Carl Fausett

Greg and others - you echo our thoughts exactly.

That fatigue crack or material failure was already in that spring.... we decided to do one more pull, this time on "driving simulation" setting, where we would spend more time in each RPM band. Instead of a 6-second pull from 3000 to 7000 rpm, the eddy current dyno would load so that the engine would spend about 20 seconds going from 3000 to 7000.

Given that we were going to Bonneville and would be spending approx 2 minutes at steady WOT, we thought this long pull might uncover something that the short pulls were not showing.

It did.

As I said, that spring was already bad, we just did not know it yet. Had we hauled it the 1531 miles from here to Wendover, it would have broken on our first run. This sucks, but that would have been worse.

Still I can be pleased at how the composite intake runners behaved, and we can see the potential is really there for us to win the thing with a 928 engine.

atb

Mike Simard suffered a broken valve failure, if you haven't already it might be worthwhle to contact him to see what he did. I'm sure he would've done a re-design of some kind before putting it all together.

jorj7

Very impressive hp. When you said you wanted to go to Bonneville, I thought you wanted to do
the Bonneville 100 Open Road Race. But this is another venue that the 928 can show it's stuff.

danglerb

Are the time line logistics totally hopeless, or is there some chance that getting ahold of suppliers first thing Tuesday, and using FedEx etc, maybe having some parts shipped directly to whatever site you would prepping the car at near the race?

Its not giving me a lot of confidence in the box of only slightly rusty used 944T springs I have sitting waiting for my next motor. I never considered springs as that fussy, maybe I should.

slate blue

Danglerb springs are super fussy, they, the good springs used in Nascar and the like, such as PSI are made from super clean Japanese Kobe steel, they are often mirror polished. Springs are highly stressed and I would be concerned about the rest of the springs in Carl's engine.

I am also concerned about why Carl said he had valve float at such low revs. It is like they are too weak or something? If they are weak maybe it was valve bounce with associated spring resonance that broke that spring. From the info given it seems more investigation is needed.

Greg

atb

I've always wondered about this.
I could see a boosted motor more likely to valve float if the boost pressure tended to suspend the valve off the cam lobe as it was closing.

BC

Correct me if I am wrong, but I believe that I read that with adding boost, the highest effective rpm before valve float will DECREASE.

So if you have a NA motor, and have valve float at 7000, then if you add a determined amount of boost, the valves will float at say - 6300. Because now what you have, depending on valve overlap is boost attempting to keep the valve open while its attempting to close.

RKD in OKC

When I had my 944 Turbo Web Cams did a computer model of the valve train for Lindsey Racing. Lindsey Racing was trying to make as much hp as they could and were trying to determine why running more than stock boost was cratering hydraulic lifters. Web Cams found that with the stock springs the intake valves started floating at 16psi of boost. The stiffer "Racing" springs started floating at 18 psi of boost. The stock boost a 944 Turbo ever should reach is about 14 psi. The 944 Turbo only has 1 valve per cylinder and they claimed with the area of the intake valve the pressure differential across the intake valve at the higher boost levels was forcing the valves open.

Since valve spring stiffness effects mechanical power loss I would assume Porsche sized the valve springs only stiff enough to run in the stock operating parameters with just a bit of safety. And therefore stiffer springs would be necessary to run boosted intake pressures.

The evidence of valve float on the turbo was that as you cranked boost above the valve float limit it would make more power, but not nearly as much per psi increase of boost as below the valve float limit.

Discovering the designed in power limitations of the 944 Turbo through mechanical failure is one of the reasons why I sold my 944 Turbo and got a normally aspirated 928.

Carl Fausett

You are correct that boost can reduce the clamping force on the valve and contribute to float at lower RPM levels than a NA motor. You are also correct that this is more previlant in a 2V motor than a 4V motor, as the sq in of the valve under face is larger.

Again, we did not engineer and build so carefully this motor and then "throw in" springs we just grabbed off the shelf. They were tested for stack height, strength in several heights, and inspected.

But I do think we got our strength calculation wrong and they are too soft and we saw some valve float.

I think what got us here was our lift is quite a bit higher than stock (.442), combined with higher boost levels (10-14 psi), and higher RPM's (7000). Any one of these 3 items will contribute to valve float and we had all 3 in unison. The valve spring rate was open to conjecture on this first application like this, and we missed it.

I know more now, and will not get it wrong a second time. We also learned that a 900 HP 928 engine is possible, and I am excited to demonstrate that.

ptuomov

What fuel are you running?

Are you using stock S4 valve springs?

EAP can compute the required valve spring loads if you input the engine data in it. It's my understanding that those simulation results are fairly accurate. Especially so if one has the cam profiles digitized, EAP can ingest digitized profile info.

Some will disagree with me on this one, but boost is not very relevant to the valve spring specification. If you think it thru stroke by stroke, float basically happens off the lobe nose or bouncing off the seat. In either case, once you take into account the increased pressure in the port AND in the cylinder, boost turns out to be not very relevant. The reciprocating mass, cam profile, and rpm are the things that really matter.

smith 928

He say,s he is using 91 octane fuel... But i thought this was a race car it might be more like 108 to 112 octane race gas

Iwanna928

Carl Awesome job on your efforts! That baby will be a beast!

mark kibort

560 then 720 and then 560 again from 4500rpm to peak hp and then 6700rpm? valve float? probably unlikely even with stock springs. Anderson has been running 7000rpm for years on stock stuff. (I think ) and I certainly have had no issues running at 6600rpm for 10 years of racing. I remember scot having a broken valve spring on his car, but we dont know how it happened. they were "special" springs and replaced the one of the two springs with a stock one. no issues since 3 years ago. could you have coil bind? valve springs are nothing reallly to experiement with if you dont know exactly what you are doing. lots of dynamics, especially with high lift cams. I dont think there is any reason to run anything but stock stuff , unless you are running 7500rpm or higher. the bmw guys run 8500rpm regularly and have no issues with stock stuff.

If you want to go 238mph, what is that in RPM? I would think its going to be near 6500rpm with the 2.2. if thats the case, you are only at 560hp, so you really cant take advantage of the 700+ hp which is sad. the easy way with this much power at 6000rpm is the GEAR the car with tires and trans to make that power hit at the top speed. am I missing something??

very cool thought

what is with this car? another ground up effort, or is this the same old white car? It looks different.

jpNcos

That's what I was wondering.
Sure looks different then the Pikes Peak looks of last year.
Real thing of beauty though.

928mac

Great work Carl, good luck on your R&D.