[max8791]

how much of power gain and FE will one can get after fitting 2 exhausts on 4 cylinder engine.
i.e. 1st exhaustpipe on cylinder 1 and 3 and
2nd pipe on 2 and 4 cylinder

_____________
iam car dumb

[kausu]

Originally Posted by max8791

how much of power gain and FE will one can get after fitting 2 exhausts on 4 cylinder engine.
i.e. 1st exhaustpipe on cylinder 1 and 3 and
2nd pipe on 2 and 4 cylinder

_____________
iam car dumb

dude i think it will make no more difference than a normal freeflow would performance wise..(correct me if am wrong guys)!!! but infact will reduce the sporty grunt that ull get from a single exhaust!!!

[chetanhanda]

Originally Posted by max8791

how much of power gain and FE will one can get after fitting 2 exhausts on 4 cylinder engine.
i.e. 1st exhaustpipe on cylinder 1 and 3 and
2nd pipe on 2 and 4 cylinder
_____________
iam car dumb

The horsepower/torque is not dependant on the number of pipes.

Basically depends on the combination of cam profile ,valve overlap and back pressure of exhaust system at different rpms.To define the entire mechanism would be very exhaustive. In short if you are looking for low end torque there should be slightly high back pressure so the valve overlap controlled by cam profile does not allow too much charge to escape. this would give a torquey feel at low rvs but you will lose out on top end at higher revs. If you are looking for torque at higher speeds then a slightly more free exhaust system would be recommended with a lower back pressure. But lowering back pressure to get high end can be done only to a certain extent, the cam profile which controls how much the valve opens and how long and at what degree also needs to be altered when dealing with exhaust gases. Other factors such as header design also need to be taken into considration depending on the firing order as they have to evacuate the gases from the cylinders by providing a slight amount of lower pressure to some extent during the exhaust stroke.
You can have 8 pipes if you want but if they dont provide the correct evacuation/back pressure they are of no use

All the team BHP motorheads plz feel free to correct me.

[ananthkamath]

Chethan's right except for the backpressure part. The Backpressure theory is like an old wives' tale among hot rodders.

Exhaust velocity is a much more accurate measure of how well the system evacuates the cylinder. Engine output at a given RPM is solely a function of BMEP (Brake Mean Eff. Pressure) and therefore is dependent on how much charge can be burnt. The exhaust aids in "supercharging" the cylinder by creating a negative pressure thereby sucking fresh charge and evacuating the spent charge in larger amounts and shorter times.

However it is not possible to harness the full effect at all engine speeds because the exhaust length is specific to a certain engine speed. Header tube diameter must therefore be optimised so as to maintain good velocity at low speeds, and flow enough gas at target speed.

The normal 4-2-1 headers are designed to harness wave phenomena in the exhaust tracts. Meaning that one cylinder is "supercharged" by either bouncing the wave off of a closed exhaust valve of the other cylinder, or by using negative pressure from the exhaust flow of the other cylinder.

Therefore it is imperative that cylinders 1&4 and 2&3 (or 1&3 and 2&4 in some cases) be joined into a single pipe. So max, that answers your question.

[Psycho]

The key use of headers is to scavenge and to reduce the load on the pistons, hence allowing it to rev more freely.

Also the back pressure theory might hold true for most of the cars but on some engines there is a need for slight back pressure due to the cam profiles. The dia of the pipes can also negate the free flow principle by robbing you completely of lowend torque.

You can however run 2 pipes or 4 if planned right will defn improve performance. The easiest approach is to merge into one pipe and then redistribute it to 2 pipes, here one pipe would work as the collector thus causing the least of the worries.

[raajks]

It is the same approach that normal exhausts use. Difference being, it is routed to 1 pipe instead of 2.
I am sure you will not have a gain in power by doing so.
If you go for a performance exhaust then you would have 1 exhaust for 1 cylinder, which would give you a performance boost

[ananthkamath]

Originally Posted by Psycho

Also the back pressure theory might hold true for most of the cars but on some engines there is a need for slight back pressure due to the cam profiles. The dia of the pipes can also negate the free flow principle by robbing you completely of lowend torque.

You can however run 2 pipes or 4 if planned right will defn improve performance. The easiest approach is to merge into one pipe and then redistribute it to 2 pipes, here one pipe would work as the collector thus causing the least of the worries.

So you're saying that if you have a high-overlap cam, there's a need for backpressure?

On a different note, Mr. Baban Khan was running a 4-into-4 pipe on his Formula 1300 at the drag....I dont know about the other mods on his engine, but if he was running a very high-lift, high-overlap cam with carburetion/compression to match (I am guessing he wasn't) that's the right setup given the ultralight weight of his car.

