Black51

Ahh but see, we don't have E85 yet in Canada.

I guess the case I'm trying to bring up is at what point would upping CR, over other means become a more cost efficient/reliable option?

Paulyy

(insert canada joke here)

It'll probably cost a little more then you think to bump the compression. you'll need to change the pistons unless you shave the head that much.

I don't recommend shaving the head because there's limits on the head. Then if something happens you're in for a new head because it probably cannot be decked again. and the 8v head isn't to good of a high comp design as the spark plugs are on the side and in the middle.

Rogue_Ant

Displacement > CR

For example Sid's ~8.5CR 2.8L engine makes more power & torque off-boost than my 9.5CR 2.5L 944NA does. The added stroke especially helps torque (more than increasing the bore).

Ok, while some of the current tuner-themes might be to raise the CR, but they are generally quite a different engine. Most of the real high HP builds have the benefit of closed-deck, iron block, with small bores, penti-roof combustion chambers w/ centered plugs... Unfortunately, taking the same tuning approach as, say a 4G63, or M50 variant, will break the 951 engine. That said, the typical tuner-car is generally put through a different set of challenges than the typical built 951 motor. Building a 4G63 to be powerful for the street or drag is quite a bit simpler than building it to survive on the road-course, but I digress...

Getting back to the point. What we are seeing with high-hp, sustained load motors is a tendency of the cylinders to walk-around, or simply crack the cylinders. Obviously an answer is to build the motor stronger. So, sleeve and deckplate or fill the motor. Then you move the weak-link somewhere else. The next problem is the head lifting, and the cylinder head actually flexing under combustion pressures... This is quite a bit tougher to "fix" and we have tried quite a few approaches. The studs are spaced quite far apart, and the cylinder head needs added thickness. Fixing either of these is not trivial. So, what else can we do?
Well, lowering compression will help keep peak cylinder pressures lower. Even though the peak is lower, we can still keep the average cylinder pressure high. What I will confess is that a higher CR engine is mechanically more efficient. A 10:1 engine, cylinder volume change is 25% greater than an 8:1 engine (ratio of expansion, TDC:BTC volume). The 10:1 engine can make better use of the expanding combustion & gasses due to this advantage. However, because the 10:1 engine has a greater expansion, it also means that cylinder pressure drops off faster than it does in an 8:1 engine.
For example:



Here is two theoretical combustion pressure curves. Lets let Engine 1 be our 8:1 CR, and Engine 2 be the 10:1. The high-CR engine definitely makes more cylinder pressure. But due to the greater expansion rate of the 10:1 engine, pressure also drops off faster. The area under the curve is positive pressure - the pressure which makes TQ/HP. Even though one curve has a much higher peak, both of these curves have the exact same area under the curve.

Yes, the higher-CR engine will not need as much timing advance, and therefore have less 'negative-work' of resisting the piston's upward movement, and can take advantage of focusing peak cylinder pressure at the most beneficial crank-angle. However, that power loss can generally be made up with running a little more boost, which the lower CR engine will be much more friendly to.

It is a bit of a trade-off, but in these engines I can tell you which one is going to live longer under extreme hardships and power: the lower CR engine.

Fluidplay

Joshua - If you raised the CR of a turbo engine wouldn't going into boost offset the drop in cylinder pressure?

The thought was to try and gain in throttle reaponse/power "pre" boost in the lower rpm range, but really the question then shifts to how much over 8:1 to you need to go to really feel a difference and is it at that point it gets risky on long term engine life.

Rogue_Ant

Not sure I understand your first question... can you re elaborate a bit on what you are asking?

The biggest issue with power pre-boost is how the DME software controls ignition. Going to proper 3D timing tables will change how the car drives.

Even a full point in CR isn't going to net you that much power, probably only a few percent. For just a little better drivability, then I would just advance the camshaft 2-4°, that will pickup low-end power, and help boost come on quicker.

Fluidplay

My original question was based on the idea that since I'm ordering custom pistons through Wossner, that...is it possible to gain some power "pre" boost by increasing CR. Pros & Cons.

It sounds like from what you're saying is that there really isn't any appreciable gain without putting the engine into extreme stress. That my best option would be to go with a more aggressive cam. I'm currently using a stand alone ECU so I can definetly play with all the variables for tuning.

lee101315

I dont know, what type of driving do you do? Are you always on the track?

I shaved my head .020" and added a cam when I rebuilt my engine, results of increased compression are pending. But then again, im not looking for a peaky high boost dyno queen, I want something powerful but comfortable to drive around.

Rogue_Ant

What is your end hp goal? And on what fuel?

Fluidplay

When all said and done i'd be happy (for the time being) with 300-325hp on pump 93oct @ 18# boost. I have the ability to change maps and run 100oct to push boost to 20# on the track, which hopefully will increase the hp to 340+.
Upgrades include:
LR Super 61 turbo TO4E w/8cm
Bores @ 101 w/Wossner pistons
Stock rods
Knife edged crank
3" exhaust
48# injectors
Link ECU
A/C delete

Rogue_Ant

Those goals are no problem on stock 8:1 CR. I wouldn't bother raising CR. If you are looking for more low-end power, adjust your cam-timing.

333pg333

I think Josh makes good points here. Dennis, with those goals in mind you should be fine with what you have. If you want to give the car a little more 'pep' you could even consider changing out the ring and pinion to the S2 version. This really livens the car up a lot. Does make hwy cruising a little high revving, but it all depends on what you want from the car.

Fluidplay

Thanks for the input guys! Appreciate the advice..

LUCKY DAVE

Yep. On our land speed Aprilia (bike) engine, we are running 10.5:1 compression with 13 psi and a heavy load of nitrous, and water/meth spray all controlled by a custom ECU. RPM ceiling is 10400, fuel is VP MS109. The engine is very carefully setup and mapped on a research grade dyno at gradually raised power levels.
This engine is a 1060cc twin, so the cylinder sizes aren't too far from those of a 951. To give adequate strength for these loads we ended up using CP forged pistons with a ceramic coated crown and Micro Blue coating on the skirts, Ferria SS valves, and Carillo rods. The stock coated aluminum cylinders weren't strong enough (they were barrel shaped after high power running allowing ring seal to fail) so we installed thick steel sleeves from LA Sleeve.
These part$ were selected the hard way
As our design is supercharged and physically space limited, we are getting all we can out of the biggest Rotrex blower that will fit, spinning it at the top of its delivery curve, and making the rest of the needed power with nitrous.
We log 100 F intake charge drop with our water/meth spray setup, it prevents detonation and boosts power ~30 hp BEFORE nitrous, more like 50 hp with nitrous.
Our RWHP is 330 on an eddy current dyno fully heat soaked.
Bottom line......making a forced induction engine produce big power reliably isn't cheap, and water/meth spray is an obvious no brainer.
I'm only guessing here, but I bet you could get away with a 10:1 951 on **** poor pump gas with water/meth spray and a good ECU properly mapped.

Rogue_Ant

True, but we can't all be driving around in flat-12 cylinder engines!

We have been systematically testing the limits of the 944 engine, and working to improve things (both in quality and in cost). But regardless, there is a definite point of diminishing gains regarding investment vs output. (true for any engine)

LUCKY DAVE

Rogue has it right in an earlier post that higher AVERAGE cylinder pressure is the key, not higher peak pressure.