Cam choice

[Geneulm]

I'm doing a "all from the stash" engine and have a choice of two cams: Rsr sprint and dc60. Here are the engine specs

Hot street engine.
Redline: 7800
Sandcast alum case
Rsr crank
Carillo rods
Elverude/Weiner (RIP) heads w titanium keepers and asse springs. 38mm ports.
2.9 andial p and c. 10.3 compression. Lightweight pins
930 pump. 935 oiling (centerlube and spray bar)
Twin plug

Again the two can choices are dc60 and Rsr sprint.

[Geneulm]

Sorry. Need to add if important. This is high butterfly mfi.

[stownsen914]

You don't mention the use of the car. I'm not a professional builder, but for any street use at all, between those two I'd choose DC60. The factory RSR are very long duration, and I'd expect would need a lot of babying for around town driving.

For race only, perhaps a pro builder or someone with experience with both grinds can chime in. I'd think with a 7800 redline DC60 may still be a better choice since factory RSR grind peaked at 8000 in the 3.0L RSR.

For a point of reference, I have a 2.7L race engine with similar specs overall and running Elgin 310-296, which is a little hotter than DC60. Lower RPM manners would be barely acceptable for any street use. It comes on the cam at 5000.

[Geneulm]

This will be a hot street engine

[safe]

Is your MFI pump matched to any of those cams or are you getting it reprofiled anyway?

[Rivet]

I have a similar build in the works but with modern EFI and ITB and I went with a DC60 for my engine.

[Geneulm]

Two votes for dc60. None for sprints.

[356RS]

I have the RSR Sprint Cams in my 72 street rod. Engine is a 3.1 liter, 66 x 100 on a 77 930 case. Compression is 11:1 twin plug, twin Bosch CDI's, 41mm intake ports with matching High Butterflies. Transmission is a 915 with the 8:31 later R&P and I have CN36's which are the 60 series so not very much low end torque.
Fun car to drive but stop & go traffic is No Fun at all.
I am thinking about changing the cams to something between a DC-60 and DC-80.
Go for the DC-60's Gene.

[Mixed76]

Originally Posted by 356RS

I have the RSR Sprint Cams in my 72 street rod. Engine is a 3.1 liter, 66 x 100 on a 77 930 case. Compression is 11:1 twin plug, twin Bosch CDI's, 41mm intake ports with matching High Butterflies. Transmission is a 915 with the 8:31 later R&P and I have CN36's which are the 60 series so not very much low end torque.
Fun car to drive but stop & go traffic is No Fun at all.
I am thinking about changing the cams to something between a DC-60 and DC-80.
Go for the DC-60's Gene.

Hi Mark,

Did you ever dyno your 3.1? I just read your build thread, I don't think I saw a follow up describing performance.

Best

Sent from my Pixel 7 using Tapatalk

[Geneulm]

Mark

Your comment means a lot. The success that some were reporting with sprint cams in short strokes was what got me thinking about using them... since I had a set in the stash. Sounds like dc60!

[rswannabe]

I have a 2.8 short stroke I ran with RSR Sprint cams for years. I found them surprisingly tractable on the street. The motor idled well and was easy to drive around town. The issue was that for that low speed tractability I had to run 38mm chokes or smaller in the PMO 46’s which prevented the motor from making the peak power it should have. It still spun to 8,000 rpm and was a blast, but in an effort to extract more of the motors potential I swapped to DC62 cams. The goal was to get a stronger idle signal through the carbs at low rpm’s to allow me to run bigger chokes and get more power up top. I had to drop redline from 8,000 to 7,500 but the motor was still a blast and more responsive in the 4-5000 rpm range.

Frankly I think both are great cams and are a lot of fun. That being said, now that I am converting my 2.8 to efi, and I don’t have to worry about compromises to get the carbs to work well on the street, I’m considering going back to the RSR cams to get that glorious rush to 8,000.

[Geneulm]

Don't see things going to 8k+ with a long stroke. Just below, yes. Above no. Given Mark's comments, my transmission is relatively short. 7:34 r&p (954 part). Combined with SC 4th and 5th. Also have 7:37 in stash. Don't know if changes thoughts?

[rswannabe]

Gearing would definitely be an issue. I ran mine with a stock geared ‘69 901 transmission (AFMSZ with 7:31) which is relatively short gearing. I definitely would not pair it with an 8:31 trans unless it had a short gear stack in it as well.

