Compression

[RussellCParker]

Just rebuilt my 91 120 Force. Runs like a champ. Compression is only 105 (all 4) bored at 30 over. across the board. Thoughts?, I have noticed other people getting 150PSI. Look forward to hearing thoughts on this.

R

 

[Frank Acampora]

Re: Compression

Could just be your gauge. Cheaper ones tend to not be accurate. Give the rings a couple of hours running time (With double the oil for break in) and see if it comes up some. Also, you must crank the engine over for at least 4 turns to allow compression to build when checking.

Getting back to break-in: When an engine is rebuilt, no matter how well you have pre-oiled it, it still is not the same as oil that has been heat treated in the running engine, and rings have not seated in the cross hatching of the newly honed cylinders. Thus: for the first few hours running time, you use double the oil and avoid prolonged full throttle running. Full throttle bursts are not detrimental but continued running can be. Note that I said can be --not will be. Note that racing engines are not given a break-in, but they also are not expected to last for years.

 

[MikDee]

Re: Compression

Having a bigger diameter piston has opened up the combustion chamber (made it bigger) without extending any further into the combustion chamber then the old piston = lower compression. The piston would have to be longer from the center of the wrist pin to it's surface, to quench more, & regain that lost compression. Hence the reason for pop up pistons. After your rings fully break in, & seat, your compression should come up some.

 

[RussellCParker]

Re: Compression

Thanks Frank,

I will double the oil. Next question is my top speed right now is 35 MPH. I was expecting 40 to 43. Thoughts?

 

[RussellCParker]

Re: Compression

I will try the double oil ratio. My top speed is 35. I was expecting 40-43. Thoughts?

Russ

 

[bowhuntrrl]

Re: Compression

Having a bigger diameter piston has opened up the combustion chamber (made it bigger) without extending any further into the combustion chamber then the old piston = lower compression. The piston would have to be longer from the center of the wrist pin to it's surface, to quench more, & regain that lost compression. Hence the reason for pop up pistons. After your rings fully break in, & seat, your compression should come up some.

Actually it doesn't open up the combustion chamber any. It actually gives higher compression from stuffing a wee bit more volume into the same size combustion chamber.

 

[bowhuntrrl]

Re: Compression

Thanks Frank,

I will double the oil. Next question is my top speed right now is 35 MPH. I was expecting 40 to 43. Thoughts?

You should be breaking it in slowly and not making top speed runs !!! Putt around slowly, accelerate at part throttle briefly, then shut throttle down. That pulls extra oil into the cylinders. Hold off on the high speed runs until you have some hours on the motor of break in time.

 

[Kajun37]

Re: Compression

I will try the double oil ratio. My top speed is 35. I was expecting 40-43. Thoughts?

Russ

lol umm take it easy for awhile speed racer..you will undo what you just did.

 

[MikDee]

Re: Compression

Actually it doesn't open up the combustion chamber any. It actually gives higher compression from stuffing a wee bit more volume into the same size combustion chamber.

Aha yes! thanks, giving it more thought, the increased volume of the bore, along with the stroke, would give it more quench, sorry.

 

[Frank Acampora]

Re: Compression

Top speed depends on the hull and size. For example I had a 125 on an 18 foot closed bow deep vee runabout. Top speed was about 37. I would expect a little more on an equally sized bass boat. Same engine on my 14 foot flat bottom did 60 with a higher pitched prop.

Knowing that your lower unit gears are 1.78 to 1, You can figure out the number of RPMs the prop turns at a given engine RPM. (prop turns about 3/4 the speed of the engine) Then you can multiply by prop pitch to find inches per minute and convert to theoretical MPH. HINT 60 is 88 feet per second, 30 is 44 feet per second. Factor in about 10% slip (Less) to figure out what actual MPH should be.