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Scotty C

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Registered: 6th Nov 05
Location: Kidderminster Drives: 1.6 16v Sport
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26th Oct 10 at 13:00

Got Boring all figured out as that's pretty simple...

But i'm curious to find out out the 'stroking' side of things.

I am aware the lengh from BDC to TDC in the cylinder is increased, but how is this done?

I mean, if you fitted longer conrods the piston wouldn't be able to go as low before reaching BDC, right?

And visa versa, if you fitted shorter conrods it wouldn't go as high before it reached TDC...

So how it is done? New crank?

And, with the lengh from BDC to TDC increased surely you'd have to fit new cams to get any benifit from the newly stroked engine? as the piston would still be travelling down when the inlet valve closes?

Discuss.

mike56gte

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26th Oct 10 at 13:10

it involves changing crank, rods and pistons. i have only heard alot about it being done on alot of v8 stuff, usually the small block v8's. and think you actualy use shorter rods.

Scotty C

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26th Oct 10 at 13:15

What about changing the cams? does it make sense what i'm trying to say about that?

AlexW

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26th Oct 10 at 13:19

Longer conrods means the piston goes to a higher TDC so the air/fuel mix is compressed more thus bigger bang, yes the BDC is still higher too but it works out ok.

Like for me, i was looking into 1.6 rods in a 1.4 engine, would up the compression from 10.5.1 to 11.5.1 roughly.

And the piston is traveling down when the valve opens anyway? Hence suction.

tom_simes

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26th Oct 10 at 13:23

Technically the cams wouldn't need changing, as they are still timed to the crank.

As you know, the engine speed is measured in rpm of the crank - if the crank has a bigger 'throw' of the conrods, it just means the relative speed of the conrods is faster, but the crank still turns at the same speed as before.

Due to that, the cams still only have to turn at the same speed, and it will work if the cams aren't changed.

After saying all that though, I guess that even though it would work, the engine would benefit from cams with a different profile.

Scotty C

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26th Oct 10 at 13:23

But when at TDC the piston is level with the block? so how can it go higher?

Yes the valve is open anyway, but if BDC is increased, say by 10 degrees then the piston will still be travelling down for 10 degrees when the inlet valve is closed, so no mixture is being sucked into the chamber...

[Edited on 26-10-2010 by Scotty C]

AlexW

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26th Oct 10 at 13:25

BDC would be increased with longer conrods so the inlet valve would still shut at the right time, Your thinking about it wrong.

And the piston can come out of the block a couple of mm, but this is presuming the shorter conrods dont go right up to the top anyway.

Scotty C

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26th Oct 10 at 13:25

quote:
Originally posted by tom_simes
Technically the cams wouldn't need changing, as they are still timed to the crank.

As you know, the engine speed is measured in rpm of the crank - if the crank has a bigger 'throw' of the conrods, it just means the relative speed of the conrods is faster, but the crank still turns at the same speed as before.

Due to that, the cams still only have to turn at the same speed, and it will work if the cams aren't changed.

After saying all that though, I guess that even though it would work, the engine would benefit from cams with a different profile.

Right, that makes sense. Just unsure on what AW06 said about increasing TDC even though when at TDC the crown of the piston is level with the block..

Scotty C

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26th Oct 10 at 13:27

quote:
Originally posted by AW06
BDC would be increased with longer conrods so the inlet valve would still shut at the right time, Your thinking about it wrong.

And the piston can come out of the block a couple of mm, but this is presuming the shorter conrods dont go right up to the top anyway.

Ok.

I just can't see how, if the rods are exchanged for longer ones how they can lower than the standard BDC...

[Edited on 26-10-2010 by Scotty C]

AlexW

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26th Oct 10 at 13:28

If you go for shorter rods, the BDC would be lower yes, but so would the compression and so would the original TDC.

Longer conrods would mean the BDC is higher up the bore slighty obviously.

[Edited on 26-10-2010 by AW06]

Scotty C

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26th Oct 10 at 13:32

So to sum up..

