DC-40 Mod S cam, need new springs?
 

I am having my 2.7 rebuilt and was going to use an S cam, but found a local DC40 Mod S cam locally.

'74 7R case
9.5:1 JE Pistons
Recoated Nikasil Cyls
Redone valves
Weber 40 carbs
1-5/8"Heat Exchangers
2-in-2-out exhast

My question, if this will be a 90% street car and won't go over 7000 RPM, do I need new valve springs or can I use my stock ones? Not sure how much springs will add to the rather large $ already.

Thanks,
Nathan

Throw a set of Eibach springs on (the beefier of the two). You'll need them due to left specs of the cam you're going to run. You need about 85lbs at the seat and 250-260lbs over the nose with that cam.

Stock springs are good enough to control the valves up to 8,000 rpm. But don't give much margin at those speeds, and may well break sooner than a performance spring would.

At 7,000 occasionally I'd not worry. On the street, really - how often do you just wind it out?

For the track, what you need to do is do a chassis dyno run once it is together to see what your torque curve is. Then use the torque curve and your gears to calculate the optimum upshift points in 2, 3, and 4 (on the track you only need to worry about the 1st to 2d upshift if you do racing standing starts). The optimum point is where the torque delivered to the rear wheels (actual torque at various RPMs multiplied by the gearing) in the "old" gear is the same as it is in the "new" gear you shifted up to. The only time you might want to exceed that RPM is if you can stretch a gear to avoid an upshift just before a corner where you have to downshift.

And that is only for racing, really. So stock springs should be just fine, as I suspect you will find the optimum upshift point is below 7,000 anyway.

Randall Aase in CA makes some superb race springs - I've just fitted them on my 2 litre race engine, along with his mtching spring seats and Titanium spring caps. Engine now runs all way to 8000 without any sign of spring surge or valve bounce. Much better than the standard roadcar valve springs...

Have some pricing? Weighing options now myself.

I believe they are referring to the Aasco springs. Just noticed that our host sells Aasco valve springs - just search "Aasco".

[QUOTE=lvporschepilot;9468557]Throw a set of Eibach springs on (the beefier of the two).
[QUOTE]


I didn't realize there were two versions. Do you happen to know part numbers? I know of Eibach part # 20444.212. Not sure which version that is. Thank you ...

Edit - found it:

20701.212 (Set of 12) Dual Spring 31.6 mm 17.6 mm 55 lbs @ 36.5 mm 183 lbs @ 25.5 mm 11 mm 20.5 mm

20444.212 (Set of 12) High Lift Spring 31.75 mm 17.3 mm 90 lbs @ 36 mm 277 lbs @ 23.3 mm 12.7 mm 20.7 mm

use 20444.212. They were developed by Eibach in conjunction with Andial

The argument is a street car doesn't need them because you don't see track RPM's and it will wear valvetrain items faster.
To me, coming from an aircooled VW performance background, I don't really see this, as we use much heavier springs and the exact same intermediate shaft bearings (cam on a T1/4 VW, only difference is the oil hole location) as a 911 engine. The intermediate/cam gear is also the same, again just bolts up different.
We also use 911 adjusters in performance, so to me the wear faster claim could only be the chain, chain gears and the cam and rocker pads. I haven't seen unusual wear in these items, except the cam sprocket which wears even on a high milage stock engine.

The amount these cars will be driven I doubt it will ever be an issue.
At 60-100K you should check these item's, but in my book by this time you should be doing a top end job anyways.

I'm using a 901 trans, one blown shift and the heavier springs could save your arse.

Crazy! I paid $78 for a set of the Eibach springs from summit in 2014...now they are $300. Porsche tax finally hit.

Does DRC still sell the sport springs? Thats what he advises for DC-40 in between stock and race about 90-100# of seat pressure:

DRC just told me that if I plan on shifting at 7000RPM it should be good with stock springs.
Suggested maybe doing the Ti retainers though.

novice question: What's the benefit of the Ti retainers?

Ti retainers are lighter. Lessens chance of valve float

But on the downside some peeps say Ti retainers wear faster than steel.
This is OK on a race engine which likely comes apart often and can be checked, but in my opinion a couple grams is not worth it in a street car.

Its only $.. just spend it while your there doing it. As mentioned, 99.9% street use. Just leave it stock.

Walt, normally you and I agree on most things Porsche. I think you might be over simplifying things here. The camshaft in question here has steeper ramps than normal Porsche cams. Porsche was always careful with their cams to keep from floating valves. 906 cams for instance had lots of overlap and duration but shallow ramps so that the valves could follow with stock springs. I would suspect that the GE40 cam could float the valves at 7000 RPM with stock springs and retainers, or at least provide minimal over rev protection. In short the question of valve float depends on more than just RPM.

I would recommend either a step up on springs or titanium retainers or both.

-Andy