white924s

Exactly - raising the compression ratio doesn't change the amount of air or fuel used, so there is a limited amount of benefit to be gained. An engine uses combustion to convert chemical energy into mechanical energy, increasing CR only allows you to do so more efficiently (in other words, to capture more of the chemical energy). Most engines have an efficiency ratio between .6 and .8, meaning that they are able to capture 60-80% of the total potential energy in gas. The point of diminshing returns is closer to 10:1 than you think - even tiny superbike engines (which use almost F1-like tolerances) run 13:1-13.5:1 tops. Even then, they're only like 75% efficient. My point was that raising compression alone won't get you to 195hp - you need to have more air and fuel in the cylinder to do that. Fuel is easy - just get larger injectors. Its getting more air in that's the trick

The diesel comparison isn't really apt here because diesel is a different fuel with different properties which allows it to work in a compression engine (eg no spark plugs). Just because you can do something with diesel doesn't mean you can do that with petrol (don't believe me? try filling up a diesel truck with 93 octane...).

white924s

Oh and my issue with the Motronic is largely that the AFM is a restrictive way to measure airflow into the engine. I think that modern aftermarket EMS allows for finer control of fuel and spark as well as for total closed-loop fuel metering (as opposed to Motronic's open-loop operation at full throttle). Not to mention that standalone systems use MAP sensors to measure load on the engine, which allows you to meter air coming into the engine without putting anything into the airstream.

I'm not saying its an awful system (my car has a stock Motronic system), but if you want to make big hp from an 8v N/A (eg 951 numbers) the motronic will almost certainly have to go, or be so thoroughly modified as to be almost unrecognizable (I don't consider MAF+piggyback+motronic w/ custom chip to really be a motronic system as theres so much that's aftermarket there)

white924s

I don't mean to sound like I have the be-all, end-all answer to then old question of "How to get power out of an N/A?", I just want to add my knowledge of engines to the conversation so that hopefully together we can get the ball rolling on some more development of the N/A motors.

Lorax

ICE engines using gas are far far less efficient than .6 or .8

.6 would be the most efficient gas engine ever made by far.

it doesn't matter whether you are talking about diesel or gas motors, ICE are wildly inefficient with plenty of room for improvement by raising compression, IF you can keep it from knocking. The only reason that most N/A engines don't run higher compression is because it would eat the motors, not because it wouldn't yield better efficiency. The point of diminishing returns is when you start blowing cylinders apart because the combustion is so powerful. That would happen before any other theoretical decrease in efficiency.

MAF+PB+motronic is still motronic, and still cheaper and easier than a full aftermarket EMS.

86 951 Driver

What is all done to your 924? It sounds like you may about 170 to the wheels or something?

ritzblitz

Didnt the 924 cup cars used to make over 200hp??

V2Rocket

they were factory backed which means they had a basically infinite budget to do such things.

you could get 200hp out of the na motor.

you just need to rev the eff out of it along with all kinds of other mods.

i did some rough calculations earlier and the stock 2.5 8v seems to be about 88% volumetric efficiency (at peak HP at 5800RPM). i think with a cam ground for higher RPM it could make good gains, along with software tuning to match. (interesting and possible related fact: peak HP occurs at 5800RPM. redline is 6500RPM. 5800 is about 88% of 6500)

knifed crank, custom high-rev (7500+RPM) cam, balance shaft delete, dry sump, 951 springs, solid lifters, light-weight high-compression pistons, probably stronger custom rods, big/lightweight valve head, ports matched, short runner/large plenum intake manifold, custom software..

Lorax

Hell if you are doing all that just go with ITB's

with say a 13.5:1 CR that motor would be capable of a lot more than 200hp

67King

Compression ratio affects the thermal efficiency of the engine. The formula for the thermal efficiency of an engine is basically defined as: 1-CR^gamma. Gamma for air is 0.4. So you have diminishing returns, but you NEVER reach 100% efficiency. However, the thermal efficiency of an engine is often above 0.6. Now, there are other factors that affect it. One of them is spark plug location, so fo folks thinking a 2V is only about airflow......think again. Other things, FWIW, are chamber material, surface area, bore size.

