140/145mm conrods for 81mm crank?

[E30 Wagen]

135mm rods seem to be the popular choice (I already have a set of s52 rods and eta crank) but I think it's worth getting the best rod/stroke ratio as possible. I see s54 are 139mm, m50tub25 are 140mm, m52tub20/b25 are 145mm. Has anybody played around with these at all?

[digger]

with a 145 mm rod the piston comp height needs to be 20.5 mm which wont work well with a proper 3 ring pack.

with 140 mm rod it is 25.5 mm which is doable but getting tight and you will need a smaller piston pin and oil ring rails as the pin interferes with bottom ring.

it would be worth using a comp height that small if you had to because of a long stroke but with 81 mm crank using a 135 mm rod has much more flexibility and you shouldn't need a oil ring rail. you wont notice any extra hp as the differences are small. you certainly wouldnt want to compromise ring seal by using an excessively small compression height IMO

MM use a 140.5 mm rod with 84 mm crank which has a 23.5 mm comp height and 138 mm rod with the 89.6 mm stroke which is 23.2 mm comp height. None of their strokers make impressive power for all that is done to them. lots of reasons for this but dont buy into all their marketing.

BMW never worried about rod ratio, they went to as low as 1.44 AFAIK. 1.67 with the 81mm/135mm combo is hardly "bad". they used a longer rod in the taller deck motors that had shorter stroke because while you dont want a very small comp height you dont want a very tall height either

if you are set on using a longer rod then you need to talk to a piston guy so they can tell you what is possible.

[SkiFree]

I concur with digger, when playing around with longer rods on the 81mm stroke the ring packages get rather compact and greatly affect longevity without much power gain. You could maybe pic up a couple ponies, but at too great a cost.

[nando]

Use the s52 rods.

[E30 Wagen]

Thanks for the reply, digger.

I'm wondering now if running the 145mm rod with the stock 75mm crank would have any noticeable benefits. I'd go with 145mm because they are a stock BMW part that can be found cheap, and aren't a bulky/heavy design. With a good piston I think it could make the engine rev smoother and be more responsive, which is a quality that I'm starting to favor over brute power. It's better for long term engine health, and with a well modified cylinder head will still make power comparable to a stroker. Am I right?

I also get to wondering if I could get my hands on a m20b23 crank, what would be the longest rod I could run with that.

[nando]

Why? Seems like so much effort for no gain. 323 crank is 76 mm. The eta crank will hardly give you brute force power.

Really, in the context we are talking about, the extra long rods aren't worth the trouble.

I've put almost 90k miles on my 81mm crank with s52 rods. Realistically, there's no long term difference.

[E30 Wagen]

Why? Seems like so much effort for no gain. 323 crank is 76 mm. The eta crank will hardly give you brute force power.

Really, in the context we are talking about, the extra long rods aren't worth the trouble.

I've put almost 90k miles on my 81mm crank with s52 rods. Realistically, there's no long term difference.

323 crank is 76.8mm. I believe Hartge, Alpina, or ACS built some engines using that crank. Seems like a good option for a mild bump in displacement for a b25.

Having no experience with a m20 stroker, I'm just exploring the possibilities and characteristics of each combination. Even though BMW has built engines with worse r/s ratios, I think it's absolutely vital to consider how the rotating assembly will affect the overall behavior of the engine. I want my engine to leap to 7k rpm with a very smooth power delivery. I also want it to last 490k. I'm not entirely convinced the popular 135/81 can do that, but what do I know.

[digger]

the biggest issue with life on the m20 is not the bottom end of the motor, its the top end. i doubt you will see any tangible benefit with a longer rod. the doomsday greenies/climate changers will have all the old cars off the road before you do another 490k.....

if you were building a very high rpm motor and had to stay at 2500cc or below then it would make sense to see if a longer rod was beneficial for power right at the topend as all you will notice ( if anything) is the engine might hold onto the power better and not nose over as fast. you need to probably do a couple iterations of camshaft profiles with a custom cam grinder to get things sorted enough to benefit.

if going to a custom piston you want a 84mm crank as a minimum IMO even then its hard to justify the cost for pistons when you can do it in a proven way with basically all OEM parts.

why not build a M20B22 using the B20 66mm crank then you can use a 145mm rod relatively easily