911pcars

"...The more energy that is allowed to escape through the exhaust, less energy is available to drive the pistons and therefore the crank."

The amount of energy wasted in the exhaust (and cooling system) is more a function of the dynamics of the internal combustion process. Just "closing the door" and restricting the exhaust doesn't necessarily translate to more thrust on the piston (BMEP), nor would reducing the efficiency of the engine's cooling system.

I would suggest a reference book on the order of Smith or Ricardo might be in order to understand the intricacies of IC theory. I'm not a combustion engineer but I once stayed at a .......

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

944CS

that was someone else's quote to which i was arguing against in the rest of my post.

James-man

Perhaps on a tangent, but when I put in a free flowing exhaust, it seems that I lost about 75% of engine braking effect (ball park seat of the pants estimate, but quite noticable - I was VERY surprised by it).

Is it possible that the lion's share of engine braking is generated from shoving compressed air through the exhaust? I know on throttle and off throttle are two opposite conditions, but if it is so hard to push air through the exhaust in the first place, I wonder why that isn't a huge mitigating factor on the low end torque loss question.

As a non techie, I am a little suprised that low end torque isn't a wash and am curious about how much effect fuel ratio has on the torque question. If it were purely a function of fuel, low end torque loss would be nonexistent because people would be selling modified fuel maps left and right.

Skunk Workz

I have a Suzuki GSX-R 1100 with a T04 turbo....and the engine braking I got if I ran at 20 psi without a dump valve could toss you over the handlebars... so there's something to the pumping work done. Without the dump,the turbo stalled,and the engine braking I got for not letting the exhaust out almost locked the back wheel at SPEED. No fun....

1999Porsche911

On a N/A engine, the exhaust flow does directly effect the torque of the engine as well as when that torque peaks. Since energy cannot be destroyed it either is used as power to drive the pistons, lost to heat or lost out the exhaust. If nothing else changes and you increase the flow of the exhaust, this will draw more of the air/fuel ratio out of the combustion chamber than normal. This will cause a lean condition in the combustion chamber, potentially costing you power, but definately creating more heat. This increased heat is a result of the energy being transferred from what was used to drivie the pistons. By default, you engine has less power. So combined with, effectively, an increase in valve overlap using a bigger exhaust and increased cylinder heat, you have, by definition, lost power.

However, these losses can be overcome in many cases. Forced induction eliminates this problem as in most cases, the amount of F/A mixture being pumped in is greater than can be blown out the exhaust. You cannot thow more enegry out of the exhaust and expect the same engine power without equally increasing the food to the engine. It works exactly like the human body.

944CS

add more fuel and you won't have any lost power...as I stated before, when you leave less residual exhaust gases in the CC, you are allowing more fresh AIR to come in. In order to take advantage of this, you must add more fuel, which stock ECU's will not do. This increase in fresh air/fuel will create an exponetially greater pressure down on the piston when ignited compared with keeping exhaust gases in the CC.

1999Porsche911

You are correct that you would need to increase fuel AND air to offset any loss in power caused by a bigger exhaust and the stock comuter will do this to a small degree. However, as I stated above, without fuel/air adjustment, your car will lose power. There is no way around it without modifications. Also, so now you have increased your air/fuel injection and have the bigger exhaust. The fact remains that is you start restricting that exhaust (up to a point) your bottom end power will increase. The is a limit to the amount of air and fuel that can fit into a NA engine and most modern engines fill each cylinder close to there max right from the factory.

If there was no limit to how much air and fuel you could pump into an engine, then the elimination of all exhaust componants would provide you the most power. This is where forced induction has the advantage with unrestricted exhaust systems. The only way to eliminate this loss on a NA engine is to internally reduce the valve lash. This will reduce the scavanging effect of the alrger exhaust caused by the exhaust valve opening to early.

944CS

i believe the calculated volumetric efficiency of the early 944 is around 85%

MorganRock

what he is saying is - yes, you are right about if you have a less restrictive exhaust, you will allow more fresh air in. however, on NA engines, there is only SO MUCH air that CAN come in, because there is nothing forcing more air than available in the surrounding atmosphere in.

the effect of this is sort a curved function on a graph. you can unrestrict the exhaust and gain power only up until the point where you deplete the available air. after that, you will begin losing power because as the exhaust vents gases there will not be an equal amount of gases to replace them.

however, if you add forced induction, you suddenly have this HUGE new possibility of volumes of air coming in. that's why almost all turbo cars can benefit from a vastly less restricted exhaust than an NA engine can.

NA engines require much more careful fine-tuning to get the maximum efficiency out of them. they must be tuned to proper lengths and diameters to get the proper scavenging effects out of the cylinders, and be just restrictive enough to provide torque, but open enough to provide high-end power.

this is a common argument i have with small-disp. NA engine import friends of mine, who believe that sticking 3" turbo exhaust on their 4-cyl will help performance, but there is NO way a 2L engine is going to fill THAT volume of exhaust, without FI. It takes a LARGE V8 to flow enough air to require low-restriction exhaust (like big-block), and that muscle car true-dual 2.5-3" exhaust is what a lot of us grew up thinking was so cool, but it REALLY doesn't work on any of our NA's, because we dont have 455 cu in. of engine to fill/vent.

dbanazek

Its all about velocity

Larger exhausts have less veloicty and do not scavenge as well at lower RPMs

bruinbro

Interesting thread, it probably explains why I keep getting a O2 check engine light after I removed my muffler from my 2000 S Boxster and have just straight pipes from the secondary cats. Sounds like my A/F ratio is out of whack and causing the engine to run rich, eventually triggering the CEL and going into "limp mode". I think I'm going to try smaller pipes to see if this phenomena disappears along with knocking down the hideous resonance.

Bro

944CS

the car is actually probably running too lean and that is why it goes into limp mode

bruinbro

I stand corrected. I misunderstood what the stock ECU is trying to do.

Anyway, still going to try out smaller diameter pipes., if nothing else to try and shift the resonance frequency.

Bro

ehall

I LOVE my TURBO!