Hello, ive just done a compression test on my rover v8, the heads are Sd1 type and it has a comp head gasket, when tested i got 110psi on all 8 cylinders, it sounds abit low. What should i be getting? would a different heads gasket cause a change in my results?
Thanks.
Compression test results
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If you've fitted composite gaskets and standard SD1 heads, you will get a drop in compression. You realy need to skim the heads by about 0.5mm to make up for the extra gasket thickness.
Also the preload on the hydralic tappets may be a bit marginal with the extra distance between tappet & rocker.
Chris.
Also the preload on the hydralic tappets may be a bit marginal with the extra distance between tappet & rocker.
Chris.
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Series IIA 4.6 V8
R/R P38 4.6 V8
R/R L405 4.4 SDV8
Series IIA 4.6 V8
R/R P38 4.6 V8
R/R L405 4.4 SDV8
Re: Compression test results
Paul V8 wrote:Hello, ive just done a compression test on my rover v8, the heads are Sd1 type and it has a comp head gasket, when tested i got 110psi on all 8 cylinders, it sounds abit low. What should i be getting? would a different heads gasket cause a change in my results?
Thanks.
Blimey, I've just come in from the garage after doing a comp test on my 3.5 lump. Mine are also low. (best is 125, worst is 105). I don't think my motor is knackered as it does not burn much oil (no smoke). The heads were all done by V8 Developments about 2k miles ago so the valves must be OK. I squirted oil into the worst and best cylinder, each went up about 10 psi which isn't that much more. (The carb was fully open during all the tests and the motor was up to temperature)
I think that the cam is having a big effect on my readings, I've got a typhoon fitted which is 220 degrees at 50 thou but more interestingly is 320 from seat to seat which is a hell of a lot!
What cam have you got fitted?
Is your motor burning much oil?
Does the readings go up if you squirt some oil down the plug holes?
The answers to these question may help you workout if you've got a problem.
Personally I don't think I've got a problem other than the cam is a bit too hot for my motor. (To much hassle to change it!)
A leakdown test would tell you for definate whether your motor is knackered. I'll do one on mine when I work out how to! (You need a compressor, that much I do know!)
Pete
Last edited by sidecar on Sun Jan 27, 2008 8:28 pm, edited 1 time in total.
I did a compression test on my motor before I pulled it out.
I had a std 3.5 with std cam and tin gaskets.
I got 165 to 170 psi over the 8 cylinders.
I have since fitted a comp gasket but had the head skimmed to give me the same pressure, I hope!!
Dave
I had a std 3.5 with std cam and tin gaskets.
I got 165 to 170 psi over the 8 cylinders.
I have since fitted a comp gasket but had the head skimmed to give me the same pressure, I hope!!
Dave
1973 MGBGT
1975 MGB V8
I thought I could see the light at the end of the tunnel but it turns out it was a train comming!!
1975 MGB V8
I thought I could see the light at the end of the tunnel but it turns out it was a train comming!!
The type of pistons that you use will influence the compression readings.
It is my understanding that the camshaft will have no bearing on CR.
The CR for your engine is typically stamped just above the engine number.
That CR will take into account the type of head gasket and head chamber volume as was originally fitted to the engine, and of course the type of pistons that were fitted into the engine at the time of manufacture.
With all things being equal, using composite head gaskets with standard 36cc heads (heads made before 1994) will lower the compression ratio.
Heads made after 1994 had 28cc combustion chambers, and were designed to be used only with composite head gaskets.
Ron.
It is my understanding that the camshaft will have no bearing on CR.
The CR for your engine is typically stamped just above the engine number.
That CR will take into account the type of head gasket and head chamber volume as was originally fitted to the engine, and of course the type of pistons that were fitted into the engine at the time of manufacture.
With all things being equal, using composite head gaskets with standard 36cc heads (heads made before 1994) will lower the compression ratio.
Heads made after 1994 had 28cc combustion chambers, and were designed to be used only with composite head gaskets.
Ron.
4.6 Rover 3500 P6B
@ RoverP6B
The camshaft has a big influence on what compression reading you get. The more overlap a cam has the higher up the cylinder the piston is before it starts compressing because the valves are still open. This is partly why a race cam produces little power at lower revs. When the revs rise the compression has less time to escape and the figures start to rise. Obviously like I said this is just one of the factors and there are many more why different cams produce power at different places.
