906 cams Q - compatibility with 2.0s 67/68 carbs.

[gilbert911]

factory 906 cams in - factory 2.0s cams out.
Will it work retaining std 2.0s heads, P&B's?, rods?
Can the cams work in the std spray bar towers?

Thanks for all advise

[edmayo]

I've not done it, but I don't see why the spray bar won't work. What you will need to check however is the piston valve clearance and the diameter of the valve compared to the valve pocket. I'd suggest doing a trial assembly with clay checking of the valve piston interface.

[BURN-BROS]

Hi Gilbert,

The heads would need to be ported to effectively use those cams.

With ARP rod bolts you could use the stock rods provided you keep the rpms under 7800 rpm. They can see 8200 rpms, but I would be concerned about failure at that point.

If it is a centerlube set of cams then you will need to plug the nose of the cam to prevent pressure loss.

Spray bar cam towers are recommended and centerlube/spraybar would be ideal.

The stock pistons will not work and the compression is quite low for such a big cam. A set of 69S pistons may give you the clearance but clearance would definitely be checked with clay. 906 pistons may very well be required.

The carburetors are too small, but can be made to work ok with larger venturies.


A better cam for what you have would be a Mod S with mild portwork. The Mod S can make 40 more HP @ 4500 rpm than 906 cams, which really do not start working untll 5000-5500 rpm.

[gilbert911]

Aaron, Ed
Thanks for taking the time to reply - appreciated.
My question was what would happen if I swapped out the cams. My GOAL is actually to try and get some 46IDA's on tall manifolds to work on a 2.0s 67/68 engine.
I figured I would need the 906 cam to cope. But the knock on changes seem significant.
So my goal - what would be the implications of running the 46's on an otherwise stock 2.0s engine?

Thank-you

[andyjboy]

I think that they would be overkill - in addition 906/911R tall manifolds have 38mm lower ID to match the inlet ports on those engines so wouldn't work with a stock 2.0S engine.

I have been speaking with Mark Allen @ Vintage Excellence about the repro tall manifolds that he has advertised on this board but they are some way from 'production' & SG Racing in Belgium (who had been offering similar manifolds for a few years) seem to have disappeared (certainly their website is now inaccessible).

[BURN-BROS]

Hi Gilbert,

You would need to explain what your intended usage of this cam/carb decision.

2 liter motors do not have a very broad power bandand It would definitely be a soft motor if compression and twinplug were not done.

There are more modern cam choices today that provide very good top end without sacrificing low and midrange performance.
A high compression twinplug ModS engine can get 90-100 hp/liter with a broader midrange with proper tune.

[Mr9146]

Hi Gilbert,

You would need to explain what your intended usage of this cam/carb decision.

2 liter motors do not have a very broad power bandand It would definitely be a soft motor if compression and twinplug were not done.

There are more modern cam choices today that provide very good top end without sacrificing low and midrange performance.
A high compression twinplug ModS engine can get 90-100 hp/liter with a broader midrange with proper tune.

I am running 911R cams (because I had them) in my 2.6L, high compression, twin-plug motor and I'm still wary of the bottom end power/around town drivability issues associated with the 911R cam. And this is despite the fact that I have the extra ".6L" of displacement to compensate.

Listen to Aaron, Gilbert, he's spot on.

HOWEVER, if you do choose to use the 906 cams, you might consider doing the gearbox as well in order to help keep the car in the meat of the power band.

[BURN-BROS]

Hi Raj, This can be a very long subject so I will condense it and focus on the moderate builds.


The 2.0 Liter, IMHO requires RPM in order to be competitive.The improvements we benefit from are
good aftermarket rods, higher rate valve springs, and Ti retainers.

I focus on cams and compression. The 2 liter engine has a problem with compression ratios over 10.5/1 so they will be soft on the bottom and mid range when running the bigger cams.The availabilty of quality aftermarket springs and Ti retainers have allowed for more agressive cams. Modern cams have more aggressive ramp rates, less overlap, more lift than the race cams of the period.(906 cams were designed to use the stock production springs) The updated profiles keep dynamic compression up, allow more airflow, giving you both midrange performance without sacrificing the top end. The DC60 cam could be considered am updated 906 cam giving good power to about 8000-8200rpm in a 2.0 liter. When given the opportunity I ditch the old cam grinds and update.


Once you step up to the 2.2 liter motors the compression ratio problem goes away. 12/1 compression ratios really wake up these motors.

[Mr9146]

Aaron and marco


would you guys be willing to share with me what we have learned and improved upon since the 68 circular for a small displacement motor(2.0litre).What advances have we made-materials,technology,machining,etc and how do these advances effect the recommendations made in the circular?-just want to pick your brain.sorry for the diversion.

raj

You must have misunderstood my posting, Raj. I was merely agreeing with Aaron in his assertion that there are better options (obviously the idea of "better" is personal and wholly subjective) available in terms of what cam choices are available for primarily street-driven cars. The 1968 factory circular appears to be all about getting more power from the 2.0L engine. However, "more power" is not so important as "usable power". Aaron's and my recommendations and postings were not saying that the factory is wrong but, rather, that it is single-minded in its approach to upgrading the 2.0L.

Additionally, when the factory circular was released some 40+ years ago the assortment of cam profiles available was rather slim compared to what is currently available. The passage of time and people's experience racing and modifying Porsche engines have opened the door to numerous cam profile options that, for many reasons, are more desirable than the 906, S, Solex, etc. choices that were once all that was available.

Now, in regard to carburetion, it has been my experience that 46mm Webbers are, for the most part, overkill on a primarily street-driven car. Obviously there are exceptions to this rule, but the majority of the time a set of properly tuned 40mm Webbers will be more tractable and make more usable power on a street car. Sure, 46s can be made to work, but there are concessions that the user would have to make.

Also, in terms of what advances have been made in materials, technology, machining, etc. I can't offer much as I'm merely a layman. But I do know that computer aided machining processes and advances in metalurgy have led to more horsepower through the development of better, more efficient and (in some cases) less-expensive parts. Add to that the knowledge acquired over 40+ years of racing and performance modification and you have more efficient port designs, more efficient headers/exhaust and on and on and on.

And, conversely, I offer that we have actually taken a few steps backwards in terms of what fuel is readily available at the pump. The increasing amounts of ethanol blended fuel is wreaking havoc on the performance car community. Modern cars with their knock sensors, 13:1 static compression ratios and variable camshaft timing have it easy; but trying to make horsepower with a carburetor and a mechanical advance distributor is getting more and more difficult because the fuel is getting less and less like gasoline and more and more like alcohol.

So I once again state that, perhaps, you misunderstood my posting. I was agreeing with Aaron's assertion that, depending on how the car will be used, there are better camshaft profile choices currently available for a 2.0L engine than there were 40+ years ago. I also offered my own circumstance in which I have some slight concern that my 2.6L engine with 906 cams will be a bit too radical for the street; however, I'm in a better position than our friend Gilbert in that I have 600 more CCs to work with "off cam". So while I might not make any real, significant horsepower below 5000RPM, I have the added displacement to help mask the engine's deficiencies in the lower part of the rev range.

Did I answer your questions?