How to calculate added HP??

[Swampmouse]

As some of you may have seen, I have rebuild my '83 115hp Johnson.

2 of the Cylinders are .20 over and 2 are .30 over.

Does this add any horsepower and if so, how much more?

Thanks,
SM

 

[Silvertip]

Re: How to calculate added HP??

Increasing the bore of an engine adds very little to its displacement so there would be very little increase in HP. Certainly not enough that you would notice it in the seat of your pants.

 

[Scaaty]

Re: How to calculate added HP??

Interesting..V-8's punched out will create more, but ya only have 4 cylinders, stroke wasn't touched...hmmm.. might have added a couple cubic inches tops..I'd take a guess and say only a couple?

 

[Texasmark]

Re: How to calculate added HP??

Cylinder buildup may be larger than any change due to a diameter change of that nature.

Additionally, the "production" aspect of the engine may contain adequate variables to wash out your concerns.

Mark

 

[Dhadley]

Re: How to calculate added HP??

Interesting. We're sure you mean .020 and .030 over. If you're serious about .20 and .30 over it'd be interesting to know what pistons and head gaskets you used. Boring .020 & .030 over does very little if anything but going .200 & .300 over should.

 

[kynolan2183]

Re: How to calculate added HP??

I would say you will see nothing even if they were bore .2 and .3 and not .02 and .03
On any engine if you increase the bore but do nothing to help the air and fuel get in easier the gains are so minimal that they are normally lost in the effort by the engine to now get fuel and air.

This is why boat manufacturers have 20 25 30 and 35 horse power motors with the same bore and stroke. They all have different carbs or restrictor plates or exhaust tubes.

or 60 65 70 and 75 hp.

I once had a 20 evinrude that I put a 30 hp carb and plate on and it would scoot.

Do a search on the net for theorical engine horse power calculators and put in you bore and stroke for a 2 stroke engine.

What you will see is faster rev up and better hole shot.
Why? because you have not reached the the max air flow of the carbs and the hp curve has changed (very small change but a change)

Most boat motors are Over Square ( high HP) you are increasing that over square thus HP will increase quicker but will still top out at the same point because the air flow just is not there.

Kyle