Wide Lobe Cams

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Alan Winer
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Wide Lobe Cams

#1 Post by Alan Winer »

I'm rebuilding a stock C engine with a big bore 86mm PC kit. I have a new wide lobe cam that only has C1 & D1 cast into the cam shaft. There are no other markings on the shaft or on the end. The lobe heights are 1.338(+/- .0005 on the four lobes) as compared to the original .102 #15 cam which has a lobe height of 1.350 (+/- .001 on the four lobes).
Can anyone help identify the wide lobe cam from the markings and the lobe height and secondly would this be usable in my rebuild? I understand that the wide lobes are recommended when installing big bore sets.
Any info would be appreciated.
Thanks,
Alan

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C J Murray
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Re: Wide Lobe Cams

#2 Post by C J Murray »

Hi Alan,
The camshaft situation is very fuzzy. If you read the data in the little spec books for both the 356 and 912 it can be deciphered that from 1956 through 1969 Porsche used 2 timing specs and lifts, a total of two cams. One cam was used for the Normal and C. Another cam was used for everything else, S/S90/SC/912. The "wide lobe" cam was introduced late(912?) to give more area for wear between the cam and lifters. There is no difference in timing or lift. I don't think they widened the lobe in time for the C but I am not certain. The specs for a '57 S is the same as a '69 912. BUT, the rocker ratios changed from one to the other and that is why the specs in the book are different! The Normal and the C have the same cam but also have different rockers.

I have read occasional comments saying that there is a "wide lobe cam" that Porsche made that is different from the 912 cam but the books don't bare that out and I think that is BS.

Most of the "stock" SC/912 cams our vendors sell are wrong, too much duration, so be very cautious.

If you want it to run like a C then install a C cam. I recently did that for my Speedster and I am very happy with the torque it offers.
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Re: Wide Lobe Cams

#3 Post by C J Murray »

Forgot... to tell lift you need to rotate the cam under a dial indicator, you can't measure the entire lobe because the base circle can vary.
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Re: Wide Lobe Cams

#4 Post by DonCichocki »

Ahhh, here we go again. The lift on my worn stock '65 "C" cam was .295" @ the lobe for both intake and exhaust. It was NOT a wide lobe cam. I recently sent it to Web Cams for an evaluation and they reground it to their 1187 grind. The lobe lifts are: intake .302", exhaust .293" @ lobe. The duration is still suspect as I'm waiting until I have the engine out and can verify the cam timing #'s. Why somebody can't duplicate the stock '64-'65 "C" cam to the book specs is a mystery.

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Re: Wide Lobe Cams

#5 Post by Martin Benade »

So the difference between a N cam and a C cam is only the rockers?
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Re: Wide Lobe Cams

#6 Post by C J Murray »

Martin Benade wrote:So the difference between a N cam and a C cam is only the rockers?
That's the way it looks to me. When the rocker ratio is increased not only is lift increased but duration is also increased because the rocker gets the valve open sooner and closed later by making the rate of opening and closing faster. The duration "from seat to seat" remains the same but the duration from .050" to .050" is longer.

If you drive your 356 under 5000 rpm as a matter of habit then the C cam and rockers is the most powerful choice. I wouldn't go bigger than 28mm venturis with that cam though.
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Re: Wide Lobe Cams

#7 Post by Jacques Lefriant »

Hi
Wilhoit states the Normal cam has .285 lift 220 deg dur C .303 lift 215* and S/912 .330 lift 233* these are all at the lobe and they are identical intake and exh. and they share the same canters 107.5* Most replacement cams have 240*-250* duration and 105 centers Nutek gets cute with dual pattern cams by using different masters for the int an exh most are single patterns like stock. With the exception of German aftermarket cam grinders use the same semi-finished chill iron wide lobe blank to make new cams. regrinding cores is not practical any more since the availability is terrible. Racers have experience breakage of the new blanks and may be forced to go to steel billets.
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Re: Wide Lobe Cams

#8 Post by C J Murray »

Hi Pops,
Some of the Willhoit numbers are wrong. I think that is why his chart is no longer easy to find. I know he told me that one cam was wrong but it is best that I not speak for John.

