Would running a V8 with no exhaust cause this? (apart from hearing loss)

**Slunnie** · 15th May 2007, 07:55 PM

Originally Posted by Blknight.aus

the loss of back pressure means that your not getting the scavanging effect as much as you would with it...

the nuts and bolts of your exhaust cycle...and this is mainly applicable to low (say sub 2500) rpm

(all speed terms are relevent here a piston moving slowly aint really moving slowly but its moving slower than when its moving at its max speed in a stroke)

1. exhasut valve opens, piston is moving up slowly the gasses begin to exit from their own pressure, as this happens the exhaust port(the head and very first part of your exhaust manifold before it joins to another exhaust) gets a bit of pressure in it.

2. The piston begins speed up and to push up and force the gasses out, exhaust port pressure rises and the gasses begin to get serious about flowing. (now is the point when your back pressure begins to become important)

3. The piston passes the mid point of its upstroke and is now going flat out both exhuast pressure and flow are at their peak.

4. as the piston slows down past the mid point of the stroke the pressure begins to fall but the flow rate remains about the same due to the inertia of the gasses.

5. the valves begin to rock, exhaust pressure can be at less than atmosperic pressure in a well tuned system and the exiting gasses suck the last of the burnt gasses out of the combustion chamber leaving it clear for the fresh duel/air to come in...

Now with no back pressure you cant get the scavanging to effect properly. This will leave burnt gasses in the combustion chamber wich robs you of fuel/air mix...

This of course all goes out the window when you get the revs right up there as the relative inertia and motion of the gasses just cant keep up with the mechanically flung parts....

so the really really short version..

back pressure in a well designed exhaust at lower rpms is a good thing but bad if you want to rev the nipples off it as it slows down the gasses movement.

So from this I assume that back pressure isnt actually air pressure becoming negative, but its more about an imbalance between the dropping pressure at the end of the enhaust stroke in comparison to the outgoing pulse from the next cylinders charge. It also seems that there is no time where there is no pressure or a vacuum created in the exhaust from extreme pulsing. But based on this the it should always still be easier to pass the exhaust through without backpressure.

Also, if the fluctuating pressures are a feature of the extraction, then I assume it also means that to work properly the engine by be in a piston configuration that is divisible by the number of strokes otherwise the addition of pistons will interfer with the pressure cycle in the exhaust.

**rick130** · 15th May 2007, 07:57 PM

the other thing with 90* crank throw V8's is that the firing order on each bank aren't even and both systems need to be joined somehow, either with a balance pipe on a dual system or large single system to balance the firing pulses, which aids the reflected waves which aid scavenging and torque.

A 180* crank V8 (think F1 engine or Ferrari) don't need the systems joined as firing pulses are evenly spaced on each bank.

Way back in the mid nineties when I was building and playing with race car engines we spent several nights on the dyno trialling different merge collector/cone/megaphone/dump pipe setups on Formula Ford engines, all the way up to a 3" cone and pipe. The big system was unbelievably loud, much, much louder than anything else we trialled and astonishingly down in power and torque compared to our baseline system ( conventional merge style collector, 2" dump), let alone our optimal system.

**rick130** · 15th May 2007, 08:04 PM

Originally Posted by Blknight.aus

Now with no back pressure you cant get the scavanging to effect properly. This will leave burnt gasses in the combustion chamber wich robs you of fuel/air mix...

naa, I disagree. It's not a back pressure thing, it's harnessing reflected waves and balancing velocities with tube diameters.

**Slunnie** · 15th May 2007, 08:09 PM

It seems that with extractors you shouldn't go too high in the tube diameter to keep the gas velocity up which helps improve the scavenging effect from the extractors, the outlets into the collector as you say going in firing order. When you get too big you lose the pressure and velocity to make the extractors work.

But these all run exhaust systems, not stubs etc.

The other thing which is interesting, probably particularly with race engines, is that the length/volume of the exhaust will move the max torque position around in the rev range.

**Slunnie** · 15th May 2007, 08:12 PM

Originally Posted by rick130

naa, I disagree. It's not a back pressure thing, it's harnessing reflected waves and balancing velocities with tube diameters.

Thats what I think the game is all about too. But this is all about maximising the power/torque from engines with full exhaust systems, something where I also believe there is no need for back pressure - the easier flowing the better.

Just tell me once I've annoyed everybody with these questions, but they're sincere.

**rovercare** · 15th May 2007, 08:21 PM

Originally Posted by Slunnie

Thats what I think the game is all about too. But this is all about maximising the power/torque from engines with full exhaust systems, something where I also believe there is no need for back pressure - the easier flowing the better.

