Hot Wire AFM plumbing
Discussion
Hi,
I'm relocating the airbox which feeds an engine with EFi. I'm aware that the plumbing of air before and after an airflow meter is important to ensure turbulence doesn't cause it to misread.
With that in mind if packaging requires a bend is it best before or after the AFM?
How much straight run should I aim for as a buffer between AFM and bend (e.g. 1 x diameter)?
I'm relocating the airbox which feeds an engine with EFi. I'm aware that the plumbing of air before and after an airflow meter is important to ensure turbulence doesn't cause it to misread.
With that in mind if packaging requires a bend is it best before or after the AFM?
How much straight run should I aim for as a buffer between AFM and bend (e.g. 1 x diameter)?
Both upstream and downstream pipe geometry affects the flowstreams, ie the velocity distribution across the orthagonal cross section of the pipe. However, at the velocities and reynolds numbers asscosiated with MAF meters, it is the upstream geometry that is much, much more important.
If you have no way of recalibrating the MAF, then you need to recreate the original geometry as best you can, but any change will likely cause a measurement offset (which will mostly get learnt out in the closed loop fuelling trim, so you can use that trim value to see what affect your change has incurred)
If your MAF has a flow straightener grid in front of it, that'll help things, and generally you'd want at least 3 diameters worth of straight pipe upstream, and ideally, 6 to 10, but realistically, no MAF has ever had that because it is impossible to package. The MAF cal held in the ecu will have been done with the original hardware, so will have been used to linearise and characterise that system, so what the MAF does and doesn't measure is largely irrelevant as long as you copy the OE setup!
If you have no way of recalibrating the MAF, then you need to recreate the original geometry as best you can, but any change will likely cause a measurement offset (which will mostly get learnt out in the closed loop fuelling trim, so you can use that trim value to see what affect your change has incurred)
If your MAF has a flow straightener grid in front of it, that'll help things, and generally you'd want at least 3 diameters worth of straight pipe upstream, and ideally, 6 to 10, but realistically, no MAF has ever had that because it is impossible to package. The MAF cal held in the ecu will have been done with the original hardware, so will have been used to linearise and characterise that system, so what the MAF does and doesn't measure is largely irrelevant as long as you copy the OE setup!
The outlet from the breather is going to get pretty oily. I wonder whether there's a danger of that running downhill through the AFM and into the filter - it's not common in my experience to have intakes sloped down like that. May not be an issue, but I suggest you keep an eye on it to be on the safe side.
GreenV8S said:
The outlet from the breather is going to get pretty oily. I wonder whether there's a danger of that running downhill through the AFM and into the filter - it's not common in my experience to have intakes sloped down like that. May not be an issue, but I suggest you keep an eye on it to be on the safe side.
Interesting point..the rocker breather is below the base of the inlet bore on a tangent but I'll keep an eye on it all the same.Sloping down was the best package option - everything else ended up with elbows, bends or longer hose runs.
Also makes it less likely to ingest water when paddling!
Next bit of the puzzle..
I've packaged this and had to chop the inlet trumpet off the "front" end else it would protrude beyond the bodywork and look crap.
Reading about inlet trumpets I understand they are better for airflow than a conventional tube - so I am planning to re-use this:
Plan is to blank off the old inlet and make a new one tangential to the main body.
This is because it's higher up so reduces the chance of water ingress. I don't do crazy wading in the vehicle but would like a metre of freeboard all the same - water sometimes conceals holes.
Something like this I was thinking:
I've packaged this and had to chop the inlet trumpet off the "front" end else it would protrude beyond the bodywork and look crap.
Reading about inlet trumpets I understand they are better for airflow than a conventional tube - so I am planning to re-use this:
Plan is to blank off the old inlet and make a new one tangential to the main body.
This is because it's higher up so reduces the chance of water ingress. I don't do crazy wading in the vehicle but would like a metre of freeboard all the same - water sometimes conceals holes.
Something like this I was thinking:
Steve_D said:
An improvement (but perhaps only minor) would be to replace the pipe between MAF and plenum with a smooth bore pipe.
TVR guys claim this makes a worthwhile improvement.
Steve
Given that this pipe is, oh, about 70mm diameter, and the engine makes about 280 bhp tops, nah, i think you can safely leave it in place!TVR guys claim this makes a worthwhile improvement.
Steve
GreenV8S said:
The outlet from the breather is going to get pretty oily. I wonder whether there's a danger of that running downhill through the AFM and into the filter - it's not common in my experience to have intakes sloped down like that. May not be an issue, but I suggest you keep an eye on it to be on the safe side.
Prop the throttle butterfly open & have a look in the inlet of the plenum.On mine (3.5EFi) the breather passage into the plenum turns 90deg to the right (viewed from the driver's seat looking forward) from the metal tube the rubber hose pushes onto then after a short distance turns 90deg towards the rear of the car where it opens into the plenum so the breather fumes enter the chamber on the inner side of the butterfly. Unless the 3.9 is different!
Does need cleaning out from time to time. Pull the rubber breather hose off, prop butterfly open & push a clean lint-free cloth in to absorb the spray & dirt & give it a good blast of aerosol carb cleaner.
Mine was blocked solid when I bought the car. This causes the crankcase to pressurise & oil will leak out of all sorts on interesting places.
paintman said:
Prop the throttle butterfly open & have a look in the inlet of the plenum.
On mine (3.5EFi) the breather passage into the plenum turns 90deg to the right (viewed from the driver's seat looking forward) from the metal tube the rubber hose pushes onto then after a short distance turns 90deg towards the rear of the car where it opens into the plenum so the breather fumes enter the chamber on the inner side of the butterfly. Unless the 3.9 is different!
Does need cleaning out from time to time. Pull the rubber breather hose off, prop butterfly open & push a clean lint-free cloth in to absorb the spray & dirt & give it a good blast of aerosol carb cleaner.
Mine was blocked solid when I bought the car. This causes the crankcase to pressurise & oil will leak out of all sorts on interesting places.
It's got a new engine (hence improved air filtration) and all the ancillaries are clean 😉. On mine (3.5EFi) the breather passage into the plenum turns 90deg to the right (viewed from the driver's seat looking forward) from the metal tube the rubber hose pushes onto then after a short distance turns 90deg towards the rear of the car where it opens into the plenum so the breather fumes enter the chamber on the inner side of the butterfly. Unless the 3.9 is different!
Does need cleaning out from time to time. Pull the rubber breather hose off, prop butterfly open & push a clean lint-free cloth in to absorb the spray & dirt & give it a good blast of aerosol carb cleaner.
Mine was blocked solid when I bought the car. This causes the crankcase to pressurise & oil will leak out of all sorts on interesting places.
blitzracing said:
Get yourself an ITG ram air filter, they have a decent bell mouth that can butt up against the AFM to give a smooth airflow. It's very noticeable on the 14CUX if you get turbulence in the AFM as the idle is poor.
Thanks for the suggestion - I'm sticking with a paper element from now on.Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff