CP car but has a CR camshaft

[jojim65]

Good afternoon all,

 

I have a 1970 CP which has always suffered from running rich and more than just a little smelly, plugs are pretty black and seems to use more fuel idling in the garage than on the open road. It starts easy enough, no misfires or hesitation.

 

In my quest to cure the rich running I have been through the PI system time and time again.

 

Thinking a little left field can anyone enlighten me on the effects of having a CR cam in a CP with an MU set up for 150bhp. It might be possible that at some time before I owned the car a CR cam was fitted.

 

And what is the internal difference between a 150 & 125 MU.

 

Thanks

 

Jim

[Peter Cobbold]

Jim,

The horse power, hence air consumption, of the 125 is nearer 135 when compared on the same calibration basis, so about 10%.

Sounds like your is running really rich, more than that 10% difference in air consumption would cause.

Peter

[TriumphV8]

Although MU is the same the setup inside differs due to different

manifold pressure. Springs above the diaphragm are different so

every attempt to find a proper setup with the wrong MU might fail.

 

The MU seems having been tuned earlier so must be set anyway

whatever the next cam will be.

Alternatively an exchange MU for installed cam maybe bought ready to fit.

 

CR nearly has no lift in TDC compared to CP that opens the valves a bit.

[OAF939M]

What are the technical differences between the two cams?

Timing, lift and overlap etc?

[Tom Fremont]

CP is 35/65/65/35, 280 degrees duration where CR is 18/58/58/18, 256 degrees duration valve timing-wise. Don't know if there's any lift difference.

 

To check, the #1 inlet valve should just start to open at 18 degrees BTDC if it's a CR and 35 degrees if a CP. The stock pulley has enough calibration to show which it is.

 

Cheers,

Tom

[jojim65]

CP is 35/65/65/35, 280 degrees duration where CR is 18/58/58/18, 256 degrees duration valve timing-wise. Don't know if there's any lift difference.

 

To check, the #1 inlet valve should just start to open at 18 degrees BTDC if it's a CR and 35 degrees if a CP. The stock pulley has enough calibration to show which it is.

 

Cheers,

Tom

Thanks Tom,

That's very interesting, I should be able to check which cam I have without pulling it out...

 

Jim

[OAF939M]

Thank you Tom for a simple and technically understandable answer.

Lister.

[TriumphV8]

CR nearly has no lift in TDC compared to CP that opens the valves a bit.

 

Seems this was too short to be understood:

 

Simply turn engine to mark TDC at front pulley and look at the rockers

of cylinder 1 and 6. If 1 fires take 6 and vice versa.

This is the so called "overlap" where both valves are in use.

If this cylinder has valves a bit open it is a CP cam, if they are closed it is CR.

An easier way to check that is not possible.

 

As cams have a ramp at start and end of lift where over a long period

not much will happen in valve lift it is not the best idea to take that area

for measurement.

The Americans do that better and advertise cam duration with 0.5" valve lift

what in practice puts the area for reading the data into a field where

much difference in lift happens with small change in degree.

The result will be much more precise and can be used for judging a cam.