906/Carrera 6/911R motors - characteristics

There are a few of you on this BBS that own the above mentioned cars (any variant of the 2 liter, twin plug, 906 cammed motor). What I am interested in getting is a nice, first hand description of what the motors are like in these cars. I've been going back and forth with Tadd trying to abstract what these motors are like, but all we have is conjecture.

From idle up to redline, I am curious. How does it behave in traffic versus on the track; do you have to slip the clutch a lot? Would you even attempt to drive it in traffic? Does it even idle without serious bucking and popping?

I have wanted to build a 906 clone motor, but would really like to hear how drivable these motors are on the street. From there, I believe I could comfortably make a decision on configuration.

If you guys have one of these cars, please indulge me with a description of how the car behaves. I might even be able to muster up a cigar in thanks.

I can provide you dyno results:D

Kenik:

Having driven quite a few 2.0 race motors with 906 cams and all that, there is no way I'd ever call such combinations anywhere near streetable. :) :)

Now, I realize that many folks have differing opinions about what constitutes "streetable", but this setup comes in last.

Now, take all those components and install them into a 3.4 to 3.6 and it becomes a docile, easy to drive, street motor that still pulls hard to 7800 and can be raced. Even a good 2.8 with a 906-type cam is FAR easier to drive,....:)

IMHO, small-displacement, powerful engines with race cams are simply not fun in any kind of traffic.

Hi kenikh,

I just joined the registry and have posted a picture of my 68 TR on the show my ride thread. I bought the car 4 years ago when its original 901/02 engine was installed. This had the factory rally kits 1 & 2 but still had S cams, 40 IDAs, S port / valve dimensions and single ignition. As chance would have it I then came across a genuine 901/20 engine that had been taken out of a 906 prototype. The temptation was too great and I embarked on installing it in the TR. What seemed like it should be a fairly simple exercise ended up taking a year to complete. For example one thing I had forgotten was that 906 engines were installed the other way round so the throttle linkages, inlet manifolds, timing chest covers and tinwork all need replacing. Anyway we ended up rebuilding the engine from scratch and the anticipation of driving it for the first time was immense.

Am glad (and relieved) to say that the result was spectacular. After running the egine in carefully for 500 miles I took the car down to Classic LeMans in 2004 for its first outing. Although I was still limiting myself to 6,000 revs the gain in power and torque over the 901/02 engine was very noticeable all the way through the rev range. The engine was also remarkably tractable - it would pull cleanly on full throttle from 4,000 and if you were careful with the gas was quite driveable between 3,000 and 4,000 in traffic. Needless to say it was happiest with the throttle wide open and the exhausts blaring! I drove it round the full LeMans circuit during the parade laps before the racing started and the noise of the megaphones bouncing of the main grandstands on the pit straight had to be heard to be believed! I have a bit of in car video which I will try and see if I can post somehow (any tips anyone on how I do this?).

The engine now has about 2,500 miles under its belt and has freed up nicely. I've had it dyno'd at c.210hp - we didn't ring it's neck to find out exactly how much it would produce at the top end but the power curve was still near vertical at 7,750 and it should run to 8,200 or more. Because of the lighter flywheel and internals the engine loves to rev and consequently it is easier to drive as it was intended - flat out. Having said that it is quite happy in traffic and only complains if you try and run on part throttle below 3,000. The only situation it becomes difficult in is when the car in front is doing 25 mph - too slow for 2nd gear and too fast to get the 901 box back into 1st without a very nasty noise!

I had some trouble initially with setting the IDA46s up. It is running with small jets in the interest of driveability rather than ultimate power. It is also worth mentioning that while the engine has 906 cams and 906 valve sizes the ports are 911S dimension. This just happened to be how the engine was configured when I bought it but this combination seems to be responsible for producing better tractability than the 901/20 is known for. I have a friend who has driven my car and a 911R and he noticed the difference.

I could ramble on for hours but in short I have no regrets. The 901/02 engine was great but the 901/20 is in another league altogether.

