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Twin-scroll exhaust header primaries lenght?

J

Jussi Alanko

Member
Joined
Jun 5, 2020
Location
Finland
So, does the length of the primaries in twin-scroll exhaust header make any difference? I have a pipe 58cm long from the exhaust valve to the turbo flange and everything is the same length.

I can't obviously anymore change them in any direction, but does it matter at all, are the longer ones better, or should they be as short as possible, but still the same length?
 
I was told a long time ago everybody thought longer the better for turbo setup. But, now that has changed, they say shorter the better. My guess is on a long tube header you loose heat which is energy. How much, I have no idea. I have a long tube header as well and I have plenty of power. Im sure someone will chime in that knows more than me.
 
90volvo said:
I was told a long time ago everybody thought longer the better for turbo setup. But, now that has changed, they say shorter the better. My guess is on a long tube header you loose heat which is energy. How much, I have no idea. I have a long tube header as well and I have plenty of power. Im sure someone will chime in that knows more than me.
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like most things in engine building there's tradeoffs and it's not as simple as "better or worse".

Super simplified shorter runners will improve turbo response and long runners will have more outright power output potential.
 
Stiggy Pop said:
like most things in engine building there's tradeoffs and it's not as simple as "better or worse".

Super simplified shorter runners will improve turbo response and long runners will have more outright power output potential.
Click to expand...

Might be worth bringing up that these differences are the last 5% of possible optimization. Having smooth, clear ports and shaping in the manifold is far more important. Cylinder head flow, camshaft, turbo choice, and a good intake manifold are all areas that would take priority in consideration before runner length becomes a factor.

OP is likely putting cart before horse here.
 
Regardless of primary runner length, on a twin-scroll manifold you want to keep all of the runners equal length and keep the 1-4 and 2-3 pairs completely separated. It's all about evenly timed exhaust pulses helping the turbine spool up at low engine speeds, so if you do anything to make the "firing order" inside the turbine uneven, or allow the two volumes to cross-talk, then you are being counterproductive.

All of the negative pressure wave pulse tuning theory from NA engines absolutely still applies to a turbo engine; the basic physics don't change. But usually you won't need to eek out that final 10/10ths of performance from a turbo system as you can generally just turn up the boost to make more power. Dedicated turbo racing engines have very carefully designed (usually long-runner) exhaust manifolds, but they spend big bucks and tons of hours optimizing using analysis tools and engine dynos, which most enthusiasts won't practically be able to do.

In general terms I always recommend two completely separate external wastegates from the two sets of primaries on a twin-scroll setup. I've seen lots of attempts at dividing flow into a single wastegate and it seems like more trouble than it's worth, IMO. The nice thing about the twin wastegate setup is that the valves themselves can be smaller since they only need to flow half as much as a larger single (that doesn't mean 1/2 the valve diameter, since flow is not linearly proportional to diameter).
 
I'm not sure why you decided to personally attack me and my header in that way, but I'm too overspent on the 242 this year to address it and I feel like you should respect that by not reminding me of conflicting pulses. :-P
 
Forced Induction Performance Tuning
Book by A. Graham Bell

This book is a great source of info and has the dimensions you are asking for.

Lots of other great info on designing your turbo set up too.

Links in post below
 
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Stiggy Pop said:
I'm not sure why you decided to personally attack me and my header in that way, but I'm too overspent on the 242 this year to address it and I feel like you should respect that by not reminding me of conflicting pulses. :-P
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At least you didn't intentionally BUILD your header incorrectly. :-(
 
Stiggy Pop said:
I'm not sure why you decided to personally attack me and my header in that way, but I'm too overspent on the 242 this year to address it and I feel like you should respect that by not reminding me of conflicting pulses. :-P
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Tfrasca said:
At least you didn't intentionally BUILD your header incorrectly. :-(
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:cool:

I wouldn't shame anyone - there are lots of reasons why things don't work out ideally when building cars and/or designing parts. Maybe I should rephrase and give my twin scroll advice as a standard to aspire to, as much as possible given constraints of packaging space, budget, tools, etc.

On the other hand, we should all be personally attacking each other for spending so much time and money on these old worthless bricks :)
 
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