Compression ratio in two-strokes

labougie

Seaman Apprentice
Joined
Oct 7, 2010
Messages
40
From my engine tuning days as a youngster, skimming the cylinder head (by a precisely calculated amount) could give a worthwhile (10%) increase in bhp. Anyone know whether the same rules apply on two-strokes? I've read all about two-stroke tuning and it mainly seems to be about tuning the exhaust, rather than the combustion chamber. Any thoughts?
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
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12,345
You can polish head piston, cylinder head, intake and exhaust ports to a smooth cond, that way fuel delivery, combustion and exahust will work more fluent, more efficiently if it's the right wording compared to factory delivered. To get the max out of any engine, must be correctly OB/transom height seated, well trimmed and running a maximized prop for engine to achieve it's max wot rpm range or slight bit more while combo rides on water with well distributed deck weight.

If not all the extra power gained done whatever to increase HP will enevitably be lost there..

Happy Boating
 

Faztbullet

Supreme Mariner
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Mar 2, 2008
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15,590
If its a looper you can machine head and increase compression, crossflows need larger internal deflectors in head. You rough up the intakes to keep fuel/air mixture tumbling and prevent puddling areas and smooth the exhaust.Dont expect a lot of noticeable increase....
 

robert graham

Admiral
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Apr 16, 2009
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6,908
Some folks used to modify the size/shape/location of the intake and/or the exhaust ports to help increase the power curve....it works but can make a 2 stroke engine stronger in higher RPM's but less tractable in lower RPM range....if you decide to do it, best to know what you're doing....
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
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Old folks that raced old wooden zapcats used to file powerhead a tad to increase compresion along other stuff described on previous posts, seen outboards engines blow during racing due to installing less pitch props for fast planing without running a previous wot rpm test with a tach.. Now a days if plan going for a less pitch prop to pull wot revs up on any portable/medium range OB's is a must have an induction tach..

Happy Boating
 

labougie

Seaman Apprentice
Joined
Oct 7, 2010
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40
Thanks for the input guys - I think I'm going to leave well enough alone!
 

jimmbo

Supreme Mariner
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May 24, 2004
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12,944
Thanks for the input guys - I think I'm going to leave well enough alone!

It is extremely easy to turn a good engine into one that is no longer any fun to operate:blue:. Great top end, but little else:violin:. Or into a pile of metal that has value only by the pound:eek:
 

Chris1956

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Mar 25, 2004
Messages
27,036
A bit off topic, at least for now, but, Infinity is planning to produce a 4 cycle auto engine with a variable compression ratio. Anyone know how they plan to do it?
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Thanks for the input guys - I think I'm going to leave well enough alone!

If want to get the max HP output of current engine without mod anything, go for a prop maximization along an induction tach for engine to rev towards its max wot rpm range factory stated. On light boats it's usually one less pitch if being a 3 blade prop. It's night and day performance difference..

Happy Boating
 

Cadillac-ack-ack

Petty Officer 2nd Class
Joined
Dec 1, 2017
Messages
104
A bit off topic, at least for now, but, Infinity is planning to produce a 4 cycle auto engine with a variable compression ratio. Anyone know how they plan to do it?

Do not know about Infinity's plan but years ago Volvo was experimenting with this too. As I recall they designed an engine with 3 major components, the lower block, the cylinders, and the head. The OHC head was conventionally bolted to the mid section cylinders but the cylinders were connected to the block in such a way as to allow them to raise up some thereby increasing volume and reducing CR. I don't remember how this was accomplished mechanically - hydraulic or electronically. The constantly varying movement between lower block and cylinders was sealed with a thick rubber accordion style gasket.
 
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