[chetanhanda]

Originally Posted by ananthkamath

Chetan's right except for the backpressure part. The Backpressure theory is like an old wives' tale among hot rodders.

hey thanks man for the reply ..
If backpressure theory is not correct can you plz explain what must have happened to my car.. I have tried out 6 exhaust systems for my Maruti MPFI 800 (the 4 valve per cylinder Alto engine version with 47 bhp not the new 30BHP version ..this version is not in production any more )..
I noticed the following :
(1) I retained the original header and cat .. only put a free flow muffler
Result: Nice exhaust growl but hardly any change in torque

(2) I retained only the header but removed the cat + same free flow mufler as in step (1)
Result: Nicer exhaust growl with a louder sound and noticeable loss of low end torque in 1st and 2nd gear

(3) I retained the header , but removed cat + replaced the pipe between cat and muffler + same free flow muffler
Result: Not much change in exhaust growl but car definately lost low end power but felt very rev happy at higher speeds .. accelarating from 60 to 80 KMH was definately quicker , felt as if my top end power was much much better.

(4) I still kept the header but removed cat + same large dia connecting pipe + new muffler which was slightly more restrictive as compared to the one used in the previous steps 1, 2 and 3. The outlet dia was reduced slightly and the perforation in the muffler was reduced slightly
Result: my low end torque in 1st and 2nd came back , the exhaust note became slightly muted and the exhaust note also changed slightly

(5) I kept the header + removed the cat + same larger dia connecting pipe + new free flow muffler (this is the 3rd muffler ) with same perforaion in step 4 but had 2 exhaust outlets from the main can
Result: Exhaust growl came back but was not so loud, low end torque reduced + tope nd 60- 80 KMH came back

(6)I kept the header + removed the cat + same larger dia connecting pipe + new free flow muffler (this is the 4th muffler ) with same perforation in step 4 but had 1 exhaust outlets from the main can
Result: Exhaust growl came back and was nice and loud , low end torque in 1st and 2nd became hopeless + top end 80- 110 KMH felt very nice. I love this cobination and still keeping this

What I felt was that the key factor must have been the backpressure as all the other factors were constant which determined the low and top end torque... this was my experience over 2 years with 6 combinations , I am planning to use the same combination on the OHC also.. can you please explain what other than backpressure altered the torque on my car if "backpressure is an old wive's tale" . I'd love to hear an explanation becoz im confused now !!

[ananthkamath]

Could it be something known as MHP- Mental Horsepower?

I dont believe anything unless I see the numbers. Or quarter-mile times. Or in-gear acceleration times. Or dyno readouts.

Since you want an explanation:

(1) to (2) - you lost velocity by removing the cat. Ergo, you lost torque. Notice the cat has tiny perforations like a honeycomb. That increases velocity.

(2) to (3) - you lost velocity by going to a bigger center pipe. Hence, you lost torque.

(3) to (4) - you say that dia was smaller. You gained velocity, so you gained torque.

(4) to (5) & (5) to (6) - can't comment unless i saw the muffler myself.

If only you had changed the header along with all the other parts. You would have ended up spending less money & gotten more power. Unless your aim is to experiment, not unlike me.

The muffler is the least important part of the system. Although a bad design can still undo all the gains from the header.

[Psycho]

The cat has tiny perforations yes but it acts as a baffle hence increases backpressure, remove that and your low end torque goes if you do not replace it with a correct dia pipe.

Yes it is important to calculate your exhaust system before you install it for the best benefits.

[chetanhanda]

Originally Posted by ananthkamath

Could it be something known as MHP- Mental Horsepower?

I dont believe anything unless I see the numbers. Or quarter-mile times. Or in-gear acceleration times. Or dyno readouts.

Since you want an explanation:

(1) to (2) - you lost velocity by removing the cat. Ergo, you lost torque. Notice the cat has tiny perforations like a honeycomb. That increases velocity.

(2) to (3) - you lost velocity by going to a bigger center pipe. Hence, you lost torque.

(3) to (4) - you say that dia was smaller. You gained velocity, so you gained torque.

(4) to (5) & (5) to (6) - can't comment unless i saw the muffler myself.

If only you had changed the header along with all the other parts. You would have ended up spending less money & gotten more power. Unless your aim is to experiment, not unlike me.

The muffler is the least important part of the system. Although a bad design can still undo all the gains from the header.

but as per what u have written "you gained velocity so you gained torque" doesn't it mean the same thing .. since I had the free flow muffler I had a lower back pressure so gasses could flow freely and unrestricted so they could flow at a higher velocity .. doesn't it mean the same thing ?? let me know what you think ..

you also mentioned "cat has tiny perforations" that increases velocity ... this statement holds true only if you are comparing it to a badly designed gutted cat (like an empty can).. but in my case I had removed the cat and put a straight thru pipe.. so in my case the
straight thru pipe allowed a drop in pressure as compared to cat with tiny holes which blocks up the exhaust system and increases the pressure build up... so in this case as I had no cat there was low pressure and so the gases could flow with a greater velocity .. isnt it the same thing ?

u also mentioned "dia was smaller. You gained velocity, so you gained torque"
doesnt it mean the same thing that since the dia was smaller the exhaust system allowed a bigger build of pressure so the velocity at which the gases flows reduced..

isnt this statement right for my case : gasses can flow with greater velocity if the muffler + straight thru large dia pipe is allowing a lower back pressure build up..

