2.7RS alternative cams

[pieterk]

After a great day at Streets of Willow today, and after a very informative conversation with our competition committee director, I've realized that as I prepare the track car for a higher class, there's room to make two changes to my motor, if it's worth the expense. I can't afford any points to change my compression, port or valve sizes, displacement, induction, or bump up to twin plug, but I can change my cams, and run open exhaust, the latter of which is an easy change.

So if I'm currently running Mod-S cams, and everything else remains exactly the same except for open exhaust, what are my options for a more aggressive cam that hopefully won't render everything below 5k useless, and how big a bump up top can I realistically expect? My power band is roughly 5k to redline, which is 7,300, and it is an MFI RS. The car does get driven to/from the track, and/or to TRE, but that's it. Otherwise, it sees no street duty whatsoever.

The ideal scenario would be a cam that allows as much useable power below 5k as the Mod-S cam, but also allows for more power up top. However, I might be willing to make a modest sacrifice below the peak if the increase up top is significant enough.

[kenikh]

I am using DC44 cams on 102 lobe centers. They make more power everywhere than Mod-S cams. The only thing you might need are hi-rev springs. You might not.

[pieterk]

Thanks Kenik. Any idea how much more power?

Too, if I keep my redline at the same 7,300 RPM, I'm not sure why it would be necessary to have high-reving springs. Longer duration putting more stress on the springs maybe?

[kenikh]

John Dougherty can answer these questions best. He goes by camgrinder on this board, if you've never spoken to him. My cams were designed for a higher rev range than you are looking for, even though they they do meet your power criteria. He may be able to cook you up a custom grind that would be even better. His prices are very reasonable, too.

My cams require high rev springs because they lift .490" and have very, very fast lope ramps. At 7300 RPM, though, it wouldn't be necessary. My motor redlines at 7800 RPM.

[Jeff Alton]

I use the 44's (with a wider lobe spacing) on my 3.4. They are great for that motor too!

Cheers

[jluetjen]

Originally Posted by pieterk

Thanks Kenik. Any idea how much more power?

Too, if I keep my redline at the same 7,300 RPM, I'm not sure why it would be necessary to have high-reving springs. Longer duration putting more stress on the springs maybe?

Two reasons:
1) The DC44 may have more lift, which will require a spring with more travel before the coils stack up and bind. This goes hand-in-hand with different seats which aren't as thick, to provide the extra travel at full lift. When you buy Camgrinder's competition valve springs, you get both items.

2) While your engine isn't running any faster, your valves will be accelerating significantly faster in order to achieve the potentially higher lift and longer functional duration while still maintaining a reasonable seat-to-seat duration (without making excessive overlap). Think of it this way, if you want more lift with the same seat to seat duration, then the valve will need to travel further in the same amount of time, which means higher acceleration. Secondly, if you can snap the valve open to full lift faster, it will provide more flow within the same amount of crank rotation -- which can mean more HP. Once again this means higher valve acceleration. Since energy increases with the square of acceleration, you're talking about significantly higher forces involved.

You see these forces in two areas:
- Peak lift. If the valve springs aren't strong enough, the valve and/or follower will loft off the cam causing valve float. NASCAR engine builders do this in a controlled fashion to increase lift in engines where cam-lift is limited. In a 911, it means that the valve will stay open for longer then it is supposed to (robbing HP) and will come crashing down on the seat rather then following the cam down in a controlled fashion. This second phenomenon can destroy the valve and the seat.
- Higher ramp pressures as the valve is accelerated off of the seats as they are opened. If these forces get too high, it will cause premature cam and lifter or follower wear, if not broken rockers.

When Porsche designed their camshafts in the '60's, they designed them within the context of the valve spring technology of that time. The result was that the forces generated by a CIS cam, an S cam and a 906 cam are all roughly within the same range. The good news is that this means that with the smaller valves of a 2.0, it's even possible to use 906 cams with stock springs because the valve accelerations are not that much different then those generated by S cams.