Longer rods will mean the BDC will be higher up, but also so will TDC, which means the mixture can be compressed more, bigger bang, higher compression ratio etc etc?

AlexW

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26th Oct 10 at 13:33

Yep

tom_simes

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26th Oct 10 at 13:34

You'd use shorter conrods to allow for the same distance between valves and pistons at TDC, as you've said Scott.

You'd use a crank with a larger throw though, so the the BDC will be a lot lower than as standard.

[Edited on 26-10-2010 by tom_simes]

Scotty C

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26th Oct 10 at 13:36

Right got it, thanks

What would be a typical compression ratio increase on an average 2.0 4 cylinder engine then?

AlexW

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26th Oct 10 at 13:39

Depends on the engine etc. Also the std compression.

tom_simes

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26th Oct 10 at 13:40

I don't know, but I'm unsure if you'd get a massive difference in CR - the idea of stroking an engine is to increase its capacity, and therefore the volume of air/fuel mixture it can utilise.

tom_simes

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26th Oct 10 at 13:51

Scott - it might help you to read this:

http://www.hotrod.com/howto/69883_stroke_any_engine/index.html

Scotty C

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26th Oct 10 at 14:09

Thanks Tom. I'll look into it

ianofbhills

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26th Oct 10 at 14:16

quote:
Originally posted by AW06
BDC would be increased with longer conrods so the inlet valve would still shut at the right time, Your thinking about it wrong.

And the piston can come out of the block a couple of mm, but this is presuming the shorter conrods dont go right up to the top anyway.

What you talking about here???

Scotty C

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26th Oct 10 at 14:29

What's wrong mate?

ianofbhills

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26th Oct 10 at 14:33

Stroking an engine is all to do with the crank. Generally you'd fit a crank with a longer throw to increase the engines capacity. You then have to change the rods to keep the piston correct at tdc.

So take the original crank say the distance between the centre point and where the rod is connected is 5cm when this rotates 1 revolution the piston will travel 5cm up and 5cm down the bore.

If you fit a crank with a larger throw say 7cm from centre point to the rod then the piston will then travel 7cm up and 7cm down thus increasing the stroke. You can then fit more fuel air mixture in this space so your increasing the capacity of the engine.

Because the crank diameter is bigger you then need to decrease the rod length to keep the piston in the correct position at tdc. The piston still travels more because of the longer throw on the crank.

If that makes sense

Phillips_91

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26th Oct 10 at 14:58

quote:
Originally posted by ianofbhills
Stroking an engine is all to do with the crank. Generally you'd fit a crank with a longer throw to increase the engines capacity. You then have to change the rods to keep the piston correct at tdc.

So take the original crank say the distance between the centre point and where the rod is connected is 5cm when this rotates 1 revolution the piston will travel 5cm up and 5cm down the bore.

If you fit a crank with a larger throw say 7cm from centre point to the rod then the piston will then travel 7cm up and 7cm down thus increasing the stroke. You can then fit more fuel air mixture in this space so your increasing the capacity of the engine.

Because the crank diameter is bigger you then need to decrease the rod length to keep the piston in the correct position at tdc. The piston still travels more because of the longer throw on the crank.

If that makes sense

Now that is the first one that i've read that i understand and actually seems* to make sense

*I only say 'seems' to make sense as i dont know anything on this subject but that does sound right

tom_simes

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26th Oct 10 at 15:20

Isn't that the same as I've said above, just without the numbers to aid the explanation?

ianofbhills

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26th Oct 10 at 15:35

quote:
Originally posted by tom_simes
Isn't that the same as I've said above, just without the numbers to aid the explanation?

Agreed pretty much yes it is. I knew you knew what you were talking about but i think AW06 was making it confusing

Phillips_91

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26th Oct 10 at 15:55

quote:
Originally posted by tom_simes
Isn't that the same as I've said above, just without the numbers to aid the explanation?

Might of been what you where saying but i just didnt understand the way you put it, whereas the other way i personally found it really simple to understand and helped me learn

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