930 crank case replacement needed!

[tom1394racing]

I'll chime in to say that the build-up on the pistons looks normal to me.

[simsalabim]

Thanks Tom. Should I worry much about clean-up in your opinion?

Also, I spent too much time looking at old forums on changing to a 964 cam profile.

It seems like with my motors 9.3-1 comp. ratio, this is a realistic upgrade.

HOWEVER, this car will be for the street (mostly ), and I do NOT want to sacrifice low-end torque like the other half of the forum folks say will be the result.

John at camgrinders thinks it will work out fine without any other changes to my motor and I am tending to agree with him.

Anyone out there that can settle this thing???

Thanks as always,
Bill

[KTL]

I agree with Tom that the carbon on the pistons is completely normal. My 16K pistons actually looked a bit heavier on carbon. BTW the pistons I have wouldn't work for you, as the Carrera 3.2 is the same diameter as yours (95mm) but the pin height in the piston is different. You'd have to offset bore your rods on the small end to use them. The displacement gained by the 3.2 was in the crank (stroke changed from 70.4mm SC to 74.4mm 930/Carrera) and the bore stayed the same. The cylinders would work for you but I can't give those up. The nice cyls are why I bought the set!

My friend Jay and also MotoSook here on the forums, both local to me, both have the 964 cam profile in their "stock" 3.2L Carrera engines and neither of them found any downside to the cam regrind. Going by seat of the pants feel mind you. They were pleased with the change and had to do something anyway, since they were redoing the top end and cams had some pitting on them.

Forgot to add, you can clean up the pistons and the "carbon ring" around the tops of the cyls if you want. But it's just going to come back again.

[simsalabim]

Thanks Kevin,

I knew about the mods to make the 3.2's work, but thought - wait for it - while I was in there, it might be a worthwhile effort.

Also, something about knowing I have Alusils that bugs me.... no matter their condition! Believe me, I know how silly this must sound to some. I will probably stick with what I have unless there is a great deal out there somewhere???

Should know something about the condition of my heads today or tomorrow and maybe get to the bottom of the torn seals etc. issue.....

Cheers to everyone out there,
Bill

[simsalabim]

The word on the heads is good. No guide work needed. Also, after measuring everything out, it is indeed the opinion of the machinist that the fact that the guides were a bit long (especially on the exhaust side) was what was causing the valve seals to tear. He will trim all the suspect guides down and call it good.

Also, tested the installed spring tensions. All measured out at 60 (ish). I am wondering about the recommended tension for the updated 964 cam profile I am strongly considering? Should I look at changing out the springs?

fun, fun, fun...

[KTL]

911SC & 964 use the same valve springs. So the installation specs are the same for either cam

[simsalabim]

Will stick with the springs I have. Pressure is good both seated and open. John suggests shimming to 70lbs and this will be done.

Now I need to find another fan cover and probably go ahead and change out the alt and seperate reg. to the new combined units.

Anyone out there with a good used (no stress cracks etc.) fan cover and/or a good alt/reg.??? Mine is an 81sc.

cams and rockers sent to John today and heads should be done next week along with some miscl. bead blasting.

Hope to have new case by then as well....

[Walt Fricke]

I've gone the other way with regulator location on my race motors, because I found I was replacing regulators on wife's 3.2 and my SC more often than I thought I should have to, definately more often than wear on the brushes necessitated. If yours is external, I'd keep it that way.

I'm pretty sure that the replacement external regulator is solid state, not the old coil and points setup the 2.7s had. On my race motors I just use a 914 external regulator - same 3 pin connection, and it seems to work fine. One I suspected, whether rightly or wrongly, of not working fine I separated from the potting material, and it is for sure all solid state, and not many components at that. I mount it externally on the motor, but that's because with race motors you want to reduce the number of things you have to unplug when you pull the motor, because you do it so often compared to a street motor you hope will be good for 100 to 200K miles minimum. But changing the internal regulator is an order of magnitude or two or three harder than changing out an external one.

