Case and crank mods: are they worth it?

[Rob 930]

I have my 930 engine apart. I'm converting it to EFI but I'm also making some significant mechanical upgrades "while I'm at it."
The engine will be primarily for the track, running between 4K and 7K rpm, with the potential for 600 hp or more at 1 bar. I wasn't going to re-do the bottom end, since I'd just been through it a few hundred miles ago, but I'm thinking it's worth a look, since I've got it down to that point anyway. I want to do what's necessary and advisable for this engine without overkilling on these options (and of course, I really want to avoid something that's actually detrimental). From various sources, I've had suggestions for doing some or all of the following mods:

1) Cross drill the crank center main bearing and groove it for better oiling of No. 2 and No. 5 rod bearings
2) Machine rod journals .001 undersized (to increase bearing clearance from .0015-.002" to .0025-.003," for better high rpm oiling)
3) Have crank re-heat treated and clean out oil galleries afterward if the above mods are done
4) Shuffle pin the case
5) Boat tail the main journal webs and knife edge the crank (to reduce windage)
6) Moon the cylinder spigots where they protrude into the case (to reduce windage)
7) Replace oil squiters with 993TT or 964 oil squirters (more flow?)

I'm not interested in mods that only benefit high rpm engines, but it may be that a high horsepower engine also puts stresses on the oiling that may make some of these sensible. I'll do what needs doing but I hate to waste money on mods that produce only bragging rights. For example, if the windage mods only make a 5-10 hp difference above 6000 rpm, I'd probably skip it, unless there's some other fundamental reason why it would be good to do.

Does anyone have any solid advice on how valuable (or risky) these mods are to my specific application? Any suggestions for who should do them (or who should be avoided)?

[snowman]

Rod journal clearences. Are you shure about the numbers? Some recommend scrapping anything that has 0.002" or higher clearence in the rod journals. The NASCAR guys use 0.0015" for the rods and they run a constant 9000 RPM. At least thats the info I have. ANyone else that knows whats really going on on the lube side of things? I know that it gets complicated by bearing velocity and things like this that may make the Porsche a differen't animal.

[Tinker]

My $0.02.....save your money.

If you just did the bottom end 'a few hundred miles ago' and everything is standard and in spec. you should be fine. You did replace the rod bolts and head studs?

Your proposed RPM range is entirely within the range of the stock parts. As long as your A/F ratio and timing are healthy you will be fine.

Save your money for track fees and tires.

Tinker

[Rob 930]

Yes, I had the crank checked out reasonably well 600 miles ago, and I replaced the bearings, resized the rods, and installed ARP rod bolts and head studs. But as I go through the engine again, I'm up a notch in horsepower and intensity since the last build, and I don't want to risk a lower end problem. There are lots of people who think that cross drilling the No. 5 main bearing is a good idea for a "road racing" engine such as mine. I understand the principle of doing it but I'd like feedback from those who have done it -- or who have suffered the consequences of not doing it -- to offer their opinions. Does the fact that this is a high horsepower engine, with greater bearing loads, up the ante for doing this mod (or any of the mods I proposed)?

[DonE]

High RPM's are really the problem with a reasonable oil source. Its at lower RPM's/high torque (or so called "lugging" the engine) where the pistons hammer the crank and squeeze the oil from the surfaces which starts the problem.

[Rob 930]

But what's the definition of high RPM, when does it pose a problem, and how does it manifest? Even if you never exceed 7000 RPM, but you spend a large proportion of the time running between 5500 and 6500, all the while pushing, say, 500+ hp, does that constitute a strain on the lubrication system that is beyond what a normal 930 crank/rod bearing/lube system can handle? If so, does cross-drilling (or any of the other mods) benefit an engine that's used this way?

[snowman]

I strongly suspect that it not only would be a waste of money to increase your bearing clearences from 1.5 thou to 2.5 thou, but counter productive, ie more likely to fail. at high, say 8000 to 9000 rpm.