[stownsen914]

If you're running an SC 4th and 5th on a 7:31, I vote for this is fine for street use. A 28:23 fifth on 7:31 will give you about 3600 RPM at 60 mph. A little short if anything.

[Geneulm]

Sc 4th and 5th on a 7:34. Shorter still. It's a hotrod; all about the acceleration.

[Henry Schmidt]

Cam choice is perhaps the single most important choice when it comes to driving characteristics.
First decide what you want your engine to do, then build an engine with components that best offer those characteristics.
If the choice is DC60 or RSR Sprint, your compatible components become pretty highly restricted.
That said, you can use sprint cams with reasonable success if you're looking for a spirited street engine.
Believe it or not, they can perform very well with reduced compression and advancing the cam timing. This will broaden the power band for more enjoyable street driving.
25 + years ago we set out to make a 2.8 liter short stroke engine with slide valve work on the street. We built a twin plug, with 9.8:1 compression which allowed us to do some tuning with timing advance. Remember, we were working with Ca. street gas 91 octane.
The end result was a crazy fun 7800 rpm 300+ house power rocket. It was called the Red Sled.

You might remember this engine from the cover of Wayne Dempsey's first addition Engine rebuild book.

Slip Sliding Away in a Carrera RS Replica
Article and Photos by Randy Wells

“The combination of sprint cams and slide-valve injection is not really a street-friendly combination,” Jerry Murray explains as I prepare to test drive his blood red 2.8 liter shod early 911. Undeterred, I strap myself into the perfectly positioned race bucket and turn the key. The motor awakens from its afternoon nap accompanied by a raucous swell of air-cooled flat six goodness. I blip the responsive throttle knowing that a lot of revs will be needed to get rolling thanks to the Kevlar clutch and aluminum flywheel out back. As we gently ease into traffic the engine sneezes a bit, almost as though it’s out of tune. “It took us a long time to get the car dialed in for the street,” Jerry remarks as we pick up speed. “I sometimes need to lubricate the throttle bodies before getting started. If I don’t, the slides can stick open.“ This is not exactly a comforting thought as we are already accelerating down the backstretch leaving Laguna Seca Raceway. But resistance is futile, and it’s foot to the floor at the first clear opening. That’s when everything starts happening very quickly.

Forget about being melted into your seatback with the long steady pull from a modern engine. This beast snaps your spine so hard it will pop the buttons off your lederhosen. Even with 8500 rpm available, constant vigilance is required to hit shift points that come up uncomfortably fast with the close-ratio gear set. “I probably inadvertently over-rev this car more than any other I’ve owned,” says Austrian born / LA based Murray. “Thankfully, Supertec built me a bullet-proof motor”. It’s at this point that I’m grateful they also added big AP brakes on all four corners. As I throw out the anchors for a rapidly approaching bend it becomes abundantly clear - the only thing that parallels this car’s acceleration is its ability to stop.

“The Can/Am style calipers with 12” rotors and twin master cylinders were used on 917’s, but eventually made their way to 911 racers seeking an edge.” Jerry’s lips are moving, but the screaming racket coming from the open intake stacks and sport muffler is drowning out everything except the one thought that I might die and go to heaven today. After another attempt at the land speed record it’s hard on the binders again. This slows our pace just enough to start the ride all over again at the next corner. A quick turn of the wheel, roll on the throttle, skate along that thin line between traction and no traction, catch the slide with some opposite lock, then be glad the limited slip differential does its job. Cars that were right beside us are dispatched to the rear view mirror in a heartbeat as the tractor-like pull from this impossibly torquey motor propels us continually forward. I tell myself I could get used to this - all the while fully aware that I’m dangerously close to breaking the speed limit and expanding my carbon footprint beyond what is considered politically correct.

We are rapidly approaching our destination, so I make one more sub-sonic pass to ensure the experience is indelibly imprinted on my brain. When we finally slow down enough to catch our breath, I realize the car’s ride has been firm in the corners and reasonably supple on the straights. On smooth surfaces it’s supremely planted and sticks like glue, but irregularities like cracks, seams and pavement scoring can make this 911 a handful. Jerry’s understated delivery continues, “It definitely reacts to everything on the road, for better or worse, so you need to stay on top of this car.” As we unbuckle, he tells me more about what went into his special 911, “I bought the car about ten years ago as a 1972 RS replica built by Mark Kittinger at Black Forest. It had the upholstery, RS flares and bright red paint job you see today; along with a 2.7 RS spec MFI motor. The first thing we did was take the car to Henry Schmidt at Supertec in Fallbrook, Ca. I bit the bullet and asked Henry to build a motor that was out of the ordinary. We went with a short stroke 2.8L with twin plugs and MFI.”

