Corridor Electric Power Generation and Distribution

[prr60]

By the way, at most utilities 69kV is defined as a transmission voltage. Sub-transmission is usually 34.5kV when used on a network, not radial line.

...However, "most utilities" makes the above excerpt accurate because so many utilities are relatively small and may consider their 69kV as transmission, and indeed for them it may perform such function to at least some extent. But "most" in terms large majority percentage of utility infrastructure ownership, define transmission as "everything above 100kV" relegating 34.5 and 69 to subtransmission. The Federal Energy Regulatory Commission agrees, as its two major annual report requirements for transmitting utilites define transmission as 132kV or greater (Form 1) and 100kV or greater (Form 715), and in both cases, network, not radial facilities.

Of course, causing confusion, is that transmission service is sometimes provided over subtransmission, or even distribution, lines, most commonlly in the case of interconnection of a small generator to those classes of lines. But there is a distinction between "transmission service" and "transmission facility."

With all due respect, you analysis of what is transmission and what is “sub-transmission” is wrong. And I state that as a 33-year transmission design engineer employed by the largest electric utility in the country. We operate 5000 miles of transmission lines up to 765kV. At my company, and at most companies I deal with (and that is most companies), 69kV is and always has been a transmission class voltage.

The reason for this is simple. The engineering, tools, and skills required to design and maintain a 69kV line are dramatically different than the requirements for a 34kV line. The requirements set by the National Electrical Safety Code (NESC) change as you move above the 34kV voltage level. The power transmission at that level typically requires larger conductors which require more rigorous structural analysis. The structures are bigger, taller, and the code requirements are more severe. While 34kV and lower lines are usually installed using cookbook standards and minimal engineering, 69kV and higher typically are engineered on a line by line basis with custom structures, full line surveys, and far more detailed specifications and drawings. Then, when all is said and done, the line is dispatched by the transmission board, not the distribution board, and is maintained by transmission line crews, not distribution crews.

The term “sub-transmission” is typically used to describe lower voltage network lines (not radial feeds) that are designed, installed, and maintained by the distribution side of the T&D operation. Basically these are distribution lines performing a transmission function. Lines at 69kV rarely fall into that category and when they do it inevitably turns out to be a colossal blunder. I say that from painful personal experience.

One further clarification. The Federal Energy Regulatory Commission does not specify a voltage in its regulatory definition of “transmission line”. To FERC, the definition is purely functional. If a line moves power from one point to another with the intent to move the power to still another location, then that line is a “transmission line” and must be placed in the FERC transmission asset category. If, on the other hand, the line only moves power to end users (radial feeds to distribution substations, for example), it is by FERC definition a “distribution line”. A line may be 765kV or 2.4kV: it does not matter. What matters for FERC accounting is what it does. We have FERC-designated "transmission lines" as low as 13kV. And, we also have FERC-designated “distribution lines” operating at 230kV. This is strictly a regulatory and accounting nuance. It has nothing to do with our design, maintenance or operation. To us up to 34kV is distribution, and 69kV and up is transmission. That is also, by the way, the legal definition prescribed by Pennsylvania Utility Code.

FERC Form 1, the annual report filed by most utilities, is a statistical summery of a utility’s assets and operation. For that form, on Page 422, FERC wants a listing of all FERC-designated transmission lines constructed for operation at 132,000 volts or higher. Note that it is the design voltage, not the actual operating voltage that determines inclusion in FERC Form 1 (a line designed for 132kV but presently operating at 69kV is included). This does not mean that they do not consider voltages lower than 132kV as "transmission". They certainly do. All it means is that for the purposes of that particular report they are only requiring the utility to list those transmission lines designed for 132kV and up.

[Bill West]

In jest, Jersey Mike’s recent Thorndale trip pictures marked Bryn Mawr as a “Mystery Sub” http://acm.jhu.edu/~sthurmovik/Railpics ... nails.html The brick building shown received Arsenal 44kv lines on the roof and transformed the power to 11kv fed out through the upper square grey openings to the catenary. PRR’s westward expansion in the 30’s appears to have lead to the outdoor rebuild we see in front. The recently developed high speed circuit breakers would have been installed and the square lattice tower on the right would have handled 44kv for larger outdoor transformers.

