8 cylinder front engine iconic vehicle
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By Herr-Kuhn
#151832
We have been doing some head and valve train work on the 928S4 heads and I have come to the conclusion that any moderately boosted 928S4 should have a valve spring upgrade. We have identified three viable twin spring options and as well, some retainer options for these engine that are mostly "off the shelf". All of these options stem from the spring options which are available for Audi I5 (AAN,3B, 7A) and include springs from Supertech, Ferrea and Autotech. There are some tricks to making it all work properly which in my case will likely include custom spring perches. For the record, on the black twin turbo car we ran the PAC1223L springs on top of custom made perches with over 90 lbf on the intake seats and over 100 lbf on the exhaust seats. While this may seem high that engine was spun all the way to 8,000 RPM and never missed a beat. Prior to this we were running with about 68 lbf on the intakes and that was inadequate for manifold pressures over about 16 psig where the engine would nicely lay out 725-740 RWHP on pump gas. With the upgraded seat forces and more manifold pressure that engine put down 850 RWHP on 21 psig, again on pump gasoline. That engine was running an enhanced S3 profile done by Elgin. I think the key here is to really know where you plan to run the engine and how much manifold pressure is going to be run before you select a cam and spring setup. The spring forces become real important to know when higher lift cams are utilized. In this regard we can learn a lot by looking at what sort of springs are being run on the I5 and I4 stuff from VW/Audi. I'm embarking on a full engine build for my personal car and we will be doing some mild porting, potentially a semi custom set of cams and probably a bit on intake work. I've learned over the years that a pair of GT3576R turbos can really get things breathing super heavy and one has to be careful on the setup. One of the most overlooked areas on any high performance engine build is the valve train. More information as it becomes available.
worf, JMW928, MattiasH and 1 others liked this
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By Strosek Ultra
#162261
This is what I once posted on Rennlist about valve spring pressure.

How to calculate the required valve spring pressure for a modified engine.
image.png
I am not a specialist in this area but this is how I do it. The calculation is made for an 928 engine modified for higher power output. The camshafts acts directly on the lifter. Calculations for an engine having rocker arms will be more complicated.
For the calculation, the maximum deceleration at the nose of the cam lobe, a figure that every camshaft grinder can and should supply, is needed. Through careful measurement and calculation, the maximum deceleration can be found, but that requires measuring equipment which I do not have access to.
However, the maximum deceleration is usually between 0,0000045 to 0,0000075 mm per degree per degree (=per degree2) for most camshafts.
For the below calculation I assume a deceleration of 0.0000075 mm per degree2.
The total mass of the valve train is 150 grams or 0,150 kg. which includes 37 mm light-weight solid motorcycle lifters and oversize 42/36 mm valves.
Maximum safe rpm is sat to 9000 rpm which is 75 rps at the cam.
Gravity is 9.81 m/s2.
The rate of deceleration at 9000 rpm will be 0.0000075 x (75 x 360) 2 = 5468 m/s2.
Open valve spring pressure is calculated as (0.150 x 5468) / 9.81 = 83,6 kp.
Add 20% as a safety margin = 100,3 kp (open spring pressure)
Closed spring pressure is obtained by the spring rate times the valve lift. The beehive valve springs for my engine have an open pressure of 98 kp at 12,5mm of lift. The spring rate is 4,91 kp per mm.
Closed spring pressure is calculated to 98 - (12,5 x 4,91) = 36,6 kp which is very close to the measured value of 38 kp.
I make my on titanium retainers weighing 9 grams. A steel retainer would be about 15 grams. Not much of a difference but the stock steel retainer do not fit the beehive racing springs.
The 928 stock valve train including hydraulic lifters filled with oil is appr. 183 grams.
I have measured the stock S4 valve springs and found the open spring pressure to be 72 kp for both intake and exhaust. The closed spring pressure is 32 kp for intake and 36 kp for exhaust respectively.
Åke
https://rennlist.com/forums/928-forum/1 ... ngine.html
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By Herr-Kuhn
#164568
On the black car we used about 95-105 lbs on the seats. We did this to make certain the springs got enough on the seats plus got somewhat close to coil bind to make the beehive concept work. That really worked quite well as the engine was taken all the way to 8K rpm without valvetrain float. Those were Elgin cams.

