Which Cams?

[tharbert]

I'm in the middle of rebuilding my 72 2.4T. I have the case worked over by Ollies and a new set of 86mm 9.5:1 P/C's, bumping my displacement up to 2453, with the added benefit of raising the c/r from it's paltry 8:1. I have two major concerns left to address. I'm hopefully queued to have Mark go over my MFI pump this fall. I assume I need to give him the particulars about the cam and c/r so he can dial in the pump.

The car will be a weekend driver with the occasional autocross/D.E. It has a 915 4 sp. trans.

I was planning to go with an "E" profile but started wondering, since the MFI pump can be made to match the changes, are there more modern/better options available for the intended use? What cam grind is best for me?

[Flat6pac]

I have always likes the Solex grind because its more than E but not as peeky as S
Bruce

[larrym]

you can get away with solex at your CR - but in my experience they are not very much "less peaky" than s cams

running mostly on track - go solex
runnning mostly on street or AX - go E - you want torque to come on at lower rpm

.

[tom1394racing]

I am building the same engine for a '70E I am restoring. I will be using the DC-40 Mod-S cams from John Dougherty. Nearly the same lift and duration as the S cams but with the E lobe separation. Should make for good driveability on the street without too much sacrifice in top end power.

[Henry Schmidt]

I build the 2450 all the time. Maybe 8-10 in the last 3 years. It's the engine I've built for my last two personal projects. Our experience is that with a 36mm ports, 9.5:1 compression and the "mod S " cam, this engine will make 180 rear wheel horse power @ 6400 rpm. The best small engine combination for dual use I've seen in 30+ years.
Great pull out of the corners and real horse power.

[cmcfaul]

I have used stock E cam and the DC 30 cam on a 9.5 CR 2.4 with MFI. The DC 30 is basically an E cam with more lift. This results in higher VE and better air flow. Net result is the DC 30 pull strong to 7300 RPM redline. The E cam pulled a little harder at low RPMs but ran out of pull before redline.

The Good - DC 30 pulls all the way to redline hard and good all around performance
The bad - Pay close attention to fuel ratio. I burned two pistons running too lean and too hot. It is very difficult getting the MFI mapped correctly to the new cam profile. Been chasing a partial throttle lean spot for a while. Lean is really fast but has a huge cost.

I installed an LM2 AFR meter which I love and is the most useful instrument in the car (well second to the tach). New cam, higher CR and MFI you will most definitly need one.

Chris
73 911 E

[tharbert]

I took these numbers and notes from drcamshaft's web page:



The notes indicate the DC40 possibly needs better valve springs. My heads are already rebuilt so I won't go that route. So, I'll start looking at the DC 30. I'm starting with T cams. I don't know if that grind is possible...

Thanks for the feedback.

Tom

[Henry Schmidt]

Real/ factory 2.8 and 3.0 RSR race engines used stock 911 valve springs.

They were 8,000 rpm race engines.

The advent of a plethora of aftermarket valve springs has everyone believing that bigger/better valve springs are necessary.
On sub 7200 rpm street engines, Not so much................

[Eagledriver]

Originally Posted by Henry Schmidt

Real/ factory 2.8 and 3.0 RSR race engines used stock 911 valve springs.

They were 8,000 rpm race engines.

The advent of a plethora of aftermarket valve springs has everyone believing that bigger/better valve springs are necessary.
On sub 7200 rpm street engines, Not so much................

Those engines used cams that were compatable with with stock valve springs. The DC-40 has more aggressive cam ramps. The RPM is not the issue, the slope of the cam rams is. Cams like the DC-40 (mod-S), only came about when people started using higher pressure valve springs. I would use Johns Street plus as a minimum with these cams.

-Andy

[Henry Schmidt]

High pressure springs create heat, friction and excess wear. Our goal is to always use a spring that provides appropriate pressure based engine use as well as cam specification.
The lowest spring pressure for the intended use is paramount. More is not always better and indeed that "more is better" attitude creates more problems than it solves.

