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Author: Subject: MX5 NA air intake position
Slimy38

posted on 5/2/24 at 09:03 AM Reply With Quote
MX5 NA air intake position

For those of you running/using an MX5 engine (NA or NB), is it necessary to have the air flow meter and airbox on the exhaust side of the engine? I think I have enough room to put them either straight on the throttle body or at least on the front of the engine, but I believe the length of the pipe sometimes has an impact on engine behaviour.

From a packaging perspective I'd prefer a shorter intake length as it fits better and hopefully will get better airflow. I have seen some MX5's with the airflow meter moved but they're usually in combination with a different intake manifold.






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Mr Whippy

posted on 5/2/24 at 10:22 AM Reply With Quote
Brace yourself for 101 different opinions.

Just move it.





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Slimy38

posted on 5/2/24 at 10:47 AM Reply With Quote
quote:
Originally posted by Mr Whippy
Brace yourself for 101 different opinions.

Just move it.


Yep I've already gone down that rabbit hole on the Miata forums, but they're focused on how many horses and torques are gained/lost. As long as it still works I think I'll JFDI...






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coyoteboy

posted on 5/2/24 at 10:49 AM Reply With Quote
What kind of an air flow meter is it?

In general you won't notice any seat-of-the-pants difference if you shorten the pre-TB intake. Theoretically yes it does affect it, but can you tell 1hp here or there? The biggest issue I can see is making sure you have enough clear flow upstream or downstream of a mass flow sensor or flapper to ensure that the full rev range yields close to laminar flow through the sensor - nominally something like 6 inches either side would do that though.






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SteveWalker

posted on 5/2/24 at 11:07 AM Reply With Quote
I'm not sure with the vibrations of an engine and its intake system, but in industrial processes, the straight length before a thermal mass flow instrument is usually 10 pipe diameters, with 5 more following it (reductions reduce accuracy). I don't think that most cars could fit that in, so in all likelihood, their ECUs have pretty wide tolerances on accuracy and changes to the system won't matter.
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Slimy38

posted on 5/2/24 at 11:11 AM Reply With Quote
quote:
Originally posted by coyoteboy
What kind of an air flow meter is it?

In general you won't notice any seat-of-the-pants difference if you shorten the pre-TB intake. Theoretically yes it does affect it, but can you tell 1hp here or there? The biggest issue I can see is making sure you have enough clear flow upstream or downstream of a mass flow sensor or flapper to ensure that the full rev range yields close to laminar flow through the sensor - nominally something like 6 inches either side would do that though.


Perfect, that's the sort of thing I was considering but I wasn't sure of the details. The airflow meter is a flap type rather than hot film or wire, then the throttle body is then another flap. If I fix those back to back will there be some funny things going on, and what you're saying is that a few inches would be preferred?






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coyoteboy

posted on 5/2/24 at 11:38 AM Reply With Quote
Yeah you want to maximise the distance between the TB and the flapper as much as is reasonable. Back to back I'd be worried about interactions. 6 inches I'd consider an absolute minimum. Anything more would be better.

Bear in mind that turbulence around objects like the TB will change with rpm and throttle opening so if you're too close, there may be some cases where at 4000rpm WOT you're seeing no problem but at 4000rpm part throttle you see all kind of oscillations. 10 diams is a nice safe number as mentioned but you will rarely fit that, just use it as a guide.

From memory with my own flapper type, when I went from a curved intake path about 18 inches long to a straight intake path 6 inches long, it got some unexpected flutter from the turbo stall on overrun which caused it to buck and judder at low throttle, for an example of sensitivity. But flappers are a bit less sensitive than hot wire types at least.

[Edited on 5/2/2024 by coyoteboy]






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Mr Whippy

posted on 5/2/24 at 12:55 PM Reply With Quote
I just think of my volvo, that has the airbox and air meter at the front of the engine bay, it then goes over to the back of the engine, through the turbo, then loops over the top of the engine via a big plastic pipe, goes through the intercooler before heading over to the butterfly/manifold and finally into the cylinders.
Total length must be almost 2m! Goodness knows how many seconds it takes for the air to pass through all that, it's amazing it runs smoothly at all.





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coyoteboy

posted on 5/2/24 at 01:06 PM Reply With Quote
at highway cruise, it takes about half a second for the air entering the start to reach the throttle in that setup. However the flapper is reading flow rate, which changes almost instantaneously.

[Edited on 5/2/2024 by coyoteboy]






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