compression ratio

[phil]

In trying to guess the compression ratio on Seagull 102 models and Centuries , my best estimate using several methods comes out around 5:1 or somewhat lower. It would appear that raising it to around 7 or 8:1 might produce some extra power, although I realize given the relatively low power output to begin with, that might not amount to much.
In the discussions on preparing motors for races, I notice many of the common methods used, but haven't noticed any talk of attempting to raise comp. ratio. I suppose it would require a custom-built longer con rod and or piston.
Interested to hear any comments.

[Charles uk]

Phil as the piston controls all the port timing, anything you move, changes other things & not usually for the better.

All the serious Seagull racers spend weeks modifing just about every part of their outboards, hoping to make this seabird sing.

Perhaps fitting a cylinderhead with a smaller combustion chamber might help.

The only rules are that the parts, must be parts that British Seagull used during their existance.

No one will tell you all their secrets, or your motor will be a clone of theirs.

So start taking them to bits & looking, measuring & thinking.

All you've got to do is improve the breathing in & out of the motor, raise the compression everywhere, get the best possible spark, lower the internal friction as much as possible & transmit all these power improvements to the water by the most efficient prop you can find.

You willl know when you've got it right, your boat will be faster than all the rest.
If it's a fast running hull.
But remember "To finish first, first you have to finish!"

[pistnbroke]

to find compression ratio...bring piston to TDC and with a syringe add oil through the plug hole till its full......engine capacity divided by quantity of oil gives the compression ratio......you can then see how much weld you need to add to the underside of the cylinder head to raise the comp ..volume of metal in each rod and round and round you go ..use rods for cast iron ...oops should not have said that ......6.5 :1 is normal for a modern 2 stroke ..you could of course turn some off the head mating flange but as said that changes the porting ...like charles said you need to measure and think ..make up a port timing diagram and refer to a good book on tuning 2 strokes to see which way to go ..

[Charles uk]

Lowering the head by shortening the cylinder from the head end won't change any of the port timings, will just increase the compression.

It's probably best to measure compression with a compression gauge, normal is between 45 & 55 lbs, unless all the bearings are in very good condition you won't want to raise that too much, as slack bearings get hot very quick under higher loads & then die!

[pistnbroke]

I was thinking of the type with the head and cylinder cast in one...

55 psi is very low for cranking pressure ...you would get a more usefull reading if you spin it with a drill .. 100-120 would be normal on a modern engine ...I think Phil wanted to know the compression ratio not the cranking pressure and the oil method is the easy way to do it ...

[John@sos]

I always test the compression of Seagulls here before they leave and the reading is on the data sheet with the motor. I have established the pressures were as follows,

Forty series approx 100psi when new, down to 85 psi and still going well 40 years later.

The Century and Silver Century, Model 102, from new 55psi, but they work well down to only 35psi, amazing!

The 170 and QB's were slightly up, about 120psi. when new. approx 100psi when they reach me.

As I never soup up Seagulls I could not comment other than to say certain early Century heads are in demand, they have part of the combustion chamber filled in.... to raise the compression ratio, not the compression in psi!

Regards,
John

[Charles uk]

All the Seagull model from the wartime 102's to the end of the centurys, were designed to run on the fuel of the day,
Which ranged from 85 octane during the war to 88 / 92 when the centurys were conceived.
Low compression is a given for low octane.

[pistnbroke]

It would be interesting to know john if you spin them or use a cord. The problem is with only about 3 revolutions with a cord you have the volume of the pipe to the gauge to consider and with only 3 revs on a pull cord this is significant in comparison to the head volume .

anyone got any photos of the two types of cylinder heads ..high and low compression ??/

[John@sos]

Further to my first comment.....

I use a direct, screwed into the head, tester. As with all similar testers the instructions are similar, spin motor at starting speed.

I spin with a rope cord as normal, but to ensure I get a good reading, I repeat this 3 times. Normally there is no increase in pressure after the first pull, but I like to be sure.

Remote guages are more problematic, but most I have seen in the UK are similar to mine, direct fitting, but most seem to have a rubber, conical spark plug hole fitting, which means you need an assitant to pull the cord...

You have to search for a screw in one, and one with an 18mm adapter. mine is probably 50 years old!

John
SOS

[Collector Inspector]

Yep, mine is screw in one, rubber push ins are not really that good as it is difficult with a helper and making sure that is sealing for the compressive strokes required for a good reading. Bloody rubbers split anyway.......

Mine is Sykes P.

I just go on what I can pull for a reading, nothing more than three pulls usually maxes out.

Basically that is it.

Starting comp is either there for firing or not.

Running compression gets more maths after that for exact values due to velocity and induction fuel/air density factors along the way.

Pointless any further if the Old Dear starts 1st or second pull and reaches max operating RPM on a hull within spec.

C.I..

[rosbullterier]

John, I bought a Gunson compression tester with screw in 14 and 18 mm adapters from Halfords last week . . .

[Waggles]

A question, why the difference between the 40 series and the larger 102cc models? 100 psi ( new ) compared to 55 is a fair old difference!

Fully understand low compression = good thing with low octane rating, but surely the 40 series would be running on the same octane as the centuries?

Appreciate psi does not equal compression ratio but it must be related to it? ( 5 to 1 should give 5 X atmospheric psi very roughly shouldn't it? ) That could mean the 102cc models running on a very low compression ratio comparatively