Teach me about camshafts

[jehatwhitland]

9 minutes ago, WP0ZZZ said:

Is there a mix of port sizes within the same model year?

I seem to remember that small port sizes are preferred for performance builds (which sounds kind of counterintuitive.) Is that right?

As far as I know, the small port heads were only on US-spec SCs from the 1980 MY and, no, they aren't preferred for performance builds as the inlet port is 5 mm smaller. I think they were a way of maintaining the mid-range pick-up (less air to accelerate) whilst capping the top-end performance of the US spec models even as the compression ratios was raised to improve emissions. Don't ask me why - was probably to meet some Californian anti-smog requirement! 

However, if you want some good condition small port heads I'll happily swap them for your large port ones. 😉  I can't give them away!!

Bruce Anderson book is a very good source for working out what you want to do and the SC is a very strong base to start from.

 

[Richard Bernau]

Someone will correct me if I am wrong, but my recollection is that all the later 204hp SCs were the small port/small valve spec, so not just the US cars.  So counter intuitively, its the less powerful SCs that are the marginally better pick for a performance build.  

[GaryH]

Larger port, more air but at lower airspeed so better torque ? Or is that an over simplification ? 🤔

I think RB's correct about all later SC's having small port heads, not just the US. 

Edited August 21, 2019 by GaryH

[GaryH]

Some interesting reading here.

Specifically... "The ports were polished but not increased in size to maintain low- and mid-range drivability. "

[GaryH]

More info here

Specifically...

"Porting and flow testing the heads to increase airflow is one of the most important parts of increasing engine performance. Since any engine is simply an air pump, getting more air in and out, at all valve openings, is paramount to power increases. Done correctly, you will see a torque increase throughout the RPM range that translates into more usable power, not just at peak RPM. It’s too easy to just hog the ports out and lose almost all low and mid-range torque."

and 

"The first 3.0 SC engines, ’78-’79, employed larger ports and intake manifolds than the later, ‘80-‘83 versions. These early heads are more suitable for modifications and have a good deal more airflow than the later ones. For applications requiring major camshaft and compression upgrades, both types of SC heads should be opened up and flowed for maximum airflow at all valve lifts."

 

Edited August 21, 2019 by GaryH

[Leicestershire]

1 hour ago, Richard Bernau said:

Someone will correct me if I am wrong, but my recollection is that all the later 204hp SCs were the small port/small valve spec, so not just the US cars.  So counter intuitively, its the less powerful SCs that are the marginally better pick for a performance build.  

204 bhp Euro spec cars have 39mm ports.

 

[Leicestershire]

1 hour ago, GaryH said:

Larger port, more air but at lower airspeed so better torque ? Or is that an over simplification ? 🤔

I think RB's correct about all later SC's having small port heads, not just the US. 

Torque ultimately makes bhp.

Port size changes where the torque and power are made.

[GaryH]

4 minutes ago, Leicestershire said:

Torque ultimately makes bhp.

Port size changes where the torque and power are made.

After (!?) my heads were ported I remember reading that over-porting them could reduce torque by increasing the air-flow speed too much ?

[Leicestershire]

19 minutes ago, GaryH said:

After (!?) my heads were ported I remember reading that over-porting them could reduce torque by increasing the air-flow speed too much ?

It’s a very fine balance. For reference SCRS heads had 41mm ports.

Its only a few fractions of a mm from perfection to disaster.

I am sure that many ‘DIY have a go heroes’ will have gone with the theory that bigger must be better.

 

[GaryH]

More opinions from an (old) Pelican thread,

Specifically... "Small ports move the power/torque band lower in the rpm range. Smaller diameter tubes speed gas flow but at high revs, gas flow approaches sonic limit sooner, thus the dropoff in efficiency and torque, therefore power."

and

"We had a 3.0 spec. race engine on the engine dyno with the cams dialed in at 1.5 mm.
After re-setting the cam timing to .9mm we gained 7hp up on top. With it at 1.5 peak hp was at 5900 rpm. With cam timing at .9 peak hp is at 6050. This timing change for more top end hp resulted in a similar 6-7hp loss from 3500 to4800 rpm."

Edited August 21, 2019 by GaryH

[Leicestershire]

The gains with cam timing change with the cam profile.

Maximum bhp gain is with the cam timing retarded.

If the cam timing is set at maximum retard (for the spec of cam) and the timing chain stretches then the cam timing will be retarded out of spec and performance will suffer.

Most engine builders will time cams at mid range. 

[GaryH]

14 minutes ago, Leicestershire said:

Most engine builders will time cams at mid range. 

964 cams don't seem to have a 'range' though ? 

There's only one figure given: 1.26mm (as opposed to the range given for an SC/3.2 cam: 1.4 - 1.7mm) 

Plus.... 