The engine is the soul of any car and this one is unquestionably a work of art: NOS 3.0L RSR case, 66mm crank, Carrillo rods, 95mm 9.8:1 Cosworth pistons, race springs, 964 oil pump, 40/38 mm ports, RSR sprint cams, 935 twin fuel pumps, twin plug ignition with twin 935 8 pin Bosch CD boxes, and those wild sliders which are replicas of the ones used on 910 Porsches campaigned in the early 60s. Jerry explains the reasoning behind this unusual choice of induction, “Slide valve injection is not usually seen on a street car because it’s difficult to modulate the throttle and it’s prone to binding up from road grit. I never would have thought of this myself, but I turned Henry loose to create a special motor and he came up with some really neat stuff.” Certainly, this motor looks as good as it sounds - with handmade aluminum intakes topped by wire mesh screens and twin plug wires & 935 fuel lines delicately wrapping around the fiberglass shrouded engine. To finish it off, a dual outlet sport muffler exits under the rear overriders.

In addition to the motor, Henry at Supertec spent quite a bit of time considering the transmission, suspension and braking aspects of the car. A late 915 gearbox was chosen for its greater strength and was equipped with an aluminum flywheel, limited slip and hill climb gear set with a tall 5th gear for freeway cruising. “That top gear is the only aspect of this setup that is really not suitable for the track”, says Jerry. “The reduction in RPM between 4th and 5th takes me out of the power band. But for the street, I can’t recall when I needed to keep the revs up during that transition.” The suspension was totally redone front to rear featuring 935 coil over struts, Eibach 250 lb springs in front, 380 lb in rear, Sachs shocks all around (shortened in front to receive mono balls), ERP spring plates, and highly adjustable Charlie sway bars. “The shocks were heavily reworked by the race team we got them from, so I have no idea how they are valved. All I know is that they work exceedingly well, but are not suited for those with loose fillings,” Jerry says. In addition, roller bearing spring retainers were implemented, as were turbo tie rods. Brakes are the aforementioned A.P. four piston calipers with 917 style rotors, twin master cylinders and a balance bar. Considering all the racing components on this 911, you would think it spends all its time at the track. But its primarily use is for driving to and from car events. Still, the rock chips on the front bumper confirm that this is no garage queen – even if it has been shown at the Quail Concours in Monterey, California.

Murray has a long history of owning and restoring distinctive cars since the 1960’s. His 246 Dino is a Copa Bella winner, and he recently sold one of his American hot rods to the Petersen Museum. But ultimately his heart belongs to the Porsche marque, which has been a part of the family for decades. Jerry’s brother and son own early 911s, and his Uncle Wilhelm was a German tool and die maker who worked as a jobber for Porsche in Austria. Like many automotive engineers and craftsman during WWII, Wilhelm was consigned to work on some of Porsche’s “other” efforts. Consequently, he was detained in Austria by allied forces when the war ended. Much like Ferdinand Porsche, Wilhelm was cleared of any wrongdoing and was eventually released. But it was some time before he could begin working again, since the allied command were still housed in Porsche’s Stuttgart facility. Jerry (short for Gerhardt) remembers his birthplace and bloodlines well, “Years later when I would travel to Europe to visit relatives, I would scour family barns and work sheds looking for parts. I never found anything Porsche related, but the family is always on the lookout for me.” Jerry gleefully recalls another of his early hot rod roots; “I was always into anything mechanical. When I was growing up, one of my chores was to mow the lawn. I remember my delight in replacing the old mower’s muffler with a piece of strait pipe, thinking it would go faster. It just made more noise and angered my dad, but it was worth it.”

