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A Series Rod Ratios - By The Numbers Part 2


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#1 AKat

AKat

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Posted 15 February 2019 - 05:40 AM

First part of the write up I did on this was based on G Series engines and I guess it probably didn't seem super relevant to this crowd. 

I've redone it with new data that captures the 998, 970, 1098, 1275 and for a wild card the 350 Small Block Chevrolet as a basis for comparison with an engine that is fairly well known around the world.

As before, the results are calculated at an engine speed of 4,000RPM.  This set really tells a good story about how rod ratios impact on induction, combustion and torque curve.
 

 

 

Rod Ratio

998 (1.91)

970 (2.41)

1098 (1.74)

1275 (1.79)

350 SBC (1.63)

PPS

998           3247.1 @ 76°

970           2606.4 @ 79°

1098         3591.3 @ 75°

1275         3478.9 @ 75°

350 sbc   3811.4 @ 74°

MPS

998          1977.75

970          1606.60

1098        2172.96

1275        2109.45

350sbc    2319.76

Max Angularity

998         15.12°

970          11.97°

1098        16.65°

1275        16.15°

350sbc    17.77°

 

Dwell Speed Average

998         530.12

970         412.588

1098       594.18

1275       573.17

350sbc   636.36

 

Dwell In Piston Position at 15° of TDC

998        0.064"

970        0.049"

1098      0.071"

1275      0.069"

350sbc  0.077"

 

 

The results above show Peak Piston Speed (and at how many degrees ATDC...), true Mean Piston Speed (not the short hand formula), Maximum Angularity between the Conrod and the Bore, Average Piston Velocity 15° either side of TDC, and how many thousandths of an inch the piston ascends/descends within 15° of TDC.

So it is apparent from these results that a long rod ratio is a good thing for reducing piston thrust, it provides more time in the vicinity of TDC meaning better flame propagation, a good thing for piston reliability.  Manufacturers and race teams look at MPS more than PPS when deciding on the best pistons to use for an application.  A very rough, very general rule of thumb is that most OEM style pistons will tolerate sustained MPS's of 4,000fpm, a good set of street forged pistons 5,000 and a set of competition pistons should manage 6,000fpm. 

The plus side of long rod ratios, is also the down side of long rod ratios.... depending on the application.  As stated above, a long ratio is more reliable at high rpm due to reduced piston velocity.... the flip side of that, is that at low rpm, the decreased cylinder pressure decay rate is going to reduce the speed of air entering the engine and the "port energy" (the momentum of the charge air mixture).  What this means is that it will take longer to develop torque and when coupled with a lopey cam, it also takes longer to overcome the effects of reversion, so it takes longer for it to come on the cam.

Shorter ratio's are also attractive (and this is subjective) for turbo charged applications for the same reason, the higher port energies decrease reversion from the exhaust port (which is under pressure) back to the combustion chamber during any overlap.... basically, the air is pushed out harder with more momentum so its harder for it to turn around and head back to the combustion chamber.  If you look at YB and BDA cosworth engines, they run Rod Ratios in the high 1.6's, because there was always the intention to turbocharge... same as the Nissan SR20, it was built with turbo charging in mind, and they run a rod ratio of 1.58.

When you look at the test results, the real elephant in the room among the subjects is the 350 SBC.  I did not choose this engine at random. 

In popular engine building culture, Rod Ratio first made its way into the lexicon back in the 1960's and 1970's due to one of its earliest advocates for using it for performance enhancement,  Henry (Smokey) Yunick.  If you're reading this in the UK.... Imagine if Collin Chapman was a smart ass cowboy from America.. That's Smokey Yunick.  He was most famous for his work in the NASCAR series.  Popular engines at the time were Fords, Chevs, Chryslers, and Pontiacs.  When you look at the results from the 350, they're not brilliant in the rod ratio department, but not much different to anything else that was around at the time.  But, by using 6" Rods (standard item) instead of the 5.7" that the 350 came with, a 302 SBC crank and 4.25" pistons in plus 0.060", you can still achieve 350cui... but now with a rod ratio of 2.0!  At 8,000rpm, this is worth a saving of 1,135fpm peak, adds 0.014" to the 15° dwell mark, and takes max angularity from 17.77° to only 14.47°.  Now, it's important to keep in mind that this is a long way from being the only modification that Smokey did to his engines that was unique (he was also called a cheater all the time for some of his tactics).... BUT it was A factor in how he got the extra power and reliability. 

But what if we back it up for a second.  What if you just use the 6" rod in a 350 with a normal stroke like everyone on the internet tells you to because they heard from a friend of a friend that it will guarantee you an extra 10hp... Well, rod ratio goes from 1.63 to 1.74 impressive right?... hmm keep reading.... the dwell is now only 0.001" better, the PPS only drops 32.2fpm, the angularity only improves 0.912°......... "but my friend said" -  your friend can pay for the parts mate, because that adds up to squat! 

So lets come back to the A Series results.  Looking at all 4 of these engines, and touching on what I mentioned in Part One... These engines collectively have long rod ratios.  Even the "worst" is still better than most popular performance engines.  The best is actually better than the Cosworth DFV (which had a rod ratio of 2.05) and is still on par with a lot of the F1 engines from the 90's!

Looking at the effects the rod ratio has on piston speed-cylinder pressure decay-port energy... You only need to look at those graphs and figures for a second to see why you can not try and compare a 998 to a 970 just because they're only 28cc's apart.  Yes the 970 came out with an AEG163 head which flowed many more CFM than the 12G940 or the 12G295 and this will have an effect on the way it produces torque, but it could afford to because you would need to rev the thing 1,300rpm higher just to achieve the same decay rate as a 998 (and its Pressure Decay - Pressure Differential - Δ p, that fills cylinders, not CFM!).  Ultimately a 970's MPS is that much slower than a 998 that it will tolerate a sustained 8,000rpm on even the most ordinary cast pistons penny's can buy. 
 

For anyone curious, this is a comparison between a 970 cooper and a Cosworth DFV and there is a velocity comparison in the graphs below.
 

 

 

 

 

Cosworth DFV

Bore 3.373"

Stroke 2.55

Rod Length  5.23

 

970 Cooper

Bore 2.779

 

Stroke 2.437

 

Rod Length 5.875

 

  

 

                      

So for anyone considering a long rod swap in an A Series in the pursuit of better rod geometry, I hope you find this useful.... and bear in mind the 350 SBC comparison, after all their stock rods are 5.7" -

Attached Files


Edited by AKat, 15 February 2019 - 06:11 AM.





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