so im building my first custom turbo manifold...

i've always heard you gotta make the runners as close to the same length as possible.

my question is do you have to and how close is close enough.

if your longest runner is say 8"... do the other 3 runners need to be within what an inch or withing 4 inchs?


if there not equal length whats it gonna hurt?

 

There are various theories, but my take is that Turbo systems are less sensitive to length variations overall, but flow resistance must be maintained as close as possible. This means 1 runner should not have 4 90's, and one only 1 90 to get the same length.
The equal length argument is most important when building for n/a cars where scavenging can be influenced and greatly increased by timed sound waves in the system. Sound does not see any extra resistance in a bend, so the length is the top priority since sound does see even slight variations in distance. Stepped headers work mostly on this principle, and support the principle of an exhaust system that continues to increase in size as it increases in length.

What you really want in a turbo header is for the exhaust gas to hit the turbine with evenly spaced, and high-velocity pulses.

 

hm... ok.

so something like this for example is still fine since the runners arent even but enter the collector evenly? i feel like such a newb hahah.

http://dtmfiberwerkz.com/access/images/m20mani2.jpg

 

As long as the runners are within +/- 3.0", it should be fine from the many variety of tubular manifolds I have built, tuned and experienced. Anything more, then you will start hearing audible differences (lopey sound, etc..)...

But unless the engine is on the ragged edge (insane power levels), the difference in runner length in a typical tubular manifold rarely poses a problem. It's really based on the percentage of runner length/volume versus one another. If you had a shorty tubular manifold with the longest runner being only 6.0" long, then a 3.0" runner length difference will create a much bigger impact on performance in terms equal running cylinders. On the other hand, a topmount manifold with longest runner 24.0-26.0" in length won't even make much of a difference with runners being +/- 3.0"

The merge collector is the most critical part for a turbo manifold. Low angle merge collectors work well because of the long length before it reaches the turbine. This allows exhaust gases to pick up velocity before it reaches the turbo.

The straighter you can achieve before the turbine inlet, generally it will have the best spool and response. It's a reason why topmount manifolds are always so responsive between shiifts, because the design allows a long straight run of piping and a long low angle merge collector.

 

equal length does not mean equal flow. like bells said, exhaust gas velocity should try to be balanced between cylinders.

tony makes a very good point in reference to the merge collector being a key design feature in making power/exhaust velocity. more gradual is better.

someone correct me if i'm wrong, i'm under the impression a way to monitor flow between cylinders is to have an egt in each runner very close to the head. the twin turbo bbc i did a few years ago had requested this feature for tuning. i'm no tuner, but i would think this would be a good way to monitor the engine and help balance it out and even be used as a tool to help tune manifold design.

 

Most of the v8 customers we deal with at work use the egt for tuning rather then o2. Seems to be different methodology then common with the import guys.

Personally i would suggest drive pressure gauges per cylinder over egt. To monitor manifold/header flow variations between runners. As egt sensors will tell more about whats happening inside the cylinder then what is happening after.

 

from everything ive ever seen its not really important, just make sure there arent a lot of bends, dont cheat any bends, and make your merge collector/wg a priority

look at all the unequal length subaru's that make power

 

I don't have any personal experience with anything other then v8's but from what I've read, merge collectors and wastegate placement are 2 major factors

 

What say you to a manifold of this design when compared to long tube turbo header like full race etc?

What effects does an early merge have on velocity/ flow? Does this early merge make the wastegates less efficient since their placement is away from the runner merge? Thing is basically like a mini v8 single turbo header.

I want to do a twin scroll turbo mani like this. The design looks robust, and allows for really good turbo placement and a seemingly simple construction. I have the material to do a full tubular, but iffy as to the benefits for the extra effort and more hassle to get the right turbo position while snaking runners everywhere.

 

yeah along with my first question about equal lengths i was also curious along the lines of what your saying.

whats if you went 4 into 2 into 1 instead of 4 all the way to the collector? or 4 into 1 right out of the motor and ran the 1 all the way to the turbo?... i know a lot about cars, but not much about exhaust manifold design and what affects this or that will do to velocity and power...

i will post some pics of what im thinking and you guys can tell me if its dumb or not.