short intake restriction calcs?

[Mud_]

Is there an accepted guideline for intake pipe sizing, or how much necking it takes to create significant restriction? I think I may have made an error in buying this Y piece:



My proposed intake goes 4" into dual 3" (~500fwhp NA V8 with an airbox per bank), but I fear the necking in the Y piece before the split could hurt me (it's basically 3" where it narrows). It looks worse with it in your hands than it looks in the photograph.

When referring to the size of an intake pipe it usually refers to the full length, so I'm not sure if the Venturi effect will 'sort' a short length out (within reason), or if this presents a massive restriction. I've read the restriction goes approximately inverse-proportionally to the cross-sectional area and proportionally with length, but a change of section clearly makes it more complicated.

FWIW (being pragmatic to get some numbers down) the engine was originally around 330fwhp and came with an ~3" intake pipe with an elbow - so the 4" start seems fine, as do the dual 3" bits, it's just that neck! With a short straight 4" pipe straight into a single 4" throttle body on a different intake I have no restriction (100+ kPa MAP at the rev limiter).

[KiaDiseasel]

By Venturi Effect I suspect what you mean is the pressure recovery that can take place after a restriction which increases flow over and above what a straight tube of the restriction size would flow. However, pressure recovery takes some distance to achieve and in any case is only partial. From a few percent up to maybe teens of percent. Firstly your pipe splits immediately after the restriction so there's no distance to achieve pressure recovery in down a single channel and even if there were at best it might make a 3" restriction act like maybe a 3.25" one.

So that just begs the question, what minimum size of intake pipe do you need for a given bhp with no power loss. It depends on a bunch of stuff but a rule of thumb is that what comes out must be the same mass flow as what goes in so something similar to the exhaust pipe size won't be far off. Yes, you're adding a bit of fuel to the intake air so the exhaust is coping with a tad more mass flow than the intake but not much.

Exhaust pipe sizes are here https://web.archive.org/web/20110903091024/http://... so going by those I suspect you're f***ed.

[Mud_]

As I feared really - thanks. The exhaust is dual 2.5", looks like there isn't much headroom there either.

I researched lots of people's ITB setups and found most compromised in one way or another...restrictive filters, hot air, dirty air, airboxes too small, small/bad shape NACA ducts, crinkly rubber pipes, etc.

The driver to go to the Y piece was to retain the over the radiator cold air intake I already have (which packages tidily), but I guess I'll have to go to filters behind the headlights or take it through the bonnet somehow.

I believe NA is more forgiving than boosted applications, but out of interest what do you make of branching one bank off another with unequal lengths of pipe? I'm struggling to find the photograph I'm thinking of, but it looks something like:

|     |
\_____\_____(filter)

My gut says go with two separate filters and pieces of pipe, but that would be less work.

I could also go to a tray intake like this Harrop, but I think it's hideous.

[KiaDiseasel]

My gut says go with two filters as well.

[Mud_]

I did look at this Y, but even though NA should fairly forgiving (I think...), I was never 100% sold on splitting the tract.

http://www.spectreperformance.com/search/product.a...

At least I've already moved the battery and airbox, so can use both front corners for filters...but some fabrication work shielding things off required.

[KiaDiseasel]

The design of perfectly flowing Y pieces whether for intake systems or exhaust collectors is quite complex and most people make a right mess of it. Any time that pipes are cut at an angle and welded back into a joint tends to cause a restriction. The best way is to flare the single pipe up into a bigger "plenum" and take the multiple pipes straight out of that. In your case the 4" would need to flare up to something like an oval 6" long and 3" wide so both 3" pipes could come out of that side by side with no cutting.

[Mud_]

Nuts to that then!

I thought there were pitfalls, and to avoid CFD or second-guessing, I figured keeping things simple was the way to win. Twin 3" intake tracts it is then

[KiaDiseasel]

If you look at your photo above and think about how two small pipes can possibly be cut and welded directly back into one big pipe the most you can ever achieve is to cut each small pipe exactly in half at a diameter and join them together to form a circle the same size as each started out. Makes no real difference how or where you cut and weld you get back to a flow area the same size as each small pipe. So yeah, no real surprise it ends up with a 3" neck under the 4" pipe. It's very basic and very daft but hey, that's humans for you. The only proper way is with an intermediate plenum and the small pipes going into that uncut.

