Rod lenght or the rod ratio : 140mm vs 135mm rod is 5mm going to make a difference

[retoropak]

I am building a stroked m42. My crankshafts are already on the boat from europe. I need to get the pistions and rods.

Question is should i go with 140mm or 135mm rods.

Ill be running 9 or 9.5 compression, e85 and fairly large amounts of boost. Rev limit will be set to 7.5k rpm.


longer rods will place the pin higher in the piston and probably reduce some crown thickness.


Whats your opinion?

thank you

[digger]

worry about something thats going to make a difference, i.e something else

[retoropak]

worry about something thats going to make a difference, i.e something else

Could you elaborate? Why do you think that it wont make any difference? Thanks Sent from my iPhone using Tapatalk

[ForcedFirebird]

Rod stroke ratio isn't as important as many make it out to be. Look at the s54 which is 1.53:1.

https://motoiq.com/rod-ratios-ej205-stroker-engine/

A rod to stroke ratio of 1.7:1 or higher is a good general starting point for a performance engine. Although many respectable production engines don’t meet this. For example, the Honda B18C is 1.58:1 and the K20A2 is 1.62:1. The Chevy LS7 is 1.53:1 and the BMW S54 is 1.53:1.

[digger]

Could you elaborate? Why do you think that it wont make any difference? Thanks Sent from my iPhone using Tapatalk

its not that it makes no difference but its small and its swings and roundabouts

You haven't stated what crank you are using but even with the 88 mm one based on the ample deck height of the block you should have a more than adequate compression height with either option (~28 mm with 140 rod or ~33 mm piston CH for 135 rod) for a turbo piston. On a smaller deck height platform e.g. m20 you’d definitely run the shorter rod as the piston CH will be too short for a good forced induction piston. Ultimately you want the best piston and the rod whatever out works out to be just connects the piston to the crank. We aren’t building formula one engines.

The difference in piston position vs crank angle difference due to a 5 mm rod length chance is actually miniscule and a second effect at best so engine breathing is not affect nor the burn due to the dwell at TDC the internet things is a big deal. The only time a difference may show is with a really crappy and small port with a 2V per cylinder engine as the very slight extra depression can be enough to make a bad port a POS. A 4V engine likes short rods as ports are usually oversized from the factory.

The shorter rod does have a bit of extra friction because of the angularity and taller piston skirts but its not enough to worry amount IMO with that ratio OEM bottom ends last hundreds of K’s before eating themselves.

If you were really worried about friction you wouldn’t be building a stroker in the first place as the stroke increase from 81 to 88 mm itself will increase friction by a much greater amount than the change in rod angularity of a couple degrees because of the Mean Piston Speed and ring drag of the longer stroke.

[retoropak]

its not that it makes no difference but its small and its swings and roundabouts You haven't stated what crank you are using but even with the 88 mm one based on the ample deck height of the block you should have a more than adequate compression height with either option (~28 mm with 140 rod or ~33 mm piston CH for 135 rod) for a turbo piston. On a smaller deck height platform e.g. m20 you’d definitely run the shorter rod as the piston CH will be too short for a good forced induction piston. Ultimately you want the best piston and the rod whatever out works out to be just connects the piston to the crank. We aren’t building formula one engines. The difference in piston position vs crank angle difference due to a 5 mm rod length chance is actually miniscule and a second effect at best so engine breathing is not affect nor the burn due to the dwell at TDC the internet things is a big deal. The only time a difference may show is with a really crappy and small port with a 2V per cylinder engine as the very slight extra depression can be enough to make a bad port a POS. A 4V engine likes short rods as ports are usually oversized from the factory. The shorter rod does have a bit of extra friction because of the angularity and taller piston skirts but its not enough to worry amount IMO with that ratio OEM bottom ends last hundreds of K’s before eating themselves. If you were really worried about friction you wouldn’t be building a stroker in the first place as the stroke increase from 81 to 88 mm itself will increase friction by a much greater amount than the change in rod angularity of a couple degrees because of the Mean Piston Speed and ring drag of the longer stroke.

Ill be using 88mm crank. I thought about 135 m50nv or eagles rods or cx 140mm rods N50nv are known to support 100whp per hole. They are a bit on a heavy side tho. I am planning for 400+ whp. Thanks you very much for a detailed response. Sent from my iPhone using Tapatalk

[TobyB]

Everything else being equal I would use a longer rod.

But everything else is NEVER equal.

If one is cheaper, go that way.

If the longer rod makes for a thin crown or small top land, don't do it.

If one piston or the other has a better shape for boost, do that.

See where we're all heading?

The people who were selling rod ratio had already done everything else.
(I'm thinking Smokey Yerballs, here, of course)

t