I put this here because the engine is a naturally aspirated 4 port 13B. It needs to have tremendous low end driveability so the engine is stock ported and the intake etc is all worked out. Now only exhaust remains to be done.

I'd like to present an idea that should fit within the limited space back there on the side of the engine. It consists of 2" OD thickwall pipes, correct 30° collector, mandrel bent 90° and 180° u-bends, and two Rotary Engineering glasspacks with nearly 2" ID inside diameter perforated pipe and glass packing. They're almost 20 years old but were designed for rotaries. I tested one of these on my 20B and it actually worked to keep that beast quiet!

The 2" diameter is perfect for a stock ported 13B. I'm just curious how flow and velocity will be affected by all the twists and turns. Low RPM and idle should be fine but will this exhaust be adequate for up to 7k RPM? It has to stay on the side because the radiator is directly behind the engine for best air flow.

 

Im not an exhaust expert, but if you use all mandrel bent tubing you should be fine. 2.5" tubing might be better though.

 

Care to elaborate on the 2.5"? Thanks.

 

okay you want to get use of scavenging in your exhaust system, primarly the header by a low pressure area created in the exhaust system that can be used to "draw" the exhaust out the chamber. I know how to explain how reversion works on a piston engine but I'd have to look into it further for a rotary. BUT if you reduce reversion you increase volumetric effeciency and power so you basicaly build your header for RPM range.

now you want low power? then you need to utilize equal length tubing with a maximum variation of +/- .5" the bends (as long as they aren't crush bent or 90 degree bends) won't affect the flow, the length and tubing size will. By going from a 2.5" (outer diameter) primary pipe to a 2.0" (outer diameter) primary pipe you increase the exhaust velocity speed providing more low end power since you want the smaller tubing to maintain a high enough velocity when there is less exhaust volume at lower RPMS.

have you considered thermal coatings or wraps? With thermal coatings you can use those to retain heat in the exhaust system since as the gases cool they loose velocity, velocity increases scavenging at high speeds and reduces reversion (reverse flow at low RPM) and not to mention reduce underhood temp so you can get cooler air into your intake for MORE POWA! Thermal wraps do a better job but I wouldn't recommend them for a street vehicle since they tend to retain moisture and will rot your pipes alot faster.

smaller pipes better low-mid range power, so basically from going to a larger pipe to a smaller pipe you move your power curve more over to the left hand side of the graph (X axis is RPM, Y axis is exh velocity or power) steeper slope.

you generally want to keep around 240-260 feet per second when an engine is operating at peak torque.

to calculate the pipe size you need there is a formula

1. desired RPM at which max tq is to be produced
2. multiply CID of one cylinder (or rotor)
3. divide by 88200
the result is cross sectional area of the pipe required

4. now divide that by 3.1412
5. take the square root
6. multiply by 2
the result is the necessary inside pipe diameter.

by changing the pipe size you shift the torque curve around by going from a larger pipe to a smaller pipe your causing the engine to take away torque from one RPM and add it to another.

increasw the length, adds tq below to engines tq peak, and vice versa.

the lower RPM range the header is required to work the longer the pipes need to be.

you can also use a collector to produce more tq below the peak but will decrease tq after the torque peak, typically you want your collector to be 3/4" bigger than your primary pipe diameter.

hope this helps!

 

Thanks for that.

 

That's about all I know with exhausts, glad it could help.

 

Does the same forumula apply to a forced induction application where as the same volume of air would be under a different pressure?

 

Like a supercharger?

 

for finding out exhaust diameter?

 

looking at the pic, id think that first bend right after the collector is the weak point, the straighter you can make that the better i think, total length of the system isnt a big deal, if anything longer is better.

ive done some measuring, and depending on the wall thickness 2" od is like 1.75" ID. i think it might work better with 2" ID pipe, which is closer to 2.25 OD.

anyways, it'll be a fun ride...

 

Yes, and no i guess, sorry for not elaborating.

for both super charger and turbo charger

and yes exhaust diameter, however would that also apply to manifold runner diamaters?

 

Thanks Mike! I swapped one of the old glasspacks for a newer smaller shelled one (3") and this will help round out the first bend by shifting everything over 1". The ID of the newer glasspack is about 1.75" so I think the system will be balanced and flow ok because as the gasses leave the engine they cool/contract, and loose some of their power as they pass through the silencers and approach the end of the system. Stock exhaust ports too. Yeah the system need not be long but I can see how that would be better whenever possible. Oh well.

This system's ID and flow potential is sort of tuned to match the stock intake and stock ports of the engine for practically GSL-SE-like low RPM torque while still flowing well up to at least 6k (but I'll take 7k if I can get it). Stock VW trannies don't like high RPMs and I'm not about to spend twice on the tranny as I spent on the engine.

This car previously had a stock bug engine and stock geared bug tranny. With big tires, it had very little control for climbing curbs and other small things. Plus it barely had enough power to keep up with traffic at freeway speeds, not to mention very unstable at speed due to being lifted with the aerodynamics of a shoe with wings. I'm hoping that changing to a lower geared bus tranny will help with the "control" issue and the greater rev range of even stock ported rotaries will allow it to keep its ability to go fast enough at speed to become unstable.

So, greater control and driveability at low RPM while maintaning its previous top speed-ability. Sounds like a win to me.

 

Well this is the NA forum so I don't know how much on that subject you'll get here, Mel. But as for the intake manifold size or length, I'm using a stock '76 reversed runner manifold in the hope that the increased primary runner length will naturally boost the primaries thereby increasing low RPM/primary carb barrel performance up to near GSL-SE territory (133 torque at 2750RPM!). Then the secondaries have a very short run directly into the intermediate plate, which may run out of breath by 6k; it is all theory at this point. Plus my somewhat smallish exhaust pipe ID won't help high RPM, but for this project, that is ok.

 

by having longer intake runners you shift your power curve more towards the high torque low RPM range rather than, longer runners better for low rpm torque and short ones are best suited for higher RPM

 

I finished it. Notice how I got rid of the first 90° bend after the collector? I was even able to reduce the first 180° bend to about 135°. The only compromise was having to make unequal length primaries. This was necessary in the small space back there.

 

can you do another 135 where the current 180 is? or are you running into clearance issues then?

 

I can't do another 135° due to clearance.

 

soooo pics of it installed?

equal length primaries would be ideal.

 

I'll do some pics in the next few says.

Yeah, I know equal would have been best. As it is, it shouldn't be that much different from a stock manifold where the pulses can affect each other.

 

Here it is installed.

 

Iam anxious to hear how it performs?

Have you taken it for a drive Jeff?

 

To make a long story short, the engine I built for it is too tight even for the strongest VW starter I could find (it's a high torque unit from an automatic). I'll break it in in something else and toss in another 4 port 13B that I built a few years ago. It always fires up even after sitting for a while. I need to work the bugs out of the project and a well run in engine is one less headache to deal with.

 

oooooh a baja beetle! I was thinking light duty truck baja, awesome!

push start it!

 

Have you tried a 6 volt starter at 12 volts? If you can still find a 6 volt starter any more..