Rover V8 Pressures

[350matt]

Chaps
Would anyone know what sort of compression pressure you'd expect to see on a healthy 4.0 Rover V8 (approx 10.25:1 C.R.) . I'm measuring 10.8 - 12Bar ( 156-174 Psi) and is does go up by about 1.5 Bar when I add oil, pointing towards a worn set of bores.
Its a long way from k'ned but it looks like it may be time to start saving for the rebuild.

Matt

[stevieturbo]

I checked my virtually new LS1 last night. 8.7:1 and it had 175psi on all 8.
Dont recall any numbers for my RV8

A lot can depend on cranking speed, camshaft, and make sure the throttle is open for it to get air to compress !!

[GreenV8S]

My 4.6 at about 10:1 measured between 160-170 psi.

[leorest]

I'm not sure of the exact definition of CR.
Is it true to say that (in theory)when an engine with a spec of 10:1 CR has a compression test is would return 10bar(147psi) on a day when the atmospheric pressure is 1bar?
If this is so then how does it relate to a real life engine?
In 350matt's example assuming the test condition of atmospheric=1bar and 10.25CR then 10.8 to 12 bar seems very healthy if not a little over the top? e.g. 1*10.25=10.25bar or 147*10.25=151psi.

I had always assumed that regardless of the pressure if all cylinders are in balance then there can't be too much wrong.

Leo

[GreenV8S]

leorest said:

Is it true to say that (in theory)when an engine with a spec of 10:1 CR has a compression test is would return 10bar(147psi) on a day when the atmospheric pressure is 1bar?

If the compression was isothermal (same temperature before and after) then that would be true. In practice, the compression is more or less adiabatic (no heat added or removed), and the temperature goes up significantly during the compression. The temperature rise increases the pressure, so the total pressure ratio is more than the compression ratio.

[350matt]

more to the point, the damn reading increased by 1.5-2Bar when I added oil into the cylinder, meaning it seals better, meaning the bore is worn....
4 years ago it used to read 13.5 - 14.6 Bar
Bugger

Still I wanted to change the cam for a 404 and fit bigger inlet valves anyway and it could do with line boring....


Matt

[leorest]

GreenV8S said:

If the compression was isothermal (same temperature before and after) then that would be true. In practice, the compression is more or less adiabatic (no heat added or removed), and the temperature goes up significantly during the compression. The temperature rise increases the pressure, so the total pressure ratio is more than the compression ratio.

a ha!
So to work out what a compression test on a good engine should give....

Atmospheric Pressure * CR * n =

And from the results on your engine [1(AP)*10(CR)=11bar(162psi)]
I deduce n = 1.1, this is the ratio that the pressure is increased by due to the temperature increase.
Is there a scientific way of working out n?

Would it be reasonable to assume that most engines would increase the pressure above the CR by approximately 10%

If so then 350matt's results look good 1(AP)*10.25(CR)*1.1(n)=11.3bar(166psi)

Just had a look at some old test results on my 350 11.1 to 12 bar It's a standard Rover SD1 Vitesse engine so I'm guessing CR=........Looking for the Haynes manual......still looking....... Ah here it is.....9.75

So 1(AP)*9.75(CR)*1.1(n)=10.7bar. Few

Does anyone have test figures for an engine before and after rebuild for comparison?

[leorest]

350matt said:

more to the point, the damn reading increased by 1.5-2Bar when I added oil into the cylinder, meaning it seals better, meaning the bore is worn....

Yep worrying indeed. I didn't do the oil test when I measured mine So no help there. Sorry. Does she smoke or use oil?

350matt said:

4 years ago it used to read 13.5 - 14.6 Bar

Just a thought. Was this the same test equipment?
Leo

[GreenV8S]

leorest said:

Is there a scientific way of working out n?

Do a search for 'adiabatic compression', you should find there's an exponential ratio between compression ratio and pressure ratio. The exponent varies depending what gas you're compressing.

[leorest]

GreenV8S said:

Do a search for 'adiabatic compression', you should find there's an exponential ratio between compression ratio and pressure ratio. The exponent varies depending what gas you're compressing.

Done that! Now my head aches!I need to think about this another time!

[steve_D]

The compression ratios being used above are Static Ratios i.e. the stroke, bore and combustion chamber.
When doing the compression test you will be seeing the Dynamic Compression Ratio (DCR). The DCR is a lower figure because in almost every cam the inlet valve does not close until some degrees after BDC. The result is that the effective compression stroke is reduced. This reduced stroke will result in a lower pressure reading.

Steve

[HarryW]

Prety certain so long as they are north of 140lbs and within 10% of each other there is not too much to worry about.
If you are really concerned then get a garage to do a leak down test to or give one of the major engine builders such as ray/shaun/rob at V8D, or Dom/jamie at TVR Power, a ring and ask. They are pretty approachable lot and won't sell you anything you don't need nor want.

Harry

[wedg1e]

leorest said:

Does anyone have test figures for an engine before and after rebuild for comparison?

IIRC my 3948 RV8 measured about 160-170 psi after a rebore and new pistons, rings, valves lapped etc.
I can't be more exact as my book of words is in the loft at the moment...

[GreenV8S]

What was the nominal CR on that Ian?

[tuscanbackender]

It had it mate.
New engine that is!
We have a new 2.4 for you to try!

Your mates at Cosworth!

[Boosted LS1]

I'd consider 160-180 healthy for a rover and over 200 psi for a stock ls1. If I had a blown engine in front of me I wouldn't be alarmed to see 150 psi depending on combustion chamber volume.

If you're lucky it may be worn rings and the bores could be ok.

Boosted.

[350matt]

tuscanbackender said:

It had it mate. New engine that is! We have a new 2.4 for you to try! Your mates at Cosworth!

When shall I pop round for the mock-up / installation then?
It hasn't got any of your dodgy 'hammer' cam profiles in it has it?

Matt

[tuscanbackender]

No special cams.
idless like a dream at 7000rpm.
And best of all no more that 120 dB of noise!
(at idle that is!)

[350matt]

sooooooo
what will happen to the test engines once the real castings come in .....?
I could dispose of them for a nominal fee?

Matt

[D_Mike]

leorest said:

GreenV8S said: If the compression was isothermal (same temperature before and after) then that would be true. In practice, the compression is more or less adiabatic (no heat added or removed), and the temperature goes up significantly during the compression. The temperature rise increases the pressure, so the total pressure ratio is more than the compression ratio. a ha! So to work out what a compression test on a good engine should give.... Atmospheric Pressure * CR * n = And from the results on your engine [1(AP)*10(CR)=11bar(162psi)] I deduce n = 1.1, this is the ratio that the pressure is increased by due to the temperature increase. Is there a scientific way of working out n? Would it be reasonable to assume that most engines would increase the pressure above the CR by approximately 10% If so then 350matt's results look good 1(AP)*10.25(CR)*1.1(n)=11.3bar(166psi) Just had a look at some old test results on my 350 11.1 to 12 bar It's a standard Rover SD1 Vitesse engine so I'm guessing CR=........Looking for the Haynes manual......still looking....... Ah here it is.....9.75 So 1(AP)*9.75(CR)*1.1(n)=10.7bar. Few Does anyone have test figures for an engine before and after rebuild for comparison?

you should be able to use the ideal gas law...

pV = nRT

pressure x volume = number of moles of gas x ideal gas constant x temperature.

p1xv1 = p2xv2 is also useful...