In addition to the two forces that I graphed* (velocity and it's derivative: acceleration, there is also the derivative of acceleration which is known as "Jerk", which is how fast does the acceleration forces change).

The peak intake acceleration (actually deceleration) for an early S cam is about -0.000230 inch/degree squared, or -441241 inches/seconds squared at 7300 RPM. A 906 cam at the same engine speed generates acceleration of -500713 inches/second squared, or only about 13% greater.

As time has gone one, spring metallurgy has progressed to the point where it's possible to design cams with higher acceleration profiles then Porsche used in the '60s. The result is cams with have more lift and duration, without increasing (or in some cases decreasing) the overlap, which means an engine with a wider power band. I don't have specifics for these, but Camgrinder can tell you what the acceleration forces would be for his cams.

* Note that I plotted these based on published cam data. Camgrinder has a "Cam-Doctor" machine which can precisely measure this data with more accuracy then these charts show. Tiny changes in the cam design may create significant changes to the jerk and acceleration graphs.

[pieterk]

Thanks everyone, and John, tremendous information! Great primer!

I was checking out DC's website, and I never realized that the Mod-S grind is the same as a DC40. Interesting!

So that gives me a useful point of reference. Now I understand the enthusiasm for the DC40, as, if my Mod-S is anything to go by, it's a superb cam.

If the 44 improves on that and extends power all across the band, then that's even better. I'll be in contact with Dougherty about this later, and I think the proper approach to this would be to have the heads rebuilt with his spring package, as much for insurance' sake as for maximized performance. Maybe I can even eek my redline up a little, although I don't want to over-tax the bottom end.

[kenikh]

Well described, John.

[304065]

John:

Here are my data from the card I received with my cam. Supercup on 102. Notice I don't have values over the nose.

I don't have data on stock cams, where did you get it?

[jluetjen]

To be honest John, I don't remember where I got the original. I think that John Daugherty may have forwarded it to me, or else I extrapolated it off of one of the charts in Bruce Anderson's book.

[911pcars]

John and John,
Nice information. Thanks.

Pieter,
Your claim of only trips to track and garage is suspect. Notice the coffee cup.



Hope to see you and the track car next week.

Sherwood

[pieterk]

Originally Posted by 911pcars

John and John,
Nice information. Thanks.

Pieter,
Your claim of only trips to track and garage is suspect. Notice the coffee cup.

Ummmm....errr.....yeah....that.

I was....uh.....warming up the oil. Yeah! That's it!

...

I'll be there. In the R-ish car. Now that I have the SWB, the track car really will get relegated to track and service related drives only.

I think anyway. Honestly, it's too overdeveloped to really have fun with on the street anyway, and it certainly isn't pretty, so there isn't that virtue going for it either.

[pieterk]

Revisiting the cam question, as new factors come to light on a daily basis:

If I were to ask the same question regarding a 2.2E motor except adjust the cam variable for spirited open road rather than track driving, would the answer be more or less the same, i.e., DC44? Or would the DC40/Mod-S be more appropriate? If either of these, would porting, valve job/etc. be necessary? And finally, would the gains in power and reliability be worth the expense and time, and/or, is this kind of a waste, bolting this top end on to an otherwise stock 2.2E bottom end?

[jluetjen]

Originally Posted by pieterk

Revisiting the cam question, as new factors come to light on a daily basis:

If I were to ask the same question regarding a 2.2E motor except adjust the cam variable for spirited open road rather than track driving, would the answer be more or less the same, i.e., DC44? Or would the DC40/Mod-S be more appropriate? If either of these, would porting, valve job/etc. be necessary? And finally, would the gains in power and reliability be worth the expense and time, and/or, is this kind of a waste, bolting this top end on to an otherwise stock 2.2E bottom end?

With a 2.2, porting out to S specs (36 mm) will free the motor up on the top end if your running cams similar to S spec. Doing a valve job "while you're in there" is most likely not a bad idea if it hasn't been done for a while. If you're running E cams, I doubt that porting will get you much. Note that if you put an S cam into an E engine, you quite likely will need to replace the pistons to allow ample valve clearance.

