2.7RS compression ration question

[82SC]

if I were to build a 2.7RS engine and I wanted to increase the CR...

I want to run single plug so I am thinking 9.8:1 is best?

so do I increase the length of the rods?

or

do they make a 2.7L set with a higher CR?

I know machining the heads is not suggested b/c it messes up the chanin length etc...

but if the CR is raised do the valve pockets have to machined in a OEM 2.7L RS pistons?

hmmm....

MJ

[Adam]

Normally CR is increased by using different pistons.

Matt Holcomb has a set of JE Pistons in his MFI 2.7 which equate to 11.3:1 CR!

[Bobboloo]

The rods match the stroke of the crank. You can't change the rods without the crank to alter CR. If you want higher compression then you need to get different pistons.

The common knowledge seems to be that 9.8:1 CR is the highest you want to go with 91 CLC octane and single plug.

On my project car I'm putting in JE 92mm piston's w/ 9.8:1CR. I'll be boring out my 90mm cylinders to 92mm and having them plated in Nikasil. This will give me a 2.8 liter MFI motor with S cams.

I guess some people might think that's a little crazy considering I could buy a new set of Mahle 2.7RS P+C's for a little bit cheaper ($1600ish) but in the words of Nigel from the classic film Spinal Tap. "Yeah, but you see it's got one more".

Bobby

P.S. Actually Mahle does make a 9.8:1 MFI P+C set but they're 93mm so they give you roughly 2.9 liters. They bolt up to the 2.7 case without having to cut the spigots!!!

[724doorE]

Bobby, an increase in rod length would just increase the deck height of the piston in relation to the cylinder. This would effectively raise the compression ratio due to the decrease in combustion chamber volume. You obviously would need to be careful how much you increase your deck height and as was noted, you would be wise to check piston to valve clearance when putting it together. The difficulty would be locating a suitable rod. This is much easier with american V-8's as there are many more combo's to choose from and some that will even "bolt up" most, however, need at least some minor machine work. The other option would be to simply contract some aftermarket rods to your prescribed dimensions.

All this might allow one to use a more common piston in a higher performance applicaion, but possibly at a cost in reliability if the pistons of choice aren't up to the new task.

It's probably cheaper to just buy a set of JE's and be done with it...

Good luck

PS, forgot, different rods means you should check the balence of the rotating assembly as it would most likely not be correct anymore, is this what you meant Bobby?

[Scott Clarke]

Mahle now makes a "sport" series of pistons. These are available in 90mm and 93mm, and produce a CR of 9.5:1. They are expensive, but few would dispute that they are the best.

[Bill Verburg]

If it were my engine I would only use Mahles. Come to think of it I put my money where my mouth is by purchasing a set of 102mm RSR Mahles. It's money well spent.

[Wayne 962]

There's a bit of confusion here. The rods can be lengthened about 1mm by using off-set boring. There's a picture of this in my new book...

The deck height is set - you can't change it and still end up with a solid engine. If the deck height is off, then you need to adjust it by using various thickness shims underneath the cylinders...

-Wayne

[911pcars]

"...an increase in rod length would just increase the deck height of the piston in relation to the cylinder. This would effectively raise the compression ratio due to the decrease in combustion chamber volume."

Not quite. The CR is computed by comparing the volume of the cylinder at BDC with the volume of the cylinder at TDC. A longer rod merely positions the piston at a higher point at BDC and, since the stroke is the same, goes up higher at TDC. The stroke volume, in this case, hasn't changed. The stroke volume changes by increasing/decreasing the size of the bore and/or stroke.

The only way to increase CR is to change the relationship between TDC and BDC cylinder volumes. This equates to increasing the bore and/or stroke, domed pistons, less deck height and/or smaller combustion chamber.

Sorry to get too technical, but:
V1 = swept volume of one cylinder
V2 = deck ht. volume
V3 = cylinder head volume
V4 = piston dome volume

CR = V1 + V2 + V3 - V4 / V2 + V3 - V4

Any time you increase compression ratio you should check the valve-to-piston clearance; moreso with more agressive camshafts.

Sherwood Lee
http://members.rennlist.org/911pcars

[Bobboloo]

What I meant was that all you do when you lengthen the rod is change where the piston rides in the cylinder. Similar effect to changing deck height. There is no increase in stroke from doing this and no added compression to be gained from stroking. Expensive alternative with minimal gains. Piston replacement is the better choice.

Bobby

[724doorE]

1. I miss used the term deck height. def. as distance from crank centerline to perp. surface of cylinder deck. ok no big deal.

So no your not really changing deck height per say, But I was saying this:

Theoretically, you use a longer rod ( slightly maybe 1-few mm's) and that moves the piston out farther in the bore as has been retold a few times here. This in effect gives the arrangement a greater dome area than before since more of the dome is moved out past the deck at TDC. The head is in the same place and its volume hasn't changed, but the volume left at TDC does change.Now I fully realize that if someone were to attempt this, they would need to machine the piston so the height at the outer edge of the piston would not excede the deck height when installed. The benifit to doing this wuold be that one could machine the now oversized dome to what ever configuration you want to obtain the desired CR and burn area. Reasonable with stock cast pistons... probably not, but machining down a large dome to get what you want is commonly done and all I've tried to show was that it was possible to do it here as well. So for those who say I wouldn't change CR with this method, I just disagree...

Sherwood, isn't your equation missing some components?