[Rob 930]

Jack (and others),

Thanks for your thoughts.

I have no personal experience with this, but there is a precedent for increasing clearance on high RPM motors, and there's theory to back it up why. By I'm really less concerned about the bearing clearance issue, though, because I will probably not consider increasing the clearance on my engine.

What I really want to know about is the cross drilling and grooving of the center main bearings that's often recommended on high rpm "road racing" type 911s and 930s. Apparently, because the oil in a 911 crankshaft is fed from each end, and has to negotiate a circuitous path to the rod journals as it goes from the ends of the crank to the center, there's reportedly a tendency for the middle rod journals (No. 2 and No. 5) to starve for oil. It's not a problem for a street engine in stock form, but apparently as you increase the speed, centrifugal forces tend to fight the flow of oil as it makes its way toward the center of the crank. Do you, or does anybody have any valuable experience to share that would validate whether this is a modification is worthwhile for an engine like mine. That is, an engine that will be run all of its life below the factory redline (though most of it between 4K-7K RPM), but will be modifed to produce twice the torque and horsepower of the factory engine? Does the fact that it's a high power engine produce stress on the oiling system that warrants this change, or is the need for this modification entirely driven by high RPM?

I'm not wild about cutting into a perfect crank, so I'm hoping someone will come forward with some perspective. Problem is, there aren't that many engines like mine out there being used the way I intend to use mine, so I don't have much data to draw on to make an informed decision.

[snowman]

I know one rule of thumb for a race motor is to have a mininum of 10lbs oil pressure for every 1000 RPM, e.g. 8,000 RPM requries 80 lb oil pressure mininum.

[David]

I'll throw in a few comments:

On #1 & 2, never crossed my mind, but I wouldn't have done it anyway.

#3 what's wrong with factory heat treating unless the crank has reached a temperature higher than it's original tempering temp?

#4, I didn't since I'll only do a few DE's and it's an aluminum case.

#5 & 6, I didn't do it on my rebuild because I figured a high HP 930 case needed as much material as possible. I did radius the edges of the main journal webs a little since I was in there.

#7, I didn't and I hope I don't regret it, but I do have a better intake and oil cooling system than stock.

[Rob 930]

David,

The difference may be that I'm building a road race engine, for all practical purposes. All of these mods are for high rpm, if not high load applications, as I understand it. Cross drilling is commonly done to 911 crankshafts for this use. But every engine I'm aware that got a cross drilled crank was for high rpm use -- over 7000. The reason for #3 is only if you disrupt the crank by having done #1 and 2. The shuffle pinning is the other thing I'm concerned about. I understand that 930s have been known to show fretting where the main bearing webs contact each other near the head stud. I'm not sure if this portends bad things or needs to be addressed, so I'm still hoping someone who knows more will speak up.

[350HP930]

Leave it as it is. The only thing on your list worth doing is the center bearing oil feed mod, but it doesn't make a lot of sense on a motor that is not running at redline all the time.

BTW - It sounds like you and I are headed down the same path as far as mods go. If you have not already seen it you might want to check out the thread link in my sig.

[snowman]

I also thing you are worried about the wrong things in an air cooled 600HP Porsche engine. The most significant problem you will have is melting the pistons.

[Rob 930]

Jack,

With all due respect, thanks for your suggestions, but the last few do not address my questions, which are very specific. I'll "worry" about the things that I don't already have covered. The top end is handled. Piston melting won't be an issue, or not one I'm looking for counsel on. My questions are focused only the bottom end.

