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I've seen your dyno curve and it's like damn, that thing would make insane power for another 1000rpm at least.
How often do the stroker blocks break? Is this because of the longer stroke or because of something else?
Your spacer: Are you talking about just the oil pan spacer ring or a whole web a la Simard?
I don't know how often the stroker blocks break. Marc Thomas told me the experimental engines made by Bob DeVore would break cranks because the blocks flexed and then the cranks would break. They went to an 8 cwt crank and didn't have a problem after that. Bob DeVore & Lucky Ekman were in communications with the Porsche engineers and related that information to them. The GTS came out with an 8 cwt crank. A few years back a race prep company built up a fierce 928 stroker motor with ITB intake, big cams, and intended to put it in a 944. I believe that engine made about 700 crank hp on an engine dyno. Before testing was completed, the (6cwt) crank broke and the project was shelved. I don't know enough to get into a debate over 8cwt vs 6 cwt stroker cranks. I may remember one block that broke with an 8 cwt crank. Not sure. The crack will occur at the top of one of the center main bearings and progress upward to the bottom of the piston bores. 928 blocks aren't as strong in that area as 944 blocks.
With this in mind, and desiring to keep the oil as far from the rotating assembly as possible, I made the sump spacer. I figured I could bolt the sump cover tightly to the block for better rigidity. Normally, with the sump gasket, the 6mm bolts aren't torqued tightly, but with the solid spacer you could torque them to the 6mm spec. To do it over again, I'd tap the 6mm holes out to 7mm (same thread pitch) and get even more clamping force. Use studs rather than bolts. I used Yamabond 4 sealer on the surfaces and sump/block leaking hasn't happened. About this time Mike was building his motor and took my suggestion and made better (webbed) spacers of his own. One hangs on my shop wall waiting for the next engine build. Greg Brown had a RL thread regarding block breakings. I suppose you could search for it. I mentioned the solid sump spacer for strengthening the block to Greg and I believe he uses them now too.
I think on the 928, continued high rpm use may not put a rod out through the side, but may cause the block to crack. I don't think a block has cracked that has used the solid sump spacer, but they haven't been in use a long time either.
Like John Veninger, I don't have enough money, or time, to explore the high RPM limit with this engine.
8SR-14 for 944 performance/928 race
..... Needs custom Motronic chip or aftermarket engine management....
B13-10, B13-14 for 944 race/928 race
.... Motronic will not work....
Race Solid Lifter RS 304M2/280M-10 for 944/928
.... Motronic will not work. After market engine management only...
I'm curious as to the "Motronic will not work" statement. Setting aside that the 928 doesn't use Motronic, but assuming SharkTuner-ability (fuel and timing), why would an after-market management system be necessary with some changes to cams and lifters and whatnot? I can easily see our LH/EZK system simply running out of cycles to move bits around at higher rpms, but other than that?
I'm curious as to the "Motronic will not work" statement. Setting aside that the 928 doesn't use Motronic, but assuming SharkTuner-ability (fuel and timing), why would an after-market management system be necessary with some changes to cams and lifters and whatnot? I can easily see our LH/EZK system simply running out of cycles to move bits around at higher rpms, but other than that?
I believe the issue is related to the air flow during intake and exhaust overlap, screws up the air metering, which screws up the A/F.
Also I think Jon says Motronic but means Bosch LHjet.
these are the improved lightweight hydralic lifters that many engine builders use..... 034 motorsports says they are good to 8300rpm.....assuming thats true why not use them? Just for the ease of maintainance? Cams are a limiting factor since the most we can run on hydraulic lifters is around .500... for the 32V heads
On a purpose built engine...you can design it however you want....if you want a high RPM screamer, then spend the $$ on lightweight crank-Ti rods and lightweight pistons, dry sump with a better breather....maybe solid lifters if you want 9000rpm....
