906 Cams

[andrew15]

- I'm thinking of building up a short stroke 2.5 L engine and am trying to determine the cams that would be suitable.
- This would primarily be a weekend car for a quick run through the mountains around Banff or autocross - no commuting to and from work whatsoever.

-I have a set of E cams and understand that these make a great street cam.
- I was thinking of 906 cams as these would be pretty fun at higher RPMs But.... how bad are they for street use? Would early S cams be as agressive as one can go for street use?

Regards,
Andrew M

[CamB]

Well, I've got S cams and you already have to drive it pretty hard to keep it on cam (ie about 5000rpm). I would have thought 906 cams would be better suited to track, where you can pick your shift points and are not likely to find yourself in the wrong gear.

Plus if you go with 906 cams wouldn't you have to make sure the engine is ok up around 7500/8000 rpm? Sounds like a recipe for a short (but amazingly fun) engine life to me.

Disclaimer: never driven a car with 906 cams!

[john walker's workshop]

906 cams kick ass, but they would be wretched on the street, unless you kept the Rs up real high. just comparing the lobe profile to an S is an eyeopener.

[82SC]

BA in the recent Excellence said there are more modern cams then the 906...

he plugged his buddy Jerry Woods but said there are many other cam companies out there

MJ

[Wayne 962]

Right - in two weeks, check Chapter 4 of the Engine Rebuild Book, where I talk about recommendations. I have a 2.5 with 906 cams in my recommendations section (I think). As MJ, actually, he is the only fellow on this board who has actually seen the new book!

-Wayne

[andrew15]

Wayne - The book is going to be my summer read, with a rebuild planned for next fall. The more I look at the sample pages, the more I hope the boat w/ the cargo gets here soon!

Regards,
Andrew M

[jluetjen]

Here are some comparisons that might help you out based on torque curves for some engines:

My 9.8:1 CR 2.0E:
* I can just idle around town at 30 mph at 2000 RPM in 3rd gear. This is 14.5 mkp of torque. It's pulling cleanly at 3000 RPM and making about 16.5 mkp. My engine really comes on cam at the torque peak of about 18 mkp at 4000 RPM and pull right through to 7000 RPM when the torque has fallen back down to 14 mkp again.

A 2.0S:
* This engine is also making only 14 mkp at 2000 RPM, and takes about 2200 RPM to make the same 14.5 as my E. At 3000 RPM the S is making about 15.5 mkp (still lagging my E) and doesn't generate peak torque until about 5500 RPM. But when it gets there the S is peaking at 18.5 mkp. The S will pull through to 7500 RPM at which point it is still making about 15.5 mkp. So the S will pull harder, but you have to spin the engine pretty hard to find it.

A 2.0 906 with 46 mm Weber carbs.
* A 906 engine's torque isn't even plotted until 4500 RPM. At 4500 RPM it is putting out less then 16 mkp, so less then my E is pulling at 3000 RPM. At 5000 RPM it has about matched my E at 18 mkp, but it took another 1000 RPM to do it. At 6000 RPM it is suddenly on cam and pulling a very substantial 20 mkp and trumping both the E and the S pretty handily. It keeps pulling that hard through 6500 RPM and then starts to drop off. AT the 906's HP peak of 8000 RPM it is still pulling 18.7 mkp, but that drops off to 18.25 at the redline of 8500 RPM.

Now the engine's porting will modify these numbers some, but in a nutshell unless you want to be constantly spinning the motor above 6000 RPM, an S is better then an 906, yet for real world driving an E cam could very well be better still.

[724doorE]

Ah John,
But how about a 2.7 with RS pistons, 38 mm ports and 906 cams. I think the conjecture has been that an increase in displacement will lower the power curve rpm for a given cam profile. Seems like 906 cams might be a nice little combo in a 2.7 RS'd engine in a light early car.

Sure they'd still be peaky, but how about fun!! Gee, the factory racing 41mm ports might even be better. Plug those into your equations and lets see what you get!!

[Wayne 962]

Dennis, you are 100% right, but I think that you probably need to go higher in displacement to lower and smooth out the power curve. 906 cams would probably be a great choice on a 3.5 or even a 3.2. I know that 'S' cams are good for broad range power on the 3.0L and higher engines.

I think that the 2.7 with 906 cams would still be very peaky, and not a good daily driver. Even on a 3.0L you would probably see significant drop-off on the lower end. Great power on the top end of course, but poor performance around town.

-Wayne

[jluetjen]

Hi Dennis.

Been there -- calculated that!

Basically the shape of the curve stays the same with the following adjustments...

With comparable porting, a 2.7 with a 906 cam will have the same curve as a 2.0 with a 906 cam, the only difference is that the 2.7 is 35% larger, so all of the numbers will be 35% higher. You'd be surprised how the torque/liter at different points across the rev range remains pretty consistant no matter what the size of the engine. With 35% more torque, it no longer feels so bad to be "off cam" with a bigger motor.

If the ports are a little undersized, they will choke off the torque a little bit at maximum RPM's and shift the peak torque a little lower in the rev range. This seems to have a accounted for the slightly different shape of the 2.7 RS's torque curve compared to the 2.4S since almost everything else in the engine was consistant.

