Rod to Stroke Ratio

[spens]

Hello,

I have been wondering about applying the idea of increased rod to stroke ratio to a Fiat 124 1756cc TC. Based on both verified and unverified data I think one could get close to a rod to stroke ratio of 1.967 using stock parts and custom longer rods. The idea is to use high compression 84mm pistons, 155.8mm custom rods, and a 1756cc crankshaft in a 1995cc block.

Verified data:
- 90mm 1995cc stroke
- 145mm 1995cc rod

Unverified data:
- 79.2mm 1756cc stroke (10.8mm shorter than the 1995’s)
- Late 1756cc and 1995cc crankshafts are interchangeable

If that is true then a 1756cc crankshaft in a 1995cc block would need a 155.8mm rod (10.8mm longer than 145mm). Therefore, 155.8/79.2 = 1.967.

More hearsay is that with such a rod to stroke ratio, low octane fuel can be used with quite high compression ratios. This should result in the fun of a quick throttle response and the “revyness” of a shorter stroke but I have to wonder if losing the torque of a 1995cc is really worth it. That isn’t just rhetorical. I’m actually looking for comments as well as data verification.

An American hot rod magazine published an article in 1997 where the author said they applied much the same idea to a carbureted Chevy 350 cubic inch V8. They used a shorter stroke crankshaft and longer rods and were able to change the ratio from 1.64:1 to 1.91:1. Also, their compression ratio was 11:1 and with 36 degrees of total ignition advance they claim no preignition using 87 octane ( (R+M)/2 ) gasoline. American gasoline octane is rated by averaging the research octane number and motor octane number. Most cars over here use 87 octane which is the lowest available except at high altitudes.


Thank you,
Steve

[Guy Croft]

Re:

Unverified data:
- 79.2mm 1756cc stroke (10.8mm shorter than the 1995’s)
- Late 1756cc and 1995cc crankshafts are interchangeable

I can verify that is correct. Briefly, lest anyone forget, I'll just remark that of course putting an 1800 crank in a 2 liter block will effectively make the engine an 1800cc one, not a 2liter, because it will be nominally 84mm bore and of course 79.2mm stroke..

The reduced mass of the 1800 crank will probably balance out against the added mass of the 2 liter block. And naturally, whilst it might produce an exceptional 1800 (cc +/- depending on actual bore size) it will never have the torque of the 2 liter. It will give something around max 142lbf ft in peak trim whereas a comparable 2043cc 2 liter (85mm bore) can give 13lbf ft more than that and a bit more still at 2090cc (86mm bore)

GC

GC

[spens]

Hello Again,

Does anybody have either first hand or theoretical knowledge about fuel octane requirements versus rod to stroke ratio? It seems there may be a correlation.

In my previous post I mentioned someone who claimed no detonation using the lowest octane pump gas in a motor with a rod to stroke ratio of 1.91, 11:1 compression ratio and single carburetor. Also, for the sake of testing, they advanced the ignition timing to 36 degrees BTDC and still couldn’t detect detonation and did not suffer power loss.


Thank you,
Steve