Edited August 21, 2019 by GaryH

[Leicestershire]

13 minutes ago, GaryH said:

964 cams don't seem to have a 'range' though ? 

There's only one figure given: 1.26mm (as opposed to the range given for an SC/3.2 cam: 1.4 - 1.7mm) 

964 cams time at 1.1 - 1.4.

 

[GaryH]

6 minutes ago, Leicestershire said:

964 cams time at 1.1 - 1.4.

John Dougherty's info page wrong then ? 

Edited August 21, 2019 by GaryH

[GaryH]

Anecdotal 964 cam timing results in SC and 3.2 engines here.

[WP0ZZZ]

According to Bruce Anderson's book, both RoW and US versions of the SC engine have the same port sizes. I copy here some of the values that can be found in the book:

Engine	Model	Intake port	Exhaust port	Intake valve	Exhaust valve
930/03	911 SC 78-79	49	41.5	39	35
930/09	911 SC 80	49	41.5	34	35
930/10	911 SC 81-83	49	41.5	34	35
930/18	911 SC RS	49	41.5	43	43
930/20	911 Carrera 3.2	49	41.5	40	38
M64/01	964 Carrera 2/4	49	42.5	41.5	38
M64/03	964 Carrera RS	49	42.5	41.5	38
M64/05	993 94-95	49	42.5	43	39
M64/20	993 Carrera RS	51.5	43.5
M64/21	993 96-97	50	43.5	43	39

[Leicestershire]

2 minutes ago, GaryH said:

John Dougherty's info page wrong then ? 

Who knows.

Perhaps porsche got in wrong in the factory manual.

Either way, if you set at the average of the porsche 1.25 or JD's value 1.26 your engine will be fine.

[WP0ZZZ]

7 minutes ago, GaryH said:

John Dougherty's info page wrong then ? 

Those are the timing specs in the 1991 version of the 964 workshop manual:

 

[Leicestershire]

3 minutes ago, WP0ZZZ said:

According to Bruce Anderson's book, both RoW and US versions of the SC engine have the same port sizes. I copy here some of the values that can be found in the book:

 

Engine	Model	Intake port	Exhaust port	Intake valve	Exhaust valve
930/03	911 SC 78-79	49	41.5	39	35
930/09	911 SC 80	49	41.5	34	35
930/10	911 SC 81-83	49	41.5	34	35
930/18	911 SC RS	49	41.5	43	43
930/20	911 Carrera 3.2	49	41.5	40	38
M64/01	964 Carrera 2/4	49	42.5	41.5	38
M64/03	964 Carrera RS	49	42.5	41.5	38
M64/05	993 94-95	49	42.5	43	39
M64/20	993 Carrera RS	51.5	43.5
M64/21	993 96-97	50	43.5	43	39

 

Don't believe everything you read!

 

[Chris T]

Certain I read somewhere that Porsche messed with valve timings for the 3.2 and SC, hence the 'range' depending upon model. For the uninitiated Dougherty's timing figures refer to mm of lift at the valve head at TDC as opposed to some manufactures's figures of degrees +/- TDC @ 1mm lift. Much easier IMHO.

Chris.

 

[GaryH]

Just now, Leicestershire said:

Don't believe everything you read!

Could the later model 204bhp engine you saw the large ports on have had earlier SC heads ? 

[WP0ZZZ]

Just now, Leicestershire said:

Don't believe everything you read!

 

Please note the word "according":

5 minutes ago, WP0ZZZ said:

According to Bruce Anderson's book, both RoW and US versions of the SC engine have the same port sizes. I copy here some of the values that can be found in the book:

 

Would be grateful if someone could share numbers from other sources.

[Leicestershire]

1 minute ago, GaryH said:

Could the later model 204bhp engine you saw the large ports on have had earlier SC heads ? 

anything is possible especially when these cars are knocking on 40 years old but my own car car is an '81 with 39mm ports.

 

 

[GaryH]

8 minutes ago, Leicestershire said:

anything is possible especially when these cars are knocking on 40 years old but my own car car is an '81 with 39mm ports.

Interesting, I was certainly of the opinion that the accepted wisdom was that ROW 1980+ SC's had small ports.

Although from this PP thread - "ROW SC's maintained the big ports through 83 for exports to unrestricted emissions areas." ?

Edited August 21, 2019 by GaryH

[Leicestershire]

4 minutes ago, WP0ZZZ said:

Please note the word "according":

 

Would be grateful if someone could share numbers from other sources.

I was referring to the numbers quoted for the port sizes - 49mm inlets???

SC's 34,36,39mm - trying to clarify what year, model or market has what. 3.2 has (I think) 41mm - somebody please confirm

SCRS 41mm