Thread: 3.5 high comp/low comp

Gday all

Wanting to know if there is anyway of telling if a 3.5 4.4 4.6 etc is a high compression engine or low compression by looking at it. e.g engine number etc.

thanks in advance gaz

Originally Posted by Blknight.aus

there is no need to pop the head to work out the compression all you need is a very large syringe (one of the 1l ones) or an accurately scaled 1l bottle, a drilled out spark plug some thin hose some large clear hose and some patients.

Disconnect the electrics and put the piston you want to check at BDC.

put the larger hose in the body of the drilled out sparkplug thread the smaller hose down the guts make sure its got clearance for air to get out

remove the smaller hose from the plug and then prime the syringe or container through that with 1 measured liter of oil + whatever that bit of hose will hold

re-insert the small hose and carefully fill the piston through the sparkplug hole untill you get clean oil coming up the thicker tube, reverse the flow untill the oil is just showing in the top of the sparkplug. note the amount of oil that is missing from the container. This is the bigger of the 2 numbers

from here you can go 2 ways.

suck the oil out back into the syringe or measuring container raise the piston to top dead center and then add the oil again without adding more oil to the syringe or measuring container.

or roll the piston up to TDC and capture the oil that comes out

the amount of oil missing from the syringe is now the smaller number

use your fractional math to reduce both numbers untill you get something:1

and theres your compression ratio.

disconnect the injector lead to the pot you filled with oil, leave the plug out but earthed start it and run it for a couple of minutes after getting most of the oil out and reconnecting the electrics.

(edit) this is assuming you dont trust or cant find the numbers on the block)

When doing 2 strokes we used to measure the head cc using fluid like that. We'd use ATF fluid though and a special plug that went into the spark plug hole to get a more accurate measurement. The big variation we did though is we would measure the oil going into the cumbustion chamber until it leveled at the top of the plug which was in the spark plug hole and that would be our head cc measurement which was a much easier method and does away with any mathematical problems and measuring inaccuracies from the measurement tool (pipette) and residual oils in the motor that you cant extract. We also had the spark plug come in at the top, where the V8 I don't think is like that any may very well generate an air bubble in the top of the combustion chamber which will add to the measuremen error.