[simsalabim]

Thanks Walt!

I had just discovered the fan cover stress fractures (dissassembling all tin work for a good cleaning) when I figured a little alternator preventative maintenance was too convenient to overlook (a legitamate "while in there"?) and decided I would do it. I guess I assumed newer was better, so the combo system seemed legitamate. Then, when I actually - God forbid - THOUGHT about it, a separate alt and reg made more sense.... that's when this fine forum and it's "search" function was taken advantage of and that confirmed my theory. Yes Walt, I will be taking my alternator to a rebuilder next week. I figure if it lasted as long as it has (looked to be the original one from 194k miles ago), why monkey with it. The external regulator is indeed easy to replace and inexpensive so what's the downside?

Thanks again for the validation Walt, it really helps.

By the way, have you ever repaired fan covers? Mine has a pretty good stress fracture at one of its blade/hub interfaces.

Best,
Bill

[Walt Fricke]

You mean the fan housing? The thing which holds the alternator?

None of mine, but reading here I have seen others having this problem. No one suggested repairing it (though a good welder can weld it, I suppose). The discussion tended to focus on aftermarket reproductions, and how well they fit. Should be something one could get used and not cracked.

Only trick I know of there is to get one with the right depth machined into where the alternator sits. The lower the wattage of the alternator, the shorter it is and thus the shorter the machining to hold it. Someone posted a picture of three alternators side by side, each longer than the other. Don't recall if measurements were given. The casting numbers on the housing are, I think, no help. Same casting for many years, just machined differently. Only the '78-9 SC housings won't fit at all, because their fan was smaller (225mm I think) than the more standard 245mm fan you have.

But I think this is a good thing to have caught now.

[simsalabim]

Here is the part I need.... the fracture is at the base of the interface btw the lower right "blade" and the hub. This is the fan housing/cover.

By the way Walt, my odyssey started with a post about the tearing of a couple of exhaust valve seals. The theory at the time was some possible interference with some too long guides that were installed in 1997 by the previous re-builders. After a thorough disassembly and measuring by the machinist, it is his conclusion that there was in fact some banging going on btw the seals and retainers that over time, contributed to some excess strain on the viton "rubber" seals. If you like, I can post a photo of one of them. The tear is a circumferential one at the wide end of the seal.

Anyway, here is the fan cover/housing I am referring to...

Thanks very much.
Bill

[simsalabim]

One more thing to everyone out there..

My apologies for occasionally mangleing some of my terminology. This is my first Porsche and I have not worked on cars since the early eighties when I toyed around with my old Honda civic. My learning curve has had to start from essentially zero over the last couple of weeks.

My sincere gratitude for the patience of the members of this forum. Thank You all for sticking with me.

cheers,
Bill

[simsalabim]

While I await the necessary replacement parts, It looks like I have some time to take a breath, review the events of the last 3weeks and ponder my next decisions as I push on. I hope to be driving in 2-3weeks...

Time to think is a good thing... but for some, there is such a thing as "having too much to think" I fear I may be one of those folks.

Now that the tin-work is disassembled and being prepped (bead blasting), I am considering my options for sealing and/or painting some of them.

I know there are a variety of opinions out there, each having merit, but I need to make a call in the next few days...

At this time, I am focused on just a few parts: valve covers, chain box covers, alternator/fan shroud, fan and strap. Manifolds look good by themselves.

While I appreciate that Porsche thought they were fine without any finish at all, let's face it... they also gave the thumbs up to Dilivar head studs, an airbox ill suited to the combustion system it was mated to, faulty chain tensioners, lower valve covers, cruise control (perish the thought) and I am sure, others this Porsche newbie hasn't yet been introduced to.

So.... not only do I have the metallurgical properties of both Aluminum and Magnesium to consider, but I must also factor in style and/vs. function, as well as the ever present "cheese factor". No small task for a classic over-thinker like me.

