Why Does My Car Make Less Power Without Exhaust

[sky30]

A while ago i did a quick test on the dyno where i unbolted my exhaust before the cat and it lost power (around 15rwkw). just wondering if anyone has experienced this and what would be the cause????

Motor is a 400rwkw Rb30/26 with GT35 with .82 exh housing, 3.5 inch pipe to the cat, then 3 inch from cat to rear of car,i would have thought that unbolting the 3 inch section and letting the motor breath through just the 3.5 inch exhaust would defionately make more power.

[Hanaldo]

In a word (possibly two... hyphen anyway ), back-pressure.

[sky30]

less backpressure on a turbo should make more power shouldnt it????

couple of vl turbos i worked on made more power when unbolting the exhaust.

[Moodles2]

How far apart were the before and after dyno runs done, and what kind of cat? Only thing I can think of is the engine soaking up heat from the hot gas exiting the front pipe making the intake charge hotter than it's normally due to proximity.

[DVS JEZ]

but the dyno fan should have enough force to push the exhaust away from the intake.

as said above where they back to back runs?

[XTRMTY]

In a word (possibly two... hyphen anyway ), back-pressure.

Thats debatable.

I have heard this argument many times before however in a turbo car less back pressure after the turbo = more power. This is because it allows the turbo to spool better and back pressure is still maintained before the turbo.

The car should have made more power if it was tested back to back. Maybe it would have been worth doing another dyno run straight after with the exhaust on again?

[Brezza]

maybe the tune was a little out at the extra flow made the air flow better but the map points on the ECU may have been referencing a different cell?

maybe a little fine tuning would be needed to take advantage of the better air flow?

shouldn't it only make more power if there was a restriction?

i changed my cat to a decat and lost power on a dyno, purely because it flowed that little bit better the A/F ratio was all over the place because of the extra flow.

[MrStabby]

maybe the tune was a little out at the extra flow made the air flow better but the map points on the ECU may have been referencing a different cell?

+1. That would make the most sense. Your tuner wouldn't have bothered to tune cells that the car wont see in normal operation, so those cells are probably on defaults, which would be conservative (less timing, more fuel).

[jarrod83]

Wow sky thats some awesome power, i always assumed you were ussing a 1.06.. was that on 98? were you using wmi or nos?

I seen some dynos a while ago of people putting some rediculously big exhausts on some 2litre NA cars, i think they were upto about 4 inch in size, my memorys a little foggy but im pretty sure they were gaining power in the mid range to top end but lost power down low, which goes to show that the idea of backpressure is alot more complex than most people think, involving a few factors in the fluid dynamics of the air flow.

Its more than likely due to the car needing retuning but i cant help but wonder if some of the power loss could be contributed to the greater change in the fluid velocity of the shorter exhaust, with fast moving exhaust gas hitting still air shortly after coming out of the engine, maybe a longer exhaust with few restrictions acts as a buffer in the transition between the fast moving air from the engine and the still moving air in the atmosphere, also as the velocity of the exhaust gas reduces as it cools along the exhaust pipe and contracts. Its probably not a issue when the car is actually moving as i would imagine the exhaust gas would flow out of the exhaust much easier, i doubt the fans they have there could simulate the air for of a car moving at a 100kmph. Tho my number plate gets a hell of a workout from them, id hate to see it when i was actually driving along on a highway.

Jarrod

[Harey]

As has been said you are right it should always make the same power or more with less exhaust restriction on a turbocharged vehicle (assuming everything else is consistent). The comment about this change pushing the tune out is very possible.

Backpressure is definitely not helpful on a dump pipe of a turbocharged vehicle.

[sky30]

runs were back to back on the dyno, put the exhasut back on and power went back up to were it was.

i thought the tune would be way out with out the exhast, but from memoy the AFR didnt change a geat deal (cant remember how much as it was a fair while ago)

Anyone experienced the same drop in power, then re-tuned to make more power than the original figure?

[sky30]

The ECU isnt AFM cell type, My Ecu is Map sesor based, and it didnt make anymore boost, so mapping should have been similar

[sky30]

anyone???

[lachlanw]

engine wil still flor more air regardless of boost pressure being the same. boost is only a measure of how restricve your engine is

[Kinks]

yeah your load cell will be MAP based which means it will still be on the same cell but flowing more air. lachlan is spot on. if that's the case one would think it would lean out slightly which normally improves power anyway (in addition to the extra air flow).