There are two types of compression ratio. Static compression which is the maths I.E maximum cylinder volume and what it gets compressed into. This is what is stamped on the block and the cam has no bearing on it. The other is dynamic compression I.E the engine is actually running (or on the starter) and takes into account all the losses through cam overlap. You can run higher static compression on big cams without running into detonation because of these losses and in fact you should plan on it as it will help boost low rev performance and high revs as well. As an example if you run 11:1 static compression on a standard cam you will have problems running on pump fuel and you will need high octane fuel. On a long duration cam though 11:1 could be just fine on pump fuel.
The camshaft has a big influence on what compression reading you get. The more overlap a cam has the higher up the cylinder the piston is before it starts compressing because the valves are still open. This is partly why a race cam produces little power at lower revs. When the revs rise the compression has less time to escape and the figures start to rise. Obviously like I said this is just one of the factors and there are many more why different cams produce power at different places.
There are two types of compression ratio. Static compression which is the maths I.E maximum cylinder volume and what it gets compressed into. This is what is stamped on the block and the cam has no bearing on it. The other is dynamic compression I.E the engine is actually running (or on the starter) and takes into account all the losses through cam overlap. You can run higher static compression on big cams without running into detonation because of these losses and in fact you should plan on it as it will help boost low rev performance and high revs as well. As an example if you run 11:1 static compression on a standard cam you will have problems running on pump fuel and you will need high octane fuel. On a long duration cam though 11:1 could be just fine on pump fuel.
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stevieturbo
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110psi sounds very low !!!
I had to test my LS1 the other day, which was having some ring issues. 6 were 11 bar 161psi, and the other two 10 bar, 147psi.
My cam is 232/240 at 50thou or 281/289 at 6 thou
Always perform a compression test with the throttle open. I gave mine about 5 "pumps" each when winding over.
My engine was cold at the time.
I had to test my LS1 the other day, which was having some ring issues. 6 were 11 bar 161psi, and the other two 10 bar, 147psi.
My cam is 232/240 at 50thou or 281/289 at 6 thou
Always perform a compression test with the throttle open. I gave mine about 5 "pumps" each when winding over.
My engine was cold at the time.
9.85 @ 144.75mph
202mph standing mile
http://www.youtube.com/watch?v=XgWRCDtiTQ0
202mph standing mile
http://www.youtube.com/watch?v=XgWRCDtiTQ0
Thanks for the replies guys, I guess a leakdown test would prove whether its the cam that is giving the low reading or whether my rings are leaking like seives! (The heads have just been done so I don't think its the vavles).
Paul, if you are getting 110 PSI with a standard cam then I guess you do have a problem
Pete
Paul, if you are getting 110 PSI with a standard cam then I guess you do have a problem
Pete
Hi Steve,stevieturbo wrote:
My cam is 232/240 at 50thou or 281/289 at 6 thou
You cam looks a little hotter than mine at 50 thou lift but not so hot at the seat to seat duration (Or 6 thou which must be nearly seat to seat).
EDIT... I think that I've got my cam figures messed up! The typhoon is 270 at 50 thou, (not the 220 that I stated in an earlier post), it is 320 seat to seat (or near enough 6 thou) This would make my cam quite a bit hotter than yours!
I really don't know now whether my motor is knackered or not!
Pete
Hi
Stevies cam is a roller cam, yours is a flat tappet, you have just stumbled across why people spend £280 fitting roller tappets, the roller cam can take much more agressive cam ramps so you end up with less duration at 6 thou for a given duration at 50 thou (or more duration at 50 thou for a given duration at 6 thou).
Stevies engine is an LS1.
Best regards
Mike
PS 320 at .006 and 270 at .050 sounds alot of timing for a road cam
270 at .006 and 220 at .05 sounds alot more like a road cam
Stevies cam is a roller cam, yours is a flat tappet, you have just stumbled across why people spend £280 fitting roller tappets, the roller cam can take much more agressive cam ramps so you end up with less duration at 6 thou for a given duration at 50 thou (or more duration at 50 thou for a given duration at 6 thou).
Stevies engine is an LS1.
Best regards
Mike
PS 320 at .006 and 270 at .050 sounds alot of timing for a road cam
270 at .006 and 220 at .05 sounds alot more like a road cam
poppet valves rule!
