The V8 Owners Forum -

I very much appreciate that this site is about V8s but I have found a lot of good general engine advice here and am hoping you can help me with a different sort of engine.

I have a Haflinger flat 2 cylinder 4 stroke engine that is 762cc (381cc each cylinder), revs to 5000rpm and need to install a remote air intake. The engine was originally 650cc and the original remote intake piping looks a bit small - smaller than even the air intake at the air filter.

So does anyone know a formula or how to work out what size plumbing I would need to provide enough air at max revs.

Thanks

Garry

Hi Gary
Whilst I dont know of a formula ( and there must be one somewhere ) on how to work out your intake diameter I am currently making a new air intake system for my motor
My engine is a fair bit bigger in CC than yours but I am using two 3" diameter intake tubes with cone shaped K&N filters at the end with another K&N on the top of the housing.
This will provide more than enough airflow into the motor at top end revs.

For your application I would suggest that you look at an intake diameter of 2" and as smooth a passage for the air to travel as possible to the carb.
Linear air travels much faster than turbulent air and will give you what you are looking for.
Someone fair cleverer than me may well be able to give you a formula on here

In the meantime I hope this helps .

cheers
Paul

Thanks Paul for your information.

As I understand things it is the size of the volume that individual pistons rather than the overall size of the engine is concerned as only one piston is sucking at any given time not all of them.

So at any given instant my engine is sucking as much air as a 3 litre V8 - though there would be many more input pulses of air lined up in the inlet pipes in the bigger engine.

The issue I have the remote intake is 3m away from the engine so it has to pull the air a long way and all the pieces of the pipe are joined with rubber connectors. The current pipes are 1 1/2" so your advice on going to 2" might be the go.

Thanks

Garry

garrycol wrote:I very much appreciate that this site is about V8s but I have found a lot of good general engine advice here and am hoping you can help me with a different sort of engine.

I have a Haflinger flat 2 cylinder 4 stroke engine that is 762cc (381cc each cylinder), revs to 5000rpm and need to install a remote air intake. The engine was originally 650cc and the original remote intake piping looks a bit small - smaller than even the air intake at the air filter.

So does anyone know a formula or how to work out what size plumbing I would need to provide enough air at max revs.

Thanks

Garry

How much power will it make ?

Various googling suggests even a 26mm restrictor can easily make 190hp

If you look at Formula SAE, they are 610cc engines max, breathe through a 20mm restrictor and seem to make around 100hp.

https://en.wikipedia.org/wiki/Formula_SAE#Engine

So it's easy to see how a lot of air plumbing is oversized, although obviously an unrestricted engine will make more power, but it's a good guide.

The whole subject of airflow is a minefield. If it were a simple extractor fan, it would be fairly easy to work out the trunking size needed.
But when you go to varying speed and things like a throttle valve in the flow, it becomes more like a musical instrument.

On my SD1, the air filter inlet is tiny compared to the single throttle body. It just looks plain wrong.

762cc @ 5000 rpm with a VE of 127% @ 5000 rpm = 112cfm of air required.

Dont get too deep into theory right now Garry!

The part of your induction system that meters the correct amount of air into the engine is the thottle body.

Unless you will have an air inlet pipe that is 10 feet long or more you wont need to get involved with doing fancy sums to work out pipe resistance etc.

If you have a thottle body thats say 40mm diameter then fit an inlet pipe of 60mm diameter along with a bigger air filter. ie 50% bigger. This will give all the air the enigne needs at max VE + it will give a slower air speed in the inlet pipe and make the filter last much longer.

How does the engine run on open throttle body? If it runs ok then you dont need to tune the inlet pipe I guess?

mgbv8 wrote:762cc @ 5000 rpm with a VE of 127% @ 5000 rpm = 112cfm of air required.

Dont get too deep into theory right now Garry!

The part of your induction system that meters the correct amount of air into the engine is the thottle body.

Unless you will have an air inlet pipe that is 10 feet long or more you wont need to get involved with doing fancy sums to work out pipe resistance etc.

If you have a thottle body thats say 40mm diameter then fit an inlet pipe of 60mm diameter along with a bigger air filter. ie 50% bigger. This will give all the air the enigne needs at max VE + it will give a slower air speed in the inlet pipe and make the filter last much longer.

How does the engine run on open throttle body? If it runs ok then you dont need to tune the inlet pipe I guess?

I would guess things could be optimised with a tuned induction tract, correct length and diameter.

Whether or not any budget would stretch to do such testing to yield the small gains is another matter.

But for such a small engine I'd say even a 2" pipe would never pose a restriction...even at 10ft long lol
Although it would make an interesting test

Thanks for all the information - some a little more complicated that I was expecting for such a little engine.

The engine only produces about 30hp and runs a 32mm dual downdraft Zenith Carb. The carb and air cleaner inlet are 2 1/4" diameter and I do not have the original remote air intake connected which as mentioned is 3m long and 1 1/2" in diameter - much less than the intake to the engine.

The remote air intake is needed as the engine is in the rear of the car and sucks in a lot of dust so the remote air intake is installed to allow air to be sucked in from the front in clear air.

From the discussion it would seem that 2" pipe is about the right size to put in so I will look at that - is the same size as local pvc plumbing pipe so is doable.

Thanks for all the input.

Garry