I have a sloppy "napkin calculation" that I jotted down while watching TV and reading the little spec book. Porsche measures cam timing at 1 mm lift and at the valve. The rocker ratios differ from iron stands to alloy stands. I do not have absolutely accurate data on the ratios but I think the iron are 1.1 for both intake and exhaust while alloy are 1.3 intake and 1.1 exhaust. So I did a rough conversion of Porsche's valve movement specs to the actual lifter specs with the aim to see how different the actual cams are from each other. Duration at the valve is always more than at the lifter when the rocker ratio is greater than 1.0 but how much is beyond my math abilities and I would have to do an exact measurement on the engine.

Anyway, Porsche had a total of two cams at least so far as we can go without an accurate graph of the entire path of the lifter as measured on actual factory cams. Still, the same lift and duration means they are effectively identical, either Normal/C identical or S/S90/SC/912 identical. Remember the higher rocker ratio of the intake is what makes the duration longer for the intake.
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Re: Wide Lobe Cams

#9 Post by Jacques Lefriant »

Hi CJ
From what i can see Willhoit's nos are pretty good and it looks like he made a good effort to use good samples. To get consistent ground cams you have to use a perfect master, grind to a predetermined base circle and use the perfect dia grinding wheel if you use an old school cam grinder. to make matters worse the ratio is not fixed in the arc and there could be a substantial variation between different rocker arms.
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Re: Wide Lobe Cams

#10 Post by C J Murray »

My exercise was to confirm that the Normal and C were the same cam. I already did that with the S/S90/SC/912 cam so I knew they were all the same but I wanted to contrast the mild N/C cam with the hotter factory cam so I listed the SC also.

What you need to ask yourself when choosing a cam is what rpm do you usually use as a shift point. Then you need to look at what the factory did and think about why they made those choices and if those choices are good for you.

There is no such thing as a magic cam that is superior at every rpm.
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Re: Wide Lobe Cams

#11 Post by C J Murray »

Another caution, Europe specs duration at 1 mm lift, America specs are at .050", usually, while some cam grinders give specs "seat to seat" which is the absolute end points of movement or "advertised" which is often at .020". The meaningful specs are either 1 mm or .050" since this is around the point that significant flow occurs. You must use the exact same measuring point, 1 mm or .050", to make a comparison.
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Re: Wide Lobe Cams

#12 Post by Charles H Jacobus »

CJ, Is 4500 the correct standard shift point for normal driving in a C?

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Re: Wide Lobe Cams

#13 Post by David Jones »

The correct shift point for any engine is that point where when the revs in the next gear put you in the correct point in the torque band to give you the best acceleration in that gear. If your car is stock and has the original tach with the green shaded area just try to drive keeping the revs in that area. If your engine is not stock then you may have to do some experimenting to find that sweet spot, but in general I would have to guess that shifting at 4500 rpm would mostly keep you in the torque band.
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Re: Wide Lobe Cams

#14 Post by Charles H Jacobus »

That's where it feels comfortable in general driving to me and doesn't feel like it's pushing the engine.

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Re: Wide Lobe Cams

#15 Post by C J Murray »

We all have a comfort zone, a personal sweet spot. To some extent this is affected by the way the engine was designed to be used and where it feels best. Also a factor is your personality of either being mellow or aggressive. I am not saying slow or fast, that is not the measure. Some people hustle a car down a twisty road revving moderately and shifting gently while others are past redline and flat shifting. Jackie Stewart is well known for making a passenger think the car is going much slower than it is because of his style, mellow but fast.

A stock C should like shifting at 4500 or even lower. An SC will generally be happier using a shift point that is 300-500rpm higher. If you never shift above 4500 then the C will be faster than the SC.
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