Why 78.4?

[Zach Gomulka]

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??

[neil68]

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...

[Taylor]

I always thought it was because 78 mm is to long for standard pistons and too short for strokers?

[Bruce]

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.

[deano]

Bruce, you maybe close... But, the 78.4mm stroke dates back beyond/older than the 90mm bore.

[kingsburgphil]

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.    

[TexasTom]

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 ...

[Speed-Randy]

because 78.3 wasn't enough

[deano]

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.

[Zach Gomulka]

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!

[John Rayburn]

because 78.3 wasn't enough

                                                 So, Randy was right.

[Diederick/DVK]

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.

[deano]

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...."

[Chuck Fryer]

psst.... It was Michael Keaton, Mr. Mom ,)

[Speed-Randy]

because 78.3 wasn't enough

                                                 So, Randy was right.

exactly

[Harry/FDK]

Scat 78.2 

[Speed-Randy]

Scat 78.2 

because 78.3 was to much

[stoneloco808]

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.