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  #1  
Old 08-28-2011, 01:26 PM
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.971 crank and rods with .960 pistons

Just wondered if anyone has ever attempted to make a "safe" stroker motor using a .971 crank and rods with .960 pistons. I had read somewhere that the pin height is different but I have two of the motors taken apart side by side and the measurement from the top of the pin to the piston crown appears to be the same. I need to verify this with a better measuring device since I just used a cheap outside caliper to eyeball it. I havent tried to swap pins yet so I'm not sure if they are the same diameter but it actually looks possible. I'm probably nuts but since I have a set of factory updated .971 rods it looks pretty tempting if I was to build another motor. Of course the .960 pistons are probably a bit lighter so the crank might need some counterweight removed. Just curious if this has ever been done. Would make a 3.4 liter om603.

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  #2  
Old 08-28-2011, 07:14 PM
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It should work if the rods don't contact the liner skirts.

3.3L, FYI.
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  #3  
Old 08-28-2011, 10:32 PM
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Oops, yeah, forgot the radius gets squared. I'll test fit it and see if everything clears in the next few days. Seems like this could make a pretty good combo for low end torque.
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  #4  
Old 08-29-2011, 04:09 AM
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How is this safe if the rods are still the weak link?

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  #5  
Old 08-29-2011, 04:25 AM
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Have you worked out the compression ratio? Swept volume capacity is all very well but...


...I'd like to see some pictures if you go forward with this project. I want to learn some more about these "new" engines...
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  #6  
Old 08-29-2011, 03:19 PM
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Quote:
Originally Posted by Army View Post
Have you worked out the compression ratio? Swept volume capacity is all very well but.
With the correct rods and crank the CR won't change, if the piston pin height is the same.

As noted before, MB could have changed the pin height to use shorter rods so they won't contact the skirt walls. The shorter height also means greater angle force on the rods, which is what could cause them to bend in the first place.
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Old 08-29-2011, 07:04 PM
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Probably should have said "safer" since I have the updated rods and would be running a smaller bore. Still some risk involved. Yeah, cr should be exactly the same since the .971 has the same deck height (same casting, just bored more).
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  #8  
Old 08-30-2011, 03:44 AM
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Quote:
Originally Posted by ForcedInduction View Post
With the correct rods and crank the CR won't change, if the piston pin height is the same.
How so? If CR = (swept volume + combustion chamber volume) / combustion chamber volume, increasing the swept volume without increasing the combustion chamber volume has to increase CR. The 10% stroke increase from 84.0mm to 92.4mm is accomplished without a corresponding 10% increase in combustion chamber volume. What does 24:1 static CR translate to at 1 bar boost?

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  #9  
Old 08-30-2011, 03:53 AM
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Quote:
Originally Posted by sixto View Post
How so? If CR = (swept volume + combustion chamber volume) / combustion chamber volume, increasing the swept volume without increasing the combustion chamber volume has to increase CR. The 10% stroke increase from 84.0mm to 92.4mm is accomplished without a corresponding 10% increase in combustion chamber volume. What does 24:1 static CR translate to at 1 bar boost?

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X2

That's what I was thinking... but I wasn't sure about the starting volumes / dimensions
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  #10  
Old 08-30-2011, 04:12 AM
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Quote:
Originally Posted by sixto View Post
How so?
Crank stroke has no effect on CR due to the connecting rod being shortened to match.

The same is done on all stroker engines.
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  #11  
Old 08-30-2011, 06:30 AM
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I'm still lost - someone help me out!

Would the cubic capacity of the engine be changing from 3 litres to 3.3 litres? Or are we going the other way from a 3.5 litre engine to a 3.3 litre one?

I'm reading information from here:-

http://en.wikipedia.org/wiki/Mercedes-Benz_OM603
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  #12  
Old 08-30-2011, 06:43 AM
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Quote:
Originally Posted by Army View Post
Would the cubic capacity of the engine be changing from 3 litres to 3.3 litres?
That. He is starting with a 3.0L engine with an 87mm bore and 84mm stroke.
The crankshaft and rods from the 3.5L engine will change stroke to 92.4m while keeping the stock 87mm bore, making 3.3L.

The opposite (stock crank with bigger pistons) would make 3.14L
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  #13  
Old 08-30-2011, 08:26 AM
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From my workings out made in this post

Does anyone have any nifty ways of measuring the true volume of an engine's cylinder?

And assuming a compression ratio of 21:1 same as an OM617 <= THIS COULD BE WRONG but I'm guessing it is close...

I estimate the unswept volume of an OM603 to be 150 cubic centimetres (same as an OM617)

If you enlarge the swept volume (= engine cubic capacity) then you'd get a compression ratio of

23:1

As you enlarge the swept volume but keep the clearance the same you increase the compression ratio.

I'm pretty sure this is correct...



EDIT:-

Please note ForcedInduction states later on that the OM603 static compression ratio is in fact 22:1 => this means that the calculated value above isn't quite right...

...re-doing the calculation and we get a static compression ratio of 24:1
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1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

Last edited by Stretch; 09-01-2011 at 04:11 AM. Reason: Added in some info that came later on
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  #14  
Old 08-30-2011, 09:43 AM
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What are the problems with too high a compression ratio - other than needing a mother of a battery and starter to turn the engine over?

Same problems as a petrol engine? Detonation - pinking?

When would a compression ratio become too high for Diesel? For WVO? For bio-diesel? Where could I find that out?
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1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



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  #15  
Old 08-30-2011, 12:33 PM
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I'm going to throw my .02 in...

if the stroke changes, but the pistons and combustion chamber stay the same, the compression ratio will change.

changing rod length in this instance only keeps the piston from exiting the chamber, it does not alter compression ratio.

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