Previous posts starting Oct 8 explained that the 44kv conductors were leased to Philadelphia Electric somewhere in the 60’s-70’s era. In answer to the supply mystery thus raised, the tracks are fed through the 4 track cat from the 132kv transmission supplies at Paoli and Zoo. Note that SEPTA only has 2-3 uphill afternoon trains at once on this 20 mile route, a 2 track railroad would suffice. So despite having no infeed this station still helps out because it cross ties the 4 cats. That splits the load to half on each W/B wire (or less considering the low E/B wire loading), the voltage drop becomes half and that allows double mileage between infeeds. Having the excess tracks has given us extra catenary to act as feeders. I think NH/PC similarly kept extra catenary lines on the Harlem river branch when they reduced track.

The station’s other purpose is as an intermediate catenary switching point. It provides an electrical usefulness similar to the way a set of mainline crossovers helps during track troubles or maintenance. Placing it here allows taking the cat out of service at the same points as used for doing trackwork, this is a feature of PRR’s good design. This station maybe wouldn’t have been built as merely a cross tie on a new RR but here it already existed when the 44kv was cut back. It might have been left as a supply point had it had been upgraded to 132kv in the 30’s but those lines were routed down the Schuylkill branch. Maybe continuing to step down 132 to 44kv at Paoli and Zoo is what became a problem in the 60’s. Allen Lane was fed at 44kv from Frankford in the 30’s but it too got reduced, to 11kv and is now just tapped at North Philadelphia with a reinforcing feeder up the branch.

In the past decade Amtrak has put in new catenary circuit breakers at most of the substations. They are the grey boxes with the drip hood and the little dark window and look like vacuum types. Here they are next to and across from the building, along with some older units. The left foreground box is a signal or 60hz auxiliary power transformer with enclosed power connections, the same as used in underground residential distribution. The 3’ dark green drum at mid picture is the switch heater supply transformer, the station’s similar 25hz auxiliary power supply is at the right end. The colors are an age tip off in the electric business.

Amtrak published info on the old 1990’s system here in the form of the pictures of the Power Director’s board that it provided to The Library of Congress’s Historic American Buildings Survey/Engineering Record. It is online at WWW.loc.gov under American Memory, search the Architecture HABS/HAER collection for PA-404-A and keep clicking, good data pages too. Bryn Mawr is at the top left of pict 1. If you have high speed access and a photo/graphics program download the 20 meg TIFF files, you’ll be able to zoom in enough to read switch numbers. The old Harrisburg board is also available Mike, at PA-85 picts 21, 22 and drawing 6. PRRTechFan, you might try a look at PA-404-B. Phil N. try NY-5471-A picts 19, 53, 54, 55.

Thanks for your on going photo record Mike, from 2500 miles away it tells me much about the old electrification. In the future I think it will be a very useful record of the PRR’s famous system.

Bill

[PRRTechFan]

Bill West, the photographs and data on the ex-PRR electrification at the Library of Congress link you provided are nothing less then spectacular! Quite a Christmas present for someone interested in details and the history of how and why it all works!

Does anyone know whether the 5 section Power Director's board from 30th Street still exists; has it been trashed, or has it been saved for restoration?

Thanks again!

[Jersey_Mike]

From what I know from taking with Amtrak employees the Power Dispatch office at 30th St should still be in place. I was talking with an MoW guy at Baltimore Penn Station and I know that the old power dispatch board there is still in place. Also, I have heard that the old PENN interlocking machine next to the power room on the 4th floor is not only in place but still functional and is put in service whenever the CTEC computer goes down.

Of course the most interesting bit O trivia is the section 9 power board at Harrisburg is still in operation with the original gear and manned 24/7.

To Bill West:

Thanks so much for explaining the situation at Bryn Mawr, it makes total sense now. When you say Arsenal 44kv lines, you mean they came from a power house around ARSENAL tower? I have noticed an old looking substation building down there, which appears to still be in use, does that have something to do with the 1915 electrification?

Also, I had some questions about what is going on at PAOLI. There is a 1915 Substation there, just like BRYN MAWR, but there is also a 193x substation with two large transformers (part of power section 9). Both the 1915 and 193x substations have feeders to the catenary and moreover, one of the 44Kv lines still appears to tie into the 1915 substation (although it vanishes around the east end of PAOLI interlocking (giving me the idea that the railroad distrabution line had been burried). There also appear to be a few lines tying the switching section of the 193x substation with the 1915 substation. As you seem to be a power guy perhaps you can help decipher this for me. I have posted some photos to a temporary folder.

http://acm.jhu.edu/~sthurmovik/Other/Temp/

I also put up a pic of the now out of service PALOI power board. I would also appripaite some tips on dechphering that and the power board photos from the Smothsonian collection. Thanks so much.

