Location: QLD, Sunshine Coast
Registered: August 2005
Low compression on a high-compression 4AG
Sat, 10 December 2005 01:59
My new 100kw 4AG apparently had new rings fitted about 7000 kms ago and has high compression pistions but when a mechanic did a compression test it was only 90-110 psi. I read that standard compression for these engines is about 180 psi so with high compression pistons it should be higher than this.
The mechanic said there is a good chance that having the valves adjusted could bring compression up to about 180psi but thats still only normal compression.
The guy I bought the car offs theory is that the engine only gets its power above about 3000 rpm so compression might be lower below that but the mechanic doesn't think so.
Does anyone know anything that could explain this? Are any of these theorys right? Will adjusting the valves give it full compression? Any help would be good thanks.
Is the motor stock? Are all four cylinders in the same range? The pressure readings are low if it is a stocker.
Anyway, compression pressures and compression ratios do not directly relate.
There is more to it than a direct relationship. That is pressure does not necessarily go up if the compression ratio is increased.
Other factors come into play, such as valve lift and timing.
All pressure readings are usually taken at cranking speeds, so a high compression ratio motor can have low pressure readings especially when the inlet valve lift is great or it has a long duration and late closing.
At cranking speeds a lot of air can be pushed back out of the inlet valve so there is not much to build any pressure. A full competition motor with high lift and long duration valve timing allows a lot of air back out at cranking speeds but the actual dynamics of the air/fuel gas flow at normal running engine speeds actually draws more air/fuel in even though the piston is moving up the bore during the compression stroke.
If the engine runs smoothly I would say it has some valve clearance.
Adjusting the valve clearance can change the pressure at cranking speeds as it changes the timing and lift of the valve. More clearance on the inlet will give, in theory a slight increase in pressure but it would be hard to measure a difference.
Check the inlet valve timing to see if it has been retarded which will mean the valve is still open too late in the compression stroke.
Sat, 10 December 2005 01:59
