Exhaust Manifold Trade Off

[ColinH]

Evening All,

My turbo is remote, in draw through position, behind engine. 
 

I am weighing up if the shorter manifold length of an almost log design is worth increased response, over off boost or high revs performance?

Many moons ago Imout an R1 engine in a mini and made a semi log (I’m childish so that made me chuckle) arrangement. I never pushed it hard enough to see the impact it had.

Excuse the welding, I was a maintenance welder at that point so was pretty bad!

moat draw through setups I see matched header lengths due to having space but don’t know if more direct route will boost spool but harm high end.

It doesn’t feel like it would have an impact as many blow through exhaust manifolds are more restrict than this.

I welcome your thoughts and experience.

 

[Arttu]

I don't know what your "almost log design" actually means so can't comment that specifically  But here are some general thoughts:

• Shorter manifold most likely results better response IF it doesn't mean too bad compromises with shape and routing.
• Header length matching isn't that critical. Of course extreme length differences should be avoided but if I can get better routing with moderate mismatch I would go for it.
• Collector part is critical. Try to get that smooth flowing with equal joining angles. The same applies to other branches if you are joining the pipes before the final collector. So for example in your picture those two leftmost pipes are looking a bit ugly...
• I assume that a long mid-pipe between the collector and turbo will hurt response and power but I don't have any real test data about this. If reasonably possible I would place the collector right on the turbo flange.

[Gixer1460]

In the grand scheme of things, I think it makes 9/10ths of FA Difference in response as shorter exhaust / longer inlet tract and vice versa. Its unlikely anyone has actually done back to back quantitive testing so any results are likely very subjective ie. "its so much more responsive" when they last tried it with the old system & was a year ago! Unless you are in a competitive arena and have the budget to test, test, test and then more testing - just build something that fits, works and then sleep well at night!

Arttu's points are valid, so is wrapping / insulating long exhaust pipes - turbo's are heat engines & long pipes lose heat so keep it in!

[peter1127]

 inlet and exhaust tubing all benefit from symmetry: equal length equal diameter equal resistance. That being said, lots of setups around that perform fine and have less then optimal designs. Like yours, when you insist on drawtrough   Not technical relevant but for me a log design looks a bit agricultural. Seen them made from plain steel, amateur welded  and years of rust. Better have a fairing in that case. I like the equal length design such as in the picture.

Runner and especially up pipe diameter is an important parameter that will influence spooling up. When smaller you have high gasflow which helps. Personally I would keep runner size the same as stock and I can imagine the up pipe being 2" or even a tad smaller.

Hmm thinking about this just added a draw trough turbo system on my bucket list :-)

 

Edited July 3, 2025 by peter1127

[Gixer1460]

1 hour ago, peter1127 said:

 Like yours, when you insist on drawtrough  

Who is that comment aimed at? If its me (because the picture was from me) - where have I EVER insisted on Draw through? I've built both, one with a carb, one with EFI and planning a possible hybrid draw through but with single TB and injection. So I really don't care if the work. And BTW the picture was a mock up, EFE bottom, GSXR top, 1360cc and 14:1 CR - would have used methanol - could have been fun!

[peter1127]

36 minutes ago, Gixer1460 said:

Who is that comment aimed at? If its me (because the picture was from me) - where have I EVER insisted on Draw through? I've built both, one with a carb, one with EFI and planning a possible hybrid draw through but with single TB and injection. So I really don't care if the work. And BTW the picture was a mock up, EFE bottom, GSXR top, 1360cc and 14:1 CR - would have used methanol - could have been fun!

Dude relax. Aiming at nobody. Reacted on Colins question with banter.  Really like the looks of oldskool drawtrough systems and definately going to build one in the future.

Edited July 3, 2025 by peter1127

[ColinH]

10 hours ago, Arttu said:

I don't know what your "almost log design" actually means so can't comment that specifically  But here are some general thoughts:

• Shorter manifold most likely results better response IF it doesn't mean too bad compromises with shape and routing.
• Header length matching isn't that critical. Of course extreme length differences should be avoided but if I can get better routing with moderate mismatch I would go for it.
• Collector part is critical. Try to get that smooth flowing with equal joining angles. The same applies to other branches if you are joining the pipes before the final collector. So for example in your picture those two leftmost pipes are looking a bit ugly...
• I assume that a long mid-pipe between the collector and turbo will hurt response and power but I don't have any real test data about this. If reasonably possible I would place the collector right on the turbo flange.

Thanks for your thoughts. Yes those 2 pipes carried on to collector further down but was a hash job.

That was my giess on collector as enlarging pipe size will slow gases down. That was why I thought maybe 4-2 like I’ve shown then merge them close to turbo, with enough space to fit waste gate.

would be a fun challenge to try and run 4 pipes round there.

The rev range probably changes the impact as well with regard to how much gas volume would be pushed through collector.