[Bill West]

Hi Mike, it’s sure nice to hear that the old boards are still around. I sure hope you eventually get pictures at Baltimore, it’s the only one I’ve never seen any shots of in the trade papers. Now for your questions:

1. Arsenal -Starting in 1911 Phila Elec took over supplying Phila Rapid Transit, adding considerable 25hz generation in it’s Schuylkill/A1 station on Christiana St. When PRR went AC in 1915 four 3 phase 13.2kv cables were laid across the river to a 2 story brick RR step up station at Arsenal, next to the end of the highline trestle. From here two single phase 2 wire 44kv lines went to West Phila sub then to Bryn Mawr and Paoli. In 1916 PECO added 2 phase balancer machines because the single phase was causing troubles on its side. 2 more 44kv lines, from different phases were run via West Phila to North Phila and Allen Lane subs in 1917. www.prrths.com/Phila_Images_PP2.htm is a 1928/32 West Phila station picture that shows the 2 story sub in the background. It was also the power director’s office until 30th was built, I wonder if that’s the low building next to the sub. The extreme corner of www.chesco.com/~apu/prr/images/30thconst.jpg appears to show the same building.

At some point 2 synchronous condenser machines were added at Radnor/St David’s to help the voltage. Back at Schuylkill/Arsenal, in 1927 PECO added a frequency changer to feed an outdoor RR step up for starting the 132kv Wilmington extension. By the time the PRR was done in 1932 Lamokin and Richmond fed dual/quad 132kv lines along the main, Arsenal had changed to being a large growing step down station and the 1927 supply was just a back up. At this point the 44kv was cut back to being fed from Zoo and Frankford, also the Paoli 44kv lines were doubled before or during this growth. The 1927 supply lasted to 1971.

If you feel flush some day, buy a copy of CERA’s “Westinghouse Electric Railway Transportation”. Although a lot of it is streetcars, its brochure reprints pretty well cover all of WEMCO’s mainline projects to some degree including 3-6 page summaries on LIRR and NYPenn, the New Haven extensively, full PRR 1915 and a large bulletin on the great 1932-35 work.

2. Paoli substation -This is disjointed because of its 2 projects origin. If I can make a bit of a typewriter picture, from west to east the general plan was:

*-------tall 1937 outdoor structure------*.........*-1915 building, rebuilt outdoors-*
2 132kv lines west-2 transformers-bus tie-----4 transformers-4 44kv lines east
...............................4 cat circuits west.............2 yard circuits-4 cat circuits east

Although built in 2 pieces this station is functionally one. The west circuits don’t actually connect to the catenary until after the phase break near the duck under at Green Tree mp 21.09, the west end of the yard.

3. Paoli 44kv lines -At the sub no transformers show below the terminal line switch tower, PC/Amtrak may have just not spent the money to remove the south lines more so because of the number of live circuits they cross. Your http://acm.jhu.edu/~sthurmovik/Railpics ... A_9010.jpg pict shows the south side lines returning to normal pole positions eastwards after passing over the road bridge. They could just be cut off farther east wherever PECO puts the line to work. The north lines show as Utility 3 phase west of the bridge. http://acm.jhu.edu/~sthurmovik/Railpics ... al-168.jpg shows 4 conductors approaching then crossing at 16.8, just above the signal heads and behind the lower girder, they seem to go off to the right. Now there could still be something in service here but I’m afraid I can’t find any more specific evidence so far. The lack of lines east on the tower and director’s boards argues pretty strongly against it.

4. Paoli Tower, power board -I’ve put notes on a copy of your picture and put it in your e-mail. You are welcome to include the additions in any display, don’t bother about credit. Maybe it suits your tower directory? Sorry I got carried away a bit on detail, I used to revise old switchboards like this and designed new ones for a 500kv Utility.

More later, Bill

[Jersey_Mike]

Hey, thanks for all the info, this has been most helpful. I have a couple questions and clarifacations.

