Bg86 exhaust mod?

[Eddy_t]

God knows, it's what I was told. Although I always thought the carbs had the little brass caps fitted, those carbs can be adjusted (such as the disk-saws)

[spudulike]

God knows, it's what I was told. Although I always thought the carbs had the little brass caps fitted, those carbs can be adjusted (such as the disk-saws)

 

Well I think you have been told a load of , Some of the fasstest ever racing small bore two strokes got up to around this level and to do so would have matched expansion pipes and porting - if this doesn't happen, the expansion won't have the correct effect.These racing bikes would have a very short stroke to get this sort of RPM and a bog standard chainsaw engine doing it..........under load:confused1:

 

I would imagine these fellas took their standard engine, gave the data to a pipe specialist, matched the pipe as best they could and bingo - more power but to get to 21,000 rpm, you would need to start with a blank piece of paper and not a design for a saw that normally revs to 12500rpm or so.

 

I will ask the guy who built it in April 15 when I am at the NEC again!

[Eddy_t]

Quite possibly, I think the old mac kart saws were under 20k rpm and they go like scalded rats

[spudulike]

Quite possibly, I think the old mac kart saws were under 20k rpm and they go like scalded rats

 

Modern 125cc kart engines are limited to around 16,000rpm, the latest bike engines pre the the two stroke ban in racing were around 21,000rpm.

 

The chainsaw engine will be pretty basic compared to the modern two stroke racing engine, modern racing or even road going two strokes will have powervalves able to pull 30bhp+ from a 125cc engine compared to 8-9bhp from a saw which hasn't got boost ports or powervalves!

[mowandgo]

What effects does an incress rpm on a chainsaw/strimmer/blower actualy have? Surley the powebands factory set so over rpm will be less eficent. Also does it wear engine out considarbly quicker? The machines I use I love high rpms I think they cut so much better on any machine hedge cutter, strimmer, saw. But thats factory out the box surley after higher revs after mod would have bad side effects and need lots of compensating?

[spudulike]

What effects does an incress rpm on a chainsaw/strimmer/blower actualy have? Surley the powebands factory set so over rpm will be less eficent. Also does it wear engine out considarbly quicker? The machines I use I love high rpms I think they cut so much better on any machine hedge cutter, strimmer, saw. But thats factory out the box surley after higher revs after mod would have bad side effects and need lots of compensating?

 

In two stroke terms, the power band is set by the top of the exhaust port but that is just one small part of the story.

 

When you port tune a two stroke, you are trying to maximise flow through the engine and increase the fuel/air charge in the engine. You also increasse the compression and doing all this is to increasse power which isn't necessarily to increase the flat out speed of the engine under no load but to increase the speed of the engine when under load.

 

It is a bit like revving your car at standstill or heading for a steep hill and flooring it. Revving at standstill is pretty meaningless - more important is the speed under load, the other is the torque or abillity to keep cutting when more force is exerted on the saw.

 

It is true that a ported saw will hold higher no load revs due to larger amount of fuel/air being pulled through but the speed of the saw under load under duress is of greater interest and use.