911SC using S Cams and Stock Pistons...

[CBRacerX]

So I'm in the middle of rebuilding a 1980 SC engine with the stock CIS pistons (yes, small port heads). My machine shop sent me a set of Web Cam Mechanical Injection S cams instead of stock SC cams. I didn't notice this until after I installed the cams and started timing them. The engine has Crank Fire Ignition, SSI's, Sport Muffler and Weber 46's ready to bolt on.

Am I crazy to consider actually using these cams? The overlap is breathtaking, but the intake valve lift is not much different than 964 cams which are a popular upgrade with stock pistons.

I/E Lift, Duration
S Cam - .455".399", 263/235
SC Cam - .450".395", 228/218
964 Cam - .470"/.430", 238/226


See Dougherty Racing Cams Porsche 911, 930 and 964 camshaft profiles for a nice chart comparing these profiles.

Is this a valve to piston no go? I'm not going to machine the pistons - that ship has sailed with the heads bolted on

[mreid]

You need to check clearance on YOUR engine. I'm pretty sure they won't clear, but you have to check as who knows what other things have happened to your car over the years.

[CBRacerX]

Originally Posted by mreid

You need to check clearance on YOUR engine. I'm pretty sure they won't clear, but you have to check as who knows what other things have happened to your car over the years.

Thanks Mark - I was going to get the TDC clearance next, just looking up what the valve lift timing spec for these S cams is. I thought that would be easy to find but maybe I'm just not believing the number - 5.2mm?

[mreid]

That's correct for the s cam/Carrera cam, but check your part numbers. Here's a pic from Wayne's engine building book which is very valuable if you don't have it!

[Edelweiss]

Dear x,

if your overlap is breathtaking the most important is to generate a suitable static compression ratio and a working squish.

This is essential - then next is a good and less restrictive exhaust construction with a good header junction.

To use the carbs is ok anyway.

Best reg.

Dirk

https://www.facebook.com/edelweissmotorsport

[kenikh]

If you don't pocket the pistons for the valves, you'll bends a valve. Has less to do with lift than duration. SC cams are safe because they close the valves sooner than the S cams. IIRC, pocketing the swirl dome 1mm is enough and still maintains safe crown thickness.

[JohnJL]

As mentioned you need to check clearances on your car. Put a cam in, time it, and check clearances THROUGHOUT the range. TDC is not the only or even most likely spot in which your valves can contact the pistons.

You also need to consider how this change in cam will effect the overall system of the engine you are trying to build. On another note, 46PMOs may be a bit big for small head SC. I have found 40PMO manifolds and webers liked a good amount of porting/blending to give better results.*

*I am not an expert.

[Edelweiss]

Originally Posted by JohnJL

As mentioned you need to check clearances on your car. Put a cam in, time it, and check clearances THROUGHOUT the range. TDC is not the only or even most likely spot in which your valves can contact the pistons.

You also need to consider how this change in cam will effect the overall system of the engine you are trying to build. On another note, 46PMOs may be a bit big for small head SC. I have found 40PMO manifolds and webers liked a good amount of porting/blending to give better results.*

*I am not an expert.

The key spec in coherence to any cam modification is static compression.

The valve to piston clearance is a natural thing to observe and adjust.

In a lot of cases it is better to machine the valve seats then to machine the pistons - sometimes both is necessary.

After measuring you need min. 1 mm at the inlet and 1,5 mm at the exhaust valve to piston / pocket clearance.

The 46 mm carbs are ok - and you get a good port velocity if you do not open the ports too much. If you do so consider to generate a anti reverse from manifold to port diameter.

Have fun !

Dirk

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[cmcfaul]

I believe those cams have alot of overlap and would bennefit greatly from a higher compression piston with the predesigned valve clearance. JE 9.5/1 would be my choice.

Chris

[kenikh]

Mahle Euro 9.8:1 or later US 9.3:1 would work great, too.

[Edelweiss]

If your lift in TDC is about plus 5 mm you should go with a min comp. ratio of 10,5 : 1

There is no pinging with those cams then even at 11,5 : 1 and you will see good low and midrange power too.

Idle will be very stable too.

Always take care about the squish !!!

A good squish band makes the difference between a good and a not so good engine.