[chetanhanda]

Originally Posted by Psycho

The cat has tiny perforations yes but it acts as a baffle hence increases backpressure, remove that and your low end torque goes if you do not replace it with a correct dia pipe.

Yes it is important to calculate your exhaust system before you install it for the best benefits.

right on dude .. right on ..
I agree exactly with you . thats what i was playing around with with my experiments ...
cat = tiny holes = some amount of baffle effect = incerease of back pressure = low end torque increase ....
so when I removed the cat and put large dia pipes .. I got loss in low end (I have no probs with that) and gained top end torque (love that) ... in short low back pressure = higer top end ..

thumbs up to u dude .. I also was going along those lines ..

[chetanhanda]

Originally Posted by ananthkamath

The muffler is the least important part of the system.

are you sure ? why do you think so ? why ?

[max8791]

I did some research and found few things.

--------------------------------------------------------------
In any case, the tip of the exhaust should exit at a point where there is a low pressure region to minimise back pressure. It's a cheap form of weak turbocharging.
from http://www.billzilla.org/engexhaust.htm
--------------------------------------------------------------
I would like to increase the horsepower of my car's engine. What's the best way to do this? :: Let exhaust exit more easily - If air resistance or back-pressure makes it hard for exhaust to exit a cylinder, it robs the engine of power. If the exhaust pipe is too small or the muffler has a lot of air resistance then this can cause back-pressure. High-performance exhaust systems use headers, big tail pipes and free-flowing mufflers to eliminate back-pressure in the exhaust system.
from http://auto.howstuffworks.com/question395.htm
--------------------------------------------------------------
How do exhaust headers work to improve engine performance? :: The idea behind an exhaust header is to eliminate the manifold's back pressure. Instead of a common manifold that all of the cylinders share, each cylinder gets its own exhaust pipe. These pipes come together in a larger pipe called the collector. The individual pipes are cut and bent so that each one is the same length as the others. By making them the same length, it guarantees that each cylinder's exhaust gases arrive in the collector spaced out equally so there is no back pressure generated by the cylinders sharing the collector.
from http://auto.howstuffworks.com/question172.htm
--------------------------------------------------------------

ok, iam not so good in technical stuff. so, if i add exhaust to 1&4 and 2&3 cylinders then i can gain some acceraltion with no bad effect on engine

correct me

------------
still technically dumb

[chetanhanda]

Originally Posted by max8791

I did some research and found few things.

--------------------------------------------------------------
In any case, the tip of the exhaust should exit at a point where there is a low pressure region to minimise back pressure. It's a cheap form of weak turbocharging.
from http://www.billzilla.org/engexhaust.htm
--------------------------------------------------------------
I would like to increase the horsepower of my car's engine. What's the best way to do this? :: Let exhaust exit more easily - If air resistance or back-pressure makes it hard for exhaust to exit a cylinder, it robs the engine of power. If the exhaust pipe is too small or the muffler has a lot of air resistance then this can cause back-pressure. High-performance exhaust systems use headers, big tail pipes and free-flowing mufflers to eliminate back-pressure in the exhaust system.
from http://auto.howstuffworks.com/question395.htm
--------------------------------------------------------------
How do exhaust headers work to improve engine performance? :: The idea behind an exhaust header is to eliminate the manifold's back pressure. Instead of a common manifold that all of the cylinders share, each cylinder gets its own exhaust pipe. These pipes come together in a larger pipe called the collector. The individual pipes are cut and bent so that each one is the same length as the others. By making them the same length, it guarantees that each cylinder's exhaust gases arrive in the collector spaced out equally so there is no back pressure generated by the cylinders sharing the collector.
from http://auto.howstuffworks.com/question172.htm
--------------------------------------------------------------

ok, iam not so good in technical stuff. so, if i add exhaust to 1&4 and 2&3 cylinders then i can gain some acceraltion with no bad effect on engine

correct me

------------
still technically dumb

to answer your question : it depends on how well it is done, but the basic concept is correct..
it just depends on how well you can execute it. But I feel you should go ahead only if you have an experienced person doing it who has done it before .. and not some person who has just read some books/articles about engines and gas flow etc
Also you need a proper understanding of the valves on your engine
and the torque characteristics of the engine at diff speeds

- regarding the stuff u did research on .. I suggest you can try playing with that back pressure part first.. then later on move to advanced stuff like header design (1 &3 and 2 & 4 connection)

nice to see ppl doing doing so much RnD and trying to learn ..
best of luck dude ...