15kw is less than a 4% difference at your power level, which is a small difference but it is the opposite way to what you'd expect and you did get consistency when you put the exhaust back on.

my understanding of exhausts is that the tuned length ends with the turbine and everything after that should pose as little restriction as possible. In an NA car it's not backpressure you want per se but exhaust velocity, this helps with scavenging (maintaining velocity = smaller piping = some backpressure hence what people say). exhaust velocity doesn't help after a turbine because they operate on a pressure differential across the blades. after that you just want to get rid of the gases.

[JonnoHR31]

even if boost was the same the intake charge would be colder, which would mean more air. this would be picked up by the IAT sensor and affect the fuel and timing. so ends up doing the same thing as a setup with an afm

[jarrod83]

Well i was bored so i thought i would have another crack at possibly explaining your problem since no one has been able to chip in with some real world comparisons..

As you lower the pressure after the turbocharger, you will also lower the pressure before the turbocharger, as the exhaust gas pulses out of the engine, it has a high pressure leading edge, a lower pressure body and a much lower pressured tail. Although turbo cars have small overlap to deal with high backpressure with little turbos, there is still some overlap. With a small restrictive setup you often have exhaust gases contaminating the intake charge, as you remove all the exhaust restrictions you can often have the opposite happen. As your intake valve opens pressurised with the likely 30psi above atmospheric pressure your pumping into it, you'll likely be losing more fresh air and fuel from your intake charge into the now slightly lower than before pressured tail of the exhaust pulse. With there being left usable fresh air left in the combustion chamber your engine will be using the air pumped from the turbo slightly less effeciently, the compressor will likely be working slightly harder than before to maintain the same boost pressure, which was more than likely already flowing about as much air as it possibly could, offsetting the gains to be made from a less restrictive exhaust system.

It would be interesting if you had logged the before and after exhaust and intake temperatures to see if there was any noticeable change to help explain what is happening, if you are losing fresh air and fuel into the exhaust, your exhaust temperatures will likely rise.

maybe if you adjust your cams for less overlap, you should get some gains..

Jarrod

[sky30]

makes sense i guess.

Anyone had the same issue?

[Rolls]

The fact that your ECU uses a MAP sensor and not AFM is the issue. With an AFM even though the boost pressure remains the same you are allowing the engine to breathe better hence more air is flowing, the afm will compensate for this and change the load points, the MAP sensor however will not notice the difference and will use the same load points, hence throwing your tune out.

If the dyno operator tweaked the tune ever so slightly it should at least make the same power, if not slightly more due to being able to run slightly more ignition timing from reduce EGTs and backpressure, should also spool a bit faster.

Edited September 14, 2010 by Rolls

[juggernaut1]

Not directly on point ....but explains why backpressure can help mid range torque. Also confirms that less backpressure should reduce EGT and make more peak power.

exhaust</SPAN>.htm' class='bbc_url' title='External link' rel='nofollow'>http://www.max-boost...ust/exhaust.htm

Taken from the above link.

Can a free-flow exhaust reduce efficiency?

Actually, it can. The efficiency of a turbocharged engine relies heavily on the cylinder head operating at the right temperatures - more precisely the gases between the exhaust valves and the silencer (or cat). The speed and temperature of these gases dictate the force that will drive the turbine. If the gases are too slow or too cold then the turbine isn't driven as hard as it could be, resulting in increased backpressure and a slower compressor (less boost).

An exhaust that is *too* free-flowing can result in the engine feeling 'gutless' at the bottom of the rev range. This is not always placebo, the gases leaving too early result in lower exhaust gas temps, therefore lower torque produced at those engine revs. This is only the case at low revs, because the very same exhaust design also results in higher flow at high revs, and lower EGTs there too. The only difference is that the lower EGTs are now welcome, because they are pushed below the maximum (safe point), while at low revs they were below the minimum (efficiency point).

Such an engine will produce improved max bhp figures, but looking closely it will be apparent that it's at the expense of low-down power. Once it's recognised however, it can be fixed - exhaust wrapping could help bring EGTs back up again, while the free-flowing exhaust can retain the max flow potential. Best of both worlds.