[Shifted]

   

[linderpat]

I have always thought turbo twists worked very well with the 928. Those and phone dials. My favorite are the Cup 1's however.

[Crumpler]

I weighed my solid twists today the 7 inch (smaller) was 42.8 lbs with a tire on it.
So I would estimate the hollow ones save a lot of weight, unsprung weight even better.
Those look really nice!!!
I would like a set up just like that :)

[Shifted]

Your rims might not be as heavy as you think. I looked up Pirelli P Zero Nero GT weights on Tirerack (https://www.tirerack.com/tires/Spec.jsp ... ro+Nero+GT). I don't know if your rims are 17" or 18", or which width tires you have on your 7's, but here are some sample numbers with your 42.8 lbs rim/tire measured weight:

225/45ZR17: 23 lbs
17" rim weight: 19.8 lbs
Total weight: 42.8 lbs

225/40ZR18: 21 lbs
18" rim weight: 21.8 lbs
Total weight: 42.8 lbs


Conversely, with my 8x18 rims:

225/40ZR18: 21 lbs
18x8 rim weight: 22 lbs
Total weight: 43 lbs

235/40ZR18: 22 lbs
18x8 rim weight: 22 lbs
Total weight: 44 lbs

245/40ZR18: 24 lbs
18x8 rim weight: 22 lbs
Total weight: 46 lbs


For a sanity check, I searched online for the expected weights of the hollow spoke turbo twist wheels (996.362.136.04 and 996.363.142.03). My results consistently turned up 9.65kg (21.275 lbs) for the 18x8 and 11.9kg (26.235 lbs) for the 18x11. So, my weights were in the right ballpark.

Is it possible that you actually have 7.5" hollow spokes (993.362.134.05)?

[Shifted]

I mounted new tires on my five spoke Carrera III wheels today, so I weighed them too:

Front 18x8 ET57: 22.8 lbs

Rear 18x10 ET58: 25.0 lbs

[Crumpler]

Hmmmm. Interesting.
Perhaps they are hollow, they are 17’s so I will need to upgrade anyway.

[Shifted]

If your rims are 17", then they're not 993.351.045.10.

Anyway, the turbo twists in the photos are cleaned, covered, and ready for shipping once I have something suitable to ship them in. Worst case, I can always build a crate and put them on a pallet.

[Crumpler]

I looked, they are Italian:)

[Shifted]

I updated the turbo twist wheel weights in the first post with more accurate weights from a better scale.

[Geza-aka-Zombo]

As many are most likely aware, weight is not the only physical characteristic of a wheel that impacts performance. I would argue that wheel inertia has even a larger impact. To illustrate, I created some crude CAD models of a generic wheel, a 16 X 8 and a 18 x 9. You'll note both wheels weigh about the same: 22.1 lbs vs. 22.7 lbs, but the moment of inertia of the 18" is ~40% greater than the 16". This has a direct impact in the amount of torque required to accelerate the wheel when driving and slowing the vehicle. Add to this the increased inertia from the larger tire, and this negative effects are increased further. Just something to consider.

[amdavid]

Damn you!.....was prepping a set of 18" MY02 BBS Carrera wheels for my 944S2 that's currently riding on 16's. Wheels are actually the same weight, but I see that's not the only thing that counts.

Relating this to bikes, 26" vs. 29" wheels.....29" are harder to get started on a steep up hill for sure.

Good stuff, thanks for sharing.

[Shifted]

Where the weight is on the rim definitely makes a difference. If you're comparing apples to apples, and the weight of two wheels is identical except one has a bigger diameter with the same width, the other factor to consider is the tire. Tires of the same model and same tread width can vary as much as two or three pounds between 16" and 19" bead diameters. And that weight is all on the outside of the assembly, not distributed within the cross section of the wheel.

The general guideline is that given the same overall diameter, and the same width, and same tire model, and same wheel model...the bigger the wheel and the shorter the sidewall then the lower the overall weight and the less weight that is on the outside of the assembly (rotational inertia). In general, tires are heavier than wheels per radius inch. And all of the weight is on the outside, which increases the rotational inertia the most.

Of course, you could take a small magnesium rim with really nice lightweight tires and compare it to a larger steel rim with really cheap heavy tires. That would make it seem like smaller rims are better.

There are exceptions to every rule, but that's the general guideline. There are also significant performance benefits to shorter sidewalls that can't be overlooked.

[Geza-aka-Zombo]

The general guideline is that given the same overall diameter, and the same width, and same tire model, and same wheel model...the bigger the wheel and the shorter the sidewall then the lower the overall weight and the less weight that is on the outside of the assembly (rotational inertia). In general, tires are heavier than wheels per radius inch. And all of the weight is on the outside, which increases the rotational inertia the most.

