Honda Spree and Elite 50 Forums

The Spree (all years), minimum compression is 114 PSI. This engine has a 7.2 to 1 ratio.

The 1985 Aero 50 states 114 to 171 PSI is acceptable. This motor has a 6.8 to 1 compression ratio.

The Aero 80 says 142 to 200 PSI is normal and this engine has a 6.8 to 1 compression ratio too. (FYI, I have one engine that starts and runs fine at 90 PSI).

The Aero 125 manual says 171 PSI is minimum and this engine only has a 6.7 to 1 ratio.

As you can see it is not possible to state a number that applies to all 2 strokes or 4 strokes for that matter. That's why there are such things as service manual. Never ever believe anyone who just pull a number out of his * because he heard this or that. This is why the manuals are available on this and on other sites.

i know you probably can't help being a * so I'm not offended.

Sorry, I didn't think it was against the law to open discussion without reading and retaining every single tidbit of scooter information in the universe. I don't treat people like *'s due to asking questions...I try to help them.

I tried to ask, read and do as much research in a short amount of time to have an intelligent conversation looking for help.

Maybe it's the motor and maybe it's the gauge. i won't know until I have a second gauge....just surprised is all it runs mint, screws at 60psi


I'll run along to a different forum

see ya

Kerry wrote:i know you probably can't help being a * so I'm not offended.

Sorry, I didn't think it was against the law to open discussion without reading and retaining every single tidbit of scooter information in the universe. I don't treat people like *'s due to asking questions...I try to help them.

I tried to ask, read and do as much research in a short amount of time to have an intelligent conversation looking for help.

Maybe it's the motor and maybe it's the gauge. i won't know until I have a second gauge....just surprised is all it runs mint, screws at 60psi


I'll run along to a different forum

see ya

Bye * head!

actually...i'll stick around and pin point everything that upsets you and concentrate on it

old fart

Bear45-70 wrote: The Aero 80 says 142 to 200 PSI is normal and this engine has a 6.8 to 1 compression ratio. (FYI, I have one engine that starts and runs fine at 90 PSI).

How is 142 to 200 psi a 6.8:1 ratio

atmospheric pressure is about 14.7psi

So isn't 142psi 9.65:1, and 200psi 13.6:1

90psi = 6.12:1

So what is up with that?

Am I missing something?

Kerry wrote:actually...i'll stick around and pin point everything that upsets you and concentrate on it

old fart

With your logic, you are gonna be in * puppy's class and there is nothing you can do that would bother me except being stupid and that is already a given.

Crankcase pressure.

eliteguy50 wrote:Crankcase pressure.

???? Explain this better so I can understand.

Trafficjamz wrote:

Bear45-70 wrote: The Aero 80 says 142 to 200 PSI is normal and this engine has a 6.8 to 1 compression ratio. (FYI, I have one engine that starts and runs fine at 90 PSI).

How is 142 to 200 psi a 6.8:1 ratio

atmospheric pressure is about 14.7psi

So isn't 142psi 9.65:1, and 200psi 13.6:1

90psi = 6.12:1

So what is up with that?

Am I missing something?

The bore and stroke combo can effect the numbers. But those are the numbers straight from the manual. You also have to remember the natural supercharging effect of 2 strokes.

The compression ratio is a static number. It is a ratio comparing volume to volume. The actual pressure in the cylinder comes from a compressed mass in the volume. Pressure in the crankcase will force more mass into the cylinder, a denser charge in comparison with the atmospheric pressure. If that mass was held at atmospheric pressure, you would find that it is a larger volume than the cylinder allows. That denser charge is still being compressed down to the same volume as in the CR though so the PSI is higher than what the static CR calculation shows. Basically, there is more compressed mass at TDC in a running engine than is accounted for in the CR.

Bear is also right about the bore and stroke having their own effects combined with volume. Buildup can also increase pressure. The "natural supercharging effect" is what I was talking about with crankcase pressure.

eliteguy50 wrote:The compression ratio is a static number. It is a ratio comparing volume to volume. The actual pressure in the cylinder comes from a compressed mass in the volume. Pressure in the crankcase will force more mass into the cylinder, a denser charge in comparison with the atmospheric pressure. If that mass was held at atmospheric pressure, you would find that it is a larger volume than the cylinder allows. That denser charge is still being compressed down to the same volume as in the CR though so the PSI is higher than what the static CR calculation shows. Basically, there is more compressed mass at TDC in a running engine than is accounted for in the CR.

Bear is also right about the bore and stroke having their own effects combined with volume. Buildup can also increase pressure. The "natural supercharging effect" is what I was talking about with crankcase pressure.

That makes more sense now thank you static -vs- dynamic

eliteguy50 wrote:The compression ratio is a static number. It is a ratio comparing volume to volume. The actual pressure in the cylinder comes from a compressed mass in the volume. Pressure in the crankcase will force more mass into the cylinder, a denser charge in comparison with the atmospheric pressure. If that mass was held at atmospheric pressure, you would find that it is a larger volume than the cylinder allows. That denser charge is still being compressed down to the same volume as in the CR though so the PSI is higher than what the static CR calculation shows. Basically, there is more compressed mass at TDC in a running engine than is accounted for in the CR.

Bear is also right about the bore and stroke having their own effects combined with volume. Buildup can also increase pressure. The "natural supercharging effect" is what I was talking about with crankcase pressure.

Actually there are two compression numbers, STATIC and DYNAMIC. Static is if the rings sealed perfect and there is no addition pressure from outside forces. Which does not and and will never happen. Dynamic is what happens in actual fact.

Trafficjamz wrote:That makes more sense now thank you static -vs- dynamic

That is it, dynamic. I couldn't think of the word

thanks for explaining that...learning more and more

and I got a Pm explaining the old fossils routine....copy that

haha

You also have to remember that the porting of the cylinder has an major effect on actual compression numbers.