Compression Bump w/Rods

[echrisconnor]

I've read in a few places, including Wayne's book, that you can increase compression by having offset rod end bearings put in. This effectively stretches the rod and increses compression.

If you do not have deck height issues, does this give you any significant power gain? How much can you bump compression by before you're likely to have interferance issues?

I'm planning a rebuild in the future of my 3.0 carrera. It has an 8.5:1 compression ratio and while I'd like to spring for different pistons, I'm unlikely to find the $$ kicking around.

[anh911]

do a search on compression ratios. I found some info on this. There's a rule of thumb (it might be in Wayne's book too) about how a 1mm change equals x amount of compression. I don't recall the exact number so I won't guess here. Someone else can chime in if they have it.

[anh911]

Oh yeah before anyone flames me - be sure to check your deck height, piston to valve clearance etc.

[Alan Cottrill]

i don't understand how you can gain compression from streching the rods.

does stretching the rods do anything but reduce the deck height.

if you reduce the deckheight too much you have to make up for it with more shims under the cylinders, right?

why not just run with no cylinder base gasket? wouldn't that be the same net effect?

[echrisconnor]

I've seen the formulas for increase in comression and HP. It's something like 10% per point. So If I go from 8.5 to 9.5 I could expect a 10% increase. I suspect you can't gain a full point with rod ends only.

From what I understand, all that stretching the rods does is effectively reduce deck height. You could possibley accomplish the same thing in several ways including omitting the base gasket or fly-cutting the heads.

Since this rod-stretching procedure would lower the deck height, is there any rule of thumb as to how much you can lower this from a stock setup? I know you can't go too far with stock P&Cs. If, however, you could safely lower the deck height from stock, can expect to gain any significant increase in compression, like a half point?

[Grady Clay]

Alan is right, that is equivalent to removing the cylinder base gasket. However there are several undesirable situations.
First, it is possible for the wrist pin bushing to rotate. They are usually rolled in place and staked, not keyed. The good news is if it moves it lowers the piston and lowers the CR. With a stock (round) bushing loose, it just makes some noise and you get it fixed.
Second, there is much more to raising the compression than reducing the deck (piston-to-head clearance.) Absolutely research this subject. anh911 is right on.
Third, that brass is a cushion to prevent some detonation shock from traveling down the rod to the bearing on the crank. It’s one more line of defense. When offset, it is thin and transmits more shock to the rod bearing and is itself more prone to failure.

All in all, there are far better ways to raise the compression ratio.

Best,
Grady

[dtw]

Chris,
The rules go out the window when you change deck height. How far you can come down all depends on what cams and pistons you are running. In my case I fly-cut the heads 1mm and then added back an additional .25mm by using .50mm base gaskets. I obtained a .75mm net decrease in deck height (and came in around .45mm deck, by the way). This was the most I could obtain without running into valve clearance issues - and I had to advance my cams .2mm to get these results. Bottom line is, you'll have to do a dry assembly run of your top end to carefully check clearances. I'd also make sure you get on the phone with some experienced 911 engine builders and talk over your ideas and options before you commit to machine work.

[echrisconnor]

So, if I were going to try to go down the path of increasing compression without changing pistons, is the preferred method to fly cut the heads? With proper measurement, is the omission of the base gasket a viable alternative?

Basically, I'm planning out this rebuild on the assumption that I won't have a wide open bank for it. Fly cutting the heads runs around $100 per head -- right? I could probably pick up some used later 9.3:1 SC pistons for about the same price.

The euro Carrera that I have has a relatively low compression ratio. I expect to chage the cam to a 964 or Elgin's equivalent to the Webcam 20/21 grind. But that's about the only "enhancement" I was planning, unless there was something in the compression department which could be leveraged without significant spend.

Any other thoughts?

[ChrisBennet]

Originally posted by Alan Cottrill
i don't understand how you can gain compression from streching the rods.

does stretching the rods do anything but reduce the deck height.

if you reduce the deckheight too much you have to make up for it with more shims under the cylinders, right?

why not just run with no cylinder base gasket? wouldn't that be the same net effect?

Stretching the rod effectively raises the piston closer to the head. It reduces deck height at the same time. Raising the whole piston is a little like having a bigger/taller piston dome. Either method results in a smaller combustion chamber volume. CR is the ratio of maximum cylinder volume to minimum volume. Raising compression by stretching the rods (or increasing the piston dome volume) reduces the minimum volume more as a percentage than the accompanying reduction in maximum volume.
CR = Cmax : Cmin
Before: Cmax=1000cc Cmin=100cc CR = 10:1
Now subtract 5 cc by raising the piston:
After: Cmax=995cc Cmin=95cc CR = 10.47:1
The above figures where picked for convenience but I did some calculations and for a 95mm bore (3.0 and 3.2), raising the piston .7mm would result in a 5cc reduction in volume.

