Cold air to filter

[HUB]

Hi all, Asking here cos I can't get to my car atm. Looking to put extension tube on to air filter housing to "catch" cooler air from grill area...What diameter tubing do I need??

Cheers.

[Pete Lewis]

i used 2"waste pipe on my Vitesse when i had her  smooth and very cheap

or  pond pump hose ,  as smooth inside , you dont want helical turbulance

[NonMember]

12 minutes ago, Pete Lewis said:

 smooth inside , you dont want helical turbulance

And yet the factory fitted helical flexible hose...

The surface turbulence is minimal. Smooth is nice but it really doesn't matter.

[dougbgt6]

H,

Your profile says GT6 mk2?  Canley Classics say the mk2 & 3 filter box are the same part and they sell the mk3 tubes for £6.65 each. They are corrugated, as NM points out so Pete's smooth pipe is better and cheaper. I've got corrugated. 

Doug

[Pete Lewis]

3mtrs  black 50m  £10  make loads of them Ha !

Pete

[HUB]

3 hours ago, dougbgt6 said:

H,

Your profile says GT6 mk2?  Canley Classics say the mk2 & 3 filter box are the same part and they sell the mk3 tubes for £6.65 each. They are corrugated, as NM points out so Pete's smooth pipe is better and cheaper. I've got corrugated. 

Doug

Should be Mk3....So 2" smooth rubber will be what I'm looking for?!

[Clive]

I would get a ruler and measure the inlet tubes, just to be on the safe side. 

In all honesty, corrugated tube will be just fine. Use whatever you can find easily. Personally I would avoid anything rigid or stiff And generally something like the tubes will have a much larger size than the inlet valves, so are unlikely to be the restrictive part of the induction.

[dougbgt6]

7 minutes ago, clive said:

I would avoid anything rigid or stiff

There's a joke there,  somewhere!

Doug

[JohnD]

4 hours ago, NonMember said:

And yet the factory fitted helical flexible hose...

The surface turbulence is minimal. Smooth is nice but it really doesn't matter.

Have to disagree with you NM!

An uneven inner surface breaks up the boundary layer, slowing flow for much further in towards the centre.   This has a significant effect, as flow varies as the fourth power of radius.  So a tube half the diameter will flow SIXTEEN times less air.   Your rough walled tube won't have that great an effect, but still significant.

For the same reason,it would be better to ha v e one, bigger tube into the airbox.

 

[Clive]

What is needed is a rolling road and some different hoses to see what the reality is..... But for a std GT6 which makes approx 100bhp, I doubt different hoses would make any measurable difference. 

 

In fact, we need somebody with a nice long straight driveway, and a 1/4mile timer app on their phone. That is free😉

[dougbgt6]

Interesting. For H’s benefit I can report crinkle cut is better than no pipe and easier to fit than straight cut.  

I drilled the holes in the radiator cowl with a recessed ceiling light auger. Hole dia was 2 & 1/8” and the pipes screwed in exactly.

 

Doug

[daverclasper]

If you are struggling to get exact fit on ribbed hose, then I slit the ends down about 1 and half inches and used a cable tie on each, not quite as tidy though. 

[Anglefire]

I really must get my temperature loggers lashed up to the car to measure the temperature in front of and by the filters to see how much temperature difference there actually is. I suspect very little, but more than happy to be proven wrong. 😀

[daverclasper]

 

5 minutes ago, Anglefire said:

I really must get my temperature loggers lashed up to the car to measure the temperature in front of and by the filters to see how much temperature difference there actually is. I suspect very little, but more than happy to be proven wrong. 😀

 I didn't notice any difference in how my car ran, not even after long periods of idling in traffic jams on very hot days. I'll leave them on, as can't do any harm.  

[Anglefire]

They won't do any harm - though they "could" restrict the amount of air available to the engine - but in truth I doubt its of significance unless you are running on the red line all of the time - racing for example. Perhaps @JohnD   could provide some wisdom?

[68vitesse]

Vitesse 2L MK1 didn't have them MK2 did, can't imagine Triumph would spend money on something not needed.

Regards

Paul

[Anglefire]

It probably relates more to emissions that anything else (As a thought - no science!)

[JohnD]

1 hour ago, Anglefire said:

They won't do any harm - though they "could" restrict the amount of air available to the engine - but in truth I doubt its of significance unless you are running on the red line all of the time - racing for example. Perhaps @JohnD   could provide some wisdom?

At, say, 6K, a 2L engine draws in air at  2L x 3000 (one aspiration/two revs) = 6000L/min = 360,000L/hour

How fast is the air moving?   A cylinder with volume 360,000L and with a cross sectional area equivalent to those two hoses - can't recall, are they 1 1/4"?  Or 32mm, or 3.2cms,  so 1.6cm radius and  CX area 1.6^2 x Pi = 8cms^2 - will be 360,000,000 (cms^3)/8 cms long = 45,000,000cms = 450,000metres = 450 kilometers.   Per hour. 

In fact, the  velocity profile across a tube is not linear, but maximal in a narrow region at the centre, slowing to near standstill at the walls.    The velocity curve gets blunted if the flow is turbulent, as rough sided walls will promote.   The velocity in the centre will need to be much higher than the mean flow to achieve the full flow delivery.

