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Old 04-08-2005, 07:37 AM   #1
windsonian
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Exhaust Systems

Ok, here's a question I should know the answer to, and should be able to look up, but I thought I'd probably get better and more concise responses from people here.

I understand how getting a more open, less restrictive exhaust can improve the power of an engine by increasing flow, but what I want to know is whether or not this will increase fuel efficiency?

These are my thoughts and they may be way off:
The exhaust will increase the efficiency of the engine in terms of power produced per amount of fuel burned. But if I'm just driving around the city normally (90% of my driving is to and from work in traffic), would the more open exhaust cause more fuel to be burned each combustion stroke of each piston? Because unless I start driving in a higher gear at lower revs (which isn't going to happen realistically), I'm going to be doing exactly the same number of rpm as with the old exhaust. Therefore the exhaust which causes the least fuel to enter the chamber each stroke would be more efficient.

However, then I thought, well if the engine is getting more power for the amount of fuel burned, then maybe exactly the same power is generated by just using less fuel. This makes sense from the point of view that to keep the car at 60kph in a certain gear (ie constant revs), the amount of torque/power needing to be supplied to the back wheels is just enough to overcome drag, friction and other losses. Any more power and the car will accelerate. And no matter what exhaust is used, a certain gear will have a certain rpm for a certain speed (eg 4th gear 2100rpm at 60km/h - guess). Therefore, to get the same power out of the more open exhaust, the throttle would be more closed, letting LESS fuel into the chamber to achieve the required rpm.

But then, doesn't an engine give a certain power at a certain rpm? If this is so, then you can't just say "I need 2100rpm and 120kW", because the engine might now be giving 135kW at 2100rpm. So closing the throttle would then slow the revs down, even if the power being applied was still sufficient to keep the car at 60km/h.

As you can see, I'm going round in circles. I'm sure this sounds very confusing to someone reading it, but I'd love to understand it more clearly. I'd be more than willing to clarify anything I've said (tried to say) if it helps you to give me the answers I want.

Much obliged ... thanks for reading all the way to here.
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Old 02-22-2006, 03:20 AM   #2
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*Bump*
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Old 02-22-2006, 04:21 AM   #3
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I agree with what Windsonian posted i.e. "I'm sure this sounds very confusing to someone reading it..."

If I get the gyst, there is an assum[tion a larger exhaust will result in better efficiency at low revs = "warning, warning, Will Robinson".

There are a few things to consider here and as always a balancing act. You can pretty much bet there is always yin and yang with any mods, but in this instance consider:

if you reduce impedance on the exhaust side (commonly coined reduction in backpressure) and make no other changes, at idle and low revs blowdown velocity will be higher resulting in lower velocity during the exhaust stroke, resistance to air/fuel exiting the chamber during overlap will be lower, syphoning will be lower resulting in a lower VE, swirl will be lower, BMEP will be lower, torque will be lower, etc.

Of course someone will say that less residual exhaust gas in the pots is a good thing because it makes more room for oxygen/fuel. But this is not the general case. Residual exhaust gas slows combustion, lowers combustion temps, lowers NOx, etc which can be desirable in traffic..
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Old 02-22-2006, 05:32 AM   #4
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Quote:
Originally Posted by Wally
I agree with what Windsonian posted i.e. "I'm sure this sounds very confusing to someone reading it..."

If I get the gyst, there is an assum[tion a larger exhaust will result in better efficiency at low revs = "warning, warning, Will Robinson".

There are a few things to consider here and as always a balancing act. You can pretty much bet there is always yin and yang with any mods, but in this instance consider:

if you reduce impedance on the exhaust side (commonly coined reduction in backpressure) and make no other changes, at idle and low revs blowdown velocity will be higher resulting in lower velocity during the exhaust stroke, resistance to air/fuel exiting the chamber during overlap will be lower, syphoning will be lower resulting in a lower VE, swirl will be lower, BMEP will be lower, torque will be lower, etc.

Of course someone will say that less residual exhaust gas in the pots is a good thing because it makes more room for oxygen/fuel. But this is not the general case. Residual exhaust gas slows combustion, lowers combustion temps, lowers NOx, etc which can be desirable in traffic..
Thanks for the bump Godlaus... wasn't expecting this thread to return..

Do you get what I'm trying to say in my 3rd paragraph though?

Forgetting backpressure and all that for the moment - if the engine produces (for example) 200Nm at 2000rpm ... then I make a mod (any mod) so that it now produces 250Nm at 2000rpm. Just say that in 4th gear the car is going to need to use 150Nm of that to keep the car at 60km/h on a flat straight road. Therefore the throttle would need to be more open on the original engine than the modified one to produce the requisite torque to maintain this rpm/velocity. Should this modified engine then allow LESS fuel into the engine? Or is it that more fuel gets sucked in through a smaller opening (in the case of a carby)? And for injectors ... how does air flow change fuel flow? I thought it was based on throttle valve position?
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Old 02-22-2006, 06:19 AM   #5
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Ive heard they DO increase milage because the engine breathes easier, and therefore doesnt work as hard (more exhaust escaping means free flowing engine so its better either way). Also add quite a bit of power. Try to find the one that is as straight as possible if you are shopping for it.
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Old 02-22-2006, 07:08 AM   #6
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Quote:
Originally Posted by newyorker
Ive heard they DO increase milage because the engine breathes easier, and therefore doesnt work as hard (more exhaust escaping means free flowing engine so its better either way). Also add quite a bit of power. Try to find the one that is as straight as possible if you are shopping for it.
What do you mean by "doesn't work as hard"? You mean more efficient .... and that's my whole point, I don't think it's that simple either way. "breathes easier" implies more flow - so either you're running leaner air:fuel, or you're using more fuel.