On my personal car I have plans to go with a twin spring, but I have not yet determined which one. I have options from Ferrea, Supertech, and Autotech. Pretty sure the Ferrea and the Autotech are both made by PAC racing springs. Both of those are line contact inner to outer which is supposed to ward off resonance even better than just with two non contacting springs. I will be doing some mild port work on my personal car in the coming months. The new business has me really, really busy. I have a show to attend in the UK in 4 weeks and heaps of work before and after that so my available time to tinker on cars is very limited.

I'm not exactly sure which way Tuomo will be going on the transmission. I think he is looking heavily at the Tremec option. I mentioned that if there is a way to graft a 928 Automatic's ring and pinion to something else that it might be a good way to go to avoid having to cut the rear subframe...that's a real negative to that Tremec installation in my opinion. I think he used what I call the "swinging dick" boost setting on the car a lot with very high RPMs. I need to see if he can get me images as to how the failure happened. I'm still not convinced that something in the forks didn't get broken, but I just don't know. I know the car was very hard to keep up with in terms of driving it on the street with over 800 WHP. My wife kept yelling at me saying the acceleration was hurting her pony tail because it kept hammering us back into the seats...the damn thing just keeps pulling and pulling and pulling.
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By maddog2020
#164602
the rear crossmember is actually a pretty good design in my opinion. it was just the lazy crappy assemble and welds that really make it sketchy as hell, as it was sold. I had mine rewelded and some additional boxing to make it more sturdy.
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By Herr-Kuhn
#164703
My only point is I believe maintaining the crossmember integrity is a big plus. There's a way to do it, it just hasn't been done yet. I swore off all sink parts in my shop a long time ago. :roflmao:
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By Strosek Ultra
#166868
Herr-Kuhn wrote: Fri Aug 05, 2022 4:45 pm On the black car we used about 95-105 lbs on the seats. We did this to make certain the springs got enough on the seats plus got somewhat close to coil bind to make the beehive concept work. That really worked quite well as the engine was taken all the way to 8K rpm without valvetrain float. Those were Elgin cams.

On my personal car I have plans to go with a twin spring, but I have not yet determined which one. I have options from Ferrea, Supertech, and Autotech. Pretty sure the Ferrea and the Autotech are both made by PAC racing springs. Both of those are line contact inner to outer which is supposed to ward off resonance even better than just with two non contacting springs. I will be doing some mild port work on my personal car in the coming months. The new business has me really, really busy. I have a show to attend in the UK in 4 weeks and heaps of work before and after that so my available time to tinker on cars is very limited.