Best regards,

Philip

OliveR what are those collectors going to the megaphones???? Heater boxes or some trick exhaust?

Kenik, My current application is a development of the 906 for racing only. Its awesome above 5500 and basically makes no meaningful power below it. It idles nicely at 1000-1200 but (due to the lightweight flywheel) you must revúp and slip the clutch to get it going. Judging from that I'd say that perhaps you can tame the beast by using a heavy flywheel to providde torque buyt you'll loose the fast spiinng nature of the beast (at least a bit).

I guess that Philip's car perhaps has a stock flywheel to compensate and thus provide the driveability he mentions.

My prior engine was a "stock" 906 spec and was more tractable than my current engine but it was still peaky and not for everyday driving. Again I had a light flywheel...

Perhaps John can chime in on the effects of the flywheel and how much it could compensate for what he and I have experienced on race engines.

Remember, with these low displacements, high revs are needed if you want to make power.

Cheers, Juan

Fantastic car Philip !!!

I would agree with Steve as far as a C-6 motor with an ultimate race tune...

However, after driving Stephen Childs T/R for 4 days in '04 I would completely agree with Philip....

This car was initially well set up for long distance rallying by the Swedes ... then FIA raced through the early '70s... then brought back as a vintage rally/touring car by Mark Waring...

It's set up is for speed, tractability and long distance comfort... and it doesn't disappoint

I would have to talk to Mark, but I would imagine that cam and ignition timing have been pulled back a bit, and the 46's choked down slightly..... But the result is still brilliant... Sure, you have to keep the revs up (asspeshelly driving with a passenger) ... but once you 'get it', the rewards are wonderful...

The car will idle, pull smoothly at low RPM, and SCREAM when you want it to.......

And Oh that scream.... There's nothing like in the Porsche world...
In my estimation the quintessential 911 symphony with about 4 different octaves all the way to 8000...... :eek:

Cheers Philip and welcome... and please more pictures... :)



The Childs Gulf T/R then and now...

I didn't expect to get so much feedback, both on the thread and in PM! :D OliveR, your dyno runs would be interesting to compare to these:

http://www.early911sregistry.org/for...highlight=dyno

The reason I asked for impressions is that even though I have seen the dyno figures, the vast hole below 4000 RPM left me wondering what the car behaved like in lower RPM range. Additionally, nothing beats a narrative, seat of the pants assessment, IMO. I think you guys have answered that.

BTW, the comments regarding the 901/20 motor with 'S' ports is more than I could have hoped for. Having these data points is invaluable. Now I am curious if anyone has MFI vs. carbs opinions and thoughts in terms of the driving experience.

Keep them coming. :cool:

My '75 Carrera street/track car came with a 2.8 twin plug. 906 cams, 46mm Webers, twin plug & ported heads, 2.8 RSR pistons with the tops cut down to lower compression, 90mm Mahle barrels bored to 92mm, Electromotive HPV-1. 221 rwhp. (254 @f/w) & 185 max torque.
On the street, the motor wasn't very happy, and didn't come alive until 4500 rpm and pulled to 7500 rpm. Starting it was a ritual. The problem is most of your street driving is in the lower rev range. After finding a few pulled head studs I went with a 3.4 twin plug. Even though the 3.4 is set up for racing, it's much easier to drive on the street.
I've sold off most of the parts, but still have a '73S case bored out to 92mm and the twin plug heads if anyone is interested.
Harold #334

Here are some pictures of my 901/20 engine after it had been fully stripped. I particularly like the titanium conrods - they were almost too nice to put back in the engine!

Depending on you intentions there is no substitute for cubic inches at least for a street motor. I have a 2.8 in a track 911, even this motor doesn't wake up until 5000-5500 rpm, it likes 6500-8000 even more, is is a little bit of work keeping the revs in the powerband. I have a 3.5 in a street car, it could care less where the revs are, pulls from 2k-8k like a train. I know the vintage 2L guys are getting very big HP from 2L motors, I have heard as much as 270 +, may be all BS. But these are high strung and expensive grenades.
Phil