First, as it stands today the switchgear in front of the old Paoli Substation feeds the lines east and is shares a common 12kv bus with the 1937 station, correct? When the 44kv was still in operation how would the bus tie have been worked, ie would it have mostly been open, or closed, why or why not. Also, do you feel that the original 44kv transformers in the substation would have been operational until the end of 44Kv service, or replaced with something outside in the switch yard with the 1937 project?

Second question is about this style of railroad electrical operations in general. Railroad substations usually occur in conjunction with a section break in the catenary. Does a substation feed one or both of the sections adjacent to it. If both, is there another break between substations? If not is only one substaion switched in to feed a section at a time?

Correction, since the 1937 project there were only 3 tracks wst of PAOLI and hense the need for only three 11kv circuts, not 4. I also believe the yard was fed on the main circuts, not the main one, see my more detailed e-mail reply as to why.

Comment, I am sure that the former 44kv line on the south side of the tracks is simply tied off near the east end of the interlocking. The second photo at MP 16 shows the Peco like serving a distrabution substation. I wonder what that rationale the PC had in leaving that stub in place.

Thanks again for your answers.

[PRRTechFan]

to Jersey_Mike and Bill West: Thank you for your contributions toward understanding the various stages of PRR electrification. I have been searching for information concerning PRR power and signaling for almost 40 years, and what with the explosive growth of the internet and historical information available on line; plus forum discussions like this one, I have a much greater understanding of what was done and why.

Jersey_Mike wrote: Railroad substations usually occur in conjunction with a section break in the catenary. Does a substation feed one or both of the sections adjacent to it. If both, is there another break between substations? If not is only one substaion switched in to feed a section at a time?

In general, substations will feed "both ways" toward the next substations both up and down the line. The next substation does the same. The result is that each catenary segment is fed from the two substations at either end. While there may be multiple transmission line circuits between each substation and multiple traction voltage circuits from each substation feeding each track in each direction; the multiple substations and circuits are essentially operated in parallel up and down the railroad. The section breaks and individual feeds for each direction of each track at a substation are there to isolate potential faults to as local an area as possible so as not to take down the entire railroad in the event of any single failure. Under normal operating circumstances, all (or most...) of the track circuit breakers are closed and everything runs in parallel. In the event of a fault, only the breakers and or switches associated with the failed segment, transformer, etc need to be opened, and (hopefully...) everything else continues to operate without even a blink.

The bulk of the ex-PRR NEC electification can be observed as having four 2-wire transmission voltage circuits, and each substation usually has (or had...) either 2 or 4 transformers to step down transmission voltage to traction voltage. This was usually arranged such that each track at each alternate substation would be fed from a transformer connected to an alternate transmission circuit. The failure of any one transmission line, transformer or switch would not "kill" the power to any one track segment.

In addition to the redundancy of equipment, most every substation had the capability of switching between the various feeders and equipment. The system was very remarkably robust and redundant for the time. The fact that it is still in operation essentially unchanged is a testament to the designers and engineers. I understand that there are a number of substations where transformers have failed, removed and not replaced; and it still hums along....

However, the bulk of the NEC was electrified in the 30's using the same design criteria. Transmission voltages were 132kV (+/- 66kV to ground), traction voltage was 11kV, everything was single phase and everything was 25Hz.

My understanding (...which has been greatly expanded due to these discussions..) is that the Philadelphia/Paoli area had been electrified much earlier and was done using different standards based on much less traction power experience. I suspect that is why you see more 3 phase distribution on the railroad itself; it also probably explains the earlier 44kV transmission voltages in the Paoli area. I also recall reading something somewhere that the 44kV system used autotransformers tapped at 11kV to derive the traction voltage. Very ingenious for the time!

Bill West wrote:

If you feel flush some day, buy a copy of CERA’s “Westinghouse Electric Railway Transportation”.

While afraid to display my ignorance, I'd love to see a copy of the book. Who is "CERA"?

Does anyone recall a book entitled "The Electrification of the Pennsylvania Railroad" or similar title? It was a complete description of the construction project. It had details concerning the substations and breakers. It also explained the construction of the main powerplant near Philadelphia. One of the things I remember about that was that the generators were set up to generate both three phase and single phase from the same machines. But the PRR apparently had too much trouble applying 3 phase directly to traction circuits, and the system ultimately became strictly single-phase. There were also articles about interlocking machines, the signal system and the various special circuitry designed to ensure reliable signal system operation with minute signal rail currents while ignoring order-of-magnitude greater AC traction currents in the same rails.

