Incorrectly Sized Exhaust or Sidewinder Dip?

[70BayBus]

Hey All,

I've been poking around these forums for a while now, piecing together bits and pieces here and there to build a nice engine for my Bay Window bus. Problem is, there's a definitive lack of power from 2700-3300 rpm or so. Everything above this range absolutely rips, and everything below pulls pretty well.

Engine in question is a TW 2180cc with:
Tims S1 heads, Beehives from Dan Ruddock
Web Cam 163 w/ 1.3 rockers from Brian
41mm (1-5/8") CSP Python Exhaust
Dual 40mm IDF Throttle Body EFI w/ ignition - Microsquirt
9.2:1 compression (I live at about 4500 ft above sea level)

As stated above, I'm getting a pretty nasty "dead zone" between 2700-3300 rpm with this engine at WOT and partial throttle, less so with light throttle. This appears to be in the range of the dip in power common with sidewinder style exhaust, but much more extreme.

I have not had it on a dyno yet, but I know through many data logs that the AFR is good through that range (about 13:1) and spark is pretty well dialed in, no pinging, but not overly conservative. The dip in power is so bad that I've resorted to REALLY revving out the engine to skip past 3300 RPM when upshifting. If I do end up in this RPM range, giving it more throttle seems to only make it bog down more.

Engine is confirmed to be healthy, very low leakdown, good compression, rings are cast Grant and are fully seated.

My questions:
Is this sidewinder dip or some other tuning problem?
If it is sidewinder dip, what can I do to tune around it?
What else could be causing the lack of power?
Is this exhaust even suitable for what I'm needing, or do I need a different or smaller exhaust?

I'm planning on getting this on a dyno for fine tuning of the EFI soon, but want to get this issue resolved first. Any assistance is greatly appreciated!
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1970 VW Bus
2180cc TW:
Tims S1 Heads w/ Ruddock Beehives
Web Cam 163 w/ 1.3 Rockers
1-5/8" CSP Python
Dual 40mm Throttle Body EFI powered by MicroSquirt

[Alstrup]

Sigh... When do people learn. Sidewinders, - especially Pythons are not a good exhaust for a bus. They are simply made wrong. The header is -ok - The collector is too large and the sec collector and muffler is hopeless.
If you are willing to put in some work and make your own set up from the header to the tail pipe I can give you details as to how to do it. it will cost you a day and some parts, but you can reduce the dip by abbout 50-60% and then most of the rest can be tuned away with intake length and ignition timing changes
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https://www.thesamba.com/vw/forum/viewtopic.php?t=435993

[70BayBus]

[94touring]

I'm in the ballpark of engine specs to you and have a sidewinder too. I made my own exhaust from the collector back. I can send pics how I did it but basically have a slight reduction (1.75 inch if memory serves me) past the collector flange (think venturi effect) into a 2 inch pipe, which then goes into a resonator muffler with a big size body and then into a magnaflow turbo muffler. I have minimal dip, good AFRs, and it sounds good too.

[chrisflstf]

Alot of guys run the sidewinder along with the dip. If someone has a fix for it, it would be helpful if that info was shared.

Along with tuning intake runner length. Is that velocity stack length changes?

[94touring]

[richparker]

Go with the VS. I run one on the 2275 in my ‘71 Westy. No dip, sure I’m leaving some HP on the table but it’s a bus motor.
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’71 Westy build
Adventure thread
’65 Deluxe Build
’71 Double Cab

[Alstrup]

[chrisflstf]

I used the 8x cylinder volume for my secondary. Modok had mentioned it in a thread I found, looking for what is the correct relationship. Dont know if that is what alstrup meant or not.

Don't have a sidewinder, but happy with my setup

[74 Thing]

[94touring]

[94touring]

3rd gear 3200rpm WOT mountain pull a long way to 7000 feet with the windows down. I was showing Torben the CHT temps with the water/meth going. It was a tad rich due to meth percentages at that altitude if you see it in the scan past the wideband gauge. Back home at 600 feet it's a solid point leaner. Nevertheless, you can't really hear the exhaust either.


https://www.restorationmini.com/forum/index.php?action=gallery;sa=view;id=4390

Looks like you have to click the link above to view.

[70BayBus]

[70BayBus]

[94touring]

Not a problem, give me a few days probably. I'm on the tail end of a 15 day road trip in my bus.

[Alstrup]

OK. time flies.

Chris/Modok are right with the 8 x cyl vol in the first exhaust chamber. It is however only a fraction of the story. It is part of setting up a muffler system which can support the same power as with op headers/stinger. But the collector/size and the secondary collector is even more important if you are chasing driveability and afr control.
For street/bus engines a set up like 94 Touring has built is about the bare minimum, but it works, - on his engine. Personally i like the 2 in line muffler, one on each side of the engine set up as it serves 2 purposes. 1. getting a decent length on the entire tail pipe which has benefits with both noise levels and tuning (As long as we stay within a certain window.)
If you want to chase max power but comfy street level exhaust noise uou chos ethe collector trap the size your engine needs/wants. Then you dimension the sec. collector by 2 X 1 cylinder volume, going into the chamberred muffler with 8 x 1 cyl vol. From there you route it to a free flow muffler of your choice which flows 2,3 cfm/hp.

