stretching the rods

[philippeF]

Hi all
first of all, happy new year !

did someone try to stretch the rod to improve compression ratio on a 2.7RS stock engine...
i'm considering some little improvements during my upcoming rebuilt.
new size, new datas ?
many thanks
Philippe

[TimT]

you can have the rod end bushing reamed offset to lengthen the rod a bit

[addictionMS]

rods don't make the compression ratio, cranks do, making the rod longer will also reduce the amount of air pulled in. Pistons and cranks have an effect, rods do not.

Jim

[scottc]

"Rods don't make the compression, ratio cranks do...."[/QUOTE]

Jim:
I beg to differ on this point.

While you are correct that a longer rod will in lessen the initial uncompressed volume of air in the cylinder (call this V1) when the piston is at BDC by a small volume corresponding the small change in length of the rod (call this v). We can represent this new volume as V1-v.

When the piston is at TDC the old compressed volume (call it V2) is now going to be somewhat less due to this small lengthening of the rod. We can represent this new volume as V2-v.

Since the overall (static or theoretical) compression ratio is a ratio of the uncompressed volume to the compressed volume we can represent this as;
(V1-v)/(V2-v)= compression ratio

In a purely QUALITATIVE analysis; notice that we have lessened both numerator and denominator by the same amount (v). Since V1 the uncompressed volume is many times larger than V2 the uncompressed volume the effect of subtracting the small volume change (v) from each will have a great percentage effect on the compressed volume (V2) then it will on V1.

Doing a simple QUANTITATIVE analysis might help here. Using nice round numbers to just illustrate the math we could some assign values;
Let V1=10, V2=2 and v=1

We can rewrite our formula for Compression ratio;

Old compression ratrio; V1/V2 or 10/2=5 or 5:1

New ratio: (V1-v)/(V2-v) or (10-1)/(2-1)=9 or 9:1

The mistake you made in your assumtion is a common one but I think that you can see that when dealing with percentages or ratios which involve division the assumption is incorrect.

This is the same rational that is behind putting higher domes on "high compression" pistons. If your argument were true then this time honored and proven method would not work any better than stretching the rods would since both reduce the volume of the intake stroke. These techniques both reduce the compressed volume by a larger percentage than they do the uncompressed volume.

Hope this helps.

Scott

[addictionMS]

I stand corrected.

Jim

[audi2.7t]

I guess to simplify matters you could say that the piston is still travelling the same distance, just higher into the cylinder thereby creating the same volume of vacumn but creating higher compression by decreasing the area between piston and head on the up stroke. (?)

[Eagledriver]

I don't see how stretching the rods will help. It seems to me that you still need to have the deck height meet the minimum spec. If you can get there with stock rods then you would not have room for a stretched rod.

-Andy

[Wayne 962]

Originally posted by TimT
you can have the rod end bushing reamed offset to lengthen the rod a bit

Yes, that is commonly know as "stretching the rod."

-Wayne

[Wayne 962]

Originally posted by Eagledriver
I don't see how stretching the rods will help. It seems to me that you still need to have the deck height meet the minimum spec. If you can get there with stock rods then you would not have room for a stretched rod.

-Andy

It's definitely a gray area on this one, like flycutting the heads. You're basically reducing the deck height and making sure that you're clearances are checked over and over again...

See page 48 for a photo of this...

-Wayne