I can't agree with this analysis. If you have the same overall tire diameter and width, same tire model (construction) and wheel model and width, but different rim diameters, the larger diameter rim combo will weigh more and have the higher inertia.

Break up the tire system into simple components. Assume you are comparing 16" to 17"

1) Hub section - same between both 16" and 17"
2) Spoke section - about the same but a bit more weight/inertia on the 17" due to longer spokes
3) Wheel barrel - significantly more weight and inertia on the 17". Overall volume (weight) of material (aluminum) is ~6% more and inertia ~27% more
4) Tire sidewall. 16" slightly heavier due to more section height
5) Tire Tread - same between both 16" and 17"

Or course, things get even worse because folks normally increase tire and rim width also when increasing rim diameter.

I'll update my CAD models and add a tire to confirm. Stay tuned....

[Geza-aka-Zombo]

Damn you!.....was prepping a set of 18" MY02 BBS Carrera wheels for my 944S2 that's currently riding on 16's. Wheels are actually the same weight, but I see that's not the only thing that counts.

Relating this to bikes, 26" vs. 29" wheels.....29" are harder to get started on a steep up hill for sure.

Good stuff, thanks for sharing.

I think your 26" vs. 29" inch issue is more gearing than anything else, assuming the diameter of the tires are also ~3" different.

Lets say you are providing 1000 lb-in of torque to the drive axle though your legs and bike gearing. For the 29" wheel, you would be transferring (1000 lb-in)/(29 in/2 {tire radius}) = 69 lbs of drive force to the tarmac.

For the 26 it would be (1000 lb-in)/(26 in/2 {tire radius}) = 77 lbs

[Shifted]

Run the numbers. Take a OEM 225/50R16 and compare it to a 225/35R19 tire. The higher profile tire is at least two pounds heavier. Compare otherwise identical wheels that only change diameter, but are otherwise the same width, material, design and construction method. You'll find that the bigger wheel is a little heavier, but the extra weight is often less than the the weight saved by the tire. So, you've effectively taken weight off of the outside of the assembly, moved most of it toward the center, and discarded the rest.

Your CAD drawings above have different wheel widths, and increased width adds a lot of weight to a wheel. Try running the same computations with wheels that are identical other than diameter, then add identical tires that only change their side wall height to maintain overall diameter.

Like I said, not universal, but generally pretty much how it works.

[amdavid]

Interesting, of course gearing is the primary factor. I guess an 8% differential is negligible.

[Geza-aka-Zombo]

I don' think you can get a better apples to apples comparison than this.

16 x 8 wheel: 18.43 lbs
19 x 8 wheel: 23.70 lbs (5 holes increased in size and moved further outward for cosmetics)
225 50 16 tire: 27.83 lbs
225 35 19 tire: 27.30 lbs (tire diameter actually reduced a few mm to match the 16")

19" setup is 10% heavier and has 14% more inertia.

Larger diameter, further out from the rotational axis tire bead, which has steel in it normally, also doesn't help the 19"er.

[Shifted]

That's really cool, but where are you getting those wheel and tire weight numbers from? Your own computations and designs? Because when I look at real world wheel weights for alloy wheels of the same design and manufacturing process but different diameters, I'm not seeing anything with the 5.3 lbs difference that you show. You also show the tires weighing almost exactly the same, which is also not what I see in real world specs on tires. Of course, we could probably look for wheels and tires that have those differentials, but I don't think that's normal.

I know that the odds of you getting the CNC/CAD specs for wheels from a manufacturer are about nil, but there is a lot more to their wheels than discs on cylinders. They've got safety bead profiles, tire well, flanges, etc. All of that has a profound effect on weight and rotational inertia. The nicer ones also engineer the wheels to target specific load ratings so that they can keep the weights and rotational inertia down for the very reason that you're concerned about it.

I'm sure that your calculations are right, but I don't think your wheel and tire weights and weight distributions are representative of real world specifications.

Pretty neat exercise, and I can tell that you're putting a lot of time into this.

There is no question that greater rotational inertia negatively impacts acceleration and deceleration. The question becomes, is it enough of an impact to notice or care? And does the potential advantage in acceleration/deceleration with lower rotational intertia tire/wheel assemblies offset the performance disadvantages of taller sidewalls? Maybe just as important, what effect is the unsprung weight having on the performance and ride quality? Which is not just a matter of the height of the sidewall, but also a function of the materials used in the sidewall and whether or not it is a consistent for the entire height of the sidewall, or if it is progressive.