It all comes down to $$$$$ an long term reliability......the new sleeved huge displacement motors are still being tested-proven....the exsisting 6.4-5L strokers have been around for a long time and are race proven reliable in the under 7000rpm range using "standard" parts (not lightweight Ti stuff).....since they can make nearly 600whp on street gas.....why go higher?
Some interesting background info relating to these revlimiter settings of high rpm engines.. The standard EZ-K (87+ models with knock sensing units) runs out of processing power somewhere in the 7100-7200 rpm range and reboots itself. It's interesting to be learning this in 2020 and looking back at which redline rpms have been mostly "aspirational".
It has turned out that there are in fact a large number of things that one has to do to get an S4 engine to run at 8000 rpm and even more things to do to get it do so reliably.
Originally Posted by GregBBRD
I generally move the rev limiter to 7500 when I tune, on my custom engines, although there is little reason to rev these engines this hard. We are severely "camshaft" limited and most of the engines are well past the place where it makes sense to shift, long before this.
Originally Posted by Louie928
My "soft" rev limiter is set to 6600 RPM and I hit it sometimes, but try not to. Steel rods, no special lightweight internal parts. HP curve is still climbing at 6600, but I don't want to risk breaking anything by going higher to find the peak. The "stroker" engine blocks do have a tendency to crack. The solid spacer between the engine block and oil sump I devised should help stiffen the block, but I don't have the funds to be experimental. Don Hanson with the same internals except he had Ti rods ran his to 7200 RPM.
Originally Posted by Brett928S2
Hi Standard 928 S2 engine - 310hp (no work done and 142,000 miles) Rev limiter moved up to 9710 by John Speake. Have run at over 8000 a few times on Nitrous...but only for 10 to 15 seconds a time....no problems at all.. My shift lights are set at 7500 so I can shift then if I wish... I seem to gain no more power over around 7800 so I usually shift at somewhere around 7700 ish... All the best Brett
Some interesting background info relating to these revlimiter settings of high rpm engines.. The standard EZ-K (87+ models with knock sensing units) runs out of processing power somewhere in the 7100-7200 rpm range and reboots itself. It's interesting to be learning this in 2020 and looking back at which redline rpms have been mostly "aspirational".
It has turned out that there are in fact a large number of things that one has to do to get an S4 engine to run at 8000 rpm and even more things to do to get it do so reliably.
RPM changes, in an engine, are very complex.
Bill Dailey (Dailey Dry Sump fame) points to the issues with this thought, when discussing crankshaft drilling:
"Oil supply to a crankshaft is never an automatic thing, where one thing always works. I've seen engines that were limited to a certain RPM, which lasted for years on end. The owners decide to raise the rpm limit a few hundred rpms, with internal parts and clearances exactly the same, and suddenly they can't keep their engines from having bearing failures."
Most of the "old school" big block Chevy performance crankshafts were cross drilled. These engines were designed for use in the 6500 maximum rpm range. Raising the rpm limits to 7500 rpms, on these engines, generally eliminates the possibility of running a cross drilled crankshaft.
Most of my current crankshafts, rods, and pistons are designed to be able to run past easily past 8,000rpms. (The exception being the 95.25mm stroke crankshafts, rods, and pistons.)
However, to this point, I've not found a camshaft that is able to run "productively" past ~7200 rpms. (Naturally aspirated.)
(I am currently building a slightly "shorter" stroke crankshaft engine, with a more aggressive camshaft, that we intent on running to 7800 rpms.)
That being said, when an engine has over 500 ft.lbs. of torque from 4500 to 7000 rpms, running over 7200 is somewhat moot. Even when shifting any of the manual 928 transmissions, at 7200 rpms, with an engine with that makes that kind of torque, you are always going to be right back into the middle of the "high torque range" in the next gear.
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
The stock 928/944/951/968 crankshafts have no problem whatsoever spinning reliably at 8000 rpm. They all have about 70mm /2.75” mains and cross drilled oil passages. They just need clean oil and enough oil pressure to run reliably at 8000 rpm.