[KiaDiseasel]

Like this...

http://www.bskspeedworks.co.uk/k-collector.html

[Mud_]

Understood

I did wonder about turning around a collector, but given the sizes involved it would have to be a custom fab. None of the exhaust shops around me are up to that, but I do know a place that makes tanks (for fluids) that may have a go. I'll stare into my engine bay for a bit before probably ultimately deciding to go with separate pipes

[PeterBurgess]

That BMW looks like a poor collector design, adding a spear point would help a lot, we saw in excess of 7 bhp on an MGB race ex manifold from welding a longish bolt between the pipes where they merge into one and then shaping each one as a continuation of the pipe until a point was reached. Spear point collector. The same would obtain if the flow was in the opposite direction. We see a lot of ex manifolds that look good on the outside and are poor on the inside. My thanks to Ollie at NMS for showing us how to do the spear point properly.
http://www.onallcylinders.com/2015/11/27/header-th...

Peter

[anonymous-user]

KiaDiseasel said:

So that just begs the question, what minimum size of intake pipe do you need for a given bhp with no power loss. It depends on a bunch of stuff but a rule of thumb is that what comes out must be the same mass flow as what goes in so something similar to the exhaust pipe size won't be far off. Yes, you're adding a bit of fuel to the intake air so the exhaust is coping with a tad more mass flow than the intake but not much.

Er, sorry but rubbish! You know what the exhaust MASS flow is, because it's the intake mass flow plus the AFR you are running (because AFRs are mass ratios.. So say you're running 10:1, then for each "10" of air going in, there is a "1" of fuel.

And even more importantly, what actually matters isn't MASS flow, but VOLUME flow, because the pressure loss of any fluid carrying object scales with the SQUARE OF THE VELOCITY of that fluid. And, the exhaust is rather warm compared to the intake. Typically, something like 500degC even under light load running, and climbing up to 900 or more degC under heavy loads. Of course, the exhaust gas rapidly cools as it expands down the exhaust system, loosing heat to the pipe walls, but typically, i'd take 300 to 400degC as the mean temperature. So as a result, compared to the intake air, at maybe 40degC (with a bit of upheat in the intake system) you're talking about the exhaust gas being a lot less dense, and hence having a much higher velocity.

Lets do an example:

2.0 litre engine at 7krpm WOT, AFR 11.5:1 with typical road engine VolEff:

Intake mass flow: 0.128 Kg/s
Exhaust mass flow: 0.139 Kg/s


Intake Volumetric flow: 0.117 m3/s
Exhaust Volumetric flow: 0.207 m3/s

So, for a given pipe diameter, the exhaust is going about twice as fast as the intake air.

Lets work out the dynamic head for say a 75mm ID tube:

Intake air, which is doing ~26m/s: 0.38 kPa
Exhaust gas, which is doing ~52m/s: 0.75 kPa

So, assuming you got NO pressure recovery, and lost ALL your intake velocity without recovering any static pressure (which is pretty dam hard to do, even zero length changes in diameter recover something like 50% of the dynamic head) your intake plenum pressure, given a 100kPa day, would drop to 99.62 kPa.



The other factor worth noting, is that by using two separate intakes, you loose the ability for dynamic tuning as a result of one bank of cyls ramming the other one! Secondary intake tuning, ie joining the banks together can be worth several percentage points of ManVolEff, so well worth having........

[Mud_]

Max_Torque said:

{info}

Lets work out the dynamic head for say a 75mm ID tube:

Intake air, which is doing ~26m/s: 0.38 kPa
Exhaust gas, which is doing ~52m/s: 0.75 kPa

So, assuming you got NO pressure recovery, and lost ALL your intake velocity without recovering any static pressure (which is pretty dam hard to do, even zero length changes in diameter recover something like 50% of the dynamic head) your intake plenum pressure, given a 100kPa day, would drop to 99.62 kPa.

So you'd run my Y pipe?

Max_Torque said:

The other factor worth noting, is that by using two separate intakes, you loose the ability for dynamic tuning as a result of one bank of cyls ramming the other one! Secondary intake tuning, ie joining the banks together can be worth several percentage points of ManVolEff, so well worth having........

