Stroke to rod ratio

hey guys, i apologize if the topic has been discussed before but i searched and nothing came up.

while taking notes on the rod to stroke ratio tutorial, i thought i would calculate the LS7 rod to stroke ratio which came out to 1.51 (6.067" rod = 154.1mm and a 4.000" stroke = 101.6mm) now in the tutorial he mentions that a higher ratio of 1.75 is generally accepted for a 7-8k RPM engines and the LS7 is a 7k RPM engine but happens to have a ratio that is far away from that, any explanation for that ? i am assuming the ls7 being a pushrod engine has a part to do with that but i would like to know why if someone may assist with the knowledge id be thankful.

It's simply that there are physical limitations of the deck height* - the distance from the crankshaft centre-line to the top of the bore, the stroke of the engine, and the available distance from the piston/wrist/gudgfeon pin to the top of the piston - this takes into account the material required between the top of the piston and between the rings for strength, and how close it can be placed to the rings.

There may be aftermarket specialist suppliers who have pistons that allow slightly longer rods to be used, but that's not something I can comment on.

*it's an old engine, but the original "small block" Chevrolet engines all had 5.7" connecting rods, regardless of actual stroke, excep5t for the "400 CID" variation which had a much larger 3.75" stroke and used a unique shorter 5.565" (IIRC) connecting rods to fit the stroke in the engine's physical limits. It might be of interest that specialist aftermarket parts allow one to build a 4.0" stroke engine with "long" rods and very special piston designs. To put that in perspective, they were initially designed as a 265 CID engine, the smallest OEM was a 262 CID and these aftermarket parts allow for a, very expensive, 427 CID engine to be built!

thanks a lot for the reply and intensive details! that is interesting to know to say the least.

excuse me if i havent posed my question properly, i do understand the limitations in deck height when it comes to a longer rod and also the custom pistons to accommodate by basically moving the wrest pin location.

what i meant was, having a 1.51 ratio and revving to 7k seems to be kind of a no go area according to what i have learnt, meanwhile the LS7 does it all day so is an LS an exception to the rule? it surely did not seem that way in the video which made me wonder.

It may not be the most desireable, but if the assembly has the tensile strength for the "snatch"around TDC, and the piston side thrust isn't too much of an issue, it shouldn't bee too much of an issue.

There was a video from one of the 'engine' channels where they did three different length rods and piston combinations, can't recall the actual ratios, on a V8 engine and there was negligible difference in the power/torque curves to 7k rpm. The engines were otherwise identical, even the same heads were swapped between the short blocks.

Another reason, for some engines, may be it's cheaper to use the same rods and different pistons as the latter are usually cheaper to produce - don't think it applies here, but with some engines with different displacements the same rods are used.

RS ratio is more important to turbo engines than NA since cylinder pressure isn't that high in NA engines. It's all about side load and cylinder wall and piston rings wear. The less RS is and the more cylinder pressure is the quicker cylinder wall and piston rings will wear off. I've seen hi boost turbo engine with 1.5 RS ratio having very noticeable cylinder wall wear just under 6000 miles. Since pressure in combustion chamber is very high and the leverage of short rod is small it results in very high friction between cylinder wall and piston rings so those short blocks don't last very long - it's definitely not a good option for a high boosted engine but it's much less of a problem for NA engines. As Gord said power wise the difference isn't that big- shorter rod will give a little bit more torque at low RPM range whilst long rod will provide a little bit more power up top.

If you want to find out more about RS ratio effecting on engine performance you can download StrokOrNot pdf file from here

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://pdfcoffee.com/stroke-or-not-mitsubishi-4g63-pdf-free.html&ved=2ahUKEwiY0-rLk72OAxW7-gIHHYm8MfwQFnoECBwQAQ&usg=AOvVaw1t_8S6O2JfnSFqU78xgL5r

It's not the most scientific, definitive comparisons, but this is the video I was thinking of for the comparisons - https://www.youtube.com/watch?v=lGDM7A9KtU0

Oh, and Shota is 100% correct on the killer being cylinder pressure loading the major thrust side - it's why split bores, etc, always happen on that side!