I thought I saw this book in Straussburg over 20 years ago; I have kicked myself ever since for not purchasing it. It might have been a reprint or re-issue of a series of articles from one of the railway or engineering magazines of the era. Any ideas?

Thanks for the information; please keep me in the loop as you discover more.

[prr60]

One of the better references concerning the 1930's PRR electrification expansion is a Railway Age article from February 25, 1933. It's a 34 page technical description of the project planning and installation. I have a faded but still readable copy in my office that I obtained from Gibbs and Hill, the original design consultant for the 1930's work. G&H has since been absorbed by the Washington Group.

You are correct about the Paoli electrification pre-dating the bulk of the PRR system. Both Paoli and the PRR Chestnut Hill line were electrified circa 1915. A give-away is the pipe-section catenary columns. These 90 year old structures are a ticking time bomb for Amtrak and SEPTA. They have some nasty corrosion including hidden problems under attachment collars. The later work used standard rolled steel sections, primarily wide-flange I-beams, which have proven remarkably rugged considering the almost total absence of preventive maintenance over the last 40 years or so.

[Jersey_Mike]

The 1920 electrification to Wilmington also uses steel pipe sections. The CHW branch was electrified in 1918 and there are some pipe based supports on the Connecting Railroad as well.

[Bill West]

Hello PRRTechFan, Central Electric Railfans' Association, Chicago, Bulletin B-118, 1979. Abebooks.com will give you a list of used book dealers having it, that's how I got mine this year.

PRR60, yes Railway Age's article is excellent. I walked in and copied it at my University Library after checking their online catalogue under periodicals . My City Library has some as well. Transactions of the American Institute of Electrical Engineers (AIEE, now IEEE) also has some good papers. I copied about 10" of paper at my company library. Some day I'll have to type out a bibliography to post.

Happy New Year, Bill

Late PS: PRRTechFan, is the other title you mention perhaps "Electric Traction on the Pennsylvania Railroad 1895 - 1968" by Michael Bezilla ? It covered the areas you mentioned.

[Bill West]

Hello Mike, from your Dec 29th:

1A. -Running the 44kv in parallel with the 132kv from Frazer to Zoo is a system planning question. Plus -the more ties the better the reliability, Minus -the more ties the higher the short circuit current and the cost of breakers, Iffy -the load will divide according to the impedance of each route, if this doesn’t match the ratings then one route may overload way before the other. My observations are that small lines always overload before large parallel ones reach maximum. It’s surprising because the 132-44 transformers at each end add quite a bit of impedance. I’ve never seen info on what PRR did here let alone Amtrak/SEPTA.

1B.-The original transformers were two 2000kva indoors. 1935 shows that capacity and two more 3000kva units, presumably outdoors. It would be easy for the originals to have lasted to the 60-70’s.

2. -PRRTechFan has covered PRR’s cat feed system quite well. A significant point is that a given sub only feeds the whole load when a train is in front of it. When the train is mid way between subs you only have half the load coming from each station, the line length is one half of the spacing distance and both transformers at each station are participating. So the voltage drops can be kept down when the whole RR is on the same phase, something that Utility based 60hz systems spread over 3 phases can’t do.

Amtrak’s Boston system provides an example of the multi phase approach. Each example of similar systems I’ve seen adds tie CB’s for flexibility so the layouts end up being similar. There are two ways to operate the ties, open at supply subs or open at tie subs. I’ve used phases A-B-C in pairs here because single phase RR loads are always connected phase to phase at the utility infeed.

......................................................each effective cat section is:
.....................................................”center fed”.......”2 end fed”
.......................................................tie sub open .....supply sub split
.......................................................phase=.............phase=

.....................transformer 1...............AB...................AB
supply sub 1..............tie CB...............closed..............open
.....................transformer 2...............AB...................BC

tie sub 2...................tie CB...............open.................closed

.....................transformer 1...............BC...................BC
supply sub 3..............tie CB...............closed..............open
.....................transformer 2...............BC...................CA

tie sub 4....................tie CB..............open.................closed

.....................transformer 1...............CA...................CA
supply sub 5..............tie CB...............closed..............open
.....................transformer 2...............CA...................AB

tie sub 6....................tie CB..............open.................closed

There are also 2-3 autotransformer stations on each leg.