[EDIT]: And I'm aware of the reasons for straight pipes ... by the same tokens, you want smooth pipes .... or maybe rough enough to get a thin turbulent boundary layer to assist flow.
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Old 02-22-2006, 07:53 AM   #7
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found this off a website...Cat-Back Exhaust System, True Bolt On Performance, Top Quality T304 Stainless Steel, Smooth Mandrel Bent Tubing, Turndown/Turnout Tip, Split Rear Exit, 7 To 15% Increase In Horsepower Over Stock System, Better Fuel Milage, And A Distinctive Performance Sound, Backed With Borla's 1,000,000 Mile Warranty!.
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Old 02-22-2006, 08:31 AM   #8
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with a larger exhaust sytem, more air is escaping from the enigine quicker, allowing more air to exit the engine with less "work" at a certain rpm, so i THINK that means that a less RPM with the larger exhaust will attain the same power as the higher RPM with a more restrictive exhaust

Thats just my logic, dont know for sure though
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Old 02-22-2006, 11:28 AM   #9
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Quote:
Originally Posted by windsonian
Thanks for the bump Godlaus... wasn't expecting this thread to return..

Do you get what I'm trying to say in my 3rd paragraph though?

Forgetting backpressure and all that for the moment - if the engine produces (for example) 200Nm at 2000rpm ... then I make a mod (any mod) so that it now produces 250Nm at 2000rpm. Just say that in 4th gear the car is going to need to use 150Nm of that to keep the car at 60km/h on a flat straight road. Therefore the throttle would need to be more open on the original engine than the modified one to produce the requisite torque to maintain this rpm/velocity. Should this modified engine then allow LESS fuel into the engine? Or is it that more fuel gets sucked in through a smaller opening (in the case of a carby)? And for injectors ... how does air flow change fuel flow? I thought it was based on throttle valve position?

You've never mapped an engine have you Windsonian?

Remember there is a difference between wide open throttle, where the peaks are measured and part throttle. On a carby the venturi and venturi amplifier create a low pressure region in relation to the bowl; the faster the air flow the greater the differential and thus more fuel. On an injection engine the program looks at the VE maps, enrichment, plenum pressure or air flow, rpm, gamma, engine temp, air temp, etc and meters the fuel accordingly (alpha-N looks at throttle position for more than just acceleration enrichment ).
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Old 02-22-2006, 01:23 PM   #10
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does anyone suggest "the best" brand of exhaust and type that will give you the greatest gain in hp. thats around 700 to 800 dollars for a dual. i heard bolra is very good? but then i have seen where people hated it. (also with a pretty big gain in loudness)
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Old 02-23-2006, 12:15 AM   #11
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Quote:
Originally Posted by Wally
You've never mapped an engine have you Windsonian?

Remember there is a difference between wide open throttle, where the peaks are measured and part throttle. On a carby the venturi and venturi amplifier create a low pressure region in relation to the bowl; the faster the air flow the greater the differential and thus more fuel. On an injection engine the program looks at the VE maps, enrichment, plenum pressure or air flow, rpm, gamma, engine temp, air temp, etc and meters the fuel accordingly (alpha-N looks at throttle position for more than just acceleration enrichment ).
Is it so obvious

Carby I understand .... it's straight mech .... you'd think I'd know the other seeing as how I work for a control system co. now
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Old 02-23-2006, 01:13 AM   #12
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Quote:
Originally Posted by windsonian
Is it so obvious

Carby I understand .... it's straight mech .... you'd think I'd know the other seeing as how I work for a control system co. now

Yes and you know all about PWM, etc. The trouble with a lot of the ECU programs is that they are pretty much open loop, although controllers like the Megasquirt throw an O2 sensor into the routine once it reaches a user defined rpm to maintaiin a gamma (1/lambda) setpoint . What you would call lookup tables, the auto industry calls maps.
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Old 02-23-2006, 01:53 AM   #13
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Quote:
Originally Posted by Wally
Yes and you know all about PWM, etc. The trouble with a lot of the ECU programs is that they are pretty much open loop, although controllers like the Megasquirt throw an O2 sensor into the routine once it reaches a user defined rpm to maintaiin a gamma (1/lambda) setpoint . What you would call lookup tables, the auto industry calls maps.
yeah, we use pwm to control heater outputs on HVAC systems.
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Old 02-23-2006, 02:10 AM   #14
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Quote:
Originally Posted by Fender89
does anyone suggest "the best" brand of exhaust and type that will give you the greatest gain in hp. thats around 700 to 800 dollars for a dual. i heard bolra is very good? but then i have seen where people hated it. (also with a pretty big gain in loudness)

is there a best brand? What I'm thinking is that different companies specialize in different types of cars/exaust systems. Also the design is different from brand to brand.
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