Intake port and manifold size

[ottobevis]

Hi,
I've been searching but have not been able to find information on how to calculate the ideal intake port and manifold area (diameter if round).

What I am looking for a formula, ratio or chart that determines what size intake port and lower part of the intake manifold should be used based on other variables. For example, a calculation based on carburetor venturi size (and possible the throttle body size) and valve curtain area.

I know there's other variables which would effect this such as valve lift duration, head flow rates, additional porting work around the valves etc. A simple estimate would be suffice.

My carbs are dual 48 DRLA's with 36mm venturis (may increase the venturi size to 38 or 40mm). The intake manifold tappers around half way down to 29mm diameter. The intake ports on my heads are 33mm. I have 42mm valves with 12mm of lift (0.468"). Are my manifolds too narrow at 29mm? I've seen posts mentioning intake manifolds should get narrower to increase air speed and fuel atomization, that makes sense. It may improves torque at lower RPM's too. But at what point does it become too narrow that it restricts the maximum flow of the carburetor and/or valve curtain area?

[Brian_e]

This is from the Superflow manual written years ago, and is still very applicable. I have the manual at home, but I borrowed this pic from Modok's gallery.

The first thing you need to do is figure your minimal cross sectional area based on your displacement and how high you are expecting to rev the engine. Then you will know what size valve will get the job done, without being larger than necessary. After that you can design the rest of the port.

Brian
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[FreeBug]

Check out the Automotive Calculators at Wallace Racing. It's there someplace.

[Wreck]

There is also program called "Pipemax" . It has a calculators that will give you different dimensions for port /manifold diameter (CSA) depending on low torque verses higher RPM power .
It also gives information on exhaust systems, diameter and length.

A bad picture similar to what Brian posted ,this is from David Vizard's " How to port and flow test cylinder heads " a very good book .

the bowl area under the valve is the expansion area , then there should be about 2 inches of the high speed section ,starting in the port and into the manifold .(minimum cross sectional area) then the taper out to the top of the manifold . Top of the manifold should be larger than the throttle plate to help with reversion .
You have to allow for the valve guide area in the CSA .

I've only used these theories on a type 4 motor that I'm happy with the outcome . what I'd like to know is what happens when you rev an engine past it's theoretical "choke" RPM ? i.e a 2276cc with a 42mm intake valve will start to choke just over 5500rpm according to Wallace .

[ottobevis]

Thank you both! The diagram and Wallaces calculator will be very helpful.

I know this sounds back to front but is there a way to estimate what maximum RPM my engine was built for? I purchased the engine used and the PO and builder have unfortunately both passed away.

These are the specs. If anyone would give me an estimate/guess it would be helpful.

AS21 case
94 x 82 (2276)
9.5:1 static compression
CB G04 044 heads (ports appear unmodified)
42 x 37.5 valves
60cc chambers
0.045" deckheight
1.1:1 rockers.
Chromoly pushrods.
Cam model unknown. 0.426" lift at cam (0.469" lift at valves). Approx 260 to 276 duration at 0.050"
Engine came with dual 48 DRLA with 36 venturis (can change this)
1 5/8" exhaust
Crank unknown
Engine was built for a light weight buggy with short gearing. I plan to run it in a full weight car with stock gearing using 91 octane gas

[Wreck]

I just put some guesstemits into Pipemax , this is what it comes up with , be interesting how it compares to anyone with real world experience on a similar combination .

If you click on the picture it will open to show RPM data .

[ottobevis]

Wreck, thank you very much for taking the time to put these figures on Pipemax.

Based on intake diagram (45 vs 90 degree angle) I will reduce the 0.85 factor on Brians diagram to 0.70.

The Pipemax figures are very interesting. It indicates the valve curtain is limiting HP. And it is way over camed. Maybe this was intentional and has something to do with short gearing and being in a light weight buggy (it was built by a VW shop with parts sourced from CB performance so I figure they knew what they were doing). I will see if I can swap the 1.1:1 rockers to 1.25:1 to increase valve lift.

[Wreck]

the minimum CSA to valve ratio is something I'd like to learn more on from the people that really know this field with our design engines . The straighter the port , the higher the percentage . Think of an old V8 (mostly what these programs are written for) if you look in the port you can't even see the valve .

I ended up with an 85% ratio on my type 4 engine because in my way of thinking the port would be the longest restriction . It is a large motor 2600cc but even with a cam similar to yours it still has good low torque and is surprisingly economical . I matched the choke rpm of the valves to the port choke , about 6300 in my case .

[modok]

[modok]

[Wreck]

It would be nice to have a dyno and a lot of time . I just look at some combinations that make power past what should be the valve curtain choke point and wonder how they do it .

I'm just about to strip and inspect my engine to fix an oil leak and wonder if I can get more power with slightly larger CSA , going from 40 to 41mm diameter port and moving the theoretical choke point up a couple of hundred revs .

Manifold length and tuning reversion is another thing I want to play with as well ,
With the help of the various forums and books I laugh at engines I build as a young bloke . the work I would have done ,would have lost horse power not found it

[modok]

I have seen several calculations to define choke, with varying levels of complexity.
if Hp is continuing to increase after you hit the choke point of your PICT venturi.....your breaking reality. BUT

You don't necessarily hit it all at one time. think about that.
At first it would be a MOMent only at peak piston speed, and the % of time flow was choked it would increase as rpms went up....depending HOw it flows.

skinny straight runner you might hit it like a wall, choke at the venturi would be like hitting a ceiling, but a tapered runner with the MCSA being way down, not so much.

[ottobevis]

Thanks Modok, matching the choke point of the manifold tapper to the choke point of the valve seems logical. I'm assuming my heads are what are considered good flow (they are CB G04 044 heads from 1999, I can't find specs on their flow rate). I understand the taper of the manifold should be around 86% of inlet valve diameter. My valves are 42mm, so the tapper should be 36mm. My manifold diameter tappers to 30mm, so I should widen them to 36mm right?

I was thinking the valve lift would effect the above percentage, more lift, more flow, higher percentage. Probably impossible to figure out without head flow information though.

Brians diagram suggests the venturi diameter should the same as the tapper, while the carb bore the same as the valve. I have a set of 44idfs so should I use these with 36 venturis instead of 48 DRLA's with 38 or 40 venturies? If the tapper diameter remains at 36mm (the choke point based on the valve) then am I correct in saying there is probably no power increase to be had in using larger carbs (larger than a 42 bore with 36 vent)?

As you may tell I'm working this out using the 42x37.5 heads I already have as the starting point. I don't plan to change these but can change the manifold/carb/vent.

[modok]

Manifold I would shoot for 32x34 as a absolute minimum
maybe 33x36 would be good
It will probably need to be oval in the bottom half of the manifold so you don't end up with the sides of the manifold too thin.

34-38mm venturi should work well with 42mm valve size IMO.

[Jubilee Kafer 1985]

I have a question ...

On a single carbI, dual port manifold ...

What happens if you remove some of the aluminum web,
from between the two ports
on the intake manifold end castings, just before where twin runners enter the head ?
Say ... A 1/2" hole, 1/2"up ?
Would that make for more available mixture supply above each valve,
for more intake valve flow ?
Maybe calm some reversion ?
Thanks
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