Functionality? Well it seems there is only corrosion resistance, thermal properties and co-efficient of friction to think about. As far as style? for this old engineer, it's simply form following function... as long as it is tasteful, understated and generally accepted.


Here's my third grade thinking on the subject(s).

It seems 1. keeping an engine from overheating is a good thing and that 2. excessive wind resistance is a bad thing.

Therefore, I figure the fan itself should stay as cool as possible and contribute as little as possible to the drag on the motor. I have been amazed at how far aerodynamics has come during these days of maximized speed and energy efficiencies. Anyway, the 11 blade bare Magnesium fans seem to be prone to a fair amount of surface corrosion. The rougher the surface, the more air friction right? Don't we want to reduce drag? I figure if I can make my fan as smooth/shinny as possible without unduly compromising it's ability to stay cool, this is a good thing.

As far as the other parts, only the lower valve covers and perhaps the chain box covers strike me as being appreciable heat sinks/radiators (does the fan housing or upper valve covers get very hot?), so I believe I will only blast them and leave them be, with perhaps only a sealer (for easier cleaning as long as non-yellowing).
The upper valve covers and fan/alternator shroud?.... being aluminum, I figure I will just use a self-etching primer and then a few coats of matte black (supposedly the best paint for maximizing heat conduction). Strap and fan? hmmm....

See what I mean? "too much to think"

bill

[Walt Fricke]

Many paint their fans (and also housings) for the perceived cosmetic advantages.

A fellow recently asked us why his fan belt was slipping (he couldn't turn the motor over with a tool on the fan pulley, and - given the mechanical advantage pulley sizes give - he should have been able to do that (aided by a thumb on one side of the belt to give it a bit more tension). It was noted that he had painted the pulley face which is part of the fan itself. He hasn't responded to let us know if that was the curprit.

But improving the aero of the fan? That is worthy of an engineering study for sure. Many would be delighted if doing what they already want had a practical benefit as well.

Me, I restrict myself to painting the fiberglass engine shroud.

I wonder just how much convective heat loss one can expect elsewhere? Doesn't the efficiency of that depend on the temperature delta? Is there much air circulation around the engine, considered first above the surrounding tin, and then below it with the heat of the exhaust? Radiant heat coatings? How much can one expect from that source? Where would it go to - the air, or the chassis?

Most of the discussions about engine heat center on how to lower its production, or how best to get it to a remote radiator and shed it there, or both. And there is another consideration - the engine is most efficient at a fairly high temperature. I recall suggestions of maybe 225 deg F? Something like that. Naturally, there are also those parts life vs. heat graphs, so efficiency has to be balanced against longevity. But it is not by accident that Porsche sets its air cooled thermostats at about 190 degrees F. I sometimes wonder if the oil cooling on my race car doesn't keep the engine a bit cooler than it should be for racing purposes.

[simsalabim]

Uhh-ohh, it looks like Walt is on to me... yep, just looking for some semi-plausible justification for doing what seems like a "neat" thing to do .

Then again, when I combine a few basic principles, as well as some specific - albeit cursory - research (the boys over on Samba sure looked hard at this when they explored modifying their V.W motors with Porsche style fan and shroud cooling) into the practical aspects of the air-cooled motors, the subject seems anything but cut and dry. Although, if someone out there had definitive evidence that the design, as is, is as optimized... then PLEASE, put me out of my misery...

Thermodynamics/heat transfer, when applied to the relatively complex interplay of systems and subsystems found in any motor trying to run at optimal efficiency, is admittedly the stuff of college doctorates. Having said that, the benefits provided by decades of performance and reliability data, as well as the undeniable technological advancements that have continued in most everything, makes one wonder what original designers would have done with the same knowledge.