[Eagledriver]

I'm with you there Henry. I don't like racing valve springs for just that reason.

-Andy

[burtm5]

Wouldn't a built 2.5 work as good or better than the 2.4? 66mm x 90mm, 36 mm intakes, webers, 9.5 cr E or Solex cam? best

[tharbert]

Originally Posted by burtm5

Wouldn't a built 2.5 work as good or better than the 2.4? 66mm x 90mm, 36 mm intakes, webers, 9.5 cr E or Solex cam? best

86mm is the largest P/C set my case will allow without machining the spigots. My lowly 3r case, I was assured, isn't a good candidate for that option. In any event, 70.4 x 86 is still a 2.5. 90mm would give me a 2.7.

I've got an email into Dougherty Racing (John) to see what's possible, whether my cams can be worked into the DC30 or even the DC40 profile. I included a link to this thread in the email. Hope he answers soon!

[PropellerHead]

I hate to hijack this thread.....but there is good info and overlap to what I'm working on.

Would the same basic recommendations change if one were building a 2.7L version?
Specifically, JE 10.5:1 pistons on Nikasil cylinders.
Twin plug ignition.
Head work to include 36mm ports (or larger, not too late to change this).
SSI's with a sport muffler, no headers.
Building an MFI capable engine but will begin with 40mm Weber carbs to start with as I have these ready to accept the appropriate jets, etc. PMO intake manifolds to match with head ports.

Application is a 'hotrod' dual purpose car for track use and some spirited street use. The car is a '71 911T Coupe in largely stock appearing trim but the original engine is coming out to be stored and the hotrod engine to take its place. Car is modified 'beneath the skin' with suspension and driveline modifications. I'll be unhappy if the engine doesn't display some 'hotrod like behavior'. This isn't a truck engine after all, low and mid-range torque is great but I'm happy with a 'peaky' engine just the same for this application. Doesn't need to spin to 8,000rpm however. I'll likely keep the rev limit in the 7,200rpm range.

Is the DC40 Mod-S still the cam of choice? The above thread fits well with my preliminary targets. Original thought was an early S cam but the DC40 seems like a nice alternate, especially with the added compression and displacement of my 2.7L.

Thoughts?

[burtm5]

Prophead, I have close to the same set up. 71 T. with 2.7, 9.5, 40 weber 35 mm intake, ssi. Stiffer suspension. I used my left over L/E cams. It has good power everywhere no soft spots, but doesn't have the top end rush you'd get with solex or S. I use it as a DE car as I sometimes instruct with it or an m3. and I drive it on the street. Unless you are track exclusively I would not go to a S cam. If you want to drive across Illinois to Putnam Park in ten days I can give you a track ride with the BMW club DE.

[PropellerHead]

Thanks for the feedback. My application isn't track exclusively because it will also be street driven but performance is the focus and I want it to feel strong at the top end. I have a really hard time spending the money to do the engine build right without feeling that 'rush' at the top end when it is complete. That's the biggest complaint I have about the 2.2L 'T' engine, it runs out of breath and feels anemic at the top end as a result. My hunch is the 2.7L will make up for the top end peaky behavior by displacement and compression over the 2.2L and the early gearing and light weight chassis is such that you can keep the car in the upper rev ranges pretty easily. If it idles lumpy from an aggressive cam, I'm good with that too.

I too instruct in either this car or my BMW a 330i ZHP in this case (I sold a track prepared M3 to buy this 911). Putnam is quite a drive from here on short notice or I'd take you up on it!

[tharbert]

I just got off the phone with John. My T cams can only be reground to E spec. A set of E or solex cams can be reground to DC30. So, I need to source a good/inexpensive set of E's over the next month or two in order to do DC30. WTB placed in the used parts forum... Thanks for the input!

[YTNUKLR]

I have some E's that are perfect for regrinding -