Camshaft for 86mm Big Bore engine

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Gary Campbell
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Re: Camshaft for 86mm Big Bore engine

#16 Post by Gary Campbell » Tue Feb 28, 2017 11:55 pm

So...
The 'C' cam is a great street 'feels good' w/torque and great drivability.

The 912 cam makes a bit more power but less torque, great drivability.
More duration (240) even less torque?
Why is the 912 grind better than the C for strictly street use? How many of us rev to 5 grand on a regular basis?
I seldom see the tach go north of 4 myself.

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Re: Camshaft for 86mm Big Bore engine

#17 Post by Dan Epperly » Wed Mar 01, 2017 1:04 am

I went with the Schneider cam on the advice of my machinist who said it would lower the power ban, meaning I wouldn't have to wait until 5k plus for the horsepower to really kick in, which is what I have with my 912 using, I assume, a stock cam. Maybe I need to do some reading about this whole duration thing.

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Re: Camshaft for 86mm Big Bore engine

#18 Post by C J Murray » Wed Mar 01, 2017 8:50 am

Putting together the right combination to get the desired result is complicated. The camshaft needs to be matched to the CR, port flow, carburetors, and exhaust so when any one of those is modified it may be necessary to modify the others. Car manufacturers spend enormous amounts of time and resources to develop engines that work seamlessly and we as individuals just don't have those resources. If Porsche decided today to further develop the 616 engine they could, after much effort, produce much better street performance as well as better manners. Unfortunately we are not as smart as Porsche so we need to proceed with caution when changing our engines.

In general, all other things remaining unchanged, more cam duration pushes the peak torque and HP up higher on the tach. At the same time the range of usable rpm gets ever more narrow as duration is increased until you get to a point that the available rpm range is unacceptable for practicality. For conservative drivers possibly wanting better fuel mileage and fewer repairs driving long distances or in urban use the C engine was the ultimate development by Porsche for the 616. For the customer that wanted more performance the SC was as far as Porsche wanted to go to avoid durability problems or customer complaints. The SC had a bit more duration, more lift, higher CR(to make up for the lost dynamic compression caused by more duration), and larger carburetors.

One fact about engine modification is that the further you go the more impressive the sensation of power becomes. This is because you are reducing power in the lower rpm and increasing it near redline and that results in a seat of the pants rush that can be very misleading. Most likely your car is getting slower as you drive it in normal street use. Think 911S vs 911T and check out those cam specs.

These are pretty solid rules of thumb but they are not cast in stone so your results may vary or your personality may benefit from a peaky engine. It's your choice.
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Re: Camshaft for 86mm Big Bore engine

#19 Post by Martin Benade » Wed Mar 01, 2017 8:51 am

He probably meant power band. If your 912 really doesn't do much until 5K rpm either its not stock or something is wrong with it. I am guessing that no aftermarket cam gives a lower power band than the stock one.

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Re: Camshaft for 86mm Big Bore engine

#20 Post by Jacques Lefriant » Wed Mar 01, 2017 12:42 pm

Hi CJ
the point i am trying to make is there might be a better fit than the stock cams for a particular individual. Even thought my hero Metzger may have had some input the C cam is basically a revamped Normal cam. Here in California the cam grinders have had a great deal of experimentation with cams for both the VW and pushrod Porsche engines. in my opinion Porsche did not want to address optimizing the profiles like they had to do for the 911 even for the 2.0 they had the Solex grind, the basic grind for the new headers, the 911S, the 911T, 911E, 911SMFI, 906, and tuners like Shreck offered more grinds. Also 911s can advance or retard the cams easily. If you talk Carreras they have 5 stock lobes so you can do 25 cam different combinations before you time the events.
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Re: Camshaft for 86mm Big Bore engine

#21 Post by C J Murray » Wed Mar 01, 2017 3:18 pm

Jacques Lefriant wrote:the point i am trying to make is there might be a better fit than the stock cams for a particular individual.
True! I am just saying that most of our cars are driven in a way that an SC cam is plenty. I am trying a C cam with 8.75CR this Spring and if it pulls ok to 5000rpm I think it will be a great combination for the average 356 owner because it is sure to be great in traffic. Solex carbs would be too large for that cam and spoil the low rpm smoothness and power.