350 hp 930,

Thanks, I've seen your site. Great work -- and very resourceful. I wish I had your abilities and resources for machining. Regarding your advice, though, what is the basis for your suggestions? I know others who have shuffle pinned, for example, and still others who have suggested that I do. I'm looking not just for opinions that this or that should be done (I've already gotten those!) but some sort of basis for the opinions so I can evaluate their validity -- just as you would require. I don't mean this negatively, I just want to understand the nuances before I do any cutting. I'm not going to blindly start modifying my crank or case without a clear understanding of what it all means, and I'm looking for someone with higher order knowledge to chime in, if they exist. Here are some examples: on the cross drilling, for instance, why do you say what you do? Where's the threshold where it becomes necessary, if ever? Is it based on RPM only, or on engine torque and RPM? (I *will* be running near the redline all the time.) And what about case squirm on an engine like this? Isn't an engine like this more prone to loads on the bearing webs? Does that justify shuffle pinning? Are there any drawbacks to shuffle pinning other than the cost? Do the extra loads on the case from a high horsepower engine create a problem if you start boat tailing the main bearing webs? (Does boat tailing compromise the case strength in any significant way?) Is boat tailing necssary or advisable (or inadvisable) for an engine in this category. Again, any drawbacks other than cost *for this application?*

That's the level of information and justification I'm after. Maybe I'm asking for too much and I won't find it here...

[snowman]

If you want dependable answers on your questions I would suggest visiting as many of the well known Porsche machine shops as you can and talk to the machinist. For example Ollies in Santa Anna, CA or see past postings on Porsche machine shops for a complete list. These are the only people who "know" whats what on many many engines over many years. And visit at least 3 shops because you will still get conflicting info. You will have to use your own judgement on who is telling the best story. Also talk directly to the authors of 2 or three of the high perf books that have been published, they have extensive hands on and are typically engineers that can fully explain why they did what they did. These guys tend to be available and quite talkative if you give them a call.

On shuffle pining I can tell you that most everyone running a real race engine has probably done it. I have been told by a couple of the machine shops that at high RPM and high HP (they tend to go togather) meaning over 300 to 350 HP that shuffle pining is highly advised and over 400HP required. Again not direct evidence you want but even if you can't get such and answer it seems to be something that needs to be done. If you look at the web holding the crank and ask what holds this thing togather you can see that the bolts are to far away from the crank to prevent flexing the web. The web is thin and if you have an old case(one that you don't care if you break) you can pry on it and see that it is very flexible. You could probably even do calcs on the thing to see how much it will flex, but its more practical to just to pin it at that point.

On cross drilling the crank and questions like it you are going to find that there is no specific RPM or well defined thresholds anywhere only A. its a street car, so not needed, B its a race car, so do it.

On the off subject of melting pistons, I have been on the race track with quite a few 600HP Porsches and ALL of them (including the ones with piston coatings) had a pretty silver lining on the inside of their exhaust pipes. Guess where that pretty silver came from. And no it wasn't there when they first started or some kind of header coating.

[Rob 930]

Jack,

Thank you. Your additional detail was helpful. I've come to the same conclusion you have about seeking advice from machine shops, and in fact it was a call to Ollies that started all this. That list of mods at the beginning of the thread was suggested by them. I came here to see what others thought about that list.

A philosophical perspective:

Problem is - and this is true for more than just machine shops (and I'm not picking on Ollies by any means) - it's sometimes difficult to distinguish sound advice from either 1) commercial opportunism (they sell the work and promote it as necessary so you'll buy it or 2) liability concerns (not necessarily threat of a lawsuit, but overly conservative recommendations that only the highest 2% of engines need, and yours probably doesn't, but they don't have enough experience to risk suggesting anything less than the most thorough approach). I understand both influences, and I accept that they exist; it's the nature of business in general. Either way, though, to accept such advice costs money, sometimes big money, for work that may be needless. That's why I come to this forum, in hopes of finding hobbyists (like myself) who may have more experience in certain areas and are willing to share real documentable advice, backed up by something tangible. But it's a rare person who may really be able to counsel me on this.

I can certainly call a number of machinists, seek their advice, evaluate what I think I need, and make a decision. I've done that before, and that's probably what I'll do again.

