Cam Profiles

chris_seven · 05-27-2013, 07:15 AM

I have some cams which may have been incorrectly ground and I am trying to work out if the results I am getting are sensible or if something else is wrong.

I specified 911S profiles with around 11.5mm lift on the inlet and 10.2 on the inlet.

Duration at 1mm lift was specified at 270 degrees Inlet and 240 degrees exhaust.

The inlet stroke at TDC overlap was specified at 5.2mm nominal.

We have the cams and installed them in a 3.0 litre SC and we are down on torque across the entire engine operating range. Peak torque is 175ftlbs at 5000rpm.

We have checked ignition timing at 30degrees at 6000rpm with a re-curved distributor.

The engine starts well idles cleanly and runs cleanly to 7000rpm without any huge flat spots - it just doesn't have the anticipated performance and doesn't rev much beyond 7000rpm.

It also has 10.5: pistons and compression measures reasonably at between 160 and 163psi.

The dyno runs show that the AFR is OK and we can change main ject from slighlty weak to slightly rich ithout any real improvement of change in torque.

I have spent some time measuring valve opening this morning and lift and I suspect the cam is off.

The Inlet valve duration measures at 340 degree!!! (305 at 1mm valve lift)

At 5.2 mm inlet valve opening (Z1 position), the exhaust is virtually closed.

The Exhaust valve opening also shows aronud 330 degrees duration but the exhaust valve starts to open at 30 degrees and is virtually closed by TDC.

The car also have new stainless steel headers and they have badly discoloured in only a few miles.

I think I need to pull the cams out and return them as they don't meet the specification ordered but would a little more information beforte we do this.

Any thoughts?

Henry Schmidt · Henry Schmidt's Garage

Quote:

Originally Posted by chris_seven

I have some cams which may have been incorrectly ground and I am trying to work out if the results I am getting are sensible or if something else is wrong.

I specified 911S profiles with around 11.5mm lift on the inlet and 10.2 on the inlet.

Duration at 1mm lift was specified at 270 degrees Inlet and 240 degrees exhaust.

The inlet stroke at TDC overlap was specified at 5.2mm nominal.

We have the cams and installed them in a 3.0 litre SC and we are down on torque across the entire engine operating range. Peak torque is 175ftlbs at 5000rpm.

We have checked ignition timing at 30degrees at 6000rpm with a re-curved distributor.

The engine starts well idles cleanly and runs cleanly to 7000rpm without any huge flat spots - it just doesn't have the anticipated performance and doesn't rev much beyond 7000rpm.

It also has 10.5: pistons and compression measures reasonably at between 160 and 163psi.

The dyno runs show that the AFR is OK and we can change main ject from slighlty weak to slightly rich ithout any real improvement of change in torque.

I have spent some time measuring valve opening this morning and lift and I suspect the cam is off.

The Inlet valve duration measures at 340 degree!!! (305 at 1mm valve lift)

At 5.2 mm inlet valve opening (Z1 position), the exhaust is virtually closed.

The Exhaust valve opening also shows aronud 330 degrees duration but the exhaust valve starts to open at 30 degrees and is virtually closed by TDC.

The car also have new stainless steel headers and they have badly discoloured in only a few miles.

I think I need to pull the cams out and return them as they don't meet the specification ordered but would a little more information beforte we do this.

Any thoughts?

Is this a twin plug engine?
If not dial in another 4 or 5 degrees of advance and report back.
I would expect the engine to run lazy given "S" cams in 3.0 with 10.5:1 compression.
Header size? 1 5/8" would be my preference. 40mm, 46mm or 50mm carbs?

chris_seven · 05-28-2013, 04:55 AM

Henry, Thank you, I Will do this in the morning and report back.

The motor has 10.5:1 pistons and 1.625" headers and at present 40mm carbs which I think I may need to bore to 45mm.

Does anyone know the exhaust valve lift of an S cam at the overlap position when it is timed to 5.2mm inlet lift?

racing97 · 06:40 AM

This may help it is a Elgin Hot rod S cam
regards
Spec. Value
---------------- -----------------------------------------
Checking Height 0.040 Inch
Valve Overlap 56.9 Crank Deg.

Intake Exhaust
------------------- -------------------
Centerline 97.66 ATDC 97.66 BTDC
Open 34.93 BTDC 37.03 BBDC
Close 50.92 ABDC 21.94 ATDC
Duration 265.8 Crank Deg. 239.0 Crank De
Area 39.33 Inch Deg. 31.85 Inch Deg
Lash 0.0040 Inch 0.0040 Inch
Rocker Ratio 1.00 1.00
Peak Cam Lift 0.44874 Inch 0.39826 Inch
Peak Valve Lift 0.44874 Inch 0.39826 Inch
Lift @ TDC 0.1937 Inch 0.1326 Inch
-------------------------------------------------------------

racing97 · 05-28-2013, 06:46 AM

I am sorry I can't crop and paste it out of the program and editing is an issue as well.
The last column is lift a TDC which is 4.9mm on intake and 3.3 on exhaust.
You can move the figures to what want or I can change them in my cam program if need be
I apologize for the sloppiness.

regards

chris_seven · 05-28-2013, 09:00 AM

Very helpful,

I will make some more measurements in the morning.

chris_seven · 09:26 AM

This 3.0SC has been producing 200BHP with peak torque of 175lbsft and has not responded much to changes of carburation.

It has been running on Zenith 40TINs and we initially thought that this was the problem.

It is quite clear from the above graph which was obtained using 2 x dial gauges applied simultanously and the lack of any exhaust valve opening at the overlap position is unhelpful.

We should not have 'assumed' that the cam was correctly ground and should have checked the exhaust valve opening before buttoning up the motor.

The lobe separtion angle is around 115 degrees and not the 98 degrees we specified.

If we time the cam so inlet and and exhaust valve opening is around the same we end up with about 2mm lift at the overlap and this seems like a standard cam with a slight increase in duration and lift.

Has anyone tried a profile like this as if I could ues them by advancing them them to this position it would help.

This is the profile I had expected but clearly we won't get this result without new cams.

racing97 · 05-29-2013, 08:52 AM

Chris with the displacement you can increase the duration and not suffer with a peaky engine that were characteristic of the early performance cams from Porsche 2.0 2.4 etc. I have included a Modified S and the Carrera 6 profile that has good low end performance if advanced a bit. A 40 mm choke works very well if you can get around the Zenith ancillaries. If you can stuff a longer primary pipe in the works it will really pull hard down below.

regards

chris_seven · 05-29-2013, 09:30 AM

The problem is that to use the existing cam I will only have 0.08" of lift on the inlet at TDC.

If I leave the cams we just measured in the engine and advance them to give the most overlap we can then we will have this set up.

I am not sure that this will be much better than a standard cam.

I am just about to re-bore some Zenith carbs to 45mm diameter and have new throttle valves, spindles and chakes ready. We also have a adjustable idle air corrector and emulsion tubes to try out but need to resolve the cam issue first.

racing97 · 05-29-2013, 11:51 AM

I understand it could be effective a little more overlap would be nice. It will be interesting to see how it comes out.

regards

chris_seven · 06-11-2013, 10:08 AM

We have now had the replacement cams with the correct 98 degree lobe separation and have them installed. The exhaust valve lift now measured as expected.

With the same carb set up the engine is much stronger and pulls harder in every gear.

We will put a few miles on it an run on the rolling road later in the week.

So far so good

--- eventually.