Having this argument about engine compression

[Marcq]

With two friends of mine

I say, an engine with a hight compression is easier to start then with one with low compression, for me it just make sense

Am I wrong ?

Marc..

 

[F_R]

Re: Having this argument about engine compression

Nobody can win with this one because it depends....Using the 35hp OMC motors from the late '50s for an example, they had a compression relief connected to the recoil starter. Yes, they started better/easier without the compression relief, IF you were strong enough to pull it through. If you were not that strong, the compression relief made it easier for you to pull it and therefor easier to start.

 

[Marcq]

Re: Having this argument about engine compression

Thanks FR, but lets say you have two exactly same engines with electric start ,one with a compression test of 140 per cylinder and the other one with 80per cylinder, I would think that the one with the lowest compression would be harder to start and have the tendency to flood!! no?

Marc..

 

[iwombat]

Re: Having this argument about engine compression

I would think the opposite. As long as the compression is good, combustion properties aren't going to be all that different at cranking speed. What is going to be different is the cranking speed you can achieve. The lower compression motor will crank faster, and probably start easier. Honestly, I don't think you'd notice ANY difference in real life though.

 

[wildmaninal]

Re: Having this argument about engine compression

If I remember right there is 4 grades of compression, poor, fair, good, great. Or you might leave out the great and only have 3 grades.

80 pounds of compression depending on what type of motor you got that reading from may be poor compression or fair compression.

I would think that if you have more then 10% difference in between the said cylinders then it may make it harder to start.

 

[F_R]

Re: Having this argument about engine compression

Thanks FR, but lets say you have two exactly same engines with electric start ,one with a compression test of 140 per cylinder and the other one with 80per cylinder, I would think that the one with the lowest compression would be harder to start and have the tendency to flood!! no?

Marc..

Going with these specifics, I would have to vote for the 80 being hard to start simply because there is something wrong with it to cause the compression to be that much lower. Yeah, the electric start may be able to spin it fast enough to get it running, but it won't run right.

 

[reeldutch]

Re: Having this argument about engine compression

the engines with the highest compression are diesels.
in cold weather they used to be a pain to start.
due to high compression and cold motor pudling fuel
.
no ignition system just compression and heat causing the fuel air mixture to combust.

with a gas engine the compression is a lot lower so they are fairly easy to crank over.
the starter has to overcome the compression to get the flywheel spinning fast enough to get the juice from the stator to fire your ignition system.

so i would say it depents on several things.
higher compression will take more power to crank it over.
but that doesnt have to mean it will not start easily.

if you have the right fuel to air ratio inside the combustion chamber and you have spark at the right time,(advancing at startup) it will combust even if you have poor compression.
you will loose with low compression power but it will fire, if you have the right fuel air ratio and spark.
engines with bad or low compression sometimes idle great on muffs but die out under load.

 

[Marcq]

Re: Having this argument about engine compression

Thanks guys for your feedback, guess I'll call it even

Marc..

 

[rodbolt]

Re: Having this argument about engine compression

I would say it depends on the engine design and its state of tune.
if we were discussing an early 90's 200 or 225 jonnyrude looper, 80 PSI is about perfect, if your disscussing an old mid 80's inline 6 merc 80 means its blown.
it all goes back to what the engineer designed and what the state of engine tune and mechanical sealing ability.
most small yamaha fourstrokes incorperate an automatic compression release device, takes some of the load off of the starting mechanism.
some older OMC 2 strokes had a decompression mechanism for the same reason.
having owened at least one of any of the above engines the merc had the highest compression and until I converted it from choke to primer was consistently the hardest motor to start cold.

 

[Booberdoo]

Re: Having this argument about engine compression

A question like this is difficult to answer. It would depend on which CR was correct for the engine.

If it was supposed to be 140 but had dropped to 80 because of wear or whatever, then the 80 would most likely start harder. Camshaft profiling is directly related to CR in that the higher the compression the cam is able to open the valves (intake and exhaust) at the same time for a greater duration. This is called overlap. What most people do not realize is that there are two different forms of compression; static and operating. Static is measured with a compression tester and operating is calculated using the static as well as the camshaft profile. Operating compression is regulated by the choice of cam. If it had a camshaft meant for 140 CR but was run with 80 then the valve overlap would bleed a considerable amount of psi causing poor air/fuel density and therefore make it spin faster but also start harder. Add in that poor rings or burnt valves are also bleeding pressure and you see what I mean.

On the other hand, if it was supposed to have a CR of 80 but someone modified it to 140 without changings cams, then the camshaft would cause operating CR to go through the roof. I am sure it would fire right up but detonation would be extreme and the pistons would develop pretty little holes right in the top in no time (or just blow a head gasket).

Now if each engine were designed for their respective CR's, they should both start the same (given identical mechanical conditions).