Max Safe RPM?

[len911]

Thinking about having my tach recalibrated to a higher RPM scale on my track car and trying to determine the maximun safe RPM on the following engine...

'86 3.2l with Cosworth 11:1 Pistons + Niresist
Pauter Rods, 930 crank
Ti retainers, Triple coil valve springs
Twing plug/ Electromotive Crankfire
Elgin 315/300 (like a GE 80)
Boat-tailed, Ported
46IDAs Webers

Putting out 260HP@6.6K and 194Ft# @6.2k measured @ wheels. HP curve was still going up straight...

Henry?

Thanks! Lou

[Henry Schmidt]

Nice work.
It sounds like you did your homework and built a very sound racing engine.
I would think this engine would be capable of sustained RPMs in the 72-7400 range, with bursts to 7600 when your gearing just doesn't reach. Now that doesn't mean you will make good usable hp up there. I would think an engine that size might suffer from Webers.
Spin it and enjoy the rush.

[len911]

Henry, thanks for the response... Could you clarify your statement on the Webers and engine size? Thanks again! Lou

[Henry Schmidt]

The largest race engine Porsche ever used carburetors on was a 2.0.
They used 46 mm Webers. (42mm venturis)
Your 3.2 race engine is 50+% larger than a 2.0.
It seems reasonable to assume that the same carbs won't be optimum for both engines.
Air volume at high RPM is crucial.
For this reason Richard @ PMO builds a 50 mm carb.

[JeremyD]

I think with the pauters you would be safe up to 8k easy - as Henry said though - you'll be running out of gas before that happens. = and don't want to create an over lean situation.

I would dyno before I started establishing a rev limit. Look at your afr and decide from there = I would think that would be your limiting factor.

[KobaltBlau]

What are "triple coil valve springs"? Who are they made by?

Thanks,

[jluetjen]

Henry's the Pro, but given the engine spec that you described, I'd be kind of concerned about the 70.4 mm crank at speeds above 7500 RPM -- unless you have already taken special steps in it's preparation to ensure that it can stay alive above that speed. Porsche had a heck of a time making the transition from 66 mm to 70.4 mm stroke for race engines because of crank issues. They even came up with the "Short Stroke" 2.5 liter engine to band-aid the solution until they came up with the fix which involved some redesign of the transitions adjacent to the main bearing surfaces (Henry most likely knows the details better then I do).

Assuming that you have taken steps to keep your bottom end alive at engine speeds of 7500 RPM+, it generally seems as if the next gating factors is valve float -- which it looks like you have addressed.

Then you'll just need do some dyno pulls up to 8000 RPM (or maybe a hair more -- once), and see where the HP peaks. If you want to, you can calculate the torque at the wheels in each gear based on the torque curve, and that will tell you the optimum shift points. Or you can just figure 500 RPM past the peak HP engine speed which is a reasonable rule-of-thumb. I agree with Henry, based on my calculations -- chances are the carbs will be restricting the airflow before you get to 8000 RPM. When that happens, you'll see the HP drop off with a slope of about -1, so HP will drop off proportionally with the increase in rev's. That's a sure sign of an air-restricted engine.

BTW -- For your engine spec I'd really be targeting almost 100 HP/liter. So there should really be more in there.

[KobaltBlau]

You mean 74.4 crank, John? That's what he has as far as I can tell.

[Henry Schmidt]

The 3.3 turbo crank is a full counter weighted, large journal crank capable of 8000+ RPM in my opinion.
The failures at high RPM seems to be attributed to rod failure. The Pauter rod is as good as any ( I know, here comes Snowman with Carrillo, Carillo, Carillo) and better than most so I don't see a problem other than rod angularity caused by too short a rod. Excess rod angularity generally causes increased friction (heat) and premature ring wear but not crank failure. This excess rod angularity also causes some concern when we start thinking about piston dwell but also not a cause of failure just poor combustion.
When I said 72-7400 rpm my thinking was it would not fail up there. It is also my opinion that it won't make horse power up there.

Just a point of fact:
The racing version of the 70.4 (935, 962) crank looks exactly like the 3.3 ( full counter weights) but has 3.0 journals.
The race engines with 74.4 cranks ran rod journals the same size as 3.0. That why we build all racing engine that have 74.4 cranks (3.2, 3.3, 3.4, 3.5, 3.6 & 3.8) with some version of the 3.0 rod. By the way out a single crank failure.

[len911]

Gentleman, you have all kinds of knowedge! Thanks for all the feedback! Some history on the engine... I bought it disassembled about 5 years ago from a fellow outside of Boston. Got it for$3600! All that was missing were the rods... Was recently put together and somehow it had spun a rod. Fellow provided a 930 crank with it and it also has a 964 oil pump.