You seem to be building a model to prove your hypothesis, but I'm wondering what your specific objective is? Do you have a specific wheel and tire size that you think is ideal? Or are you just trying to prove that the smallest rim you can fit will perform the best?

[Geza-aka-Zombo]

The weights and inertias just come from the CAD model of a generic type aluminum wheel and rubber tire I created. It's a good apples to apples comparison because every feature is basically the same, only the rim diameter changed (except the 19" wheel is actually lighter than it should be because, for cosmetic reasons, I increased the 5 hole diameters, which cut weight.

I did not try to skew the models to meet a hypothesis. The models are identical in all ways (except those 5 holes and their locations), so this just proves the physics. For the real world, there can be significant differences in the design and construction of the same tires and wheels in different sizes. I suspect for the 16" tires, more structure has to be put into the side walls to increase stiffness, that would potentially add weight and thickness. Perhaps the bead is even thicker/stiffer. For the wheels, I wouldn't be surprised the manufacturer spends more time cutting weight on the larger diameter wheels by thinning out the aluminum wherever feasible, just so it doesn't look too bad compared to the naturally lighter smaller diameter wheel. And they charge you for that difference.

For a real world example, when I used to track my A4 Avant, I used forged BBS wheels (15 x 7.5) from a previously owned '91 Audi 200 quattro 20V. I still have them with R compound tires mounted. I'm currently running Fikse FM5 forged 17 x 8 wheels. When I recently got new tires, I loaded the 17" into the Avant and put the 15"s on to drive to the tire shop. Wow, what a difference - just driving to the shop I felt a huge difference with the lighter wheel/tire set-up. But, at this time, my vanity precludes me from shelving the 17"s in the name of performance

[Shifted]

I see where you're coming from. Within the constraints of the models provided by your software, it does look pretty conclusive.

However, for your consideration:

Bridgestone Potenza S007A:

image.png

There are lots of other tire options, I just picked one at random. You can see that even with slightly increased diameter, the lower profile tires weigh less.

BBS Wheels:

image.png

image.png

That's 24 lbs and 26.1 lbs. in the one piece SR wheel in their "Design Line". It's hard to find one to one comparisons that span 16" to 19" with the same widths. Here's a chart of Porsche wheels that are different styles in the different sizes, but they do show how the weights of larger wheels are not necessarily significantly heavier:

http://www.944racing.de/wheelweights.php


My point is...the model may not represent the real world.


But, I'm still wondering what your point is? Are you advocating for smaller wheels in general? Or, a specific size? You said that the car drove better with smaller wheels. In what way? Unfortunately, your 15" to 17" comparison doesn't really provide a decent comparison because the wheels themselves were different makes and widths. Were the tires the same make/model and overall diameter and tread width?

Taller sidewalls will give a better/softer ride. Definitely. And wheel/tire assemblies with lower rotational inertia will accelerate and decelerate faster. No question. But smaller wheels with taller sidewalls don't necessarily mean better acceleration and deceleration. And when it comes to cornering performance, within the same tire make/model and tread width, the lower profile will always provide better handling characteristics.

So...where are we going with this? I was just posting wheel weights for general information since it can be hard to get specific data on the wheels that fit the 928. Where are you taking the thread? :)

[Geza-aka-Zombo]

I wasn't really trying to take the thread anywhere. I initially wanted to point out that wheel weight does not tell the complete story. Inertia is even more critical and wanted to illustrate it with the CAD models I built. The discussion moved on from there. Hopefully, folks reading, enjoyed.

[Shifted]

In that case, we should talk about why rotational inertia is a factor. Which is primarily as it relates to acceleration and deceleration. This is an interesting read on that aspect:

https://www.racingjunk.com/news/wheels- ... -to-strip/

My take away from that article is that they did find a repeatable difference, but not much.

Another aspect that has to be considered is the impact on cornering. Lower profile tires tend to have stiffer sidewalls, which means less rolling under and better at keeping the tread flat on the ground. Which means faster cornering with less sliding.

There is also the impact on the suspension. Less cushion in the sidewall does have an effect on the suspension. The tires should be considered their own suspension system, or at least part of the whole suspension system. Changing any of those components will affect not only ride quality, but also performance.

Low profile tires sometimes have sidewalls with progressive stiffness to simulate the effective spring rate of higher profile tires. Sounds good in theory, but it's very dependent on the weight on the tire and usually a fairly narrow zone.