I know that a popular SC upgrade is installing a 964 era "radiator style" front oil cooler to replace the trombone style originally supplied. I understand that this is to help keep the cars from overheating in this part of the world. I have also read that Porsche eventually abandoned the air-cooled concept in large part because their motors were simply generating too much heat (as a by-product of increased displacement/power demands, unleaded and leaner fuel mixtures as well as other pollution fighting modifications) to remain viable. Therefore, I figure I am in little danger of over-cooling my SC motor by making my fan more efficient. As long as the volume and/or flow of air moving over the motor is not limited by the baffles or some other shroud related design characteristics, then the faster the fan spins, the more air gets moved and the better convective cooling system we have for a given input of energy.

So..... the fan is spun via it's belt/pulleys connection with the crank shaft. It is my understanding that the fan on a Porsche will spin in direct proportion to the crank shafts rpm, until the force required to spin it overwhelms the abilities of the belt to keep from slipping on its pulleys. I may be wrong, but it seems logical that if we can reduce to as little as possible the additional drag/friction imparted on the fan by its various mechanical interfaces (of which the rather large surface area of fan blades colliding with surrounding atmosphere is anything but negligible), the higher the fan rpm is that we can generate before the associated higher back pressure causes enough drag that the belt slips. More air current equals more convective cooling equals less load on the oil cooling system and in turn a more efficient motor? A much revered Thermodynamics professor once likened convective cooling to an army of men with empty buckets trying to remove a large body of water. If we substitute water with heat, and the men with buckets with air particles, then the more men we have moving as fast as they can filling up their buckets, then the faster we are getting rid of that heat.

My old Magnesium fan has a surface like that of sandpaper. New ones offered by our host and others advertise their upgraded Aluminum design. They are less susceptible to corrosion and supposedly stronger too. One thing is for sure though, they sure are shiny....

By the way, I also swore that clothes-pinning baseball cards to my bicycle spokes made it go faster too...

[Walt Fricke]

Well, Porsche went to water cooling so it could put 4 valves into each head. They had concluded, from experience with their racing motors, that 4 smaller valves instead of two larger valves increased the volumetric efficiency significantly. I don't know if this was because the total area for air to get in and then out was larger, or for some other reason. And because each valve was lighter, it may also have been easier to control the valves, especially at 9,000 rpm for race motors. They found (per Bruce Anderson, I think that's where I recall this from) that air cooling wasn't doing the trick for the 4 valve heads they had tried.

I don't think our fan belts slip, certainly not with proper adjustment and within the factory operating limits. The optimum upshift RPM for most SCs is about 6,250 rpm. That isn't straining the abilities of a motor which can safely, if not efficiently or over thousands of miles, hit 8,000. A missed shift which zings the engine can throw off the fan belt, though this is not inevitable. An aftermarket seller of go-fasts has a flat fan belt with tensioner setup for our 911s, which he says will preclude throwing the belt, so my experience (some ugly missed shifts but belts stayed put) is not necessarily representative.

Porsche has fiddled with its fans. Your fan is 245mm in diameter. Racing fans (and the '78/9 SCs) were 225. The pulley ratios have varied some to vary fan speed (lower for race motors). And the blades have changed. Eleven, then during the 2.7 era five with a faster pulley ratio, then back to 11. With the 964s (was that the changeover), the blades changed shape quite noticably. The data I have seen say these were less efficient (moved less air per revolution or liter per time), and that the redesign was to make them quieter - the better to meet government noise regulations. For at least some model years there was a special Swiss 911, with some extra stuff to meet their stricter noise rules.

So - the fan is part of the parasitic power drain on an engine (like the oil pump, valves and springs, everything which rotates or reciprocates and produces friction, and the alternator). The more efficient the fan is, the less power you can have it consume for a given volume of air moved. And moving more air than needed for optimal cooling is power wasted. Though the magnitude of anything under discussion might be small enough to be down in the noise.

When I was riding my bicycle around with a clothes hanger in the spokes beating on a Quaker Oats tubular package - a great drum surface, and it certainly did make me go faster - the word in the hot rod world was "port and polish." We have all learned that porting is good, but too much is bad. VE is affected by intake velocity as well as port size. But what about polish?