I'm not sure what you're describing here - is this like bouncing pulses off closed intake valves, but into other runners?

[KiaDiseasel]

Max_Torque said:

KiaDiseasel said:

So that just begs the question, what minimum size of intake pipe do you need for a given bhp with no power loss. It depends on a bunch of stuff but a rule of thumb is that what comes out must be the same mass flow as what goes in so something similar to the exhaust pipe size won't be far off. Yes, you're adding a bit of fuel to the intake air so the exhaust is coping with a tad more mass flow than the intake but not much.

Er, sorry but rubbish! You know what the exhaust MASS flow is, because it's the intake mass flow plus the AFR you are running (because AFRs are mass ratios.. So say you're running 10:1, then for each "10" of air going in, there is a "1" of fuel.

Eh? That's exactly what I just said!

[KiaDiseasel]

Mud_ said:

So you'd run my Y pipe?

Lol. I missed any hint of an actual answer to that bit too. I'm not sure what the relevance of all the numbers about a 75mm i/d pipe on a 2 litre engine is to your situation either. A 2 litre N/A engine isn't going to need anything like that size on either the inlet or the exhaust.

Edited by KiaDiseasel on Sunday 9th October 20:04

[anonymous-user]

KiaDiseasel said:

Mud_ said:

So you'd run my Y pipe?

Lol. I missed any hint of an actual answer to that bit too. I'm not sure what the relevance of all the numbers about a 75mm i/d pipe on a 2 litre engine is to your situation either. A 2 litre N/A engine isn't going to need anything like that size on either the inlet or the exhaust.

Edited by KiaDiseasel on Sunday 9th October 20:04

The 2.0 engine was just an example that you can' use exhaust diameter as an indicator to inlet diameter, other than the exhaust needs to be, roughly, twice the cross sectional area of the intake!

The OP can run the dynamic head numbers for their actual engine to see how the Y piece will behave. Personally, i'd run it, see how it performs. (i suspect it will not be any significant restriction to performance tbh) There are also lots of tricks you can use to optimise pressure recovery, such as ovalise the pipes etc if you need to fit a "short" Y piece in.

[Mud_]

Max_Torque said:

The 2.0 engine was just an example that you can' use exhaust diameter as an indicator to inlet diameter, other than the exhaust needs to be, roughly, twice the cross sectional area of the intake!

The OP can run the dynamic head numbers for their actual engine to see how the Y piece will behave. Personally, i'd run it, see how it performs. (i suspect it will not be any significant restriction to performance tbh) There are also lots of tricks you can use to optimise pressure recovery, such as ovalise the pipes etc if you need to fit a "short" Y piece in.

The problem with the suck it and see approach is it's expensive and I might not even know if it's sub-optimal...

The original throttle body and MAF are known restrictions at these power levels, and are both 75mm diameter - this doesn't seem to tie up with what you're saying (and was part of the reasoning for my original concern).

[PeterBurgess]

We ran a 420 bhp V8 with a homemade twin entrance plenum chamber atop 4 twin choke downdraft Webers. The holes were 3.5" and sharp edged so around 60/63% efficient. Blanking off one hole to give a single 3.5" hole made no difference in bhp. A very slight pressure drop maybe as the afr went down a tad but no difference in bhp

By 'known restrictions' have you dynoed this for yourself and recorded the problem or is it what others say?

Peter

Edited by PeterBurgess on Sunday 9th October 21:28

[Mud_]

PeterBurgess said:

We ran a 420 bhp V8 with a homemade twin entrance plenum chamber atop 4 twin choke downdraft Webers. The holes were 3.5" and sharp edged so around 60/63% efficient. Blanking off one hole to give a single 3.5" hole made no difference in bhp. A very slight pressure drop maybe as the afr went down a tad but no difference in bhp
Peter

Thanks - is there some resource for the efficiency of different hole edges?

If the AFR went down then surely 1x3.5" was insufficient (though perhaps nearly adequate)? I don't know how sensitive my engine is to AFR, but I command variously around 12.5-12.8 as a function of RPM since I'm EFI and I can...

e: 'known restrictions' - internet lore, but based on LS engines being common as muck so good knowledge-base and lots of interchangeable parts. Not personally proven admittedly.

[227bhp]

PeterBurgess said:

Spear point collector.

I think you might mean merge collector?