The common asertion that a hotter cam is good for everybody's car is wrong.
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Re: Camshaft for 86mm Big Bore engine

#22 Post by Gary Campbell » Fri Mar 03, 2017 11:43 pm

What is a safe compression ratio to run with pump gas?
I normally run 91 octane that is 'pure gas', no ethanol in my stock engine. No ill effects to rubber parts with this formula.
Is 93 required for a 9.0 to 1?

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Re: Camshaft for 86mm Big Bore engine

#23 Post by Miguel Antonetti » Mon Jan 22, 2018 11:12 pm

Bringing this thread back alive... I have a question and/or need some advise; I just put a 912 crank in my all standard "C" engine w/Webers... all good with plenty of power, goes up in revs really fast and smooth BUT it feels maxed out around 4.2K NOT in power and/or revs it could go higher... BUT it feels like it's gona blow up... I'm concerned to take it to 5K soooooo is it me, or there is a possible mismatch, or just go for it...?

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Re: Camshaft for 86mm Big Bore engine

#24 Post by Greg Bryan » Tue Jan 23, 2018 1:27 am

Miguel - I recently built a SC type motor (63 super block; counterbalanced crank; C heads; 40mm Webers) with the Schneider cam and it is very well mannered and pulls nice from low rpms - I don't rev it much over 5000rpm because it is in my cabriolet and the top is always down. With the engine right behind my ears, it is very 'there' and sounds like it wants to be shifted at 5000 rpm. Maybe I'm just a coward ....
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Re: Camshaft for 86mm Big Bore engine

#25 Post by C J Murray » Tue Jan 23, 2018 8:56 am

All of the later cranks and rods will rev well past 6000rpm safely.

Any driver that does not care to rev their engine past factory redline should use a stock cam to achieve the highest average power output.
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Re: Camshaft for 86mm Big Bore engine

#26 Post by Adrian Rendle » Sun Jan 28, 2018 9:51 am

in those days, did even the Porsche engineers play with things like lobe shape (e.g. steeper fall to increase valve open time without increasing duration), or lobe centre angle (which is supposed to change the characteristics of an engine), or even unequal angles between inlet lobes (to cater for differences in charge inertia between cylinders caused my manifolding differences or which is the "leading" cylinder, in our case 2 & 3).....surely technology/experience over the last 60 years has improved on the stock cam....trouble is, quite understandably, aftermarket suppliers won't give their secrets away, meaning us punters can't really make an informed decision

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Re: Camshaft for 86mm Big Bore engine

#27 Post by Vic Skirmants » Sun Jan 28, 2018 8:46 pm

Hard to get a different lobe for 2&3 when the same lobe also does 4&1.

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Re: Camshaft for 86mm Big Bore engine

#28 Post by Norm Miller » Mon Jan 29, 2018 12:41 pm

Vic,

Only from J C Whitney.

Magi-cam!

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Re: Camshaft for 86mm Big Bore engine

#29 Post by Martin Bruechle » Mon Jan 29, 2018 12:59 pm

Dan Epperly wrote: Cliff, I don't speak cam
:D Funny :D

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Re: Camshaft for 86mm Big Bore engine

#30 Post by Jacques Lefriant » Mon Jan 29, 2018 1:14 pm

Hi Adrian
in the aftermarket Neutek is the leader in offering cams with dual patterns ie different profiles on the exhaust vs intake. It has not been shown to my knowledge to create any measurable difference. Since we do not have the resources of a manufacturer it may be possible to do a prediction using the software that is now available. other than that we have to depend on CJ Vic and the the Cam grinders like Elgin, Web, Norris, Isky, Racer Brown, Crower etc.
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