Anyway, rotational inertia is only one factory in the overall performance of a vehicle. And if you want the advantages of the stiffer sidewall but without the rotational inertia penalties, you could reduce the car's overall weight, which has a similar net effect.

Interesting stuff, but I don't race at a level where it matters :)

[maddog2020]

Fikse FM10 18x9 with 265/35/18 Pirelli PZero Rossa's Combined weight 42.4 lbs
Fikse FM10 18x12 with 335/30/18Pirelli PZero Rossa's Combined weight 54.2 lbs

I don't remember the exact numbers but the tires weigh more than the wheels.

The overall weight vs stock really doesn't matter since I needed 18's to clear the front brakes, and needed the 12's out back for traction.

[worf]

Gyroscopic force is also a consideration for rotating mass on a longer lever arm. In particular it can change steering feedback and ‘feel’ and also screws with ability of the suspension to damp wheel motion when turning.

Inertia of rotation is why I always counseled against giant heavy cast wheels on 928s with a purely stock suspension. If you go bigger you want to try hard to minimize wheel weight. You won’t be able to escape it completely.

Take a stock 928 with OE 16” wheels and tires and throw on fat, heavy cast 18”s and the difference is vast, noticeable, and bad. But, the 18”s look really cool.

When I went to 17” fat, wide Fikses the overall weight didn’t change drastically. Obviously the moment of inertia did. I believe that I lost 1-2% of RWHP according to on-the-same dynojet measurements. But, overall, the handling improved even when considering a bit of poorer critical dampening.

[Shifted]

Fikse FM10 18x9 with 265/35/18 Pirelli PZero Rossa's Combined weight 42.4 lbs
Fikse FM10 18x12 with 335/30/18Pirelli PZero Rossa's Combined weight 54.2 lbs

I don't remember the exact numbers but the tires weigh more than the wheels.

The overall weight vs stock really doesn't matter since I needed 18's to clear the front brakes, and needed the 12's out back for traction.

Thanks for those weights. The Fikse wheels are lighter than the average wheels of the same size, so you're as good as good can be without going fully custom. Those rear tires look massive. You must be really excited to be getting the car closer to on the road! And it looks great. Really good work there!


Vector forces from rotational inertia of the wheels/tires don't come into play when in a single plane (like when accelerating or breaking in a straight line). And their affect on steering is negligible on a four wheeled vehicle. Technically, it resists turning the wheels, including straightening them out after they are turned, but it's negligible. But, definitely a factor on two wheeled vehicles. When I first started riding motorcycles, the common rule of thumb at the time was that centripetal forces, and the related vector force affects of rotational inertia, came into play at around 6mph, give or take based on tire sizes and weights.


You mentioned before that you felt that 335's were necessary for traction reasons due to your power levels. Do you have any dyno graphs? Do you remember what gears and under what conditions you lost traction?

Since I never used a real dyno, any numbers that I have for my car are not only extrapolated from GPS or RPM readings, but also have a correction factor that is partly based on weight, CD, and frontal area, but also based on comparisons to pre-turbo power levels. In other words...there is a lot of room for error.

Combining that uncertainty with the fact that I rarely spin the tires under acceleration has sometimes caused me to doubt the power levels that I think that I have (max of ~530rwph and ~550 lb ft). However, I have some factors that contribute to not losing traction. R compound tires. 95+ degree days. And I always always always roll into the throttle. The closest that I ever come to sudden power application is the one or two times that I've started at 1,500 to 1,600 RPM with foot fully off and then just stepping all the way down to see how quickly it can go from full vacuum to any positive boost at low RPM.

So, I've become very intrigued by the torque curves on dyno graphs and comparing them to my own. And, knowing what kind of tires are being used and what attention has gone into determining the best pressure, camber and toe for them. Plus, knowing how the vehicle was driven to lose, or maintain, traction. The number of examples of those factors all being known about a car is extremely low.

Maybe it's more than you care to share, but I would love to know more about your experiences if you're willing to share. It will help to better frame my own experiences and fill in some blanks for me on the differences between NA and FI power curves on our motors.

Regardless, congratulations on your progress and I hope it all comes together smoothly for you!

[maddog2020]

with 17x10 and 275's on it I had zero traction in 1st. If I downshifted from 5th to 3rd to drop the hammer it would break the tires loose in 3rd gear @ 70mph 2nd was a little more useful than 1st but I still couldn't get full traction in 2nd.

[Shifted]

with 17x10 and 275's on it I had zero traction in 1st. If I downshifted from 5th to 3rd to drop the hammer it would break the tires loose in 3rd gear @ 70mph 2nd was a little more useful than 1st but I still couldn't get full traction in 2nd.