You basically don't hear about elaborate polishing of ports and intake runners these days. Apparently, a mirror surface creates more drag then one with some rugosity. Something to do with the boundary layer, which clings better to some roughness in a tube, and perhaps air to air drag is less than air to metal? Seems counterintuitive, but nothing like testing and data to show up intuition from time to time. Engineers who work with fans (and propellers?) must have data on all this. Flow through a tube may not be a good analogy (leaving aside the fan housing, which is a tube, even though kind of short).

The only broken fans I have seen are those damaged when trying to remove them from their shaft, or when sticking a big screwdriver through them in an attempt to hold the fan in place when tightening/loosening the big nut. Strength isn't an issue here.

[simsalabim]

Yep, but the hard part is knowing what the "optimal" air flow is for a given motor running at operating temperature and located in a given part of the world.

All I want to do is optimize my fans air-flow and/or cooling capabilities, and do it with the least amount of work imposed on the motor.

Much ado about nothing? I wouldn't be surprised, but at least we are not the only ones out there wondering. I stumbled on the link below that I thought was actually pretty interesting. I especially liked "Stripped66's" comments regarding air-flow efficiencies, blade design, belt slippage and measuring parasitic loss in fans.

TheSamba.com :: View topic - 911 shroud question

Enjoy?

[Walt Fricke]

Although they don't seem to know it, the SAMBA VW guys who are talking about Porsche fans drawing 25-30 horsepower are almost certainly referring (inadvertently) to the flat fans used on 935 and other air cooled turbo race motors. Those are gear driven, and I think I have seen figures like this for them (which immediately took them off my wish list for more modest 300 hp NA motors). The fellow who had worked in the fan industry, I suspect, was more on the right track before someone tossed in the higher figures. He certainly had the background to know what is what with fans.

[simsalabim]

I agree with your assessment Walt, though I did not know about the gear-driven fan system you spoke of. 25-30hp sounded way to high for our SC set-ups, at the same time, the more realistic 3-6hp? is still significant.

The Samba discussion about "Axial" fan design characteristics did prove helpful though, as I simply seek to discover whether the fan blades surface characteristics (esp. friction co-efficient) were important to the efficiency of the fan system, and therefore the parasitic load imparted on the motor by spinning it at such relatively high rpm's.
Remember, I am only trying to come up with a practical justification for doing (as you recently observed) what I already want to do.... paint/coat the fan

So I did some more digging into optimal axial fan systems design considerations, and I believe there is some pretty compelling experimental data available that deserves - at least some - attention.

If still interested, take a look at the BASF study via the following link :

http://www2.basf.us//PLASTICSWEB/displayanyfile?id=0901a5e180004891

While a bit dry, take a look at pages 13 and 30 in particular. They address the subjects of surface conditions (as they relate to air friction on the blades), as well as the energy needed to drive different materials and configurations of fan blades. In addition to air friction, blade edge profiles and curvature tolerances seem to be quite influencial to the discussion, and some materials lend themselves much better than others in this regard.

I may be wrong, but I suspect it might be a worthwhile endeavor for someone out there to design a more efficient replacement fan and/or shroud system for our cars. In any case, I think after a few drinks I might be able to produce a semi-viable argument as to why I decided to paint/coat my fan! and that after all, is all that really matters right?

Next week, the real fun starts...

[simsalabim]

Well, I received my replacement case and alternator shroud yesterday and took them right over to the machinist for a case hot tank and a shroud bead blast. Turns out there were a few broken studs (shipping) discovered and they will be removed and replaced with those from my old case (trans mount bolt and all four crankcase breather mount studs. Could be worse though, as the rest of the case looks good and every one of the head stud threads look clean and "unmolested". I will let the shop remove the studs and then replace with like from my old case (should I spring for new studs?).

Now I am focused on the low end rebuild details. My plan is to have both my old crank and rods checked. After a magnaflux, my hope is to simply polish the crank. The rods will be checked for resizing and/or re-wrist pin bushed. Am I missing anything? Oil pump most likely fine (after 194k miles)?

step-by-step....