I'm not exactly sure which way Tuomo will be going on the transmission. I think he is looking heavily at the Tremec option. I mentioned that if there is a way to graft a 928 Automatic's ring and pinion to something else that it might be a good way to go to avoid having to cut the rear subframe...that's a real negative to that Tremec installation in my opinion. I think he used what I call the "swinging dick" boost setting on the car a lot with very high RPMs. I need to see if he can get me images as to how the failure happened. I'm still not convinced that something in the forks didn't get broken, but I just don't know. I know the car was very hard to keep up with in terms of driving it on the street with over 800 WHP. My wife kept yelling at me saying the acceleration was hurting her pony tail because it kept hammering us back into the seats...the damn thing just keeps pulling and pulling and pulling.
Tuomo, is currently in his home land Finland and has imported from Germany a used Porsche 993 Turbo which he intends to hot rod to about 500 HP.
It seems that he has lost interest in the 928TT which I think is a pity.
Åke
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By worf
#166882
Strosek Ultra wrote: Sat Aug 20, 2022 4:34 am It seems that he has lost interest in the 928TT which I think is a pity.
He was supposed to call/text/email me sometime last winter to have me come over and pull the (shredded) transaxle. I was plenty busy (always am) and wasn’t proactive, but not a peep from T.
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By Strosek Ultra
#166916
worf wrote: Sat Aug 20, 2022 10:25 am
Strosek Ultra wrote: Sat Aug 20, 2022 4:34 am It seems that he has lost interest in the 928TT which I think is a pity.
He was supposed to call/text/email me sometime last winter to have me come over and pull the (shredded) transaxle. I was plenty busy (always am) and wasn’t proactive, but not a peep from T.
As far as I know he bought a very large house in MA and moved there. Furthermore, a couple of years ago he bought a house in Finland. Lately also a BMW motorcycle and a Porsche 993 Turbo. He is also involved in a younger relative who practices a type of small car racing. He also travels quite a lot with the family.
I suppose he is very busy in spite being semiretired.
Do keep nagging him, we all want to see the 928TT running again.
Åke
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By worf
#166951
Strosek Ultra wrote: Sat Aug 20, 2022 2:14 pm As far as I know he bought a very large house in MA and moved there.
Yup. I was there 'during' the move and took some stuff off of his hands. (Not all he wanted to get out of the house, but I'm too full up in terms of space and time for the "really interesting" stuff.)

Strosek Ultra wrote: Sat Aug 20, 2022 2:14 pm Do keep nagging him, we all want to see the 928TT running again.
Yeah. @Herr-Kuhn assuming it gets to the point of moving under its own power, one of these days he's going to want me to work on it (timing belt etc.) Did he get some WSM(-like) pages with it? Torque-specs on the non-stock stuff? Anything?
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By Herr-Kuhn
#167054
There should be absolutely no reason to be into the timing belt for another 7-10 years...that's all new in there. There is no manual or the like for the rest of the stuff. It is all just straightforward basic bolts, clamps, etc. Mostly just common sense stuff. I guess from my side the biggest thing would be to not tinker with anything. There is literally thousands of hours into that car. I had one guy's mechanic wanting to pressurize the entire intake system on a car in an attempt to find a leak...that would have resulted in blowing the air snouts apart. Turns out the guys who worked on it before me left lots of wiring problems on that car. I no longer take on half-finished projects around here...it is just too stressful to fix all the screw ups. There are some tricks to getting the air tubes and airbox out and you may have to remove the alternator, but it becomes apparent when you are in there. I will be using a Mercedes alternator on my personal car which has the fan internal to the unit...this frees up some more room. The alternator has a special bracket which allows it to be collapsed close to the block, then the alternator belt is back wrapped. I will also likely get some more silicone hoses made up for this system to cut back on all the metal work...which is quite enormous.
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By worf
#167093
Herr-Kuhn wrote: Sun Aug 21, 2022 10:44 am There should be absolutely no reason to be into the timing belt for another 7-10 years...
“One of these days…” is descriptive of a set of dates that includes “7-10 years.”
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By Strosek Ultra
#167158
worf wrote: Sun Aug 21, 2022 4:03 pm
Herr-Kuhn wrote: Sun Aug 21, 2022 10:44 am There should be absolutely no reason to be into the timing belt for another 7-10 years...
“One of these days…” is descriptive of a set of dates that includes “7-10 years.”
With a broken transmission, the car is not driven much and the need to change the timing belt is pushed way into the future.
Åke
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By worf
#167182
Strosek Ultra wrote: Mon Aug 22, 2022 4:11 am With a broken transmission, the car is not driven much and the need to change the timing belt is pushed way into the future.
Åke
You have missed the forest for the trees. The forest is “no documentation of any kind on modifications.” Don’t focus on when the timing belt system might need service. The latter was an extemporaneous example of the kind of service that would require extensive disassembly of the non-stock components. What if something actually breaks that doesn’t result in an obvious failure mode like pieces of transaxle in a puddle of gear oil on the ground?