So the load is either fed from 2 transformers in one sub with the higher voltage drop of a radial line or 2 subs share the line load but each can only use one of it’s transformers. For all of the effort in this system its 25/50kv 60hz only achieves an average supply/auto/tie station spacing similar to what the PRR has! The possibility of switching tie CB’s for trouble requires open cat breaks at all stations even though half of them are normally on the same phase. This doubles the already frequent power offs, and pushes for HEP to have ride through capability. Distributed three phase supply has made the Boston route into 4 localized single phase systems that can be stretched when outages need covering.

I have not seen info on which ties Amtrak has normally open. The phasing cannot be told from photos of the Utility connections because transmission line phasing varies all over, it’s too costly to straighten it before entering a station. The fact that the transmission lines are only 2 wire has been seen however. The center feed approach was hinted at in a Boston Edison objection and for Amtrak’s layout of starting and finishing with end tie stations it would cover single supply line or transformer failures. The even division of the load over the 3 phases is non existent because the train load is not steady in timing or location across the railroad and the 3 supply subs needed for a balanced set are 80 miles apart. This system works simply because the load is low enough that the Utility’s higher voltage lines can cope with the unbalance. I think the same is true of MNR’s system too as well as Amtrak’s Harlem River branch. NJT’s M&E lines were definitely planned this way.

Amtrak published a scale sketch of their route in Dec 1999 at http://www.oig.dot.gov/StreamFile?file= ... 000020.pdf . Some of the quality is unfortunately poor. Http://local.Live.com has good views of the MA and RI stations. Warwick has a utility connecting station 500’ west of the RR sub. Amtrak had to pay for this station when their low utilization put the Utility revenue below the cost recovery rate that the PUC had approved of. New London is novel because the track connections are raised above an embankment 1-2 blocks east of the sub, Ken W2KB has a picture of this at http://w2kb.home.comcast.net/fur_friends_and_family.htm Branford is divided by the turnpike. There are 2 sets of harmonic filters and power factor capacitors in Amtrak’s supply switchyards and 50kv autotransformers at the tie stations. Unlike PRR the circuit breakers are all indoor. Roxbury is impressive for its clean lined, compact fit in a parking strip.

Bill

[JimBoylan]

I can just imagine one of those little "postcard" electric bills.... To Amtrak, 30th Street, Philadelphia PA

I did get to tour the Reading Co's. Wayne Jct., Pa. substation and saw the Philadelphia Electric Co. meter used for billing their catenary power. The S.E.P.T.A. representative wondered if a magnet would make it run backwards. The Reading rep said it was made of alluminum, without explaining why he knew!

[JimBoylan]

are the transmission lines along the Jamesburg branch still utilized for anything?

One of the 2 wires is broken one pole east of the New Jersey Turnpike (just North of Exit 8-A).

[JimBoylan]

I'm pretty sure ANY of the Silverliners can at least tolerate 60hz

The early Reading Silverliners had a manual rotary changeover switch in a cab cabinet to do this. A PennCentral official (Gary Pfeifer) said that the top speed would have to drop (or be dropped?) from 100 to 85 m.p.h. if this was done.

[Nasadowsk]

<i>The early Reading Silverliners had a manual rotary changeover switch in a cab cabinet to do this.</i>

IIRC, that's the SL IVs that had that switch, and only in the single car units.

<i> A PennCentral official (Gary Pfeifer) said that the top speed would have to drop (or be dropped?) from 100 to 85 m.p.h. if this was done.</i>

I'm not too certain - it doesn't matter much to the propulsion system, though I do know GE recomended a downrating on 60hz 12kv for the II/III cars. This might have realistically translated into a clamp on top speed, but it would be to protect the main transformer. I don't know if the PC ever issued instructions on this. Heck, I don't know if they were ever even tested on 60hz or 25kv power (the Arrow IIs did both) The SL I (Pioneer III, MP-85...) cars were bound to 25hz because their transformers were never rated for 60hz operation and they had a few induction motors in them.

The SL II/III cars always had a top speed of 85, enforced by the speed alternators on the gearboxes. This was a result of a few incidents where a PIII got over it's TM redline of 6500rpm. They were light and had the power to get up close to 100.

The IVs topped at 100 and were designed for it.