Welcome to our resource for MG Car Information.
MG MGF Technical - Air intake improvements
|While I was looking at the under-bonnet area thinking about cool air inlets (see the thread on side air intakes/scoops) I saw how crude the actual air intake is - just the cut-off end of the flexible tube. It started me wondering what the airflow would be like trying to get into the tube. Doing a fairly rough sum I was rather surprised at the probable air velocity at high engine speeds. At 6000rpm the flow would be 3000 x 1.8 litres of air/minute. Assuming only 75% volumetric efficiency, that gives about 4000 ltr/min. That converts to 67½ ltr/sec or 67500 cm³/sec. The intake diameter is about 5cm, so the area is about 20cm². Divide that into the 67500 number to get the velocity which comes out at over 34 metres/sec or 77mph! Without a decent bellmouth on the entry, the flow is going to be even more compromised. Maybe fitting one might make a useful improvement on its own....|
|M Cunningham 1|
|Hi Mike. An interesting question! This cold air aspect and my non engineering background was causing me to fumble around. I was originally considering using Rob's Rover air box set up to keep the air cool and coupling this with a set up similar to what Fabrice has shown where he uses 'drainage' pipes to obtain larger bore dual (2 into 10) air intake. I then wondered whether the air scoops (ie. those produced by Carl or advertised on ebay) could be utilised to pull in more air and the air 'funneled'via a bellmouth into the air intake pipes? Your thread indicates that these various 'innovations' if added together could produce an interesting result. I shall try to fabricate this set up. However, I am not clued up enough technically to measure the results as you do. I guess I shall have to calculate by the width of my grin if it goes well!|
I can't fault your maths, but your inlet seems very small, especially the as inlet duct on mine is more like 100mm diameter. however that is possibly beside the point.
I've been to two rolling roads trials and although I don't have the printouts I remember that any ducted enclosure, (even the flower pot I had at one time) seemed to be equally efficient within a few %, possibly because they all drew in cold air from outside the engine compartment. A weakness discussed both times was the effect of the car being still during testing.
The viper air filter I have now came with an ali bell mouth which wouldn't fit into the side vent so I just left it off and taped the flexihose direct onto the plastic vent liner. Just looking at the shape of this liner, I thought it was probably the biggest constriction in the ducting, in terms of its narrow cross-section and sharp changes in direction, but I couldn't see any way of fitting a better shape without attacking the bodywork.
One last thought: I'm not sure when the supply of air is no longer an issue and lack of fuel takes over.
|It was just a quick calc- I've been back in to get the actual size, so it could be somewhat bigger. I suppose the ultimate possible flow would be with a bellmouth directly onto the throttle body with no filter! Anything else is going to take something away, so the clever bit would be to minimise the losses.|
|M Cunningham 1|
|Update - I just measured the intake at 55mm diameter, so my hypothetical air velocity is 63.5mph. Still worth a bellmouth!|
Looking at the resonator box below the air filter box it seems as though the manual is right about dropping the rear subframe to get it out. Does anyone know if that's the case? As it is, it's rather in the way of running a duct from the airbox to the side vent. Perhaps one could bore a fresh hole in the side of the airbox /:o)
|M Cunningham 1|
|I understand that some people have managed to remove the resonator without dropping the sub frame but it did result in destruction of the box.|
|Ted's right, the the resonator cannot be removed in one piece without dropping the subframe.|
Mine's an F so I left the old resonator in place and squeezed a 100mm tube past it.
The resonator also helps to seal off the hole in the wheel arch, but it's a moot point whether this is a good or bad thing. Some of the new TFs I looked at in the dealers had no protection under the rear wheel arch, so maybe it proved unnecessary.
The biggest constriction I found was between the two skins of the car body. There is a plastic liner that runs between the two inside the airvent. This comes out easily then you'll see what I mean.
Unfortunately there is no room here to fit a better shape - unless anyone else knows better.
|Surely there has to be more people than me that have taken out the resonator box in one piece? (97-VVC)|
Ok, it was tight and I had to rotate the box and slightly deform the inner arch, but out it came in one piece.
I added a thin sheet of makrolon cut to fit where the box had previous been in order not to get road dirt entering too easily !
|>> At 6000rpm the flow would be 3000 x 1.8 litres of air/minute. <<|
Mike, that calculation is right for a two-stoke! A four stoke means that only one engine is on an intake stroke per revolution... i.e. 1500 x 1.8 litres per minute. But that said, you are in the right ball park - and it astonishes me how much air an engine needs! It also makes a mockery of all those eBay fan-gizmos that are claimed to push more air into the engine...
>> The biggest constriction I found was between the two skins of the car body. There is a plastic liner that runs between the two inside the airvent. This comes out easily then you'll see what I mean. <<
Chris, I've not disassembled this area before - does the liner pull out when the intake trim is removed?
|Hang on a minute Rob! 4-stroke = suck+squeeze+bang+puff = down+up+down+up = 2 revs/cylinder = 8 revs for 4 cylinders therefore air mass flow = ½ engine capacity per rev, so I think I'm right, 6000rpm = 3000x1.8, or 90litres/sec. Whichever way it's a hell of a lot of air! So this year's F1 cars do 19000/2x2.4liters/min = 380litres/sec. By way of comparison, the Rolls-Royce Trent engines in the Airbus A380 do just over 1 million litres/sec - each! (Provided I've got my sums right!)|
|M Cunningham 1|
|"By way of comparison, the Rolls-Royce Trent engines in the Airbus A380 do just over 1 million litres/sec - each! (Provided I've got my sums right!)"|
I think it might be a tight squeeze in the engine bay though !!