[ColinH]

9 hours ago, Gixer1460 said:

In the grand scheme of things, I think it makes 9/10ths of FA Difference in response as shorter exhaust / longer inlet tract and vice versa. Its unlikely anyone has actually done back to back quantitive testing so any results are likely very subjective ie. "its so much more responsive" when they last tried it with the old system & was a year ago! Unless you are in a competitive arena and have the budget to test, test, test and then more testing - just build something that fits, works and then sleep well at night!

Arttu's points are valid, so is wrapping / insulating long exhaust pipes - turbo's are heat engines & long pipes lose heat so keep it in!

Thanks for thoughts and very good point about real world testing, back to back.

I have a gas turbine made from large turbo semi built and understand the heat giving power to turbine but hadn’t thought about it in turbo boosted application.

maybe a pre burner with fuel injected to boost energy   I had to put emoji as that’s completely believable with my penchant for the over complicated!

I think you’re right about build it, not a huge job to change if it doesn’t perform.

 

 

 

[ColinH]

4 hours ago, peter1127 said:

 inlet and exhaust tubing all benefit from symmetry: equal length equal diameter equal resistance. That being said, lots of setups around that perform fine and have less then optimal designs. Like yours, when you insist on drawtrough   Not technical relevant but for me a log design looks a bit agricultural. Seen them made from plain steel, amateur welded  and years of rust. Better have a fairing in that case. I like the equal length design such as in the picture.

Runner and especially up pipe diameter is an important parameter that will influence spooling up. When smaller you have high gasflow which helps. Personally I would keep runner size the same as stock and I can imagine the up pipe being 2" or even a tad smaller.

Hmm thinking about this just added a draw trough turbo system on my bucket list :-)

 

Thanks Pete, I have a habit of insisting on the design that poses the most problems and compromises 

I have seen that manifold setup from say what now!? previously and does look much nicer than a log or having a semi log on!

I was looking at 1.5” as I get this stuff good prices in dairy tube due to my work. May get 1.25” though thinking about it.

I did look at butchering my standard stainless manifold and running smaller single pipe from underneath to turbo. I think that routes a little long though.

[Gixer1460]

16 hours ago, ColinH said:

maybe a pre burner with fuel injected to boost energy   I had to put emoji as that’s completely believable with my penchant for the over complicated!

You may think / it would be logical right? and rally cars often use a similar effect on the start line to generate boost - they call it anti-lag BUT it has a huge downside. Bearing in mind where they work, turbo turbines are quite fragile and can be damaged with excess heat. The anti-lag systems add excess fuel during exhaust cycle which ignites / burns in the pipe / turbine housing to provide LOTS of exhaust pressure / heat to spin up the turbine and hence boost, super retarded ignition also means fuel in cylinder will pass through and only gets ignited late, so burning outside the cylinder. All good for spool - right? Well no! Its an uncontrolled burn with temps far in excess of 1000 - 1200+ deg C which will nibble away at turbine blades leading to failure. This is ok for a factory team that can treat turbo's as a consumable item but not ideal for the average garage, engine fiddler! 

Apparently 99.5% of whatever can be thought of as clever / new / revolutionary has already been done / tried / discarded before we think of it. Only the Einstein types come up with absolutely 'new' stuff 

Edit - Oh, in case you are wondering, the 4-1 manifold above had 32mm primaries, equal length, 4-1 collector into a 38mm up pipe to the turbo, all in stainless 304. Any lag it suffered was more down to using a S&S tractor carb 

Edited July 4, 2025 by Gixer1460

[ColinH]

43 minutes ago, Gixer1460 said:

You may think / it would be logical right? and rally cars often use a similar effect on the start line to generate boost - they call it anti-lag BUT it has a huge downside. Bearing in mind where they work, turbo turbines are quite fragile and can be damaged with excess heat. The anti-lag systems add excess fuel during exhaust cycle which ignites / burns in the pipe / turbine housing to provide LOTS of exhaust pressure / heat to spin up the turbine and hence boost, super retarded ignition also means fuel in cylinder will pass through and only gets ignited late, so burning outside the cylinder. All good for spool - right? Well no! Its an uncontrolled burn with temps far in excess of 1000 - 1200+ deg C which will nibble away at turbine blades leading to failure. This is ok for a factory team that can treat turbo's as a consumable item but not ideal for the average garage, engine fiddler! 

Apparently 99.5% of whatever can be thought of as clever / new / revolutionary has already been done / tried / discarded before we think of it. Only the Einstein types come up with absolutely 'new' stuff 

Edit - Oh, in case you are wondering, the 4-1 manifold above had 32mm primaries, equal length, 4-1 collector into a 38mm up pipe to the turbo, all in stainless 304. Any lag it suffered was more down to using a S&S tractor carb 

So that’s how they do the anti-lag. Very interesting but don’t want to swap Turbo few times a year so will leave it well alone.

you’re right about new ideas. Look at variable valve timing before world war 2 by Rolls Royce I believe.

thanks for info on manifold and that it ran well. I will stick with those sizes and get building.

first I need to weld some stuff back into frame once final turbo position decided.