Geo

I don't know how you cannot see this as cylinder filling. By definition, VE.

See, I think this is exactly the place for it. It's a chance for people to learn. IMHO, it's not just a matter of trivia either. Once you learn what things mean, you can read other text and understand more, especially more technical text.

Now, you are using a term that you won't find used by automotive engineers. Yours is a short-hand for something you defined. Someone might read what you are talking about and then read other text and wonder why others don't talk about it. Well, they don't talk about it because they define it differently.

What you are defining as "effective compression" is comparing the volume of fuel/air that is actually in the cylinder (at amospheric pressure) to the volume it gets compressed down to.

The volume fuel/air actually in the cylinder (at atmospheric pressure) vs the true volume of the cylinder is the definition of VE, however the cylinder gets filled.

The actual compressed pressure of this volume is by definition bmep.

So, to summarize for the last time (I'm not going to keep boring folks with this discussion):

If one reads a technical text they should not expect to read about "effective compression." They will read about CR as a static measure, VE, and bemp instead. They might run across this somewhere, but good technical writing will not include it.

Your use of the term "effective compression" is a short-hand of sorts. If it works for you, great. You obviously use it to good effect in your work.

NineMeister

George,
This is my last comment on the subject as we are indeed boring others to death.

Effective compression ratio is not a dynamic comparison of trapped volumes, it is a geometric comparison of the volumes defined by the piston, bore & head from the point that the valves are closed and at TDC. It is just a useful number that surprisingly has remained roughly constant over the last 100 years of petrol engine development. I did not invent the term, it was introduced to me during an engine tuning seminar (note, not an automotive engineering seminar) by two people whom I respect as being more informed than I can ever be as long as I live. It is probably not a technical term in the truest sense of traditional 4 stroke design (although it is completely accepted by 2 stroke tuners). ECR has nothing to do with VE, a 50% VE model T Ford engine might have an ECR of 7:1, whereas a 90%VE 944S engine is around 8:1 against CRs of 7:1 and 11:1 respectively (early engines closed the exhaust valve at BDC). Do you see what I mean??

Your definition of VE is correct.

Your definition of BMEP is not correct. Brake Mean Effective (that word again) Pressure is the average cylinder pressure in the cylinder after combustion when reverse calculated from a power reading taken from a brake (read engine dyno).

Have a few beers George and see if you can find a couple of standard production engines that differ by years and measure or calculate the ECR from the stated CR, bore, stroke & cam timing. I think you will be surprised by your findings. And that's it, no more from me on the subject.

M758

Ok guys lots of nice talk and theories.

Very well indeed, but I have am important question.


When is someone going to actually get 250 bhp from 2.5L 8v NA motor?

Seems like we have some folks here that apparently know how to do it or how they could do it, but nobody seems to have done it (probably wrong here) and certainly there are not parts or places offering these upgrades.

George is interested from a racing pespective, but IT rules limit his ultimate mod potential. I also run class with tight engine rules, but still am interested in it.

Skunk Workz

Working on it..

kennycoulter

im one of the laymens guys and am rather stoopid, but what i think you guys are saying(probably wrong) is that cams are the key? so regrinds? or 944s as a start? and time the cams different? are the exhaust and intake cams seperate, or does one cam control half of intake and half of exhaust? i hope you guys dont start a fight with me for misunderstanding.

M758

Cool

Skunk Workz

kennycoulter: The main problem is that the port is the wrong shape/size. Better performance can be had by making the exact cam profile you need for your engine. Off-the-shelf items are always just going to be just that; "approximate"...and not perfect for your car. Ergo,no "perfect" power production.

Mongo

It's possible to get the power, and I hope you sleeve that motor if you're gonna go with a bigger bore and not to mention get that baby up to about 12.5 or 13:1 compression, that would be one mean NA

Skunk Workz

Mine will run close to 14:1...but then we have 98 oct pump gas here. And yes,it will be sleeved.

M758

What about power from a 2.5L block. No chanages to bore/stroke or pistons.

I am thinking head work (ports, valves) , intake & exhaust manifold, and right cam.

I have seen power from overbored motors, but I'd rather see if from 2.5L stock bore motors.

NineMeister

The cylinder head, not the cam, is the highest priority to find the missing power. If you are going to tune a 2.5 engine the obvious starting point would be to retain the stock valve diameter so if you use the computer model of my previous post as a starting point this predicts @ 100% VE
Peak torque 238Nm @ 5300rpm, Peak power 218bhp @ 7000rpm, ideal port 38mm dia. & flow requirement 197cfm.

The first stage is to mock up a 38mm intake port in the head at the manifold face, attach a straight tapered intake horn to extend the port to around 300-400mm long and then flow test the head at 25". You will have to revise the port until you achieve more than 197cfm, since this is the flow demand at peak piston velocity which is somewhere around 80 degrees ATDC.

This then starts the design for the cam. Let's say that your fully developed head now flows 200cfm at more than 12mm of lift. The flow requirement means that you have to have the inlet valve open to 12mm by 80 deg. ATDC. Peak lift may occur after this point but unless you get the cam open fast enough you will not make the power.

Another scenario is that you can flow 240cfm through the port and so have an excess of flow still through the 38mm venturi. In this case you have 240/197 x 100% available filling capacity, or in other words a potential of 120% VE. A recalculation on the computer should prove that this head will yield the magic 100bhp per litre, in which case pat yourself on the back and get on with working on the rest of the engine.

I would now use another model to develop the cam, looking at all timing events, but in the real world you may just want to go with the longest duration cam available. I use a 320/305 cam in my 993 for 103bhp/litre so this would probably be a good starting point if there was one available. Don't worry about a long cam being finniky, the small port improves torque and response to an acceptable level even for a road car (don't mention emissions though).

Combustion is the next issue, you have to get as much squish as you can into the cylinder and probably aim for 12.5:1 as a starter CR. Forged pistons will be a must at 8000rpm, many comments on these above. Stronger rods will also be essential, as will oiling revisions to the crank.

Finally the intake will have to be designed around the computer model, as mentioned before a straight shot tapered port is ideal, but if you cannot achieve this under the hood wrap the runners using at least a 6:1 bend diameter to port diameter ratio. A single plenum of 100% swept volume is probably ample fed from a large single throttle body of probably 65 to 70mm diameter.

Danno

No one has mentioned piston-velocity yet and I'm not sure what limits it imposes. The large-displacement per cylinder in the 944 engine brings up a lot of issues in terms of cylinder-filling, valve-sizes & port diameters. The 944 engine has the same average piston velocity at 6500rpm as the 993RSR motor at 8000rpm. In many ways the 944 engine is closer to its limits. It also appears that stock cam is already lifting the valve at 26% of its diameter.

Mark Lue

This is all very interesting and enlightening but when you start talking about head work and replacing 45mm valves with 38mm valves and making new cams etc. I can't help but wonder how much this would cost to fix the suposed mistake that Porsche has/had made when designing the 2.5NA engine.

Forgive me guys, I know you have to spend money to make HP but all this sounds very expensive which once again brings up the cost efectiveness of spending all this money verses getting a 951.

ARRGGGG!!!! What did I just say! I'm starting to sound like the NA haters on this board.

Skunk Workz

You keep the valves...it's the port's average cross-section that's too big. The stock intake valve size is rather ok. The port size is the problem,not the intake/valves/cam/etc...of course they can be improved on,but the essence is that the port needs the proper work done to it before all the other/usual mods really can do their job of kicking VE through the roof compared to a stock car.

inactiveuser92616

so to get that port size do you need to add or remove material from the stock port? would you add material via welding or some sort of epoxy?