Anyone know how to figure out compression ratio?

[dbc112]

I bought a used motor that I think runs high compression pistons... I picked up a compression guage that reads PSI. Anyone know how to figure out what you are running ratio wise? I searched the internet and came across a formula, basically divide your PSI by 14.7.


Then yahoo answers says you need to know volume etc.... No idea what to do.

Originally posted by dbc112
I bought a used motor that I think runs high compression pistons... I picked up a compression guage that reads PSI. Anyone know how to figure out what you are running ratio wise? I searched the internet and came across a formula, basically divide your PSI by 14.7.


Then yahoo answers says you need to know volume etc.... No idea what to do.

well if i'm not mistaken volume would be the size like a 400ex is a 397cc machine so its a .397L motor. If thats the case just diving your PSI by 14.7 then i only have 7.5:1 compression or something

found this but i dont know how it works i enter the bore and stroke in mm and the chamber volume in cc. all i get is a really high number

[flyin#5]

pretty hard thing to do without taking the engine apart and measuring volume. if you want to do that i can pm you the formulas. get your compression in psi and compare it to others on this site.

[wilkin250r]

The problem is that your compression READING has many factors that will influence it. The same engine can read 180psi on a fresh rebuild, 190psi when it's broken in and the rings are seated properly, and then 160psi when it's old, tired, and worn-out.

Radically different reading all the the same "compression ratio".

Essentially, this means that there is no hard and fast rule about converting pressure readings to compression ratio, there are too many factors involved.


There IS a way to measure it without taking the engine apart, but it's kinda messy, requires some special equipment (not expensive, just unique) and also requires that you know your exact bore and stroke.

[dbc112]

Bore is +2mm over and stroke is stock 300ex 65.5mm

The engine also has port and valve job, and I think that effects the amount of overall volume, or is it just the efficiency in which air can enter and exit?


The whole reason I am trying to figure this out is because of many factors.

1. I really dont want to run on race gas if not ABSOLUTLEY necessary

2. I am installing a dial a jet, and I want to tune on 93 or aviation leaded fuel (only $4 per gallon)

3. I also hot a hot wire and Nology plug comming (sweet setup for great spark, silver plug, non resistor, and wire has built in coil)

So with the new mods to bring air/fuel in, and burn it more efficiently I really want to know the ratio, I just hate "burning" money.

I do know the engine was built for nationals, so I would think a high compression piston was used... Would make sense. Falls within the guidelines (I think) and helps get more HP with the cc limitations (+2 brings cc to 298+-)

Oh, and before anyone comments. I am also running a full exhaust and Uni with lid box on (read dial a jet tune manual causes more problems than worth)

[Bradracer18]

lol...if its a "full national kit" I'm sure it needs race gas or atleast a 50/50 mix. The easiest way would be to tell your mods to someone on here, and seek their advice about fuel.

[dbc112]

Yeah I figured so.. oh well...


I was told "full national kit" and confirmed with Lars at GT thunder.. Need race gas.



Soooo... The question is leaded or unleaded. I have a great leaded booster that legitimatley raises octane... half a bottle (20 per bottle) will get me to approx 106 leaded. Vs going 115 VP or whatever.

[Bradracer18]

I use leaded sunnuco(spelling?) gas....leaded just provides that much more lube and cooling

[dbc112]

Ok, so octane is octane then... Thought you get better bang with leaded.


I would think 108 is sufficient, but wouldnt really know unless I had the compression ratio...