big bore exhausts actually reduce power???
Discussion
i have Been told that an big bore exhaust can actually reduce the power of a car.Apparently it's got something to with back pressure.So that these little 1.2 clio'sfor example with say a 5" big bores will actually be slower than the standard car.is there any truth in this.?
regards john
regards john
jbi said:
depends on the engine...
A big 5.7 litre pushrod V8 or a turbo motor usually needs as little back pressure as possible.
Yep. Tuning the exhaust to the car is what its all about. A big 5.7 litre pushrod V8 or a turbo motor usually needs as little back pressure as possible.
IIRC there's some formula between exhaust system diameter and Cubic Feet per Minute (CFM) that can be used to make the perfect system.
Then there's balance pipes, X or H which can be used for scavenging gasses from the other bank of cylinders etc
Quite a dark art.
k...Lplatejohn said:
i have Been told that an big bore exhaust can actually reduce the power of a car.Apparently it's got something to with back pressure.So that these little 1.2 clio'sfor example with say a 5" big bores will actually be slower than the standard car.is there any truth in this.?
regards john
I wish people would read up on this.regards john
NO you don't want backpressure!!!!!!
What you want is the freest flowing exhaust with minimal backpressure that doesn't reduce the exhaust flow rate.
What confuses far too many people is fitting a free flowing exhaust (and/or tubular manifold) can seriously affect the air/fuel ratio of the standard tune. So unless you retune the car to take this into account it 'might' run worse.
Tune it and it'll run a lot better.
Not directly related to just exhausts, but tuning and modifying in general.
HP = torque x rpm / 5252
This means the more torque you make the more HP, but more importantly making the same torque at higher rpms will yield even higher HP increases.
The amount of torque an engine makes is largely dictated by it's displacement. So while you can see torque gains when modified, they are usually far less significant. The trick comes in moving the power curve up the rpm range.
Using the fake graphs above. The torque curve has been moved to the right, it's still the same PEAK but it now occurs at higher rpms.
But the profile/shape of the curve doesn't really change. So at low rpms you are now further down the curve, ie you've make gains in mid and higher rpms, but will have lost out in lower rpms.
This affect often causes people to think mods haven't worked or they have lost power. In reality it's how the engine feels and if you don't rev the engine enough (many don't) you won't see the gains. Also such tuning may require different gearing so you can keep it in the new powerband better.
As for silly big exhaust tips - pointless. But since when has the tip dictated the bore size of the rest of the exhaust?
Backpressure is the most incorrectly quoted term of all time when it comes to exhausts. Why would anyone think greater power would be created by restricting gas flow exactly?
Balanced systems with tuned length primaries and depending on the engine configuration things like X pipes and/or loops.
Balanced systems with tuned length primaries and depending on the engine configuration things like X pipes and/or loops.
V88Dicky said:
Yep. Tuning the exhaust to the car is what its all about.
IIRC there's some formula between exhaust system diameter and Cubic Feet per Minute (CFM) that can be used to make the perfect system.
Then there's balance pipes, X or H which can be used for scavenging gasses from the other bank of cylinders etc
Quite a dark art.
I believe David Vizard quotes 2.2cfm flow rate per HP for a zero resistance exhaust system.IIRC there's some formula between exhaust system diameter and Cubic Feet per Minute (CFM) that can be used to make the perfect system.
Then there's balance pipes, X or H which can be used for scavenging gasses from the other bank of cylinders etc
Quite a dark art.
SSBB said:
Is it possible that properly engineered exhaust systems can actually reduce backpressure below atmospheric (ie negative gauge pressure) due to the Berrnoulli effect of other cylinders' gasses flowing through the manifold?
Yes, it's called exhaust scavenging and is the most important part of n/a exhaust tuning.On something like a Chevy LS1, long tube headers (exhaust manifolds) will promote scavenging and can see in excess of 20hp gains.
Compare to similar design short tube headers (only have short length primaries) which then to yield more like 2-3hp gains, despite out flowing the stock log style manifolds.
On FI engines it doesn't work quite the same though, as the intake air is already under pressure when entering the combustion chamber.
300bhp/ton said:
What confuses far too many people is fitting a free flowing exhaust (and/or tubular manifold) can seriously affect the air/fuel ratio of the standard tune. So unless you retune the car to take this into account it 'might' run worse.
Tune it and it'll run a lot better.
^This.Tune it and it'll run a lot better.
I fitted a Rimmer Bros s/s sports exhaust with headers to my 90 V8. Before fitting it was lively and pulled well at all revs. After fitting it could barely get out of its own way until I took it along to my local V8 specialist and had some juicier needles put in the SU's and a quick retune. Like night and day. With the new exhaust the carbs just couldn't supply enough petrol, with the new needles not only was performance restored, it was significantly increased.
300bhp/ton said:
Yes, it's called exhaust scavenging and is the most important part of n/a exhaust tuning.
On something like a Chevy LS1, long tube headers (exhaust manifolds) will promote scavenging and can see in excess of 20hp gains.
Compare to similar design short tube headers (only have short length primaries) which then to yield more like 2-3hp gains, despite out flowing the stock log style manifolds.
On FI engines it doesn't work quite the same though, as the intake air is already under pressure when entering the combustion chamber.
Interesting, didn't realise it could have such a profound effect if it is done poorly. You learn something new every day!On something like a Chevy LS1, long tube headers (exhaust manifolds) will promote scavenging and can see in excess of 20hp gains.
Compare to similar design short tube headers (only have short length primaries) which then to yield more like 2-3hp gains, despite out flowing the stock log style manifolds.
On FI engines it doesn't work quite the same though, as the intake air is already under pressure when entering the combustion chamber.
300bhp/ton said:
On FI engines it doesn't work quite the same though, as the intake air is already under pressure when entering the combustion chamber.
Air is already under pressure on a NA by that notion....If you are including turbos within FI as your statement there is the main point thats its still not higher than the pressure in the exhaust manifold. The turbine needs a pressure differential to provide shaft torque. So turbine inlet pressure is often way above inlet manifold pressure. Hence low overlap cam timing.
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