It is currently used on a DE car so I will not be doing any racing... My question was to make sure that I did not over-rev it. Seems that keeping to 7200 will be a safe bet. No need to jeopardize my baby!

I have not been able to find who manufactured the springs. I have searched the web to no avail... They have two springs just like the OEMs, but in addition, there is a third, flat spring wound between the two... Including pictures.

Once again, my thanks. You guys are the best! Lou

[Steve@Rennsport]

Brother Henry is right on about these cranks (and Pauter's).

Properly prepared, these things hold up very well beyond 8K. ALL 911 engines suffer from insufficient oil flow to the center rod bearings and if the correct measures & preparation are completed, these can be quite durable.

FWIW, I've seen both Pauter's and Carrillo's survive beyond 10K RPM due to missed shifts and continue in service (after X-ray & magnaflux NDT) for years. You certainly cannot do that with a factory or Titanium rod. We use both interchangably depending on availabilities.

I'd also affirm that the long-stroke motors suffer from some less than ideal rod ratios and that translates to more heat & piston/ring wear, especially using JE's,.... We've done some long-rod motors that did very very well but thats another story.

[Porschekid962]

To Steve and Heny in no particular order, if you had your choice for a rather high rpm motor with close to F1 stroke to bore ratio what kind of rods would you go with, pauter or carillo? I know the valvetrain needs to be taken care of but what would you do on the bottom end short of one off titanium rods? Pauter or Carillo? I have heard conflicting stories towards both.

[Porschekid962]

What does everyone think about this though.........

930/52 case, 66mm crank, gt3r oil pump, pauter I beam rods, racing springs and Ti retainers, 100mm P and C's about 11:1 CR, twin plug, sprint style cams, MFI, bells and whistles, bla bla bla. Sound cool? Too over the top?

[Henry Schmidt]

Originally posted by Porschekid962
To Steve and Henry in no particular order, if you had your choice for a rather high rpm motor with close to F1 stroke to bore ratio what kind of rods would you go with, pauter or carillo? I know the valvetrain needs to be taken care of but what would you do on the bottom end short of one off titanium rods? Pauter or Carillo? I have heard conflicting stories towards both.

Crower, Eagle, Pauter or Carillo. The brand wouldn't matter if the company was willing to build a rod that I designed. As of now, non are willing.

[QUOTE]Originally posted by Porschekid962
930/52 case, 66mm crank, gt3r oil pump, pauter I beam rods, racing springs and Ti retainers, 100mm P and C's about 11:1 CR, twin plug, sprint style cams, MFI, bells and whistles, bla bla bla. Sound cool? Too over the top?[/QUOTE

This engine sounds cool, but if we're just dreaming why not a 961 crank in a 964 case with 102 cylinders?

[Porschekid962]

Well I am not dreaming haha, at least not completely, the old engine ideas I had were just shot out the window with dad's latest business trip, I.E. 993S on the way so no need for a hot rod motor which means I dont get his 78 SC mill. Your engine idea does sound like loads of revs but how easy is it to get hold of a 961 crank? I am sure LN could cook up some 102mm cylinders for a 930/52 case this could be fun and loads of cash that could go elsewhere in the car.

Turbo's keep sounding like much more fun haha.

Junior evil mind at work. Ryder

edit: Henry why is no one willing to build rods for you? Something too radical in the design?

[Henry Schmidt]

It's just a matter of quantity and my willingness to pay a premium for one or two sets.
If you have the money people will build anything. But if they're too expensive it would be a hard product for me to sell.

[HawgRyder]

Henry...I have in posession one rod (1 only) that was made by MJD whoever that is.
The rod is made of a composite material (fiberglass? / carbon?) for a small block chevy.
Because I only have one rod, I have never used it.
I wonder if the rod is a viable piece or just someone's idea of a joke.
It's been in my box for more than 20 years and to this day I can't help but wonder if the technology might have advanced to the point of building these things in this era.
Bob

[Henry Schmidt]

Wow, that sound cool.
I am not an engineer nor do I have any composite experience so I'm not the guy to ask.
I know that technology is moving quickly {way quicker than me} so composite rods are coming if they are not already here.

[jluetjen]

I don't know who MJD is, but I do know that a composite motor has been built and raced before. I love how they trimmed 200 lbs ( ) off of a Ford 2.3 liter 4-cylinder -- leaving an engine that weighs less then me! I suspect that the technology can't support a 100K mile engine yet which is why we're not seeing them on the road.

[HawgRyder]

I have seen composites used in several places in equipment and one of the good things is ....no stretch...
If it stays together it's fine.
If you exceed the parameters....it gone so quick it's only a memory.
Bob