Compression Test Gear?

[Niander101]

I'm quoting Phil

[phil]

H-A said, "How does this work on a twin".

Personally I prefer to use a compression gauge, as I like to have some actual numbers to write down on the motor tag which I use on each motor to help me remember it's repair details including current compression.

[Horsley-Anarak]

Nice thread by the way.

Which one, 14mm or 18mm?

Sometimes H-A , British humour gets lost in translation.

Don't worry Stelios got it, I wish I could speak another language well enough to understand jokes.

Perhaps Stelios watches too much British TV humour

H-A

[Stelios_Rjk]

I think I get it by my first response but it would be certainly easier if we were talking in a pub in front of some beer etc

[Horsley-Anarak]

Do you get British Beer in Greece?

Or is it all lager type fizz?

H-A

P.S. Just got the Guy Fawkes bit, I think?

http://news.yahoo.com/lightbox/proteste ... 56888.html

[Stelios_Rjk]

There is Guiness and Murphy's but I prefer pils or weissbeer.
Lager is the most usual to find everywhere.


Yes Guy Fawkes appeals to this and many more events. Many or most of the people here wait/wish the parliament goes BOOM!

[rcweaver]

Compression Silver Century. There are some interesting posts on this subject! Things like 125 psi! Who has a Seagull with a turbocharger - I want one?

(1) Well lets look? The bore measures about 64mm but this is not important. Take the spark plug out and measure from the plug seat to piston top at "Bottom Dead Centre" I read 70mm and then at "Top Dead Centre" and I read 30mm.

(2) Allow 15 mm for the head thickness so the swept volume is cylinder area (Say "A") times the sweep (70-30) 40mm.

(3) And the maximum cylinder volume is cylinder area "A" times 40 plus unswept volume (space at the top ). This is "A" times 40+15 mm which in total equals A times 55

(4) From Boyles Law we know that if a gas is compressed the product of pressure and volume remains constant or P1 x V1 = P2 x V2. ~(More accurately Charles Law
PV/T = C but we will ignore temperature effects for a first approximation.)

(5) We shall chose state 1 for the induction so the volume is (A x 40+15) and the pressure is atmospheric 14.696 psi or near enough 15psi. This is why the throttle must be open for a compression test or we will choke the flow and reduce the pressure. (P1 = 15 psi and V1 = 40+15 x A)

(6) We know Pressure at induction = atmospheric pressure So P1 x V1 is 15psi times volume of cylinder or 15psi x 55 A
We shall choose state 2 for the compressed gas i.e. compression pressure where the volume is now squeezed to A x 15mm (the gas space at top dead centre)
And P2 x V2 translates to P2 x (A x 15 )

(7) from 4 above putting in the derived figures into Boyles Law P1 x V1 is equal to P2 x V2 So (15psi x 55A = P2 psi x 15 A) or P2 is 15psi x 55A divided by 15. So the pressure is 55 times 15 divided by 15. Or 55psi

So 55 psi is the maximum compression pressure - unless we use a supercharger!
This assumes
(a) throttle wide open
(b) no losses in suction
(c) no leakage past piston
(d) no heating caused by compression.

May measure the 40 plus model later but it will not be far different for a final compression pressure.

Good thing Seagulls don't know about the lower compression, don't bother about it and work so splendidly!

Regards Roger

[skyetoyman]

Don't you have to allow something for the fuel vapour. I assume petrol is not very compressable.

[phil]

You can't measure from BDC to calculate compression, you have to measure from when the exhaust port is closed.

[Charles uk]

I'll tell you what Roger.

I will put 4 of my Seagulls on a stand you can pick anyone of the 4 after running them all for 1 minute on a 20 : 1 fuel mix we will measure the compression pressure with your compression gauge, for every pound of pressure over 60lbs after three pulls you give me £2.00 & every pound under I will give you £3.00, tempted?

I'd prefer this to happen the day after you've been paid, if you don't mind.

[Horsley-Anarak]

So 55 psi is the maximum compression pressure - unless we use a supercharger!

Regards Roger

I have just checked a stock Silver Century that has done little work, just over 60 psi.

My racing Century is over 100, as are lots of my 40 series engines.

I have not tested any QB engines, but would think they will be higher than a Century. When I get the Seagull 6 that I am restoring back together I will test that.

One problem that we may have is the accuracy of pressure gauges.

H-A

[david doyle]

I think we read on here pretty frequently that any 40 with over 55 PSI will run well and be satisfactory? Neat that RCweaver's caculations figure out to the same #. I do not think he was trying to raise stakes on bets nor was he trying to say that seagulls do not ever measure over 55 PSI compression.

Phil thanks for the learnin on the exhaust port. good point.

RCweaver. Do your self a favor and do not think or god forbid do math out loud on here. Show some polished chrome you'll make friends quicker

[phil]

Most Centuries and Silver Centuries I come across show compression numbers of 50- 65 psi by my gauge, my best one being 72 psi. The worst I've measured was 35 psi.
I would quite unhappy with a model 40 that was not over 75 psi. Most of my forties are between 75 and 120 psi with several above 100 psi.
My gauge appears to be somewhat on the low side in measurement judging by a sample of 40-50 old outboards of various makes.

[charlesp]

Thinking is very welcome here, we have a bunch of clever people contributing. Hard work and a lot of thought has gone into a lot of projects, resulting in a growing repository of knowledge backed by examples and hard proof.

"Math" as you folks from across the pond would have it, has its place.

We are a formidable collection of enthusiasts with an even more formidable track record of proven problem solving ability, both technical and historical. Nothing that has a basis in fact or reason is excluded, nor even the more fanciful theories. That is our strength.

[david doyle]

Charles since you bring up the formidable track record, I am still waiting to hear about where British Seagull currently manufactures parts? Last time I asked you gave a 'fanciful' story about some piece work conducted in a kitchen by a haggard house wife? As a 'formaidable' historian surely you understand that recent history can be as telling and useful as can ancient history.