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Why 78.4?
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Topic: Why 78.4? (Read 8001 times)
Zach Gomulka
Hero Member
Posts: 6992
Don't piss down my back and tell me it's raining.
Why 78.4?
«
on:
August 15, 2010, 01:14:58 am »
Something that's always bothered me. Who came up with 78
.4
, and why?? No other stroker crank was given extra tenths of a millimeter. Why is 78 so special??
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Born in the '80s, stuck in the '70s.
neil68
Hero Member
Posts: 538
Re: Why 78.4?
«
Reply #1 on:
August 15, 2010, 02:16:03 am »
Some Porsche's featured 78.4 cranks and I think it goes way back to Oettinger, SPG, etc, and others. Interesting that some Mazda's also use a 78.4 stroke. A "tenth of a mm" that is not rounded off to 0.5 or 1.0 is fairly common. I believe Scat sells a 78.8 mm crank...
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Neil
Der Kleiner Rennwagens
'68 Beetle, 2332 cc, 204 WHP
12.5 seconds @ 172 KM/H (107.5 MPH)
Dynojet Test:
https://www.youtube.com/watch?v=M9B_H3eklAo
Taylor
Hero Member
Posts: 577
Re: Why 78.4?
«
Reply #2 on:
August 15, 2010, 02:58:43 am »
I always thought it was because 78 mm is to long for standard pistons and too short for strokers?
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Bruce
Hero Member
Posts: 1420
Re: Why 78.4?
«
Reply #3 on:
August 15, 2010, 03:38:05 am »
I thought it originated back before you could get CIMA 90.5s. Back then you used NPR 90mm pistons. With that bore and a 78.4 crank you get exactly 1995cc. So that stroke is the longest you use and still keep under the 2 liter limit that is common in many racing classes.
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deano
Hero Member
Posts: 1851
Re: Why 78.4?
«
Reply #4 on:
August 15, 2010, 04:57:32 am »
Bruce, you maybe close... But, the 78.4mm stroke dates back beyond/older than the 90mm bore.
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Anglia Obsolete Guru
'67 Heaven
kingsburgphil
Hero Member
Posts: 876
Re: Why 78.4?
«
Reply #5 on:
August 15, 2010, 05:12:07 am »
Way back then you could build a welded 78x92B motor as almost easily a a stocker. With low compression, modest cam/carburation, one could venture hours out into the Badlands. Flogging the engine at will, protecting only the tires...and knowing you'll make it back to camp. At the time, the big, slow turning and cheap 2074 was a popular combination for offroad enthusiasts. As to who tacked on the .4, your guess is as good as mine.
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TexasTom
Hero Member
Posts: 1518
12.58@106, 7.89@89 Texas Motorplex 10/18/09
Re: Why 78.4?
«
Reply #6 on:
August 18, 2010, 19:13:01 pm »
My guess would be it came out of the gas classes back in the good old days.
I'm sure there was some advantage to the extra .015-.016 inch. Also, there's a near perfect 1.75:1 rod ratio when using stock 1600 length units.
True innovation back then ...
«
Last Edit: August 18, 2010, 19:40:30 pm by TexasTom
»
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Speed-Randy
Hero Member
Posts: 980
Re: Why 78.4?
«
Reply #7 on:
August 21, 2010, 05:58:15 am »
because 78.3 wasn't enough
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deano
Hero Member
Posts: 1851
Re: Why 78.4?
«
Reply #8 on:
August 25, 2010, 00:38:19 am »
Okay, according to Ron Fleming, this stroke goes back to a time when the German TUV had stricker rules and regulations for cars fitted with an engine larger than 2000cc. So, at a time before the 92 cylinder (late 1968), Okrasa/Oettinger, SPG, etc., slightly increased the stroke to bring the total engine size just below the 2000cc limit.
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Anglia Obsolete Guru
'67 Heaven
Zach Gomulka
Hero Member
Posts: 6992
Don't piss down my back and tell me it's raining.
Re: Why 78.4?
«
Reply #9 on:
August 25, 2010, 02:32:56 am »
Quote from: deano on August 25, 2010, 00:38:19 am
Okay, according to Ron Fleming, this stroke goes back to a time when the German TUV had stricker rules and regulations for cars fitted with an engine larger than 2000cc. So, at a time before the 92 cylinder (late 1968), Okrasa/Oettinger, SPG, etc., slightly increased the stroke to bring the total engine size just below the 2000cc limit.
So Ottinger/Okrasa 90's I presume? Thanks for the info, Dean!
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John Rayburn
Hero Member
Posts: 2481
Der Kleiner Panzers
Re: Why 78.4?
«
Reply #10 on:
August 25, 2010, 05:26:42 am »
Quote from: vdubdweeb on August 21, 2010, 05:58:15 am
because 78.3 wasn't enough
So, Randy was right.
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Diederick/DVK
Hero Member
Posts: 3692
They're never done till they're sold
Re: Why 78.4?
«
Reply #11 on:
August 25, 2010, 12:32:36 pm »
Sounds reasonable! 78.4 x 90 = 1995 cc
But if so, why on earth can we now get a 78.8 crank
I think only Scat is selling those, though.
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Diederick
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Proud member of:
DVK ~
Der Vollgas Kreuzers
deano
Hero Member
Posts: 1851
Re: Why 78.4?
«
Reply #12 on:
August 25, 2010, 14:08:02 pm »
Quote from: 67-indeed/DVK on August 25, 2010, 12:32:36 pm
Sounds reasonable! 78.4 x 90 = 1995 cc
But if so, why on earth can we now get a 78.8 crank
I think only Scat is selling those, though.
As Tom Hanks once said, "220, 221, whatever it takes...."
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Anglia Obsolete Guru
'67 Heaven
Chuck Fryer
Full Member
Posts: 148
Re: Why 78.4?
«
Reply #13 on:
August 27, 2010, 19:22:32 pm »
psst.... It was Michael Keaton, Mr. Mom ,)
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Speed-Randy
Hero Member
Posts: 980
Re: Why 78.4?
«
Reply #14 on:
August 29, 2010, 14:56:50 pm »
Quote from: John Rayburn on August 25, 2010, 05:26:42 am
Quote from: vdubdweeb on August 21, 2010, 05:58:15 am
because 78.3 wasn't enough
So, Randy was right.
exactly
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Harry/FDK
Hero Member
Posts: 3613
Every Rule Was Made To Break, Even Callook...
Re: Why 78.4?
«
Reply #15 on:
September 02, 2010, 18:19:41 pm »
Scat 78.2
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Speed-Randy
Hero Member
Posts: 980
Re: Why 78.4?
«
Reply #16 on:
September 04, 2010, 03:48:52 am »
Quote from: FDK/Hurry on September 02, 2010, 18:19:41 pm
Scat 78.2
because 78.3 was to much
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www.vdubspeedshack.com
stoneloco808
Newbie
Posts: 38
Re: Why 78.4?
«
Reply #17 on:
September 04, 2010, 07:23:07 am »
I thought the 0.4 or 0.8 mm extra was just for the bench racers. Example;
Guy 1; I have a 78x94, thats 2165cc's.
Guy 2; You will need some window cleaner to clean your windshield very good. Just so you could see my rear plates and figure out this 78.8x94, 2187cc's, will be spankin' that ass everytime.
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