I agree if your running carbs changing pistons and running a cam designed for carbs is a better way to go, but with a Mod s cam you need different valve springs and it gets expensive to start doing pistons, springs and it is a lot of work many do not want to do. These cams were designed for CIS but they work much better on carbs than a 964 cam. To be clear we didn't design these for carbs. We needed a solution that worked with cis domes.

Just to add some info that may be of help when trying to understand if a camshaft will "fit".

Do a valve to piston check first so you know the limits of what you have to work with. Then ask the camshaft supplier to give you the lift numbers at those same crank positions. Then its a easy math exercise to see if it will fit.

Any one selling camshafts will be able to give you the lift number at any centerline position.

In the case of the question above, would a certain cam fit a stock 2.7L piston, measure the valve drop to the piston from 20° BTDC to 20° ATDC and ask the cam supplier to give you the lifts at those same positions at the centerline they suggest.

If you give the cam supplier the drop numbers and the clearances you wish to run, they can easily see if the centerlines can be moved, or the performance will be compromised any.

Simple, no guess work and you know before you buy.

I was told my stock valve Springs would be fine with my MOD S cams. It's a 2.8 with J&E pistons at 9.8:1. Good up to 7500rpm.

I would like to hear more input on using stock valve springs. I have a feeling that using the heavy duty springs is overrated and only needed for some extreme situations

That's what I was told by a few reputable mechanics. I'm not using my car for full race. Canyon carving and some AutoX

I am also running Mod S cams with stock springs (motor is a 2.7) with RS pistons. Also told this was fine. Have rev limiter set at 7500 as well, no problems.

john

cam choice depends greatly on your exhaust and usage.

I'm running M&K headers and 997 GTR RS muffler

911 Slant, The Melissa cam makes more power than 964 for sure but if you like what you have keep enjoying it. Sometimes it is better to appreciate what we have and not focus on what we do not have. If you had stock cams the Melissa cam is the way to go on CIS pistons. I do my best to live by a quote by a really good person. "you have two hands one for helping yourself and the other for helping other people" I have my reputation for doing my best to do that, I am not out to just sell a product. William Knight

Be careful here when talking about valve springs. The choice of valve spring (fitment not considered here) is about the wire and the stress put upon the wire.

Some springs are made from really good steel, some are not. Typically a spring made from German steel is a good spring and has the ability to run at high loads close to or at the stress limit without failing. Many of the domestically wound springs cannot as they are wound from a lower grade of steel. We have springs wound for our use with steel that is imported but up goes the cost of each spring.

Once you know the quality of the spring and its characteristics, you can then calculate the nose pressures required with rocker arm, retainer, locks weights etc. The seat pressure is more about the seat widths cut on the seats and valve margins. This why I can never understand the seat pressures some aftermarket springs apply to the seats.

The cam design has a huge affect on the nose pressures as well. many designs have horrible harmonics built in, so in an effort to dampen these, more nose pressure is required. You are compromising the engines performance by making up for poorly designed parts.

One last requirement is the springs ability to fully compress without going solid or coil bind. The valve lift controls this.

Do you need to run more nose pressure with higher RPM. In most cases yes, with many cams available today, as the harmonics go crazy at higher RPM. If the cam is designed properly and all of the moving parts are weighed and used in the calculations, the answer is no. We have 2V air-cooled engines that run to 8000 RPM with a single spring with an open force of 160 lbs. Yes the Exhaust valve needs more, often they weigh more due to the back geometry of the valve, and you do build in a little more safety.

If you add more force because you are concerned about missing a gear shift, you are doing this because of your driving ability and are prepared to compromise the engines performance.

This could be one of the reasons why that other car out pulled you on the straight.

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I ran my 2.7 race motor (Elgin 315 cams, Mahle 10.3:1 pistons, Weber 46s with stock valve springs to start with. Felt strong up past 8,000 RPM. Eventually most of the spring sets broke a spring. Still ran fairly strong up to 7,600 RPM to finish a race.

I suspect no issue with stock Porsche springs at more sensible RPMs.

JP I have a set of SC grind cams for the 2.7 CIS engine. I was going to used them on my build but it was so messed up when we got into the engine we decided to go full RS+ spec.

Let me know if you're interested.

edit: OOOPS, just noticed this was an old thread bump. I'm sure you're done by now, so please post up what you decided on etc.

M1 cam is much more power over SC cam on a motor. uses stock springs.