Merc 115 hp. pistons question

[Fastjeff57]

I read that some later Mercs (I think around 115 hp---in-line 6s) have a transfer port in the piston. Does anyone have a photo of the piston and block, so I can study how this works?

Thanks,

Jeff

[mercguy]

I read that some later Mercs (I think around 115 hp---in-line 6s) have a transfer port in the piston. Does anyone have a photo of the piston and block, so I can study how this works?

Thanks,

Jeff

all the replacement pistons for the inlines have the transfer (boost) ports in them also, be it the blocks has the corresponding area or not. Not really relevant to your question though.

[Mark75H]

Basically it acts like a window from the crankcase to the transfer passage to change the streaming in the passage to the transfer port area, slightly changing the intake stream entering the cylinder.

Its part of a critically shaped passage system that includes differently shaped deflectors and a notch on the piston crown, too. I won't say it can't be done, but you'd probably have to grind 100 blocks and 400 pistons and try 30 or 40 different passage stuffers to duplicate a working "power port" piston set up on the little motor.

[Fastjeff57]

I don't have a Tower of Power--just curious how it was done.

Thanks!

Jeff

[Droll-l6]

Found some pictures .

First a power ported piston, then a non PP cylinder (Merc 1400, 1972), last picture shows a PP cylinder .
Mercury used power ports on the 1973 > 1977 Merc 1500 ( + 1500XS up to 1978 ), 1978 > 1981 Merc 1400 ( basically the same as the 1500 ), 1982 >1988 115hp ( prop shaft rated, only minor differences from the 140/150hp models ) .

Arne Kjetil

[Mark75H]

Are the transfer port covers exactly the same on these 2 motors? I thought they were a little different too ...

[Fastjeff57]

After stareing at the photos (thanks!) I think I finally get it: Obviously, no air/ fuel could enter the cylinder via the PP port in the wall since it's below the rings at Bottom Dead Center. That threw me for a while. So what does it do, and how does it work?

I'll bet the air/ fuel trapped UNDER the piston is forced OUT through the hole in the piston and into the transfer passages, then into the cylinder. But that's just speculation.

Any one have a different thought?

Jeff

PS: God how I love bench racing! Hey, it's winter, right?

[Mark75H]

The rings will be below the top of the notch in the cylinder wall at BDC.

If you look closely at the hone marks on the non notched cylinder you can see an area where the hone didn't touch the cylinder wall ... probably worn by the rings ... that shows the bottom limit of the ring travel

Mixture squirts thru the hole in the piston, into the notch in the cylinder wall and around the piston into the cylinder, merging with and adding to the mixture coming in thru the 2 transfer ports.

See that little dip in the piston crown on the side with the hole? That goes below the top of the notch at BDC. The whole scheme gives more transfer port action (just a puff) without lowering the transfer gas speed as would happen with just enlarging the main transfer port. It also directs the intake charge UP a little bit more (toward the spark plug) instead of across - stopping more mixture from being wasted going out the exhaust port.

[Fastjeff57]

Far out! Gotcha. What genius dreampt that up?

Jeff

[Mark75H]

There were similar arrangments in 2 stroke motors going back to the 1930's, it wasn't really new at that time (1970), but it is an interesting adaptation

[650xs]

Found some pictures .

First a power ported piston, then a non PP cylinder (Merc 1400, 1972), last picture shows a PP cylinder .
Mercury used power ports on the 1973 > 1977 Merc 1500 ( + 1500XS up to 1978 ), 1978 > 1981 Merc 1400 ( basically the same as the 1500 ), 1982 >1988 115hp ( prop shaft rated, only minor differences from the 140/150hp models ) .

Arne Kjetil

these are just like some of my 700x/650xs internals,but pistons have 2/3 rings.....why ..less drag???????????better seal????:cool: