Equal vs unequal length exhaust

[shnazzle]

Prompted  by a picture on the book of faces and my unequal fuel trims between Bank 1 and Bank 2, possibly neither of which is related to this question. But, would it be expected to have different AFRs per bank if you have an exhaust manifold with unequal runners?

The stock mani, and all the copies of it, seem to have unequal. 
The Zero and PPE are very much equal length.
Obviously the stock ECU can adjust fueling per cylinder. So it's entirely possible the two banks are adjusted for.

So A) Is the stock manifold actually unequal length? Particularly talking about runner #4 which has to go over the top to get into the right pre-cat.
B) Does it affect anything other than sound? (my Zero sounds much "racier" than stock.

[thetyrant]

I dont know the answer but i know on my old GT86 there was always a debate about which manifold was best, as in equal or un-equal length with the unequeal length making the car sound much more like an impreza with its distinct flat 4 rumble, personally i only had equal length manifolds on the car (both stock and aftermarket) so never had that impreza sound which was fine by me as being a long time Evo owner having a "subaru" went against the grain lol even though i like the impreza sound

Performance wise on the GT there was some differences from each but ive lost the will to retain that knowledge, i think equal could make more top end power but unequal helped a bit with low down performace ?  cant remember for sure though, i think the MR2 has a offbeat sound so guessing its slightly unequal length as you say and does look it, ive just skipped my stock manifold but will try and have a measure of my aftermarket one.

[Topdownman]

Cant help on the MR2 but I have had an unequal length fitted to my GT86 and the consensus there is, as said above,  that equal length gives slightly more bhp and the unequal length gives a bit more mid range torque and more of the subaru sound.

I am sure they are all very subtle changes though.

[Petrus]

There is no discussion needed.
It depends on what the goal is.
All equal has a better extraction at the revs it is caculated for price of a narrower band width of revs.
A 4 in 2 in 1 has less of the harmonics coming all together in a climax but more of a symphony over a wider rev band.

Yes it affects sound very much. Véry much actually: The pressure waves behave excactly like sound waves. That is why the lengths at which they interfere or reinforce can be calculated.
It also menas tgat when they interfere the sound waves are subtracted verseu added up when coming togenter.
The egual length 4-1 will be at least twice as load at the sweet spot than the 4-2-1 at its most effective.

Mind; turbo is different. Not the length cálculating bit but the harmonics you want are. With turbo you want foúr pulses evenly distributed. Four equal lengths yes, but as short as possible and the least interference possible. The tricky bit is the waves bouncing as it is just about unavoidable they will bounce negatively and the short length makes it a rather large negative at the valve. The design of the collector and the turbo entry can create an interference of the bouncing.
This too is why it can be very desireable to have a double entry port at the turbo, thus the last 2-1 of the 4-2-1 at the turbine wheel.
Soúnd wise the turbo takes the interference out of the equasion and is an efficient muffler.

[shnazzle]

I haven't done any comparison but if the mani is purposely unequal to enhance mid-range torque, that's another thing Toyota did to enhance low/mid torque. 
So... Why is the torque so absolutely pants!? 
I would say the MR2 is much much more aimed at higher rpm (4200+) fun and to get anywhere you always need to shift down a cog or two. 
But so far I know;
- crown design is to enhance low down torque 
- long intake runners to enhance low down torque 
- long stroke with (relatively) high comp to enhance low down torque
- allegedly unequal length exhaust mani, for low down torque 

You'd think it would have a decent wallop of torque but instead it's like starting a sprint standing in treacle. 
Don't get it.
But as I said, I haven't looked at other 1.8 engines to compare torque

[thetyrant]

Hmm weird as ive always been impressed with the low down/midrange torque on mine and felt that vvt and maybe manifold is doing a good job on that front (VVT is noticeable by its absence now ive mapped it off with turbo to save clutch)  and i find stock car makes for brisk progress at low/medium speeds without revving its tits off, but lacks any punch up at top end with flat power delivery and kind of like they designed it to feel sporty but its not actually that fast.

On track its happy enough to scream along at higher revs but on road its rare i ever revved it that hard as just didnt feel it was worth it most of time, totally different now with the Turbo of course and way its currently mapped it really takes off over 4k.

Comparison to other 1.8 engines ive owned and driven, first a 1zz celica and it felt flat in comparison to mine when stock and celica owner thought same when he drove mr2, extra weight no doubt a big factor in that, the 2 x DC2 Integras both a Si and Type-R felt very different, with decent low down torque but then flat midrange before Vtec kicks in and screams to redline, makes driving them a chore for my usage as i would be in the midrange and feeling flat having to cog it down 2 gears to overtake!...in MR2 i just dont bother trying to overtake haha.

[shnazzle]

Maybe I'm just used to it. And we've always had a diesel or powerful petrol as the "other" car so I guess it's a case of comparison.

[thetyrant]

Maybe I'm just used to it. And we've always had a diesel or powerful petrol as the "other" car so I guess it's a case of comparison.

Yes could just be that as depends what your used to, if you get some 3rd gear datalogs i can compare them to mine see how it compares.

