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MG MGF Technical - Smoothing Airlfow - Too anally retentive

Having spent the last couple of days redin over Dave Andrews pages on gas flowing a cylinder head its' emphasised the importance of smooth airflow in the head to result in increased performance.

Does this however apply to other areas and in particular the air filter/ intake system/ throttle body. I read a post in the archive that mentioned something relating to this.

I ask beacuse at the moment with my modified intake system, a K&N panel in original airbox with a sealed pipe connected to the hole in the bottom of the air box which runs down and under the car to scoop up cold air, is using a corrugated extendable foil one that came with a Ramair filter I had on my last car.

Now the insides of the pipe are very rough as they're corrugated and causing increased air turbulence I would imagine. Therefore would a smooth pipe help here or would any benefits of smoothed air flow be lost once it hit the throttle body and intake plenum/ manifold.

Also would there be any benefits to polishing the inside of the alloy throttle body to smooth the air flow.

Bob Millar

Hi Bob, the challenge is to ensure that the airflow is laminar through the throttle body (and subsquently through the inlet tracts). What happens before the filter is perhaps a little less important - just so long as there isn't an impedence to good flow.

Okay - that's a gross simplication, but what you are after is a good diameter hose, of reasonably short length that has a clear run to the airfilter without any tight turns.

If you are using a large diameter tube (70-100mm) then corrogations aren't going to be a significant issue.

Where an airfilter assembly needs to work best is in the design of the airbox (trying to ensure that there is no tubulent air in the area of the filter), and from the filter to the throttle.

This is one of the reasons why DIY modifications like the flower pot and 820 airbox work so well: the basics are right.

Would polishing the internals of the throttle body help? Actually, they're not bad out of the box. Vizard (he of A-series tuning fame) demonstrates that modifications [effectively gas-flowing] to a 1.25" SU can result in flow and power gains similar to that seen with a 1.5" SU. However, I seem to recall that Roger Parker tried this with the 48mm, and it didn't make that much difference... Overal, I'd say that it would be a lot of effort for fairly minimal gain.
Rob Bell

Hmmm. I've managed to pack her indoors off for the weekend giving me plenty of free time to fiddle with the car hence I'm looking for things to do!!!

Trip to the scrapyard early on Saturday to see If I can find a exhaust manifold off a 1.8 K to swap for mine after I've removed the internal welds and re-inforced it out side (whilst keeping an eye out for any rogue 52mm throttle bodies or perhaps some DHTB ones off a KV6!)
Bob Millar

LOL good luck! As Tim, Dave (Andrews) and I found, sometimes the Lambda can be a real pig to remove!!!

You're unfortunately unlikely to be able to find a 52mm TB in a scrap yard at the present time, but you never know! They're fitted to ZR160, TF135, TF160 and Trophy 160. I don't think they're found on any other car - unless someone else can enlighten us?

Apparently the TB from T-series engined Rovers may fit the K -series, and has an even larger 56mm choke. Unfortunately, someone has just reported on another forum that RR results were not good with this TB - the engine actually lost power at the top end when compared to the 52mm equivalent. :o(
Rob Bell

Bob, Your best bet is to remove your existing exhaust manifold and fettle that. at least that way you won't have to remove the lambda sensor. Make sure that you have a new exhaust gasket to hand though and some gun paste for the manifold -flexi join.

In order to get the old manifold off you'll find it easier if you remove the flexi from the CAT first then undo the head bolts then you'll be able to get to the bolts between the manifold-flexi. They're hidden a little by the sub-frame, sometimes a wiggle joint socket extension will get in to do the job. A long 'power bar' is handy to crack the nuts off first. And the number one accessory is a second pair of hands.


tim woolcott

Tim, If I remember you've already done this to your manifold. Did it make much of a difference?
Bob Millar

I can't answer for Tim, but I know that Andy Gilhooley was delighted when he had his exhaust manifold gas flowed ;o)
Rob Bell


Tricky to say actually. I initially had it done at the same time as fitting of the new head so I really couldn't say one way or the other. The new head flows more air than a VVC so I imagine that there must be some gains in there somewhere.

tim woolcott

Suppose so. Dave Andrews mentions on his website that the inside of the OE manifold is quite bad and really will benefit from some fettling
Bob Millar

Yep, there is a nice large lump of weld in each of the pipes. The manifold is made of some pretty tough stainless steel so make sure you have some tough grinding burrs, Dave suggests that a decent drill will be up to the task. A Dremel is really not up to the job I understand.

Wear glasses when grinding and allow a good few hours to do the job.

Report back and tell us what you think the benefits of the work are....

tim woolcott

This thread was discussed on 31/10/2003

MG MGF Technical index

This thread is from the archive. The Live MG MGF Technical BBS is active now.