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Group B cams. Why are they low lift?

Question of the day.

I have always wondered why Goupe B 930 Turbo cams have such low lift.

I just found out why.

Anyone know why?
Old 01-27-2010, 03:15 PM
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I thought it was for better gas mileage in the long distance racing.
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Old 01-27-2010, 03:29 PM
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Interesting!

Don't think so!

Old 01-27-2010, 04:12 PM
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I'm going out on a limb here...

With a big lopey (sp?) cam, would it create back pressure problems for the metering plate? That's what I've heard any way.
Old 01-27-2010, 05:57 PM
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Quote:
Originally Posted by A930Rocket View Post
I'm going out on a limb here...

With a big lopey (sp?) cam, would it create back pressure problems for the metering plate? That's what I've heard any way.
That would be the case if we were running something higher than 7.5:1 compression and CIS fueling. Otherwise, SC and 964 cams wouldn't work in our application. So I'm thinking it's for some other mysterious reason. Come on Keith, out with it!
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Old 01-27-2010, 06:03 PM
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I will guess that they wanted to limit the torque so that they wouldn't break the more or less stock 930 transmission.
Old 01-27-2010, 07:00 PM
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Come on, Keith.. Were you that irritating kid at School who knew everything, but just wasn't telling?

Out with it, man!..lol
Old 01-28-2010, 12:55 AM
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My guess up till now was:

a) to minimize valve float at higher rpm. or

b) to improve intake charge velocity for better CIS atomization / mixing.

But that dose not seem to be correct.
Old 01-28-2010, 11:22 AM
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And the answer is:

From what I have recently read, it was because under the Group B rules, manufactures were not allowed to increase lift. They could however increase duration.

Thus, the Group B cams seems to be a compromise or work around and unless one it trying to fit into a class that requires them they are probably not a best of bread cam.

They also did not seem to move the HP up the rpm curve much. They are reported to make peak HP at 5500rpm which is the same as a stock cam.

They were however allowed to fire ring the heads and they ran adjustable boost between 1 and 1.3 bar. Gross HP was reported to be around 360hp.


Last edited by 911st; 01-28-2010 at 11:32 AM..
Old 01-28-2010, 11:22 AM
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Talking with a guy who does a lot of Porsche porting development, flow is not improved so why do it.
Old 01-28-2010, 12:20 PM
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copbait73

Do what?

Please expand, not sure what you are saying but I am eager to.

Thx.
Old 01-28-2010, 12:34 PM
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copbait73

Do what?

Please expand, not sure what you are saying but I am eager to.

Thx.
This guy has much invested so he is typically short in explanation. His port size, lift and now cam profiles are proprietary. My comment is based on his typical response to discussions of factory port size to lift matches and by extension cam grinders who follow factory convention.

This is not my area, however he does not believe in lifting after the port has stopped flowing. Sounds logical, but according to him that is typical of most Porsche performance cams.
Old 01-28-2010, 01:23 PM
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Got it.

I am just starting to learn about cams.

I played with some numbers for our cams, ports, and the curtain area under the intake valve seat.

It seems that our valves only have to be open about 40-50% to equal the same area as our ports. Even on something like an SC or 3.2 w larger ports.

This fits with what your guy was saying.

If so, extra lift is not what gets us more into the cylinder!

I suspect total lift on our cars is more a product of duration. That is if the valve is going to be open longer, we need a larger dome on the lobe to accommodate it.

The byproduct of this might be that the valve dose open quicker with more lift (steeper ramp) and that to might help increase total flow potential to a small degree.

Just guesses and I would like to know if I am on the right track.

Many say the SC cam makes more low end than a C2. I am thinking that is because the C2 has more lift and can not be advanced as far without increasing piston interference potential.

If one wanted to retard an SC cam instead to early 3.0 specs it should move the HP peak from the 5500 we see on a 930 to about 6000rpm which should put it on par with the C2 cam.

Just a guess.
Old 01-28-2010, 01:48 PM
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Quote:
Originally Posted by 911st View Post
And the answer is:

From what I have recently read, it was because under the Group B rules, manufactures were not allowed to increase lift. They could however increase duration.

