Backpressure?

No properly designed engine ever performed better with more exhaust back pressure. That's why anyone trying to wring the last hp out of an engine uses a straight thru exhaust system.

I don't think you're right there. If you'd said "No properly tuned engine ever performed better with more exhaust backpressure", I'd be inclined to agree, though. As far as I know, changing over to a straight through exhaust usually requires adjustment to be made to the fuel delivery also, which hasn't been done in this case.

Andrew

Hmm...being a diesel the fuel is adjusted with your right foot. Unlike a petrol engine where the fuel need mixing with a fairly rigid amount of air.

However the missing silencer may have upset the tuning of the exhaust causing more back pressure.

Oh dear...Sorry guys but 20 years spent tuning & modifiying cars, reading some of the most respected authors on the subject, I feel a rant coming on.

Asbetsos Underwear Attached

I know how difficult it is to get this sort of info these days but...

Horse power is nothing more than torque multiplied by engine speed. No torque means no horse power.

For example, an engine which produces 100 lbft of torque between 1000 and

6000 rpm in flat line will produce between 19 hp @ 1000rpm & 114 hp @ 6000rpm, also in a straight line. The torque didn't change, only the engine speed.

We produce power by burning fuel and air to create an exposion forcing the piston down the bore. The more air and fuel we burn the more power we get out. Back pressure reduces the amount of air we can get into the engine so reducing power.

An exhaust system designed, for what ever reason, to give back pressure at low engine speeds and therefore low flow rates cannot be expected to suddenly reduce that back pressure as engine speed and therefore flow rates increase. Usually the reverse is true. A system giving back pressure at low flows just gives more and more back pressure at higher flows.

In message , Adrian England writes

I basically agree but it's more complicated than that. If the exhaust is correctly tuned the pressure in the cylinder can reduce even though the instantaneous pressure in the exhaust may appear to increase. Sounds rubbish but bear with me. Fluid dynamics means that the hot gas ejected from the cylinder acts like a plug and produces a low pressure pulse behind it. This low pressure pulse, if the exhaust is tuned correctly, can 'scavenge' the cylinder of gas to a lower pressure than would normally be expected before the exhaust valve closes (this can also be tuned to act across cylinders with valves closing just as others open but is more complicated). The subsequent fuel/air charge can then be bigger in the cylinder giving more power. If there is too much back pressure the exhaust is reducing overall gas flow and restricting the engine performance. If the exhaust is too big there will be little or no scavenging and reduce performance that way. Coupled with all this is some speed (ie rev) dependency of the tuning of this whole system.

Admittedly this is all theoretical when talking about an engine that tops out at thirteen chugs per second, puts out thirty sick-sheep-power and only really needs an exhaust like a straw ... :-)

Agreed.

Mathematically correct, but no such engine exists. Virtually all production engine have torque and power peaks, not lines - the relationship between the two peaks defines how the vehicle drives.

Agreed, but only in relation to power, not torque. Here are some links that underline the importance of back pressure for the production of low speed torque:

The two are always a compromise, but your tuning experience is only from the point of reducing back pressure to increase high rev power - this is not particularly relevant to a 2.25 diesel! As the above links show, there is a lot more to exhaust tuning than just putting the biggest, straightest pipe on you can.

Entirely correct. Similar engineering also applies to air intakes - ISTR reading about a Vauxhall engine that has some gubbins that changes the geometry of the intake from short to long (or t'opposite) as engine speed changes.

Tim Hobbs

'58 Series 2 '77 101FC Ambulance '95 Discovery V8i

... although some truck diesels come close to having a flat torque curve.

Quite a few engines have variable length inlet manifolds - long for torque, short for revs.

Julian

--------- = Pretentious Sig required =

Absolutely, but not over a 5000rpm range! :)

basically agree but it's more complicated than that. If the exhaust

No. Your absolutely right. I just didn,t want to go that deep.

Yes Ford Mondeo V6 uses this too but the operating solenoid for the butterflies jam open losing your bottom end power ;(

It was just an example to bring arcoss the information

Um...and the engine / chassis dyno results / comparisons are where?

The highest performance is extracted (Puh intended) from the exhaust by using the smallest pipe sizes compatible with not restricting the engine. Slappin a 6" drain pipe on the back of engine is one of the best ways to lose power. Max Power readers take note.

I guessed that.

Where are the engine / chassis dyno results / comparisons to back up your assertion that cars always perform better with no back pressure?

The highest power output is thus extracted - as mentioned by myself and others, to extract maximum low rev torque it gets far more complex.

But its the silencers that produce most of the back pressure. What else is there to restrict the exhaust gases?

And the ignition timing curve would be set up for this

Improved air flow in & out of the engine would have resulted in different ignition timing requirements. Did they hold stead power readings at 500rpm intervals to obtain the best advance readings and did they then replace the springs or weights in the distributor to achieve this curve. Did you use too large pipes on the exhaust manifold. Both of these would account for loss of power you experienced.

Well are you suprised? So far you haven't provided one single piece of proof to back up what you're saying.

Here's a not so small quote from David Vizard's How to build Horespower

Over the years, I've heard many strange claims from so called experts. One I've heard more than once asserts that a performance engine, or any conventional four cycle engine for that matter, needs some exhaust back pressure to optimize efficiency. This pronouncement was almost certainly made by someone who had never used a dyno or carefully evaluated how exhaust system flow affects performance. I have no doubt that someone, someplace, at some time produced a power curve that "absolutely proved" that an increase in backpressure resulted in an increase in power. My years of dyno experience tellme that this test must have been done under questionable conditions. I have found that reducing exhaust backpressure always improves power and fuel economy, providing, of course, the air/fuel ratio and ignition timing are carefully optimised both before and after exhaust system back pressure is increased.

This is a guy who has 20+ years experience building race winning engines, has written many books about building engines and even went to the extent of offering to pay board and lodging for anyone who claimed they could build a more powerful mini engine. In the 18 years since that boast, I'm not aware anyone has done it. Are you?

But your experience is entirely irrelevant coz his mini mate can get more power out of an A series by fitting a bigger exhaust! ;-)

Sometimes you have to let people learn for themselves!

google "performance exhaust back pressure"

and start counting...

sounds to me like your both right but don't realize it. exit claims you must have back-pressure whereas adrian believes you don't need any back-pressure. (or do i have it the wrong way around?) all road car engines are specifically designed to be used in conjunction with an exhaust system which lowers sound and noxious fumes etc. therefore, these engines would be designed to operate with back-pressure in the exhaust system. if that condition is changed and there is no longer any back-pressure, the engine would not run well. however, if the engine is designed from the ground up to operate with negligible back-pressur in the exhaust system, you could probably get more power out of a similar engine in a similar state of tune etc.

adrian, i had a similar argument with exit a while ago about differentials and torque splitting vs rpm splitting. by the end of it i had no idea wether he agreed with me or not, so i sympathise with your situation.

lets all argue the point and not how much experience we have in tuning or whatever. i am only 17 and only own a conked out mazda 626, however i believe my knowledge of engines etc far exceeds my years on this planet. there are some who have been around engines for decades and still can't explain the four-cycles of an engine.

cheers.