I like to have a slow car and faster car so i can swap and change when you get used to faster car, then drive slower one for a bit and faster one feels better next time you swap back!, problem is now both my cars similar lol

[Petrus]

@Snazzle; as theTyrant observes the 1ZZ is pretty good at low/mid range. It makes for a véry easy to drive and responsive car. What it is less, is a revvy thing that gives a push when it gets on song. Being a mid engined cabrio, you sort of expect the latter so might be a bit underwhelmed by how it doés perform. And perform it does.

Yes and also there is no comparing with modern TDs or large capacity engines. Well, on torque that it. Looking at flexibily however it is quite a gem.

Lastly, with the headers tuned for max. harmonics, it would have ´even´ less torque.

[jonbill]

The unequal length exhaust runners might give different AFRs per bank, because you might have different scavenging per bank.
Very hard to tell with the way the EMS operates the two banks independently.
Probably nothing in it.

[Call the midlife!]

Did this stem from the multicoloured manifold by any chance?😂

[shnazzle]

Did this stem from the multicoloured manifold by any chance?😂

It did! So ultimately not related to whatever was going on there, but just set my mind off.

[Ardent]

i have no idea, but fascinating reading.

[Petrus]

I just checked the OEM pre-cat manifold plus decat replacement and that is interesting:
Cilinder 2 and 3 have equal length tubes; say X.
Cil. 1 and 4 have unequal lenths; 2X and 0,5X resp.

The decat manifold I fitted has this replicated in Y, 2Y and 0.5Y with Y being 2X.

The latter is in effect the same as OEM and it gives a very wide rev band of waves interferencing.

Please note that the lengths itself have NO effect on the flow; it is the harmonics of the pressure waves of the valves opening we are dealing with. The idea is that bounced pressure wave, either positive or negative, arrives at the valve just before it closes. The negative or positive is to work together with whether extra burned gas needs to be extracted from or fresh gas needs be pushed back into the combustion chamber.
The theory behind it was already explained by Phil Irving in his 1949 published Tuning For Speed (the 1963 edition is much revised and expanded).

The flow and thus temp of the wall is affected by the shápe of the tubes; one where the gasflow has the most temp transference to the tubes.

[Beachbum957]

So A) Is the stock manifold actually unequal length? Particularly talking about runner #4 which has to go over the top to get into the right pre-cat.
B) Does it affect anything other than sound? (my Zero sounds much "racier" than stock.

A stock manifold has a number of requirements, with performance being one.  The other is fitting into the available space.  So stock systems are often compromises.

A 4 into 2 into 1 manifold usually doesn't make as much peak power as a 4 into 1 design but gives a wider power band.  Then there is a need for pre-cats to meet emissions.  Since the space is tight and a single pre-cat would either be large or choke the exhaust, running 2 makes sense.  With the space allotted, getting equal length runners would be almost impossible.

Any change to the exhaust will change the sound as exhaust tuning is all about managing the exhaust gas pulses and frequencies.  There is a lot of science in exhaust tuning as computerized gas flow analysis has refined the process in the last few decades, but everything is a compromise and often the best "solution" is as much art as science. 

Trying to provide simplified answers almost always ignores other factors.

[shnazzle]

Maybe I'm just used to it. And we've always had a diesel or powerful petrol as the "other" car so I guess it's a case of comparison.

Yes could just be that as depends what your used to, if you get some 3rd gear datalogs i can compare them to mine see how it compares.

I like to have a slow car and faster car so i can swap and change when you get used to faster car, then drive slower one for a bit and faster one feels better next time you swap back!, problem is now both my cars similar lol

I've done two pulls. Unfortunately both starting at about 4k rpm and one in 3rd and the other in 4th so a very small glimpse but they're 118whp and 123whp resp. according to Virtual Dyno. So that seems about right for my exhaust mods. Basically sitting just above stock.

I need to do this a bit better with proper full logs and preferably with my e-manage plugged in so that I can get a much higher resolution.

It's definitely a decently quick little car when you get it right but I would say the "quick" range is definitely around 4k.

[Beachbum957]

If you look at a dyno chart of a stock 1ZZ, you will find most will show the torque output is 90% of max from just below 3,000 RPM to over 6,000 RPM. I have a chart from a 1ZZ with the MAF mod and a Zero header, and the max torque was 119 ft/lbs, but was over 110 ft/ lbs from 2,400 RPM to 6,400 RPM.  That is a nice wide power range.

A flat curve doesn't feel powerful as there isn't any sudden jump, but it does make the engine very flexible.

[Petrus]

A flat curve doesn't feel powerful as there isn't any sudden jump, but it does make the engine very flexible.

The bum registers pressure; especially íncrease in G force. So indeed a spike in torque will féél quicker whereas a wide flattish torque graph will bé quicker,

[SV-3]

A flat curve doesn't feel powerful as there isn't any sudden jump, but it does make the engine very flexible.

The bum registers pressure; especially íncrease in G force. So indeed a spike in torque will féél quicker whereas a wide flattish torque graph will bé quicker,

Just what I want in a road car and for control by throttle.

[Petrus]

So you want equal length arms&legs but unequal length headers