So, how much is compression ratio on a given engine worth? Luckily, you don't have to go figure it out, these guys named Caris and Nelson already did. Improvement in terms of percent with 9.0 as a baseline is y=0.0136(CR)^3 - 0.7477(CR)^2 + 13.494(CR) -70.73. Big caveat there, though. That is indicated power, not brake power. Indicated power is power made before friction loss - remember, we're talking about thermal efficiency. Anyway, going from 10-14% will give you about a 6% increase in indicated power. As friction is constant, the realized gain will be a little bit more than just 6%.

Of course, to make the power, you have got to have the ability to flow the air. The S had two 37mm valves, and make, what, 190? The base has a single 45mm valve. The S has the same valve area as a single 52mm valve. That is a BIG difference. Can it be done? I don't know. Back of the envelope Z-factor calculations say you shouldn't be able to make more than about 6250 RPM without bigger valves and/or cams. I don't know what kind of VE that would take, and I'm too lazy to crunch those numbers right now.

Anyway, I used to do this stuff for a living. Still do it on the side a little bit. I always find these kinds of threads amusing.

Lorax

What gas powered ICE engines have an efficiency of .6?

Most have an average of 15%, even peak efficiency is only as high as 25-30%

67King

I knew I was going to regret getting in this thread.

As I said, THERMAL efficiency. That is how well it converts the air and fuel in its chamber into power. The term that is used is ISAC - indicated specific air consumption, and has units of mass/power-time (e.g. pounds per horsepower-hour). Once it does that, the engine has mechanical losses. Which is why I went on to explain that you'll see more brake power than the numbers will show you, because the friction stays constant.

There are three types of efficiencies - volumetric, or how well it breathes, and is usually over 100% on modern engines in the RPM region around their peak torques. Thermal, which is what I just went over. It is about 65% on a dedicated race BMW engine I'm doing some analysis on, but the baseline formula was posted. Finally, friction. It is exponential with RPM - rings are the biggest factor and increase squared with RPM. Around 70-75% in most productoin engines at their horsepower peak.

If you want to make a NA 944 8V engine make 200 horsepower, you need to understand this stuff.

Lorax

Why would you regret it?

I should have read better that you said thermal efficiency, because throughout the thread I have been referring to mechanical efficiency

Most cars dont even reach 30% mechanical efficiency, If they had 65% thermal efficiency they would have very very high mechanical efficiency because as you said friction is constant (obviously we would need a number for friction on a given engine). Hence my point.


Lets make sure we differentiate between peak and average efficiency as well too.

Lorax

Also... I am in nashville - Would you like to help me with my next motor?

white924s

Apologies, you're right. I was referring to specific horsepower (precisely fuel specific horsepower), which is in a range of 0-2.0 for gasoline powered internal combustion engines. As such, 60% mechanical efficiency is unrealistic, but .6 specific horsepower is quite do-able.

As for the compression ratio question, really commpression ratio is limited by whatever breaks first. You'll start seeing diminishing returns right away (whether the diminishing nature is tangible or not or what point it becomes tangible has yet to be proven), but you can keep increasing CR until you break something. I must say though, if you don't have detonation and your cylinder walls fail before your connecting rods, you have grossly over-engineered your rods and should drop the CR a bit but drop the weight of the rods substantially. Less rotating mass = more hp

and lastly, yes MAF+PB+motronic is still motronic, but you must admit that at that point, very little of the core functionality (eg metering fuel and timing spark) is still truly handled by the OEM motronic system. It may be cheaper and easier than a full standalone system, but its not quite OEM (which is neither good nor bad...indeed, what people are able to do with add-ons to 20 yo EFI systems is quite impressive)

Rock

Id just save all of the headache, trouble, time, and calculations with all of the stuff just explained, and go out and buy a turbo. Youd be making 350-400HP by the time the 200hp N/a even thinks about moving out of the driveway.