Thus, the Group B cams seems to be a compromise or work around and unless one it trying to fit into a class that requires them they are probably not a best of bread cam.
The Group B cams can be installed without changing the valve spring heights. Install an SC or 964 cam and the spring heights have to move up 1 mm.
The increased duration of the Group B cam will flow more mass over time since the valves are open longer. Same goes for the SC and 964 cams except that because of the greater lift, the cams are open longer yet.
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Old 01-28-2010, 02:33 PM
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Ran my port to valve area numbers again.


Port cross section:
32mm port is apx 1.23 sq inch area
40mm port is apx 1.96 sq inch area

Area under valve seat:

49mm valve has about 6.1" of circumference.

6.1" x .455" lift for an SC cam = 3.21 sq inch.

6.1" x .378" lift for a 930 cam would = 2.31 sq inch.

Compare port volume to area under valve seat:
2.78-3.21 sq in....area at full lift for 930 to SC cam verses,
1.23-1.95 sq in....area for 32-40mm intake ports.


Put an SC cam with a stock 930 port volume and there is way more lift than is need with 3.21 sq in. area volume under the intake valve seat at full lift for a port that can only flow 1.23 sq in of volume.

This would make the area under the valve 260% larger than the port.

Kind of cool info and not what I expected. I thought lift equaled more air flow.

It dose some but because of the increased lobe ramp angle, not because of the higher gross lift.

-----
These numbers are nominal. The area under the valve might be less depending on diameter at the seat, the ports might be more or less than there published numbers, and the lift might be less depending on how the cam was measured.

Fun stuff!

Last edited by 911st; 01-28-2010 at 02:55 PM..
Old 01-28-2010, 02:53 PM
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Quote:
Originally Posted by 911nut View Post
The Group B cams can be installed without changing the valve spring heights. Install an SC or 964 cam and the spring heights have to move up 1 mm.The increased duration of the Group B cam will flow more mass over time since the valves are open longer. Same goes for the SC and 964 cams except that because of the greater lift, the cams are open longer yet.
Are you sure about that?. Many out there have installed SC and 964 cams with stock springs.
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Old 01-28-2010, 03:00 PM
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This is cool.

930 cam lift is .378 & .343

Group B cam lift is .374 & .339

This fits what I read that the Group B lift could not exceed the 930's being 4/1000's of an inch shorter lift.

I have seen reference to a RUF Group B. I bet it is about the same duration as the Porsche cam but with appropriate lift for the duration.

Couple more clues...
Old 01-28-2010, 03:04 PM
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Are you sure about that?. Many out there have installed SC and 964 cams with stock springs.
I understood that seat pressure is supposed to be increased both with higher lift cams and or with turbo charging.

I went to race springs when I changed cams.

The other way to go is with light weight retainers.
Old 01-28-2010, 03:07 PM
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Quote:
Originally Posted by mark houghton View Post
Are you sure about that?. Many out there have installed SC and 964 cams with stock springs.
Mark, I've found conflicting information about this, for sure.
Bruce Anderson says the height must be increased; John Doughrty says no.
DonE says he didn't do it and bent all of his valves, so I'm of the opinion that it needs to be done.
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Old 01-30-2010, 04:51 AM
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Quote:
Originally Posted by 911st View Post

Compare port volume to area under valve seat:
2.78-3.21 sq in....area at full lift for 930 to SC cam verses,
1.23-1.95 sq in....area for 32-40mm intake ports.


Put an SC cam with a stock 930 port volume and there is way more lift than is need with 3.21 sq in. area volume under the intake valve seat at full lift for a port that can only flow 1.23 sq in of volume.

This would make the area under the valve 260% larger than the port.

Kind of cool info and not what I expected. I thought lift equaled more air flow.

It dose some but because of the increased lobe ramp angle, not because of the higher gross lift.

Keith, you've calculated what's known as "curtain area".

You are correct in saying that absolute lift by itself doesn't allow more mass flow in the stock head. Time is the important parameter here. In order to control valve velocity and keep it to an acceptable level, a valve must open earlier and close later if it is to open higher. Mass flow is increased, therefore power is increased.

I work in engine manufacturing. We grind our own cams. Ramp velocity is a big deal and must constantly be measured.

IMO, if one is upgrading cams for increased power, the 964 cam is the way to go, especially on the cars with 5 speed transaxles. The Group B and SC cams don't go far enough and all of these choices will degrade low-rpm performance to some extent.
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Old 01-30-2010, 05:03 AM
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