Just tell me once I've annoyed everybody with these questions, but they're sincere.

Not being rude, but all the theory in the world probably isn't going to convince you, best way would be take your petrol powered vehicle, drop the exhaust/fit something larger and see for yourself......But DON'T mistake the added noise for a feeling of power

The most noticeable time I realised this myself, was when I fitted a stock EFI 5l windsor to my rangie which had 4 into 1's, 2" into single 2.5" and changed it for a twin 2.25", you noticed a substantial loss in torque

Yet that was for later plans....351 now and supercharger when i get back from my trip in june

**Blknight.aus** · 15th May 2007, 08:21 PM

ok before you get to the reflecting waves thing and blanacing and all of that, think single cylinders...much like a 2 stroke...( i should have made that a bit clearer when i wrote it)

when you get to reflecting your exhaust pulses to assist with valving and extraction you still need to retain a certain level of back pressure otherwise the gasses just frell off out of the pipes and theres not much to reflect around..

**Slunnie** · 15th May 2007, 08:34 PM

Originally Posted by rovercare

Not being rude, but all the theory in the world probably isn't going to convince you, best way would be take your petrol powered vehicle, drop the exhaust/fit something larger and see for yourself......But DON'T mistake the added noise for a feeling of power

The most noticeable time I realised this myself, was when I fitted a stock EFI 5l windsor to my rangie which had 4 into 1's, 2" into single 2.5" and changed it for a twin 2.25", you noticed a substantial loss in torque

Yet that was for later plans....351 now and supercharger when i get back from my trip in june

If it just does there must be an explanation why though and pumping theory at low revs isn't exactly rocket science... its just that I don't know it.

High revs is different though, especially when there is synching of the waves etc. What I still don't get though is why at low revs what is essentially adding pressure can improve torque. Extraction theory is sound for a full exhaust system, but thats a relative for an exhaust system, not in comparison to a header which has no back pressure.

**Slunnie** · 15th May 2007, 08:39 PM

Originally Posted by Blknight.aus

ok before you get to the reflecting waves thing and blanacing and all of that, think single cylinders...much like a 2 stroke...( i should have made that a bit clearer when i wrote it)

when you get to reflecting your exhaust pulses to assist with valving and extraction you still need to retain a certain level of back pressure otherwise the gasses just frell off out of the pipes and theres not much to reflect around..

Yes, but in a two stroke the back pressure is a momentary reflecting pulse from the expansion chamber that is about forcing the fuel/air mixture that has escaped into the exhaust back into the cylinder once the transfer port has shut - so in a sence it's almost like a micro supercharger. The 4 strokes don;t have anything like that.

**rick130** · 15th May 2007, 08:43 PM

Originally Posted by Slunnie

Thats what I think the game is all about too. But this is all about maximising the power/torque from engines with full exhaust systems, something where I also believe there is no need for back pressure - the easier flowing the better.

Just tell me once I've annoyed everybody with these questions, but they're sincere.

I agree Slunnie. Everything that I've ever played with points me to the conclusion that the less back pressure the better. If you need back pressure to create torque, something is wrong with the design.
As you said, torque/power can be moved up/down the range with primary pipe diameter and length, then collector design and total system length/diameter.
With those FF engines, we made more torque right through the range with a 55% larger cross section dump pipe. ie, we reduc3ed back pressure, but made torque (including below the torque peak, although the engines never ran there) by increasing exhaust velocity (and hence scavenging) at a critical point of the system which also gave us increased top end.

Years ago I was playing with a Jeep 4.2l 6cyl system. Everyone reckoned you needed 1.5" primaries with these engines for torque, as the exhaust ports were small, and all the Oz makers at the time used relatively short 1.5" primaries, usually around 18" long (still have a set of Perry Headers in the shed off my old CJ).
Dad had originally put a set of pipes on this particular engine that were 1,5/8" primaries, 36" long and were designed for a dual exhaust. The bloke he bought them off (Liverpool Exhaust) had patterned them after a US system, but he advised to get them into a single pipe ASAP after the 2.5" collectors. This he did into a 2" system, through a truck muffler and exiting just behind the drivers door.
Anyway, the muffler rusted out and I'd inherited the truck, so dumb **** here thought he'd sand bend a better system

I made the secondaries 54" long (1.5x primary length) in 2" then into a 2x2" in, 2.5" single out Lukey muffler and no dump pipe.
It improved the bottom end unbelievably, made the torque curve much flatter and extended the torque peak by over 500RPM and I'd done this by reducing back pressure. The resonance with that muffler though was something else ...........
<edit>that engine was much torquier than my other Jeep engine with Perry headers and a stock 2" exhaust,/edit.

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