Thanks. That must have been a lot of fun to drive! What clutch did you have in it? And are you keeping that same clutch for the new build?

[maddog2020]

with 17x10 and 275's on it I had zero traction in 1st. If I downshifted from 5th to 3rd to drop the hammer it would break the tires loose in 3rd gear @ 70mph 2nd was a little more useful than 1st but I still couldn't get full traction in 2nd.

Thanks. That must have been a lot of fun to drive! What clutch did you have in it? And are you keeping that same clutch for the new build?

The low end torque was a blast. I had a spec stage 3+ dual disk clutches but the drivability was horrible. It's on a shelf in the garage now. I've got one of Greg Brown's Tilton street clutches installed and ready to go. Usually I would drive it, scare the crap outta myself, and park it for a few weeks..... then rinse and repeat.. lol

[Shifted]

I think that when just cruising or around town, low end torque is much more gratifying than high RPM horsepower. I look forward to your thoughts on the Tilton clutch once you've exercised it. You've definitely put a lot of thought into things.

Do you have particular power curve and max power goals in mind for the current build?

[maddog2020]

If you look at other ITB 6.5 liter builds, its pretty normal to hit over 600hp at the ground. I'm running Titanium rods, and hotter gun drilled billet cams, and also have straight 50mm ports into the heads, instead of necking down to the 50x45mm stock port like the other builds that used ITB's. I'm also running higher compression, and better injectors (injector dynamics) I also have the capability to run E85. I'm also using stepped headers, so it's going to be interesting for sure.

[Shifted]

Unfortunately, power numbers, especially dyno graphs, are very very hard to find for other stroker ITB builds. I did see one non-stroker ITB dyno on Rennlist, but if I recall the numbers weren't in the range that you're talking about.

Looking forward to you getting it running and tuned. I'm sure that it's going to be really awesome. You should be doing a build thread so the rest of us can live vicariously :)

[Shifted]

Here's an old one with 7.0L on Pelican:

http://forums.pelicanparts.com/porsche- ... 928-a.html

Looks like the power doesn't really come on until about 4,700 RPM and barely touches 500 lb ft of torque. Dropping off again by 6,500 RPM. He turns it to about 7,400 RPM, so that definitely helps the horsepower numbers. Very nice flat looking torque curve when it does come on, but it does seem a little narrow. For racing, and with the right gearing, I'm sure it's pretty awesome. And I'm imagining that it's very responsive.

You're has got to be even better than the above, given the communal lessons learned since then and your personal attention to detail and experience.

Just musings on my end, but it's pretty exciting stuff :)

[maddog2020]

as far as the engine goes, I'm wrapping up the ecu wiring, (transmission harness, and some other misc stuff. but it should be able to start and drive soon.

[maddog2020]

that was mike Simards stroker. he built it to be a track beast so he wanted peak torque to be up there so he could get higher HP.

his ITB's necked down to 45x50mm oval, and then he put in intake valves that were too large. it also idled at like 1200 rpm because the cam profile was so radical. He also only had 10.5:1 compression ratio, which his cams bled off quite a bit. He did this because he wanted to use pump gas at the track. My comp is higher, and cams are halfway between is profile and the Devek B1 cams.

He also didn't have stepped headers to help build torque in the low end. I expect my peak HP to be similar, but my torque should be larger, just as flat, and earlier. I can tune using different horns. He ran super short horns, which will loose torque and increase HP.

[maddog2020]

that was mike Simards stroker. he built it to be a track beast so he wanted peak torque to be up there so he could get higher HP.

his ITB's necked down to 45x50mm oval, and then he put in intake valves that were too large. it also idled at like 1200 rpm because the cam profile was so radical. He also only had 10.5:1 compression ratio, which his cams bled off quite a bit. He did this because he wanted to use pump gas at the track. My comp is higher, and cams are halfway between is profile and the Devek B1 cams.

He also didn't have stepped headers to help build torque in the low end. I expect my peak HP to be similar, but my torque should be larger, just as flat, and earlier. I can tune using different horns. He ran super short horns, which will loose torque and increase HP.

He also had way too much lift for the exhaust cams, which certainly hurt him for torque. His cams had the same for intake and exhaust (lift and duration) he also had more bore, but less stroke so that hurts on the torque again. I will start another thread on stroker engines

[Shifted]

For reference:

Four Fikse FM10's:

Center caps: 2.0 oz
18x9: 21 lb 0.6 oz
18x9: 20 lb 14.3 oz
18x10.5: 21 lb 13.2 oz
18x10.5: 21 lb 12.5 oz