NS Dev
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| posted on 10/1/06 at 09:09 AM |
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Exhaust manifold lengths theory
Right, a little discussion which I don't really fully grasp.......
I understand the classic "pub talk" theory on exhaust primary and secondary pipe lengths which dictates the "tuned rpm" and
therefore peak pulse extraction.
This concentrates the pulse tuning on one point in the rev range of the engine.
I also understand the ideal world solution of a variable length exhaust manifold to spread the pulse tuned rev range and therefore increase the torque
spread.
NOW, for road use in the real world, we want to maximise torque spread, even if it is at the expense of a few hp around peak power........
So surely, contrary to popular belief, we DON'T really want all the primary pipes the same length. Surely on a "conventional" 4 cyl
engine we are best linking cyls 1 and 4 at one length and 2 and 3 at a different length, thus spreading the effect??
No idea whether the increase in torque spread from doing this would weigh up well against the loss in peak power that this would cause???????
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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Surrey Dave
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| posted on 10/1/06 at 09:50 AM |
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I was thinking the same, ie. that it may not be crucial to have the headers all the same length.
As this concentrates the effect in a narrower range, is that not why 4 into 1 exhaust are more power but more concentrated power band?
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Browser
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| posted on 10/1/06 at 10:16 AM |
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Only my thre penn'orth but I'd imagine you'd get the worst of both worlds i.e. poor power at lo-mid rang and high revs as
you'd always be dragging along two cylinders which weren't working as well as the other two, together with issues of imbalance &
heightened vibration caused by the uneven power output from the four cylinders. The only exhaust I can think of which actually practices this ( I
believe) is the Long Centre Branch (LCB manifold fitted to Minis, and I think this was done more to combat the inherent problems with the centre two
exhaust ports on the A series head being siamesed. Until someone comes up with an exhaust manifold like a trombone slide we're gonna have to put
up with power in one place or t'other
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NS Dev
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| posted on 10/1/06 at 11:42 AM |
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quote:
Originally posted by Surrey Dave
I was thinking the same, ie. that it may not be crucial to have the headers all the same length.
As this concentrates the effect in a narrower range, is that not why 4 into 1 exhaust are more power but more concentrated power band?
The 4-1 vs 4-2-1 is (as far as I know) due to only 1 tuned length on the 4-1 system, therefore having only 1 point in the rev range where the tuned
length is fully working, whereas the 4-2-1 has a reflection at the 2-1 point lower down the rev range, which then fades out and the 4-2 junctions then
fade into play further up the rev range, thus spreading the torque.
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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Findlay234
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| posted on 10/1/06 at 12:49 PM |
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if you are genuinly interested then read up on "internal combustion engine fundamentals, heywood, 1988". and "introducing the
internal combustion engine, r.stone, 1992" these will give you more understanding than you need.
ultimately if you want after ive finished my exams i could run a couple of simulations on the lotus engine software im using for my 3rd year project.
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NS Dev
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| posted on 10/1/06 at 01:04 PM |
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I am certainly genuinly interested as I will shortly be starting to fabricate a 4-2-1 manifold for the XE engine in my 7.
I have a 4-2-1 manifold with staggered combined primary and sec lengths in my grasser which (qualitatively!) works much better than the old smaller
bore equal length manifold that I had (feels like a much bigger torque spread and is quicker on the track, but not been on the dyno since the change)
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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NS Dev
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| posted on 10/1/06 at 02:13 PM |
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Aha!!!
Cheers Dave! Exactly what I wanted to hear!
Peak torque on an XE engine in a very lightweight 7 is going to be pretty meaningless as it will just spin the wheels up.
A good spread will be just what is needed, so unequal lengths, based around those on your very nice spreadsheet, will be great!
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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colintvr
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| posted on 12/1/06 at 11:18 AM |
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I probably know less than any of you about this but where does the inlet tuning come in? I'm having an engine built by Schollars and have asked
them to pay particular attention to getting the torque curve as flat as possible, and certainly not to worry about peak hp. They felt it was worth
their time tuning the inlet lengths on the atmosphere side of the throttle bodies. My car has lots of room to accomodate long (up to 18"
inlets.