It’s always really fun to be brought a heavily modified 928 on a flatbed because it isn’t running, with part numbers scraped-off important parts, non-stock add-on wring, etc., and not one scrap of paper that describes what was going through the mad scientist’s head that did the experiment.

And yes, it’s all just bolts and stuff. And a couple of electrons and some basic physics here and there.

As someone who managed teams of engineers for decades I can guarantee you that it’s always the height of intuitively-obvious simplicity to the designers when they are designing. A couple of years after, or to those that are tossed into the mix mid-project or later, it’s an unfathomable opaque nest of snakes.

And yes, it’s all “figurable-outable” if the client pays, and understands why they are paying, for the reverse engineering.
Last edited by worf on Mon Aug 22, 2022 9:29 am, edited 2 times in total.
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By worf
#167186
worf wrote: Mon Aug 22, 2022 9:12 am … what was going through the mad scientist’s head that did the experiment.
@Herr-Kuhn take no umbrage at the above please. You are not a - or ‘the’ “mad scientist.” I’ve followed the progress on T’s 928 since inception to the extent Rennlist allowed. I have a ton of respect for the *engineering* that went into that project.

The paragraph surrounding that phrase is the result of a true story where a 928 was delivered to me, almost not-running, with someone’s “kit” installed on it and no design documentation of any kind. Unraveling the mess consumed 110 hours.
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By Strosek Ultra
#167520
worf wrote: Mon Aug 22, 2022 9:26 am
worf wrote: Mon Aug 22, 2022 9:12 am … what was going through the mad scientist’s head that did the experiment.
@Herr-Kuhn take no umbrage at the above please. You are not a - or ‘the’ “mad scientist.” I’ve followed the progress on T’s 928 since inception to the extent Rennlist allowed. I have a ton of respect for the *engineering* that went into that project.

The paragraph surrounding that phrase is the result of a true story where a 928 was delivered to me, almost not-running, with someone’s “kit” installed on it and no design documentation of any kind. Unraveling the mess consumed 110 hours.
I understand your point but don't expect technical documentation to be available for custom builds.
Expect a lot of extra work time to figure out electrical connections and other things for a custom build. If you work for a customer, he should be informed about the problems in advance to avoid discussions about costs.
Åke
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By worf
#167523
Strosek Ultra wrote: Wed Aug 24, 2022 5:17 am I understand your point but don't expect technical documentation to be available for custom builds.
I provide documentation in some form for my clients for every modification that I do to a 928.

I’ve written voluminous - 120 pages of - documentation for a ‘kit.’

There’s a difference between expect, hope, and asking a question. Should my standards be so low that I don’t even ask?

Strosek Ultra wrote: Wed Aug 24, 2022 5:17 am Expect a lot of extra work time to figure out electrical connections and other things for a custom build. If you work for a customer, he should be informed about the problems in advance to avoid discussions about costs.
You can assume that it’s not my first day.

I’ve been unraveling the mistakes and the poor modifications done to 928s on behalf of their owners for almost 25 years; the temptation to respond with sarcasm is almost overwhelming.
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By Strosek Ultra
#167610
worf wrote: Wed Aug 24, 2022 8:02 am
Strosek Ultra wrote: Wed Aug 24, 2022 5:17 am I understand your point but don't expect technical documentation to be available for custom builds.
I provide documentation in some form for my clients for every modification that I do to a 928.

I’ve written voluminous - 120 pages of - documentation for a ‘kit.’

There’s a difference between expect, hope, and asking a question. Should my standards be so low that I don’t even ask?

Strosek Ultra wrote: Wed Aug 24, 2022 5:17 am Expect a lot of extra work time to figure out electrical connections and other things for a custom build. If you work for a customer, he should be informed about the problems in advance to avoid discussions about costs.
You can assume that it’s not my first day.

I’ve been unraveling the mistakes and the poor modifications done to 928s on behalf of their owners for almost 25 years; the temptation to respond with sarcasm is almost overwhelming.
Welcome to the club, this does not only apply to the 928, it applies to almost everything else. Bad rebuilds and repairs can be found everywhere.
Åke

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