 

And as I mentioned before, and for the above reason, flow resistance increases markedly as radius decreases.    There are two such tubes on the GT6/Vitesse airbox, so the total flow would need to be half the toptal, or 225mph.     But even faster along each, narrower tube.

John

PS This isn't rocket science!   It's not even quantum chromodynamics!  It's arithmetic!     Please check my working anybody!

[dougbgt6]

1 hour ago, Anglefire said:

I really must get my temperature loggers lashed up to the car to measure the temperature in front of and by the filters to see how much temperature difference there actually is. I suspect very little, but more than happy to be proven wrong. 😀

I suspect not a lot of difference, when stopped for a while. But........I replaced my original radiator cowl because it had sagged in the middle and look and naff. I found that on a familiar run the temperature gauge never got up to it's previous top reading by 1/2 a division. This tells me that more cold air from the radiator grill is going through the radiator than previously. The carb pipes are connected to the grill and I would expect a substantial difference at sustained speed particularly considering the engine bay heat behind the bulkhead (see Gearbox tunnel thread)

Doug

[dougbgt6]

John's sorry, you'll have to spell it out, what do your graphs and calculations mean? Conclusions on your workings please.

Doug

[Clive]

I will work in cm3 and seconds....

2000cc x 6000/2= 6,000,000cm3/min Divide by 60 = 100,000cm3/second.

Area of tube is fine at 8cm2, multiply by 2 =16cm2.

100,000/16=6250cm/s or 62.5m/s. 

(convert to km/h and it is indeed 225km/h)

 

So yes, Johns figures do check out. Faster than I would have anticipated!

 

My intake uses 10cm diameter tube, area 78.5, about 5x that of the above. So speed in mine will be about 45km/h, much more sedate! But the tube is recommended 

 

BUT I still think to get answers it needs real-life testing. The tubes are probably not the limiting factor to performance. Reminds me of a friend who used to build very high performance V8's, for people who wanted to do the 200mph thing. He built a twin turbo engine, made just over 900bhp on the dyno. He was worried that the small looking K+Ns would be restricting power, so removed them. Engine made exactly the same power, no change to fuelling either. So K+N's are possibly super efficient, or more likely, not the limiting factor. And yes, the car did exceed the Vmax target!

 

 

[Anglefire]

52 minutes ago, JohnD said:

At, say, 6K, a 2L engine draws in air at  2L x 3000 (one aspiration/two revs) = 6000L/min = 360,000L/hour

How fast is the air moving?   A cylinder with volume 360,000L and with a cross sectional area equivalent to those two hoses - can't recall, are they 1 1/4"?  Or 32mm, or 3.2cms,  so 1.6cm radius and  CX area 1.6^2 x Pi = 8cms^2 - will be 360,000,000 (cms^3)/8 cms long = 45,000,000cms = 450,000metres = 450 kilometers.   Per hour. 

In fact, the  velocity profile across a tube is not linear, but maximal in a narrow region at the centre, slowing to near standstill at the walls.    The velocity curve gets blunted if the flow is turbulent, as rough sided walls will promote.   The velocity in the centre will need to be much higher than the mean flow to achieve the full flow delivery.

 

And as I mentioned before, and for the above reason, flow resistance increases markedly as radius decreases.    There are two such tubes on the GT6/Vitesse airbox, so the total flow would need to be half the toptal, or 225mph.     But even faster along each, narrower tube.

John

PS This isn't rocket science!   It's not even quantum chromodynamics!  It's arithmetic!     Please check my working anybody!

Hi John,

I wasn't thinking that deep actually - but what you have written is interesting - I do like these sort of sums! -  I was more thinking of the practical "feel" when driving hard!

I don't do them as it's not my area of expertise, but similar calculations are done in air conditioning ductwork for maximum speeds - though ultimately everything is rule of thumb with maximum speeds for given noise ratings (NR) - occupied spaces are normally between 2 and 10m/s (400-2000ft/min) 

[Nigel Clark]

This is getting interesting! Volumetric Efficiency also needs to be considered.

For a normally aspirated 4 stroke engine, with fixed valve timing, 2 vertical valves per cylinder, bath tub combustion chamber and modest cam timing, VE will be somewhat lower than 100%, perhaps 75-80% as a guesstimate.

Air speed may therefore be lower than calculated above, but it's still going to be moving pretty fast, well in excess of 150mph through those 2" tubes at peak revs and wide open throttle.

So a large diameter air feed tube will work best. The standard-fit flexi tube feeding cold air from the filter to the plenum on an injected TR6 is 4-5" diameter. That's more like it!

Nigel

[NonMember]

Of course, one should never take a figure like that without asking "is that actually a lot?" Given that the speed of sound is 760mph and the weather can achieve 100mph winds with only 50mBar pressure difference, it's maybe not as extreme as it first sounds.

Air flow through a restriction hits the "sonic" limit at a pressure ratio of about 0.57, and we're looking at 20% sonic. Given the shape of the curve, that's achievable with a pressure ratio somewhere above 0.95 - or less than a 50mBar drop.

If every last ounce of power matters, if you really care about that last couple of percent, then you need a big bore, smooth walled intake. For us mere mortals the factory solution is fine.

[JohnD]

Thnak you NM!

Which is why SofS has a Pi plenum that is TWICE the normal 3" diameter.   The orifice from the air filter is still 3", but orifice flow is always turbulent and flow then varies only by the square of radius.