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NS Dev
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| posted on 12/1/06 at 12:49 PM |
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Oh the inlet tuning is critical in exactly the same way as the exhaust is, for the same reasons but in reverse, you want to get a positive reflected
pulse on the inlet and a negative on the exhaust.
Again what is less clear is whether to have one inlet length or two different ones (or 3 or whatever)
Obviously variable would be the best but not so easy to achieve!
All engines I have used have had even induction lengths, but that is not to say that is the best layout for a flat torque curve.
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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MikeR
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| posted on 12/1/06 at 06:49 PM |
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Well now you've started it,
what about tunning inlet and outlet?
Would you tune the same or opposite, ie if you have a long exhaust primary do you want a short or long inlet?
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NS Dev
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| posted on 12/1/06 at 07:34 PM |
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Long inlets with long exhaust and vice versa
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JB
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| posted on 12/1/06 at 08:22 PM |
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Exhaust
As always Vizard is a good source.
I have not read this one but it looks good.
http://www.popularhotrodding.com/enginemasters/articles/hardcore/0505em_exh/index.html
This article is good and will tell you what you need to know, the file sizes are big though.
http://www.geocities.com/MotorCity/Track/6992/vizard.html
John
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MikeR
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| posted on 12/1/06 at 08:28 PM |
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quote:
Originally posted by NS Dev
Long inlets with long exhaust and vice versa
Ah, but if we're trying to flatten the torque curve - is that still the case?
(no idea, just posing questions to make you think)
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MikeR
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| posted on 12/1/06 at 10:59 PM |
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haven't been in the garage in months. Been doing house stuff / working hard again.
Once i've saved up some more money i'll go back, want to try and finish it this year. Know what parts i need - just got to save up, get
them and fit them!
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NS Dev
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| posted on 12/1/06 at 11:36 PM |
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Come on Mike!!!!
Even I've got my engine in now!!!
I'm on a roll for a bit, just spending too much!!!
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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MikeR
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| posted on 12/1/06 at 11:41 PM |
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yeah well - give me some motivation, figure out my fuel tank :p
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NS Dev
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| posted on 12/1/06 at 11:43 PM |
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Oooh sh1t yes!!!
Forgot about that! Sorry Mike, must suss that float unit and how to fit it on somhow, but answers aren't currently forthcoming!
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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MikeR
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| posted on 12/1/06 at 11:47 PM |
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and don't forget to think about the next item on the list - unless you'd rather I go else where for that (although money for that is a
little tight at the moment)
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NS Dev
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| posted on 13/1/06 at 12:01 AM |
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No no, we can sort something out there no probs!
I have a load of steel in again now so actually have some materials to work with!
Bought an abrasive chop saw finally now as well, fed up with the bandsaw!
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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ettore bugatti
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| posted on 13/1/06 at 11:40 AM |
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So basically you consider an 4 cylinder engine as 2x twin cilinder engines with different optimum torque rpm.
Would this theory also work for parallel valves (non crossflow)?
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NS Dev
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| posted on 13/1/06 at 12:27 PM |
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Yes, and yes!
The crossflow design is more efficient in promoting scavenging, but the non-crossflow design still scavenges in the same manner, just a bit less so!
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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stevebubs
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| posted on 13/1/06 at 01:02 PM |
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Interesting monologue on this here
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crbrlfrost
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| posted on 13/1/06 at 09:34 PM |
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Just a thought, but I have seen...and heard actually of stepped pipes. The steps are quite small, going from perhaps eg 1.5 to 1.625. The theory being
that you get small pulses at the steps, each step set for different rpm. Usually there is only one step, or maybe two, but haven't seen one.
That way you can broaden the curve slightly, and still use equal length headers. Also, I think 4-2-1 usually use equal lengths as well, its just
coupling the cylinders on complementary strokes to improve scavenging. Cheers!
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quattromike
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| posted on 13/1/06 at 11:06 PM |
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Okay so if you had a 4-2-1 manifold and wanted to have the different length primarys and secondaries would you make it so that the volume of the
2-1's was the same as each other but was joint in at different points,to get the different length primarys, if you know what i mean?
Mike
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