Interesting reading guys.
unfortunately I can't get into my car at present, but from memory the plastic liner is held in by a couple of self tapping screws and comes out easily once the outer grill is off.
The area between the inner and ouer skins is completely closed off with meal panel so the only purpose of the liner would appear to be to smooth the air flow. Both ends of the liner are quite big, but the cross-secton in the middle is a lot smaller. I couldn't see any way to improve this without cutting out part of the internal bodywork.
|Took off the side vent this afternoon and the plastic liner is fixed on with 3 self-tappers along the front edge and hidden by the outer plastic trim. The inner edge locates against the inner shell. What I had forgotten was there is a further inner part of the shell and the airway leading to this has no aerodynamic path and no easy way of sealing an air pipe to it. I used duct tape and its lasted over 12 months without peeling off, but there's no getting away from the awful shape of this final metal path. Also the panel appears to be structural, so I am not going to start cutting into it - unless anyone round here knows better?|
|>> 4-stroke = suck+squeeze+bang+puff = down+up+down+up = 2 revs/cylinder = 8 revs for 4 cylinders therefore air mass flow = ½ engine capacity per rev <<|
Nope, it'll be a quarter. I look at it like this - four stroke engine, as you describe. Therefore one cylinder in four per revolution is on the intake stroke, and therefore each revolution is 1/4 of 1.8 litres x volumetric efficiency (which admittedly varies according to rpm)...
Chris, I reckon that the best solution is a glass fibre funnel to replace the existing liner?
|Rob, I'm disagreeing again! 2 pistons going down, one on power, one on induction. The other two pistons are going up, one on compression, the other on exhaust. For one cylinder, one induction stroke every 2 revs, therefore for 4 cylinders, 4 inductions per two revs, or 2 inductions per rev = half swept volume. I was interested to hear from Carl that he'd managed to wangle the box out. I could be tempted, as that leaves to elbow into the air cleaner pointing in a rather useful direction to add a bit of ducting. Anyway, it's only two weeks to go and I'm back on the road - whoopee! I might even go out at 00.01 on the 1st if it's not wet...|
|M Cunningham 1|
|A glass fibre shape would be certainly allow one to make a smoother shape between the various holes in the body. Setting up a 3D former could be tricky and I don't have the time, which is why I ended up taping the inlet pipe direct onto the inner metalwork. |
Next time I have the NSR wheel off, I'll take a further look at the metalwork, but last time I looked, I thought the old resonator was an access problem rather than an aerodynamic bottleneck.
|Bolt on a leaf blower to blow air into the ducted intake...Or, one of those MR2 snorkel pipe thingies???|
I was gonna suggest a ram-scoop but thats on Captain Kirk's ship ;-)
|>> Rob, I'm disagreeing again! 2 pistons going down, one on power, one on induction. The other two pistons are going up, one on compression, the other on exhaust. For one cylinder, one induction stroke every 2 revs <<|
Nope - one induction stroke every 4 revolutions ;o)
One revolution is only half a 4-stroke cycle... i.e. the crank shaft has to rotate 2 revolutions for one complete cycle... or one induction stroke every FOUR revolutions :o)
if there are 2 crank revolutions for one complete cycle, there has to be therefore 2 revolutions between each induction stroke?
does this help?
|For one cylinder -|
Down for ½ a rev = induction
Up for next ½ rev = compression
Down for next ½ rev = firing
Up for last ½ rev = exhaust
Total 2 revs, 1 induction = ½ swept volume per rev
there's a nice animation at http://commons.wikimedia.org/wiki/Image:4-Stroke-Engine.gif
Now try and figure out how much swept volume per rev you get with this one! http://www.keveney.com/W*nkel.html
Or even this one -
Thank goodness the K is a nice simple engine!
|M Cunningham 1|
|4000 litres of air per min, at a fuel ratio of 14/1= 285 litres of fuel!!|
Back to the drawing board!
an Intake (I), Compression (C), Power(P), Exhaust (E) cycle takes two revolutions, but there are four cylinders, so for any one revolution there are two out of the four cylinders inhaling:
1 I C P E
2 C P E I
3 P E I C
4 E I C P
So the total amount of air gulped at 100% efficiency would be 1/2 engine capacity per rev.
I think you'll find the actual amount of air is significantly less than that though!
|Ah, but 14:1 air/fuel ratio is by mass; air = .00122 Kg/ltr, fuel = .673 Kg/ltr, so 1 ltr of air requires .0000857 Kg of fuel. So at 6000 rpm, 90 ltrs/sec requires .0115 ltrs fuel, 41.3 ltrs/hr = 9.1 gal/hr. If you imagine driving flat out in a straight line for an hour, you'd cover about 125 miles at 13.7 mpg, which sounds about right (amazingly). Actually, you would probably run dry first, because in a straight line you wouldn't get the fuel out of the right-hand side of the tank!!|
|M Cunningham 1|
|I thought it would be somthing like that, but trying to avoid too much time surfing at work :)|
I couldn't find much wrong with you calcs otherwise.
Found this which might be interesting (generic, not k):
# Factory engine: 65%-75%
# Mild street performance, flat-tappet cam: 80% - 83%
# Mild street performance, roller cam: 90% - 93%
# All-out race engine, 95%-105%, depending on cam type and operating RPM range. Long, high cams operating at higher rpm's yield higher VE.
This thread was discussed between 13/04/2007 and 25/04/2007
MG MGF Technical index
This thread is from the archive. The Live MG MGF Technical BBS is active now.