to BDC, but the real volume for a 2 cycle is TDC to top of exhaust port which is less.
HCV = head combustion chamber volume

all units the same like cubic inches.

So if you increase the CV with bore job by 0.030 and leave the HCV the same there might be a slight increase as example will show.

Example: Say the bore is D = 3", stroke is 3" and HCV = 2.5 cu. in

CV = stroke x pi x D^2/4 = 3 x (3.14 x 3 x 3)/4 = 21.21 cu. in

CR = (21.21 + 2.5)/2.5 = 9.48 :1

If you bore 0.030 over then D = 3.030"

CV = 3 x (pi x 3.030 x 3.030)/4 = 21.63 cu. in

If HCV is the same at 2.5 cu. in

Then CR = (21.63 + 2.5)/2.5 = 9.65:1 and increase of 1.8%

Weigh that against the increased weight of the oversize piston. A low revving engine will be better
tolerate the heavier piston assembly that a high revving engine. More rotating mass affects acceleration.

Run specific numbers with your engine.

Do you have a blind bore engine with cast on cylinder head or is it a removable cylinder head? Is it legal to machine the head to reduce the HCV? That would be a better way to increase compression ratio.

[fs5]

yeah mate its a blind bore 59 ci merc motor.
i just use it for fun fanging round with my mates
standard bore is 2.993 stroke is 2.795

yeah i pull the plugs after each trip,
thanks

[zul8tr]

Blind bore machining takes one that knows how to do it, not just any machinist.

[fs5]

yeah mate , thats one thing i've heard a lot of.
thanks for the info

[tyst67]

Fs5 email me tyst67@hotmail.com