3.9 overheating yet more

Are these readings wet or dry, and did you have the throttle fully open when taking them?

Its a bit strange that all the left cyls (on yr diagram) are low compared to the other side. But if that head has been replaced then it could be a reason. What makes the difference between the Hi and Lo comp engines? Head or Piston, or both?

from the manual: Compression pressure Test snip

1. Expected readings, throttle fully open, battery fully charged:

8.31:1 = 10.2-10.9 bar, 150-160lbf/in 2 9.35:1 = 11.5-12.2 bar, 170-180lbf/in 2

not sure what you mean by wet or dry (engine was warm). thrttle not open, as it didn't mention throttle in the service manual under compression testing.

could someone have put on the wrong head, therefore low compression readings and therefore again, overheating?

Thanks, Paul

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1992 RR Vogue SE "Arthur" 2005 Baby Girl "Abigail"

Right, here goes. Compression is controlled by the size of the piston bowl, not the head. All the heads have equal chamber volumes, assuming they are the same heads to start with. 14 bolt heads (3.5, most 3.9, 4.2) are all the same, and 10 bolt heads (late serpentine 3.9, all 4.0 and 4.6) are the same. The compression can also be affected by the thickness of the gasket, the original steel shim gaskets (crap, IMO) are only 0.45mm thick when compressed, the aftermarket composite type gaskets (excellent, IMO) are nearer 1.1mm compressed, a difference in compression ratio of around 0.6 to

0.8. Some of the aftermarket composite gaskets have only the 10 bolt holes, so the outer row of 4 bolts are omitted on reassembly, as they really aren't needed (They actually cause premature gasket failure if torqued to the same value as the other 10 bolts, due to applying an uneven clamping force across the head), hence landrover themselves doing away with them on the later engines. Do another compression test, but this time, do it as follows. Remove all 8 plugs and disconnect the low tension wires to the ignition coil. Squirt a couple of cc's of engine oil down the plug hole of the cylinder under test, fit the compression tester, press throttle fully down and crank for a count of 5 seconds then record pressure. Repeat for the other 7 cylinders and see what figures you get. As an aside, one or two cylinders showing a lower value 'may not' be the head gasket or even ring/bore wear, it is common on the V8 to find worn camshaft lobes, causing some of the valves to not be opening fully and hence causing the pressure differences. No. 1 cylinder is the front passenger side (assuming right hand drive!), no.2 is the drivers side. It is possible that you have a tin gasket on one bank and a composite on the other, not good for smooth running or power delivery.

Badger. B.H.Engineering, Rover V8 engine specialists.

According to a book I've just been reading at work, the head is what determines the compression ratio. It even mentioned the different sizes of combustion chamber. Our service manager confirmed this.

I can't remember the producer of the book, but it listed all the V8 varients and their catalog of rebuilt engines and parts.

But you have undoubtedly ripped apart and refettled far more V8s than I have, so I'll take your word for it.

It would account for the OPs difference in readings if one Cyl head were Low comp and the other Hi comp. Ho hum.

No, the combustion volume is what determines the c.r., as a percentage of the swept volume. The combustion volume is the total volume above the piston at TDC, this volume includes the piston bowl capacity and the head chamber.

Sorry, your service manager is talking crap. That is bollox on a rover V8 engine, which is what is being referred to here. There are 4 different head castings, but only 2 different chamber volumes, one being 28cc and I can't remember the other off the top of my head (no pun intended!) but they are effectively grouped by 10 bolt type and 14 bolt type. The 10 bolt variants were skimmed extra to account for the thicker composite gasket that landrover finally switched over to, so if you change from 14 bolt to 10 bolt heads, and fit composite gaskets at the same time, the compression ratio is unaltered.

Just 'cos something is in a book, it doesn't mean the info is proven gospel. There's a lot of factual inaccuracies in the 2 Hardcastle rover V8 books as well, but then if you read them carefully, it's pretty obvious what companies sponsored the writing of them. Good books as they are, though.

There are different pistons for the following compression ratio's 10.5 (P5 and early P6 rover), 9.75 (SD1 Vitesse), 9.35 (Carb SD1, Rangie EFI, some markets of carb landrover), 8.13 (Carb rangie, 90, 110, 101 etc). There are also minor variants on the old P6 engines of 8.25 and 8.5, but the chances of you stumbling across them are relatively remote nowadays, most people rebuild the early engines using later high compression pistons. Go check out the part numbers using EPC, you'll see the heads are the same part numbers for many different compression engines but the pistons differ. All the variants I have quoted are either 3.5 or 3.9, all with the same 14 bolt heads.

Yes, but due to different gaskets maybe, not the heads themselves as he already stated it was 14 bolt heads with the outer row not installed.

Badger.

Dry: without oil on the bores. Wet: with oil in the bores. It gives you a good impression of the state of the piston rings.

You need the throttle open to get a full charge of air in.

Maybe, but I cant see a link to overheating. My guess would be a furred up radiator or stcking thermostat.

On or around Fri, 08 Jul 2005 14:22:07 +0100, Colonel Tupperware enlightened us thusly:

such V8s as I have seen differ only in the pistons. The later heads have longer spark plug threads, but the same combustion chambers. The only difference, as has been said, is the head gaskets - later head gaskets are thicker, which has a minor effect on the C.R.