I was recently involved in a discussion about rod ratios. The usual arguments for Long and short ratios were coming up and I just had to stop myself and ask.... what difference are we talking about. So I put together an excel spread sheet to calculate what it all meant.
We all know the benefits of long rod ratios in performance applications... reduced angularity,increased dwell time at TDC, a lower Peak Piston Velocity and more time with the rod under pressure from the piston.
But what does it take, how much does it actually change things?
First off, Rod Ratio is the length relationship between the stroke of the crankshaft and the length of the conrod. To calculate it, you divide the length of the conrod by the length of the stroke. Engines in general (petrol, diesel, 2 stroke and 4 stroke) will run ratio's between 1.5 and 2.7. With a long rod ratio, the piston will reach lower peak speeds and will spend more time at top dead centre, a shorter ratio means less time at TDC and higher piston velocities.
The faster the piston moves down the bore, the greater the demand it places on the port, the faster the pressure drop and the faster the port velocities and the faster the cylinder fills. This is attractive for street applications because it means you don't need to rev the life out of the thing to make torque (also attractive with big cam street applications as the port energy remains higher and it overcomes low rpm reversion quicker). The draw backs with a high velocity short rod ratio scenario are the increased total rod angularity (side thrust into the bore wall) and the decreased dwell at TDC, which can effect combustion efficiency if all other aspects of the combustion space are in order... combustion space being the geometry of the chamber, the piston crown and the amount of quench area... the example I bought up the other day was the 239 flathead ford which has a rod ratio of 1.9 but the second worst combustion space geometry in the history of engines so it still only made like 70hp lol. The other consideration is that at high RPM the piston acceleration will be higher and piston reliability will suffer (also more prone to ring flutter which is potentially fatal).
So, for an engine designed to spend its life under 5,000rpm, a rod ratio on the shorter side starts sounding appealing right? Well, there is the argument that the long rod ratio will actually provide better torque through improved combustion efficiency and reduced mechanical losses through reduced thrusting.
The examples that were bought up were NASCAR and F1 engines that run 1.9 to 2.4 (these days in excess of 2.7... the specs I could find were on engines running 2.56 with a 1.565" stroke). NASCAR engine specs are set by their formula (358CUI max and a max bore diameter of 4.185 meaning a stroke of 3.25..... less than a standard SBC or LS by 0.230" to 0.370".... they're a short stroke engine)
When I ran through the numbers, I compared a standard G200 (this is a 4 cyl, 2 litre, 8 port engine with an 87mm bore, 82mm stroke and 133.5mm rod) to a G200 with a modified rod ratio of 1.8 (from 1.628), and an F1 engine with a 1.565" stroke and 4.015" rod (2.565 rod ratio). The results of these comparisons are in the images attached to this. I've put together a graph showing total piston movement and velocity at crank degrees between TDC and BDC, there is one there which zooms in on the dwell events around TDC showing 15° either side and what the piston speed is, and one comparing rod angularity. With any rod ratio combination max angularity is always at 90 degrees. This is the point where side thrusting is at its worst. The graph shows how many degrees of thrusting are occurring 10° either side of 90°ATDC.
All examples are provided at 4,000rpm for comparison.
The things that are immediately obvious are the slower peak piston velocities with the longer ratios. In a high RPM application this is fantastic. It means that you can rev the engine higher, with more reliability. Reliability is determined by MPS (mean piston speed) and Piston acceleration (the G Force it experiences... which I would compare to the ballistics of a cannon ball!). So with a lower Peak Piston Speed, there is less acceleration and lower MPS's (when they're calculated properly and not just by using the short hand formula). How much lower? Well, at 4,000rpm there is a difference of 27.2 fpm.... when you consider that the PPS is ~3,500 fpm at 4,000rpm, 27.2 is nothing. The difference between the stock G200 and the F1 engine however is 1,834.2 fpm...... now THAT is worthwhile, and what that means is that you would need to rev the f1 engine to 8,500rpm before that piston was feeling the same g force as a G200 would at 4,000rpm..... in other words, the F1 piston is more reliable at 8,000rpm than the G200 is at 4,000rpm! That is a big plus when the redline is set at 18,000rpm.
The next thing that's obvious on the graph that focuses on dwell is that there is a change in dwell by going to a long rod ratio. Between the 1.63 and 1.8 dwell figures, it is obvious that the approach and departure speeds from TDC are slower meaning it spends more time there.... between 15° before and after TDC its average velocity is 2.1% slower . In the same 15° before and after TDC zone, the 2.56 rod ratio'd engine is 55.12% slower than the standard g200 1.628. In the case of the F1 engine, that is precisely what it needs because at full noise the piston is only at TDC for 1/3rd the time it will be in a G200 at full noise (18,000 vs ~6,000rpm).... it has 2/3rds less time to ignite the fuel, burn it, develop pressure and start sending the piston on its way again. A difference of 2.1% between the 1.628 and 1.8 rod ratios with a max engine speed of probably only 8,000rpm is going to equate to very little.
The next point is rod Angularity. It is apparent that there is less thrusting with the long rod combinations. The maximum difference between the 1.63 and 1.8 is 1.7° and the 1.63 and the 2.56 is 6.49°. As percentages, that is 9.6% less between the 1.63 and the 1.8 (not bad!) and 36.6% between the F1 and the standard G200. In this instance I would say that a reduction of nearly 10% is a good thing and worth it.. though it should be kept in mind that this does not represent 10% of all frictional loss in the engine, just 10% on the pistons skirts in isolation. As we all know there are far bigger proponents for friction (rings, rear main seals, valve springs, oil pumps, compression strokes etc). The actual amount of ft/in (not foot pounds.... feeet inches) that could be saved by reducing 10% from the skirts which thrust at a MAX angle of 16% would need to be calculated by someone with a more time than me (it's taken a day and a bit to put all this together! let alone working out coefficient of friction from the skirts!). Would it amount to 1hp? Look, lets be real, probably not, not when you still have far worse sources of friction in the engine... but as we all know , its the factions that get you over the line... and as Ricky Bobby would say "If you ain't first, you're last!”
The next factor is the $/hp ratio. The cost of getting custom pistons and rods is going to set you back the best part of $2,000 (Australian dollars, about 10GBP) if not more. Typically brands like Argo and Carrillo are going to charge around $300(AUD) per rod and pistons will be around a grand on their own.... so that you can pick up a fraction.... is this going to be worthwhile for an aussie battler? Hell no. Buy a decent manifold, cam and exhaust, you'll actually be able to measure the difference!
We're all guilty of hearing and repeating the same information.. I'm just as bad as the next guy. In fact, several years ago I nearly replaced the rods in a Holden 253 with SBC rods I'd been convinced that the Chev rod (being 0.080" longer) would actually make a measureable difference! lol
It pays to slow down, and ask yourself... What are we actually trying to achieve, and is it measurable!
Edited by AKat, 13 February 2019 - 05:52 PM.