Removing the base gasket or shaving some material off the head will increase the compression. It will also make the heads and the cams closer to the crank which will result in more chain slack (possibly requiring a special idle gear to compensate) and requiring the chain boxes to be machined so the cam is centered in the hole in the chain box.

-Chris

[dtw]

Chris is right on.

Incidentally, I paid significantly less than $100/head for fly-cutting - I forget how much but definitely less than $200 for all six.

There are pros and cons to any of the aforementioned tricks. Machining your heads and chain boxes down are both non-reversible processes. Bolting up the shroud and tinware was a pain for me too, as the dimensions of the entire engine had changed. I had to offset drill a few holes.

[Wayne 962]

Originally posted by Grady Clay
Third, that brass is a cushion to prevent some detonation shock from traveling down the rod to the bearing on the crank. It’s one more line of defense. When offset, it is thin and transmits more shock to the rod bearing and is itself more prone to failure.

That is not a concern in this case, because the rod bushing is displaced so that the thinner side is facing outwards. Any pressure or force from combustion would be pushing on the larger part of the bushing, not the smaller part.

-Wayne

[anh911]

I just double-checked the CR on the 76 3.0, surprising that this is a low CR motor.
Chris C, any chance of looking for a set of good used later 9.3 pistons? Seems like a better alternative.

[Wayne 962]

Why not go with higher compression JEs?

-Wayne

[ChrisBennet]

Originally posted by anh911
I just double-checked the CR on the 76 3.0, surprising that this is a low CR motor.
Chris C, any chance of looking for a set of good used later 9.3 pistons? Seems like a better alternative.

I looked it up and it has 8.5:1 compression - same as the early SC's. Somehow they managed to get 200 for the C3 vs the SC's 180hp. Anyone know how?
-Chris

[echrisconnor]

I'd love to score some later 9.3 SC pistons but expect them to carry a somewhat hefty price tag. Anyone know about how much vs. JE's?

As for the output of the early C3 motors being higher than the SCs, there are a bunch of reasons. I'm sure Wayne can answer much more definitively, however here are some of the reasons that I know of:
No emissions gear (smog pump, air rails or cat)
Ligher crank and rods -- better revving
Different cam timing- not sure how much this counts for
Different HO fuel distributor - may not make power but enable it

Finally, should I simpy forget about the eccentric rod ends as a means to power gains?

[ChrisBennet]

Originally posted by echrisconnor
I'd love to score some later 9.3 SC pistons but expect them to carry a somewhat hefty price tag. Anyone know about how much vs. JE's?

As for the output of the early C3 motors being higher than the SCs, there are a bunch of reasons. I'm sure Wayne can answer much more definitively, however here are some of the reasons that I know of:
No emissions gear (smog pump, air rails or cat)
Ligher crank and rods -- better revving
Different cam timing- not sure how much this counts for
Different HO fuel distributor - may not make power but enable it

Finally, should I simpy forget about the eccentric rod ends as a means to power gains?

The ROW SC didn't have any smog gear either. I wonder if the ignition advance was different?
-Chris

[echrisconnor]

Possible, although then everyone with an SC would be changing their advance curves... Honestly, the biggest differentiator I belive is the crank weight. Whether that accounts for 20 hp, I dunno.

I'll tell you though, my C3 is amazingly different from the SC's I've been in and driven. The whole car is lighter as well, and with early exchangers on it, it really moves.

The smoking is driving me crazy though and those valve guides have got to be replaced in the near future.

[jluetjen]

It's something like 10% per point. So If I go from 8.5 to 9.5 I could expect a 10% increase.

BTW, I doubt that it will be a linear rule. I suspect that it is going to be more like: (Change in CR)/Original CR. The affect will be that there is a decreasing return to increasing the CR. For example, you'll gain a lot more (% wise) by going from a CR of 6.0:1 to 7.0:1 then you will be going from 13.0:1 to 14.0:1.

The impact will also be affected by the engine's ability to fill a cylinder, so the porting and cam selection will also make a difference. Bottom line, I doubt that there is any simple formula to suggest the HP gain beyond the concept that more is better until your engine starts to ping.

[echrisconnor]

John,

I'm sure you're right and these gains are not linear-- it was a ballpark figure I saw somewhere.

So, my final re-posing of the original question: If you have sufficient deck height, can offset rod end bushings increase compression and therefore engine output? Would anyone recommend this as a procedure to pursue any further?

[Wayne 962]

Originally posted by echrisconnor
As for the output of the early C3 motors being higher than the SCs, there are a bunch of reasons. I'm sure Wayne can answer much more definitively, however here are some of the reasons that I know of:
Ligher crank and rods -- better revving

Reducing your rotating mass increases acceleration, but doesn't increase HP. Think of it like reducing the weight of the car...

-Wayne