Stretching Rods

[Fishcop]

G'day Fellas

Can someone tell me by how much I can "stretch" the rods on my 2.7 RS spec engine? I'd like to bump compression a little above the standard 8.5:1. Reading Wayne's book tells me I do it by offset boring the new little end bushes, but by how much?

Cheers

[Bobboloo]

I would think that since it all comes down to deck height clearance that if you measured your deck height with the RS pistons you would know how much deck height that you have to play with. Then you could offset the rod bushings accordingly to leave the suggested factory minimum clearance.


P.S. The machine work on your case and heads should of course be done before you take the measurement. Install a copper base gasket and the new bearings as well. After you've done this you'll still need to check valve clearance as well. If I've left anything out I'm sure Wayne will chime in.

[Fishcop]

Cheers Bobby

I guess with my cases decked and heads rebuilt, I may not have to to any "stretching" at all? Just how much change in compression does a few millimeters make in these engines?

TIA

[Bobboloo]

I see we were typing at the same time. So ignore my P.S. As for the compression difference I have no idea. Maybe Wayne or Steve with Rennsport could answer that.

[Bobboloo]

John,

You should read this thread on deck height. I found the info from Steve W. very interesting.
Deck Height Numbers

[MoreGAS]

Gents,
A few MM in a motor is a mile!

With a moderate piston dome of say a std fuel injected motor in the 9:1 range- from memory, .010" (read 1/4 mm) nets out a change in compress of a .2-.25. So 1mm would be nearly 1 full point. Changing dimensions beyond 1mm usually is a problem with things sealing up on the cam housing to chain hsg areas(the chain hsgs can then be machined), but more limiting is idler sprocket troubles due to changing chain length-slackness(then you can buy custom sprockets). Beyond these small improvements, one gets to buying pistons with bigger domes, or heads with smaller volumes(ala early RSR, or 935 heads) or both for the BIG #'s. Once any changes like this are made, all bets are off and every clearance has to be double checked to see if one is going to have a parts crusher or saavily configured engine.
Std deck hts are usually in the .050" range, thus one cannot go too far in attempting to gain more here without a real machining excersize!

Happy Tweeking,
Kevin Roush
GAS Motorsport
Performance Porsche
909 985 2909

[Fishcop]

Thanks Kevin. It's certainly not my intention to machine my way to compression. I just thought that whilst getting my rods done I could perhaps cheat a few Hp!

Thanks also Bobby, I have been watching that thread with interest!

Cheers

[Tim Walsh]

I'll second Kevin's post. dtw took 1mm off his heads for compression reasons and it made a huge difference. Everything from chain housings didn't line up to the shroud's holes were off to a loose MFI belt. 1mm is a HUGE difference.

[kstylianos]

For example with a 3.0, a change of .25mm in deck height is about a .27 change in CR. PM me if you want a CR calculator.

[Fishcop]

Thanks Charlie, I'll wait until I get my case and heads back from the shop and then I'll have some real numbers to post.

[KTL]

Originally posted by MoreGAS
Gents,
A few MM in a motor is a mile!

.......... Once any changes like this are made, all bets are off and every clearance has to be double checked to see if one is going to have a parts crusher or saavily configured engine.........

Parts crusher Great expression!

May everyone's motor build NEVER result in becoming a parts crusher.