Cylinder Pressures Rover 4.6 V8

**p38arover** · 14th May 2008, 07:30 PM

A friend has just had his 4.6 rebuilt with tophat liners. It is the standard low compression engine sold here in Australia. 7 cylinders are measuring 125 psi, the 8th is measuring 100psi during a cranking test.

The engine is idling roughly and removing the injector drive on that cylinder makes very little difference, whereas removing others does. They've swapped injectors and the are sure that it's not an injector problem.

Would anyone know the sort of cylinder pressures one should expect from a 4.6 LC engine. The engine builder says the above figures are Ok and that a leakdown test is all that's important, not cylinder pressures.

I can't find any details on cylinder pressures in the workshop manuals or on the 'Net but I'd expect 140-155 psi.

Oh, the chap was going to compare a 4.0 engine with his but the rebuilder says that it's not a comparison as the 4.0 hovers over TDC for longer than a 4.6.

I can't go and check mine as the dash is out for a heater core replacement and he can't wait for me as he has to put the car onto a ferry from Tassie to take it back to builder in Melbourne tomorrow.

**rovercare** · 14th May 2008, 07:36 PM

I wouldn't worry myself with the actual pressures, I mean have you ever calibrated the gauge?, but the 25psi difference is not good news

**rovercare** · 14th May 2008, 07:37 PM

Just re read, its not yours

**justinc** · 14th May 2008, 07:46 PM

G'day Ronaldo,

100psi is no where near the mark. 125Psi is almost the bottom end of the scale, as my 8.13:1 ratio 3.5 was specced worn badly with 120Psi in all cylinders

Also, The standard low compression 4.6 in australia from memory was 8.37:1, not 8.13:1.
140 to 150 is what you would expect from a 8.37:1 comp engine with light wear, un oiled at cranking speed I would've thought..?

Either way, sounds too low

JC

**p38arover** · 14th May 2008, 07:52 PM

Yes, I told him mine was 125 when it was pretty worn.

I think it's too low for a new engine.

**djam1** · 16th May 2008, 07:12 AM

If we are talking a new motor has it had time to bed in a little??
Or has it just been started maybe it needs a good run, over the years I have found new motors both Petrol and Diesel can sometimes do with a good workout before they get to thier optimum.
I remember 1 Nissan Patrol engine that you would throw out because of the way it ran after a season of towing field bins and other heavy objects around it ran perfectly.
All this is said assuming there is not something obviously wrong with the engine 125 Psi is marginal and 100 Psi is pretty low.

**Lucus** · 16th May 2008, 08:11 AM

What heads are on it? Im lead to believe the 3.9 heads (32cc combustion chambers) on a low comp bottom end give you around 7.0:1 static comp.

Have the lifters preloads been checked?

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