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).

[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.

[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.

[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

[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

[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?

[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).

[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.

[Mud_]

I'll reread all the comments later, but to ask a naive question: neglecting any tuning effects, is moving to oversized pipe just equivalent to running without airboxes? NA ITBs remember.

E: edit to add, I appreciate the responses, and yes, fwhp = flywheel horse power... I assume that is the more relevant metric.

Edited by Mud_ on Monday 10th October 12:28

[Mud_]

Reread all the posts and nothing to add, just appreciate the comments

Stan Weiss said:

The idea is to find the area in the intake system that is the restriction. Increasing air box size or piping size will do very little if the throttle body does not have the needed flow.

Stan

That's full circle on my fears from the opening post

Given I'm running a relatively mild cam for my supporting mods I will probably put in a bigger cam later...so I will see how big the airboxes end up, but I could probably go 2x4" with no major issues. Sounds like overkill given the numbers in this thread, but I'm struggling to see a downside - there's no MAF, and of course the throttle bodies are close to the intake valve so lots of lazy air outside the trumpets doesn't seem like a terrible thing...?

Max_Torque said:

Precisely. And go and look, and you'll find an awful lot of proddy engines running throttles or around about 60 to 70mm diameter.

And of course they work fine, with very little inlet restriction, because as i highlighted previously, the dynamic pressure of a flow at around 25m/s is still rather small, as air really isn't very dense.

Unfortunately, the aftermarket "bigger is always better" approach tends to be applied to lots of after market tuning, resulting in oversized slow flowing, and poorly optimised intake systems......

As above, are you talking generically, or do you think it's true of my particular system? 5.7 litre cross-plane V8 spinning to say 7k rpm with individual throttle bodies, making around 500hp at the crank.