As far as power, keep in mind that a 2.2S was putting out about 180 PS at 6500 RPM, and 147 lb-ft at 5200 RPM, and a 2.7RS with the same cams and heads made 210 HP, so you'll most likely be in that range. A more modern cam may bump the HP a little, but more importantly provide more torque across the rev range. I doubt that these changes will gain you any reliability, and may shorten the life of the engine a little depending on how long you keep the engine in the upper half of the rev range.

[kenikh]

LOL, I was just going to say we need to ask John what the CFM those heads flow in comparison to what the cams allow at a given RPM.

[jluetjen]

[camgrinder]

Technically the DC44 cam ( on 102 lobe centers) is slightly more agressive than the DC40 (mod-S). I would expect a little more top end rpm with the added exhaust duration.
For a similar powerband to the DC40, I would choose the DC43 cams on 102 centers. Similar duration, and more area under the valve lift curve due to the added valve lift.
The duration and lobe centers have a greater effect on rpm band than valve lift. Adding lift will give you better cylinder filling accross the entire RPM band.

For the 911 E, you should look at my DC30 cam. The "mod-solex" profile. It fits the E cams as a regrind, doesnt need crazy spring pressure and the rpm band falls in between the factory solex and S cams.

[pieterk]

John (L), thanks for reminding me about the possible problem with valve/piston clearance if I were to consider more agressive cams; I forgot about that. Nix that idea I suppose.

My idea about increasing reliability wasn't so much about extending the life of the motor as it was buying a little top end insurance, since I'd most likely go with John Dougherty's spring set to complement the rebuild.

John D., thanks for the additional information on your 44. It sounds enticing. So it would be a safe bet to install those with your upgraded springs and retainers without having to worry about piston clearance in a 2.7RS? Would porting the heads and/or working the valve sizes buy me any additional advantage (yeah, I know I said that's class prohibited, but I'm curious)? And to what RPM would I have safe, useable power? I'm currently at 7,300. Another three or four hundred RPM with power wouldn't go amiss!

For the E motor then, the DC30 would be a straight drop-in, without having to worry about piston contact? Any reason not to do a port/polish/valve job while the heads are off, and if so, would that suggest an even more agressive cam that still remains somewhat streetable, or should I really leave it at that so as not to loose too much low end torque?

[camgrinder]

Originally Posted by pieterk

John D., thanks for the additional information on your 44. It sounds enticing. So it would be a safe bet to install those with your upgraded springs and retainers without having to worry about piston clearance in a 2.7RS? Would porting the heads and/or working the valve sizes buy me any additional advantage (yeah, I know I said that's class prohibited, but I'm curious)? And to what RPM would I have safe, useable power? I'm currently at 7,300. Another three or four hundred RPM with power wouldn't go amiss!

The racing springs will control the DC44 cams to 7500 rpm easily. Above 7500, will require a DC60 or larger camshaft. I would do a proper valve grind and do some bowl blending.

Originally Posted by pieterk

For the E motor then, the DC30 would be a straight drop-in, without having to worry about piston contact? Any reason not to do a port/polish/valve job while the heads are off, and if so, would that suggest an even more agressive cam that still remains somewhat streetable, or should I really leave it at that so as not to loose too much low end torque?

The DC30 will clear the factory E pistons. Going to a bigger cam will hurt low speed power, and require a bump in compression (9.5-1+).
With the E heads, I would imagine making the intake ports bigger would help, as long as it was done right.

[kenikh]

Originally Posted by camgrinder

The racing springs will control the DC44 cams to 7500 rpm easily. Above 7500, will require a DC60 or larger camshaft. I would do a proper valve grind and do some bowl blending.

There's always Ti retainers for higher RPM. That's what I am doing with my DC44s. If memory of our correspondence servers me correctly, power falloff on the DC44 on 102 lobes happens at 7800 RPM.