In the O.P.s case I would install a 2" cone and go with a 2- preferably 2 1/4" tubing to the first muffler and 2" mufflers from there, and then do a more simple 2 in line muffler set up. My reason to go 2" is that good 2" mufflers will support 165-170 hp. The O.P. is probably around 150, so there is room to work with. 2. by using 2" you keep the exhaust gas velocity up which helps in so many other ways. 3.The 2 inchers generally make less noise.

Next. Intake length. The generic out of the box intake length is rarely correct for an engine, but it sort of becomes a default. When we are dealing with a set up like that we want to be on the longer side of what the book says with intake length. That may not be possible, but then we take what we can get. Such an engine probably peaks around 5500-5600 rpm. and you favour usable torque over the last hp. So dependant on which intake length calculator you use, you would want to be around 15" or a little more. An out of the box set up is typically around 15,6" from valve to top of stack. But, as said you want to be on the upper side of that in order to increase signal strength (Not quite so important with F.I. but still. It all helps.)
In spite of what many will tell you, you can go as close as 25- even 20 mm from stack to top of fliter without it hurting power more than 1-2 hp. but you can gain a lot of lower rpm drivebility. I would shoot for about 16" intake length. and use a 60 mm straight stacks to get the best possible ramming effect and reversion control.

Finally. I assume you are using a set of "slider" 1,3 rockers. as they are the preferred solutiuon for many, due to various reasons which we will leave alone here. That said, there is a reason to why I often prefer the VW style 1,25´s. That is that it gives me the option of siameezing the valve lift. If i had a situation like the O.P´s I would gladly sacrifice say 0,1" valve lift on the intake in order to be able to reduce valve lift on the exhaust to reduce scavenging in the lower rpms. I´m getting a little fancy here, - because I can. If we go with say a set of CB 1,25 rockers on the intake (which are typically around 1,28.) and then a set of Empi/Rage 1,25s on the exhaust, (which are typically 1,25) you can effectively have say 0,505" lift on intake and 0,495 on exhaust. - Or if you are really in trouble you can go with stock rockers on the exhaust- reducing valve lift to about 0,420" - Would probably go with CB 1,25´s and stock on the exhaust because almost all out of the box heads flow too much on the exhaust anyway. Also, if you do that you will get a 30% L/D on both intake and exhaust. Thats not bad at all for an engine where torque is more important than hp.

Hope this helps.
T
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https://www.thesamba.com/vw/forum/viewtopic.php?t=435993

[94touring]

At a rest stop and decided to look up the parts I used. Also I'm using a 1.5" AA sidewinder fwiw. The reducer is a 1-7/8 id to 2" id, then it goes to 2.25 id piping into a 2.25 id in-out "1320 ultra quiet muffler" found on ebay, continue the 2.25 pipe to a dyno max, part #17736, super turbo muffler 2 in 2 out. I wanted velocity on flow, which is why I chose the 1.5 headers for a bus engine for the power band I drive in, as well as the reduction at the collector, then into a 2.25 muffler for some expansion. Note I say velocity and not the term "back pressure". You want free flow "open headers" to reduce as much back pressure as possible. The key to it is the same as the intake side, which you have with the tims stage 1 heads, great velocity without massive ports too big for what you need.

Due to all the weight on the side I have a beefy hanger afixed to my custom tow hitch, which lifts up on it slightly to alleviate stress. Because of the tow hitch setup I am limited to the sidewinder as it is.

[70BayBus]

[70BayBus]

[Alstrup]

No. Sec collector and the first chambered muffler are 2 different calculations.

If you do not use a chambered muffler but a straight through muffler the sec collector has less impact because you keep going in a straight line so to speak. The chambered 1st muffler set ups - are - mostly for nippy engines where we chase all we can. If you want to go all out your calc is correct. But there are no mufflers on the market as such. They have to be built. The closest you can get is the Flowmaster 40 series. They are easy to open and modify. But they are darn big to install under even a bus.
With the collector. You can buy a 2" cone for the Python which fits right in. Then you can build from thereon.
Wrt the V clamp. Well, opinions vary. I DEFINITELY prefer the 3 bolt flange over the V clamp for anything but racing. The V clamps never stay sealed. The 3 bolts do 99% of the time.

If you go with the easier route an "just" use 2 free flow mufflers in line, you will get a very good result which will be close to what you can get with the all out solution in your set up. you can use mufflers like Apple, Simons a.o. quality mufflers which flow- and dampens well. (Some time ago I built a 2160 cc with type 4 main bearing 5,6" rods, CB 2239 cam, ported 041 heads (40 x 35) 9-1 1,25/1,1 rockers, Italian 44 IDF A1 BB ignition, 1 5/8" Sidewinder with exactly such a regular 2 straight through in line muffler set up (Simons) It pulls a healthy 138 hp @ 5300 and 205 Nm peak torque with more than 150 Nm available already from 2000. Yet it is so quiet that you don´t hear the bus until he is 100 yards away.)

Wrt rockers. If you use the regular 2 in line set up I think that should be the last resort if it turns out that you still have an issue. If you do the chamber set up I would do it.

I would increase length after the butterfly. you do not want too much mix in motion below the butterfly. That only increases the risk of a soggy thhrottle response (somewhat dependant on your injector position)
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