What is enough oil pressure? Turns out, the very high stock 928 oil pressure is enough as long as there’s a constant supply of clean oil. This is based, first, on centrifugal force computations and, second, on observing what has been done with normally-aspirated 968 engines with offset ground crankshafts:
The above 968 engine runs at 8200 rpm in this experiment with 87 psi oil pressure.
“Here are a few more pics of the engine on the dyno, and a few from inspection after a ton of pulls to 8100 and one to 8500. I don't think you can tell the rod bearing condition from the photo, but they look new.”
As a reference, the 928 nominal oil pressure spec is 8 bar / 116 psi at high rpms and can be shimmed to 10 bar / 145 psi if one wants the engine to live at the border of an aneurism and stroke.
If there is a clean supply of oil, all that is needed from the 928 S4 crankshaft is what it already has completely stock.
With clean oil supply, there’s no power or reliability benefits from drilling the crankshaft “straight shot like Chevy” unless one simultaneously re-engineers the whole engine to run a lower oil pressure and smaller volume oil pump to pick up a couple of hp. Even the dry-sumped 928 race engines that I’ve seen run the usual nominal 928 oil pressure and therefore would be indifferent between stock-style, straight shot, or nose-drilled F1 oiling system. No benefit whatsoever.
As far as the crankshaft oiling is concerned, 100% of the 8000 rpm 928 engine required improvement are related to supplying the block main oil valley with clean oil at the standard oil pressure or dealing with the consequences when this supply falls short.
Another conclusion that we've come to, at least for our use cases, is that usable rpm band that ends at 8000rpm requires 1.2mm or 1mm rings even with stock S4 stroke. With longer than S4 stock stroke, you'll be looking at 1mm rings (since there isn't that good of selection of sub 1mm width rings yet). The physics of the ring issue are in principle simple, the intertia of the ring can't overcome the gas load. Working it thru, each stroke and rpm combination has a maximum ring width assuming conventional materials.
The next issue is that if you are going to make any sort of power with the 1mm rings in a 100mm (or larger) piston, there's a piston cooling problem. Our conclusion is that high rpm engines that need thin rings also need piston oil squirters. John Kuhn fabricated the tools to drill the piston oil squirter housings to any S4 (etc.) block.
The squirters create two interesting issues. First, in order to get light but stable pistons, one needs "Evotec" or box-in-box style pistons with very short and stocky pins. The stock 928 squirter angles aren't ideal for those, so one needs adjustable squirter location and angle tooling. Second, one needs to control windage very well at 8000rpm. One key to windage control that we've found is that the cylinders (4&8) next to the bell housing have to be bored in a certain way with a fixture, otherwise the end cylinders distort when the head bolts are torqued. Another issue is that there's a conflict between the piston to bore clearance and piston ring end gap objectives, high rpms and heat want them loose, the windage control wants them tight.
It's an endless string of little things impacting other little things with an 8000rpm 928 engine. The variable cost for another such engine isn't a stratospheric, but the required R&D amortized over a small number of engines tends to be.
Like how do you think the air conditioning compressor pulley and belt system responds to sustained moves up to 8000 rpm?
You definitely will want to run the smaller A/C drive pulley. (Most GT's had them, sometimes find one on a regular 5 speed.)
The Denso A/C compressors don't seem to like 7200 with the bigger "standard" pulley.
We blew up a couple of compressors (even a brand new one), learning this lesson.
And alternators - there's a lot of spinning rotor winding that doesn't want too many excess RPM's before it will lose its shape & self destruct against the stator. Yet we need it to spin fast enough to make decent power at idle.
And alternators - there's a lot of spinning rotor winding that doesn't want too many excess RPM's before it will lose its shape & self destruct against the stator. Yet we need it to spin fast enough to make decent power at idle.
Alan
My "updated" high output alternator starts life as a 997 unit. I've had no issues with them turning high rpms.
08-15-2009, 02:39 PM