It changes the fuel consumption and gives you mid pressure efficiency from all what your volumetric circumstances can offer.

Dirk

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[Walt Fricke]

Edelweiss - you may have better benzene over there in Deutschland. Pump premium stateside is typically no more than 91 octane, using the RON+MON/2 protocol. And in many places there is up to 10% alcohol mixed in.

Aren't European fuels rated only with RON? Which shows higher than r+m/2, I think.

Here the advice is don't go over about 9.8/1 with single plug pump gas. Engine builder friend says he has to mix with race gas to avoid breaking the CE rings. But with all that S overlap, tht lowers the effective CR, so you might be fine at 9.8.

I'm here to tell you no problems with nominal 9.3:1.

I've run nominal 10.3/1 on pump fuel in race motors, but they are twin plugged, which the early heads require. The new water cooled cars have one plug right in the middle, and that seems, along with knock sensing, to cover a multitude of sins.

[Edelweiss]

Dear Walt,

yes for sure the gas quality must be kept in mind.

Here we can chose from 91 to 102 octane and 5 to 10 % bio ethanol.

But:

It is simply and totally wrong to say from upwards 9,8:1 you have to go twin plugged.

It is maybe right to say don't go over 9,9:1 with a std. cam and a TDC lift range of 1,5 to 2,5 mm without a good working squish.

A good squish means you go as close as possible with the piston to the head and form a suitable squish area, shape and size matters !

At a Porsche Engine you can go 0,8 mm.

Then you bring all the fresh gas load into the combustion centre and there it will be ignited under control of the spark plug.

The area where pinging has its source is then more or less without any gas.

Where you find no gas you will not see pinging.

A cam needs its suitable static compression ratio and vice versa.

We run 13,8 :1 single plug air cooled engines with pump gas.

Good squish, TDC lift 6,8 / 7,2 mm no problem.

A cam with reasonable TDC lift needs a free flowing absorber exhaust.

Not every inlet plenum is - single tb - suitable for radical cams, ITB and carbs are.

Whenever someone ask me what compression ratio should I go for my question is:

What cam data ?

So twin plugging is a suitable and simple way - but often used when basics are not understood right and 100%.

The cam we discuss here can take much more then 9,8 :1 and to work well I even would say it needs more.

Best reg.

Dirk

[Walt Fricke]

Dirk - the standard approach one reads of in US 911 discussion circles points to the high dome of the early S pistons, and how it is a problem for the flame front to travel over the dome, and (if I remember this part right) to the need for more spark advance than perhaps one might want in order to deal with this.

Twin plugging (done in all the factory air cooled race motors, and in the 964s and 993s) solves that flame front distance issue, as does the top center plug in the 4 valve motors.

As to the cam, the more overlap the lower the effective or dynamic compression, so you can run a higher static compression. The problem for most of us there is that we can measure static compression, but you can't do that with home shop tools, can you, for dynamic compression? So you are left somewhat guessing as to how much overlap drops the effective compression how much - that is, to quantifying this.

If squish area cured all these ailments, would not Porsche have done that? Cheaper than twin plugging. Is Chris' cam more like the 906 cam Porsche ran in the 2.0 race motors? They all ran twin plug at 10.3:1 CR, didn't they? The first 10.3:1 I see for a street Porsche without twin plug is the 3.2 Euro Carrera. And doesn't the ECU on that support, and use, knock sensing? Lower ratio for US due to gasoline quality concerns? Or were they being too cautious all along?

[Edelweiss]

Hi again Walt,

we use twin plugs in all our 3,0 RSR race engines for example.

But for different reasons !

When your mid pressure gains, as on race engines, you are in need of more igniting energy - and with the second spark plug you have a more effective process, you can retard ignition timing a bit and the torque will see also some gains. A retarded ignition will cause a colder combustion chamber, head, piston which is also healthy.

The reason Porsche was using twin plugs at the 964 and 993 engines is very simple explained:

When the bore went up to a diameter of 100 mm and the emission rules where going to be more strict on a world wide basis Porsche was using the second spark plug to generate two flame fronts that would burn the complete load or if not complete to a higher degree. It was an approach to prepare the air cooled engines for the 90s.