Continued

[Henry Schmidt]

Many years later, when the decision was made to have an early 911-based hotrod built, he wanted to take a different approach. Creating a cool sounding RS clone would not be enough – the ideal would be to use original factory or vintage racing parts when possible and blend those with a few more modern ones to create a unique experience. Jerry reveals his reasoning, “It’s all about driving the cars, and making modifications to enhance your enjoyment. It’s the same now as when these cars were new. Enthusiasts and racers were always experimenting, trying something different. It doesn’t matter to me if all the parts weren’t factory options, as long as the majority of them are period-correct.” The building of this car continues to be an ongoing process rather than an end point. Jerry and Henry would start work on the ‘72 911, then find a new component and adjust their direction accordingly. It wasn't until the motor finally took shape that they decided the old suspension and brakes were not going to handle the horsepower. Additional hood lighting is just another example of a treatment that illustrates the underlying principle behind this car’s distinguished build.
After one too many close-encounters over-driving the headlights five years ago, Jerry came up with a solution: add a pair of Oscar hood lamps. When that wasn’t enough he came up with a better idea - add two more. “When I turn them all on, you can definitely see what’s ahead at just about any speed,” Murray says grinning. “They give off so much heat, you could probably toast marshmallows in the light beams”. To handle the extra current draw a 70-amp alternator was modified and added by Supertec. The Cibie hood lamps Jerry added are now being remanufactured, but at the time he sourced the hard to find parts. Beneath the hood lamps is an RS fiberglass bumper with a Mazda front cooler fed by through-the-body oil lines mated to a ’72 oil tank, Mocal thermostat and custom oil filter. The front trunk houses a larger capacity SC gas tank, RS style space saver spare on a 6x15 Fuchs, Optima battery in the smuggler’s box, ERP triangulated strut brace and ERP mono ball strut mounts. Moving to the back of the car you will find more tasty bits, like an authentic RS ducktail and fiberglass RS rear bumper. Inside the cabin are a houndstooth clad original Recaro race bucket and sport seat, a Heuer Rally Master Pair of stopwatches and a Halda TwinMaster. “The Trip Master is a period correct rally item that I picked up from Rolly Resos, same for the clocks which the late Barry Williams sold to me. It’s a collection of rare parts, most with race history, blended together into one homogenous package of rolling fun,” Jerry remembers. The instrumentation is what you would expect: 180 speedo, 10K tach and clock delete plug. The dash is a radio-delete version, and the steering wheel is a 380mm 1969 Momo Prototipo. “I could have used one of the newer Prototipos, which are really very nice. But this old fellow, while a bit worn, fits with the theme of the car.” Other trick pieces include H1 headlamps and Plexiglas rear & quarter windows. The wheels are 8 and 9x16” Fuchs that were sent to Al Reed for refinishing to achieve the look Jerry wanted – a subtle silver that complements the custom Carrera striping. The rims are wrapped in Bridgestone S03s: 205-55 up front and 225-50 in the back. “I switched from race rubber to these tires because at club events the sticky tires threw so much debris in the air that nobody wanted to follow me.” With a smile befitting a cheshire cat, Murray adds “and where’s the fun of having all this and being at the end of the line?”

Jerry freely admits that with an estimated 300 hp in a 2200 lb. chassis his custom 911 is over-built for the street and the skills of a novice. “I graduated from Skip Barber and a few other schools, and thought myself competent to handle what this car has to offer,” he confesses. “That was until we ran into the late Milt Minter at Buttonwillow Raceway. He was gracious enough to take this car out for a shakedown lap with me in the passenger seat. It was absolutely terrifying, and equally humbling when he later admitted only driving 8/10ths since it was an unfamiliar car.” Awareness of the capabilities of his one-of-a-kind coupe has only made Murray more appreciative of the German marque, “I’ve been very fortunate to have had some wonderful cars, but my favorites have always been Porsches”. Clearly this wickedly fast and beautiful “Red Sled” is a testament to that loyal devotion, and air cooled goodness is in Jerry’s blood. Uncle Wilhelm would undoubtedly be proud.

...

[Billiam 911 2.8]

Thanks for sharing the story of the Red Sled, Henry. Milt Minter (RIP) was so good.

[stownsen914]

Originally Posted by Geneulm

Sc 4th and 5th on a 7:34. Shorter still. It's a hotrod; all about the acceleration.

Go for it! I love a good close ratio box!

In the race car (the one with the motor I mentioned in my earlier post), I have a 901 with GA, KA, O, S, U. Wouldn’t be very suitable for the street, but man is it glorious to drive.
GA 18:32
KA 22:31
O 23:28
S 25:26
U 26:25
7:31 R&P

[stownsen914]

Quick note on the 2.9 93 mm P&C. I have those in my motor. When inbuilt it, I was advised that the cylinders are too thin for the standard CE style head gasket, and to have the heads and cylinders cut for ni-resist interlocking rings.