Thirdly, the timing (advanced or retarded) of the cams can often have a significant affect. In the case of my estimates, I'm assuming the cam timing is set according to the factory specs.

So 906 cams in a 2.7 RS will result in an engine that pulls like an RSR below 6000 RPM (by this I mean that it will trail a comparable engine with E or S cams). With the configuration that you described, I'd expect the torque to peak at around 207 ft-lbs at 6100 RPM (compared to 6200 RPM for an 906 or an RSR) and the HP to peak at around 270 HP at 7600 RPM.

So from 6200 to 7600 RPM you'll be having a ball (assuming that your engine is built to survive at those rev's.) Keep in mind that it a fairly tight rev range for street use. Depending on your gearing, you may never be "on-cam" at street legal speeds in anything higher then 1st gear! Below 6200 RPM, you'll actually be faster with either an E or an S cam.

[CamB]

My S cams are in a 3.2, and it seems to be like John says.

Basically, it is super torquey down low, but so it should be - 3.2 litres, 10.5:1 c/r and only 2300lb. But it still comes on cam pretty hard at 5000 or so rpm. The bottom end is just better because of the displacement (and compression, I guess).

[724doorE]

I knew you'd come through, John!

Well, that was the answer I expected, still not docile enough for everyday street use.

The question then becomes would one be willing to handle that type of engine for a car that would see track time/ street time.

Would still be fun!

[Chuck Miller]

See if you guys agree with what I've been told by a couple of old Porsche hands…

C-6 cams were designed for carburetion not partially for MFI. RSR cams were designed for MFI. Then you ask, the 67'-68' carbureted S's have the same cams as the 69'-73'S's - the answer I got is that the S cams work for either, but the C-6 cams work for carbs. Now, there were a couple of Carrera 6's with MFI and I suppose the pumps were reworked to run with the C-6 cams.

The other thing is that you can retard the C-6 cams to give you a fairly docile idle and a bigger midrange. I have driven 2 cars set up like this, a 2liter and 2.2liter that had carburetors, C-6 cams, and were a pleasure to drive on the street.

For what it's worth…

BTW… The Gulf car with the Minilites at the R-Gruppe event was one of those cars….

[jluetjen]

The copy of the 906 (aka C-6) owners manual that I have lists the cam timing for the 901-20 as BA has it in his book, and does not describe any alternative cam timing in the section detailing the changes for the MFI'd 901-21.

As far as the cam timing, I'd like to learn more about the affects of that. Unfortunately I don't have years of experience building and testing these motors as some do. I've drawn my conclusions from analysing Porsche's published information which assumes factory spec'd timing.

Maybe someone like TimT, John Walker or one of the guys with experience in this area can weigh in.

[kenikh]

Can anyone tell me more about the effects of cam timing? What is the general result of advancing or retrading cam timing? What are the trade-off? Anyone?

[ficke]

My .02 cents
I have "S" cams in my 2.0l 67 911S I find them to start pulling at around 3500RPM and torque starts falling off around 6500 RPM. They seem pretty liner in power, a little flat at idle speeds, I find them very tame and streetable and they will idle easily at 1000 rpm. My wife was kinda disappointed when she drove the car for the first time after hearing about "explosions of power" etc.. We like to have to blip the throttle and down shift to pass on the freeway, 'S' cams do not require that maybe 906 cams would? When I drive a sports car I like to have to drive it like a sports car.

[kenikh]

I think S or 'mod-S' are probably best for the 2.0 cars. 906 seem like they'd be too much work; but in a 2.5? That 25% increase in displacement might be enough to make a 906 cam in a 2.5 feel like an 'S' cam in a 2.0. My guess is that the RSR cam would be even better.

[Steve@Rennsport]

Kenik:

Within the limits of each cam profile and the piston-to-valve clearances in every individual engine, advancing the cams adds some low-end torque to the detriment of top end power.

Conversely, retarding the cams adds more upper-end HP, to the detriment of low-end torque.

Although this is a simplification of a complex subject that has MANY variables, I do hope this helps,....


About 906 cams,.....

In a 2.0 to 2.8, they make best power from 5800 to 7900 (depending on several variables). Their main drawback is how they run from 3000-5000; these have a big flat spot caused by reversion that can be quite difficult to get through.

Nowadays, there are FAR better choices that offer the same power with much improved drivability.

[kenikh]

Yes indeed; thanks Steve.

The only other question: is adjusting cam timing similar to the "fine tune" knob on old short wave radios? It will tune your station (power band) mildly, but not so much as to cross over into the realm of anotehr station (cam). Is it just better to just get the exact cam your motor needs when neutrally adjusted?

I just found this thread today and it has helped dramatically:

Camshaft Selection -- First Draft

Nonetheless, you can't beat getting answer from experinced mouths.

[Steve@Rennsport]

Originally posted by kenikh

The only other question: is adjusting cam timing similar to the "fine tune" knob on old short wave radios? It will tune your station (power band) mildly, but not so much as to cross over into the realm of anotehr station (cam). Is it just better to just get the exact cam your motor needs when neutrally adjusted?
[/B]

Thats very well stated,....

Camshaft selection is VERY important; especially for street use and one should take the time to do all the pertinent research to find the best choice for you.

Remember, in the genre of camshaft choices, "Less is More",....