On the subject of piston melting, I'd like to think that with a conservative A/F ratio, modest boost level, adequate intercooler, sufficient octane, and a properly (dyno) tuned EFI system, there will be no issues with piston melting. The key is in the tuning, which is something that I won't scrimp on at all. I know of a number of EFI turbos on the race track, turning lots of hp that run year after year with no such problem. It's the wanna-be racers, with maxed out CIS systems or seat-of-the pants tuning that get themselves in trouble.

[350HP930]

If you were trying to squeeze a lot of power out of an old mag case motor there is a good argument for shuffle pinning. Those cases are notoriously weak and like to warp all to hell.

Shuffle pinnin a 930 case is overkill IMHO.

Here is the reason why the center oiling is suggested on high rev porsche cranks. Normal 911 motors feed oil to all the rods from only the first and last main bearing. Thats normally not much of a problem since the passages can provide more oil than the rods will ever need.

The problem at high revs are the centerfugal forces the oil experiences inside the crank that makes it harder for it to take the long zig-zag route to the 2 and 5 rod journals. Since the rods need more oil the faster they spin its a pure RPM related problem.

Without the extra supply of oil from the center bearing an engine that spends a lot of time above 7K RPM is likely to toast those bearings. A cheaper solution if you are not going past 7K RPM is to just bump your oil pressure a little by shimming your oil pressure regulator spring.

So thats the reason why I say this is a reasonable mod to suggest, but not neccessary either. The other suggested mods I think can be attributed to paranoia or bill padding.

2 & 3 on your list are really bad advice IMHO. If you have an in spec STD/STD crank don't mess with it with the exception of getting it cleaned and polished. I would avoid the shop that made those suggestions.

[dtw]

Rob,
I won't weigh in on your questions as my experience is just sorta 'textbook'. But I can provide names of two people I'd trust to give solid advice without taking advantage of you. Henry Schmidt of Supertec Performance has proven his experience and ethics time and again right here on this board, and he's been a very straight shooter with me on our handful of phone conversations. Do a search on him here on this board.

The other guy is Dave Brown of Speedwerks. He's done a lot of work for me and several close friends of mine. His work is outstanding and he won't sell you work you don't need. His number is 336.475.6570.

[snowman]

If you got advice from Ollies be assured its not to sell anything. They do not do that, period. THey get more work than they can handle, and probably turn down work because they are very very busy. I have personally talked to George and can assure you he is purly a tech kind of guy, if he says it needs it , it needs it.

On the subject of piston melting. Its simple physics, so much HP makes so much heat. Air cooled engines CANNOT get rid of that much heat in the size constraint that the Porsche is built in, so something has to give. THe Porsche factory refused to ever make an air cooled engine over 300HP, why? because they could not cool it, once they went to water cooled heads its another story. You can build it and you will soon know the truth. But as long as you accept the frequent engine rebuild required by a 600 HP engine, you will know the other truth, you get what you pay for. And if any shop says otherwise , get it in writing, cause your being suckered.

[beepbeep]

Originally posted by snowman
ITHe Porsche factory refused to ever make an air cooled engine over 300HP, why?

930S had 330hp, 964 3.6T had 360hp, 993TT had 404hp etc. All of them were street aircooled engines. I won't even go into aircooled race engines ...

Yes, aircooled heads are hard to keep cool, but 3.3 930 engine has very conservative C/R of 7:1 and it won't knock as long as timing, fuel and boost are in margins.

This "there are no aircooled engines over 300hp, all turbocharged race engines knock and subsequently require rebuild" are myths.

Properly tuned engine will never knock. If it does, it won't put "silver lining" inside exhaust system but probably cracks first compression piston to begin with...next step is eroded/burned piston edges.

Aircooled 911 engine will ping sooner than 4-valve pentroof watercooled engine and thus cannot be boosted as much...BUT engine should be tuned so it won't ping under worst possible conditions. Failure to do so will destroy the engine...aircooled or not.


Unfortunately, I don't know enough about 911 oiling system to be able to give any advice to thread starter. I appologize for hi-jacking the thread.