Stroke rod ratio

[CleverHans]

I have been asked to build a BMW performance engine (low boost turbocharged) using the M10 block and the crank from the 87mm stroked 2.5 liter M3. This is a little out of the ordinary for me and have been doing some research on the Bimmer engines. With the 135mm M10 rods, the standard r:s ratio is 1.69:1. The 87mm stroker has a r:s ratio of 1.55:1. Ugh!

Using the 144mm M3 rods, the ratio is 1:66:1. Better, but hardly optimum.

I note that the old school M30s had a r:s ratio of 1.57:1 (135mm rods - same as the M10 and 86mm stroke).

Rather than source custom or M3 rods, as this is a budget build up, the owner wants to use M30 pistons and standard rods with the 87mm crank. I'm a little wary. I'd appreciate views.

 

[Metalguy]

Low ratios can provide pretty high torque, because the rod gets the best leverage on the crank when the piston is a little higher (higher pressure). Just make sure he keeps the revs moderate.

 

[SMOKEY44211]

If you have the option of being able to use a longer rod I think you would be ahead of the game to do so. Every engine I have ever tested always made more torque at every RPM level with longer rods all else being equal. There was a fair amount of disscussion on thread # 71-86266 slightly different question but the principles remain the same. Might be worth a look see.-------Phil

 

[patprimmer]

1.55 is marginal, but should be OK.

I believe Top Fuel Dragsters run shorter rod ratios than this and they run very big engines at 8000 rpm at huge boost and up to 6000 hp, but they only last 4 or 5 seconds between top end overhauls and inspection.

I think 1.8 would be ideal for an M3 that needed long term durability at higher rpm's, but 1.55 will still work OK

Regards
pat

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[CleverHans]

Thanks all for the advice and thanks SMOKEY44211 for the link to thread # 71-86266. That is an interesting thread.

Cheers

 

[PowerDubs]

How do you get to that thread mentioned?? <~~ I'm a noob

 

[patprimmer]

Try this link
thread71-86266

Regards
pat

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[patprimmer]

To post the link you type &quot;thread&quot; then numbers, with NO space.

I made the same mistake for quite a while before it was pointed out to me

Regards
pat

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