antiekeradio wrote:why would the turbo choke on 2,5 inch tube??? sound like more than enough!
Turbo it self creates such a huge restriction for the engine that best solution would be running an open downpipe. Of course that's not usually possible in a street car.
And of course it depends of an engine, and how much HP are you putting on the wheels, but basically bigger exhaust diameter is better with turbos..
Easy way to find out if you have too much back pressure in the exhaust and also if turbo is running in it's limits, is to put couple of pressure sensors on to the manifold collector and on DP after the turbo..
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Sorry...I didn't make that quite clear...I'd expect the power reduction will be experienced with the standard engine, prior to installation of the turbo lump.
I'd say a 2.5" 'sports' system is plenty big enough for the 1.7 turbo in road tune. We've just finished putting a full 3" system (right from the turbo back) on Ryan's 740 Turbo with the T3/T4 hybrid, and that made quite a difference.
James is correct 2.5" is a little big for a 1.7. I have just put a 2.5" system on the 2ltr and that is ok - I intend for more power that is why I went 2.5".
There is a little problem over the axle at the mo but that will soon be sorted.
The 1.4 should be fine as I've run tuned 1275 minis on 2" with loosing any over a 1.75".
2.5" should be fine on the 1.7 turbo - even good on a mildly tuned 360 turbo.
Fuse is spot on the turbo does not need any real exhaust as you do not get scavanging as you would on a na engine but you do need somthing to carry the gases away and keep things quiet.
because the 'back pressure' thing is falsely interpreted.
the author starts with explaining that a number of complex processes interact, but then fails to realize that one of these processes is the reason why exhaust tubings affect performance in other ways than just trough 'back pressure'
(i.e. there is a big difference between a steady gas flow and the pulsating 'bursts' coming out of a gasoline engine, in terms of what is the ideal system. the 'bursts' create so-called reflections and shockwaves, and with a wrongly set up exhaust these reflections end up as "peak back pressure bursts". time these wrong and your engine runs like crap.)
Well in a sense I think he's right...but I get the feeling he's trying to justify fart-can exhausts...which can never be right... All engines need a 'tuned' exhaust system, the same way they need a 'tuned' intake in order to work properly. I don't pretend to understand the intricacies of the fluid dynamics involved...but it does matter.
Many race engines don't even have a collector - they just run to an open pipe per pot...but this wouldn't make for a nice drive 'on-road'...both in terms of low-down grunt and noise. If you're making a machine which rolls out to crazy rpm, you'll never operate it at low rpm, and noise isn't a concern, the 'dragster' exhaust is just fine as far as I can see.
However, 'scavenging' effects are crucial to making an engine run nicely below where it 'naturally' makes good power, and these effects take place because gas speed is high due to the high pressure in the pipes (typically thanks to a narrow-bore pipe). Unbolt the DP from manifold slightly on a B14 and marvel at how it suddenly idles like a evil-cammed V8...and runs like turd!
open pipe per cilinder is performance-wise the best solution.
BUT even then, the length of the pipe must be equal for every cilinder, and of a certain calculated length (to avoid mismatch with reflection - and yes the spot where the open pipe ends into 'nothingness' creates a reflection!)
Open pipe is the best solution for a high reving race engine.
An exhaust has a big effect on how a engine runs and what rpm max power is produced.
I will not pretend to know it all but in general terms an exhaust needs to be sized to the caristicks of the negine it is going on and geral rules can be followed to give you close to best first time.
A drain pipe is great for a constant high reving engien but usless around town.
I see people were discussing this a while ago but...
intake and exhaust tuning is pretty similar in principal.
Basically, if you think of the inlet pipe or exhaust manifold as straight tubes then the opening and closing of the valves will create pressure pulses ie ~~~ waves in the pipe. At certain engine rpm and hence valve opening and closing frequencies, these waves will reach a resonance condition in the pipe ie a full number of waves in the given pipe length.
You can prob see where this is going but the pipe is onlt ideally tuned for one frequency of valve opening / closing hence engine rpm.
The inlet / exhaust tuning is useful as if tuned correctly, the pressure pulse of inlet air will arrive at the valve as it opens and the peak rarefied section in the exahust gas will arrive at the valve at it opens, helping to fill and evacutate the cylinder respectively.
Manufacturers would tune the inlet and exhaust manifolds differently. The inlet ( for petrol cars ) need to behave such that equally mixed and equally composed air fuel mix arrives at each cylinder. Exhaust needs to allow equal gas flow from each cylinder. There is a theory (as yet not proven either way ) that creating turbulance in the inlet is a good thing but porting and polishing the exhaust is good. However, excesslve turbulance in the inlet affects flame propogation so too much isnt good either.
Anyway, the tuned inlet or exhaust in terms of pulses only works exactly for one engine rpm but offers benefit still for engines speeds above and below that region. Manufacturers would tune the system for the 'normal' operating rev range ie 2000-2500 rpm or 1500-2200 etc.
Cars like the 850 with the N series engines introduced variable geometry manifold. Bascially, you have two inlet pipes for each cylinder and which one you use is separated by a throttle / butterfly valve. According to engine speed and load, the vlave switches from one to the other therefore giving two tuned systems eg one for 1500- 2000 and 2000-2500 for example. I dont have the exact figures to hand but you get the idea - two tuned regions....
Ideally, engines should have continuously variable inlet and exhaust (manifold ) pipes but getting a system to respond fast enough has been a challenge and the gains versus the cost put off most manufacturers.
A well tuned inlet and exhaust manifold can add to an engines performance. In the induction side acts like a mini forced induction system.
Backpressure is created by the silencers in the exhaust, catalytic converters, diesel particulate filters etc. The higher the pressure the more work the engine needs to do to loose the exhaust gas. If I remeber right the open end of the pipe also adds to the reflection of pulses so even an open pipe needs tuning.
Something Ive not looked into but heard through a VOC article is that a certain degree of backpressure is needed to maintain the turbo main seal - turbine seal ie back pressure created by the silencers etc.
anyone know about that? apparantly this chap had probs with his 940 turbo due to insufficient back pressure.........
a few guys have mentioned a turbo engine not needing any back pressure for assisting cylinder exhuast gas emptying. My guess is the turbo turbine blade upsets formation of any pressure pulses but isnt the twin scroll system introduced on the s40 V40 T4 and 2.0T not based on using the pressure pulses to spin the turbine up faster at low revs and reduce turbo lag?
Of course that's not usually possible in a street car.
All engines need a 'tuned' exhaust system, the same way they need a 'tuned' intake in order to work properly. I don't pretend to understand the intricacies of the fluid dynamics involved...but it does matter.
