StavFC

As title really.
Obviously keeping a manifold divided is optimal, but if you had no choice but run an undivided turbine housing (ie Tial etc), does it matter when the primarys join in to one, as long as the pipe stays big enough and the merge is smooth?

Should I just keep them divided as long as possible, or doesnt it matter?

Due to the unconventional setup I potentially could have very long primaries, and Racing Beat list what they suggest for N/A engines, from 10 to 125in depending on the engine, but obviously due to backpressure (and therefore reversal) turbo engines would be different I'm sure, possibly negating any usefulness these long primaries may have in an N/A setup.

I doubt anyone has tried super long primaries due to the lack of space, especially as 90% of 13B setups run divided housing for obvious reasons, but does anyone have any opinions providing space wasn't an issue?

In my mind I am in two thoughts-
Keep it divided as long as possible OR it doesnt matter when in the slightest.
I'd love to know if there would be an ideal primary length, but I kinda think, as theres backpressure and reversal with turbo engines, and I can't keep it divided, it doesn't make any difference. But I really don't know.

Opinions appreciated

Jobro

iTrader: (2)

I don't think length tuning is near as important on turbo cars. I have a feeling its something to do with the large flow restriction at the turbo.

When you look at the entire NA rotary race exhaust system, i've noticed

1,7/8 primary pipes into 15 degree merge / expansion chamber increasing very slowly to 3"-4" pipe no mufflers etc.

StavFC

Hi, and thanks.

Yeah I agree length probably isnt such an issue, but I do think 'probably' they should be kept seperate for as long as possible, as I guess the further downstream they are the weaker the pulses are so the less it matters if they finally mix? I dont know to be fair.

Does anyone agree or think that makes no difference? To be honest itd be easier to make if it didnt matter too much.

Anyone with other input, please fire away

arghx

iTrader: (3)

Besides my underwhelming experience with the FC HKS log manifold, I don't have much firsthand knowledge of undivided manifold design on rotaries. As you said, running a divided setup on a rotary has big benefits because the pulsing and backpressure effects are so strong compared to a piston engine. I've looked at some header designs on boxers and industrial diesels. The ones built for top end tend to have longer equal length runners that run into a collector pipe before the turbine inlet. The ones optimized for low and mid range response have shorter runners and collect right at the turbo.

Here's an aftermarket equal-length header for a USDM 2.5L STi which bolts to a collector pipe (up-pipe) and then goes to the turbo. The design follows the basic factory architecture on most turbo Subarus:



Here is an OEM equal-length header for a 2010 USDM 2.5L Legacy with short runners that collect right at the turbo. Subaru used this design over the traditional design with a collector pipe for better low and midrange torque and better cat lightoff.



In some of the literature I have on industrial diesels the authors mentioned that at full load and higher rpm, the turbine operates most efficiently when it has steady exhaust pressure. That would imply to me that longer runners and a collector pipe of some length would be optimal. For part throttle transient operation the pulsing effects help with response, even on an undivided setup.

This is somewhat related but there are experimental 5 cylinder diesel engines that use valves to run separated pulses during transient operation, and then during higher loads the valve open and the exhaust collects earlier.


Codan, "Controlled pulse turbocharging of medium speed 5-cylinder diesel engines," 2005



bet you've never seen a triple-scroll turbo before...

FCNAred

iTrader: (2)

The longest runner setup i've seen was on the twins turbo car



you'd prob have better luck asking drag race guys since some of the full chassis cars don't have the space limitations that most people have to contend with. Depending on your application, power level, porting, etc etc it may or may not make a difference. I think for 99% of people the standard style of short or long runner manifold won't really make a lick of difference. People will still argue passionately in one direction or the other based on either their need to prove something or their desire to sell you something.