So as a little conclusion: race engines with plus 100hp / L and a mid pressure in relation are a complete different story.

The 3,0 L domes on the 95 mm street pistons are relatively flat and so in general there is no need for the second spark plug.

Porsche was experimenting with the squish a lot - you can see it best on some 3,8 RSR works pistons.

Nower days the knowledge is on an other level and as a tuner or modern thinking engine builder you can go more consequent ways even staying on a conservative road without taking any risk.

There is a good basis of experience and so there is no need to calculate the dynamic processes. By the way they are changing over the rev band due to resonance, reflection and all phenomena which is influencing the degree of fresh gas delivery. So even works engine departments get close by calculation and the they try and test the reality.

So Walt, please don't get me wrong being a big fan of twin plugs - what I want to point out is:

There is more then this and to build the better engine you have to picture the complete package including all relations. A lot is easy to do when the engine is opened anyways.

So drilling the heads for twin plugging is one thing, it is alright and cheap so no reason not to do it - the same for adjusting the deck height and build out a good squish band.

One more time as a clear statement: there is nothing influencing the tendency of pinging more then the cam.

You have to balance static compression and cam data first.

Have a wonderful sunday !

Dirk

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[CBRacerX]

A wonderful discussion has ensued! I find it all very interesting.

However, after degreeing the cams, they are stock SC grind. Whew! The box was marked wrong.

I still have a problem, one of the oil fed tensioners is missing the little pressure relief (control?) spring, ball and cap. Doesn't appear to be sold separately so unless I can "ignore" this I'm looking for another tensioner... But I guess that is a subject for another thread.

[Walt Fricke]

Chris - the only way I know to get the tensioner part you are missing is to get a shop or friend to give you a tensioner which has crapped out for some other reason, and take those parts from it.

The formed metal cap is just held in by the friction of the edges or the what - 3 or 4 arms which stick down into the hole. I know from experience that, once it is out because of some clumsiness on someone's part (like me), if you just push it all back in it will hold for a while. But not forever, as it should.

I took a small Dremel diamond abrasive tool, which fit into the hole, to cut a crude groove inside the hole. This allowed the edges of the strip parts to be expanded a bit into the groove. Seems solid enough, but I haven't run it. Should work.

A guy used to machining small stuff might drill the upper hole a bit wider, tap it with a blind tap, fabricate something which would screw in, had holes so the oil could get out when the ball moved up, and whose upper end was closed, or at least enough to hold the spring and keep the ball from being spit out. That would work also.

This part's function is to make sure there is pressure against the bottom of the piston part next door, but not too much.

I think you should move this part of the discussion into its own titled discussion. There seems to be a way to do that, but I don't know if there is a short cut other than copying the relevant parts of these two posts, and using them as the input to a new discussion titled something like Carrera Tensioner pressure regulator Spring Missing.

[mikedsilva]

Originally Posted by Edelweiss

The key spec in coherence to any cam modification is static compression.

The valve to piston clearance is a natural thing to observe and adjust.

In a lot of cases it is better to machine the valve seats then to machine the pistons - sometimes both is necessary.

After measuring you need min. 1 mm at the inlet and 1,5 mm at the exhaust valve to piston / pocket clearance.



Dirk

de-de.facebook.com/edelweissmotorsport

Is 1mm intake, and 1.5mm exhaust sufficient or should it be 1.5intake and 2.0 exhaust? of course, more is better, but I am assembling a motor that seems very close to the former, not the latter..

[Eagledriver]

Originally Posted by mikedsilva

Is 1mm intake, and 1.5mm exhaust sufficient or should it be 1.5intake and 2.0 exhaust? of course, more is better, but I am assembling a motor that seems very close to the former, not the latter..

1mm is adequate. Factory workshop manual specifies.8mm minimum for the exhaust valve. Each other source seems to add a bigger margin for safety.

-Andy

[mikedsilva]

Originally Posted by Eagledriver

1mm is adequate. Factory workshop manual specifies.8mm minimum for the exhaust valve. Each other source seems to add a bigger margin for safety.

-Andy

cheers thanks, I don't have the factory manuals.
If 0.8mm is the Exh valve min recommendation, where does "the book" get the values of 1.5mm intake and 2.0mm exhaust?