Exhaust port = exhaust size..?

[Dalland]

I start this thread with a sentence I read on the STP forum.
"Larry Widmer once said something along the same lines, that if you port the exhaust out to match the header, throw away the heads, it will kill torque."

Right now I am running a 2275, with 42-37 valves self ported heads, fk-8 and 45 drla, with the BAS SS exhaust.
I am planning to buy a new exhaust from Turbo Thomas, the "python" version with the "peeshooters".
This will be with 42mm header.

Well the question is about matchporting the heads to the exhaust, why shouldn't you do it?

[Bruce]

It doesn't matter why, it's been proven countless times that a tiny exh port still makes great hp.
If you take a stock exh port and smooth the walls a bit, then radius the short side corner, that port can make over 200hp.  Grinding any more gets you nowhere.

[Dalland]

But which magic voodoo power is it that does so that you loose torque if you have a smooth transition/no turbulence..?

Much more interesting to actually learn something.

[Dalland]

It can't be that no one knows why?
Where have all the head gurus gone?
This must be one of the more interesting things about head porting..

[Shag55]

It's all in the airspeed! A good exhaust port should have the least turbulence and air speed should be kept up around 300fpc.
Regardless of header used it's not the port match that kills bottom end torque. It's the slow gasses moving out that can't create a vacume effect to draw all the gasses out. Most headers are to big for the application. On a lower HP street  motor like 200-220hp it would be  best would be to use a 1 1/2" at the head and a port to match. Then open up to 1 5/8" after 5-6" this will keep air speed up and turbulence down. You could even bump up to 1 3/4" after another 5-6", this will help the top end and pick up a few extra ponies.  One can then tune header length for the RPM desired.
Another note would be that an open header can be on the smaller size port as it doesn't have the back pressure from a muffler.

[Dalland]

Thanks!
Just one question, regarding thermo dynamics, when the exhaust gases is cooling down they take up less space, so why bigger and bigger pipe?
Is it as easy as that the gases don't got the time to change temperature more than, lets say 100 degrees Celsius?

So the ideal solution would be to port the heads to the exhaust size (1 3/8" or 1 1/2") then a smooth transition up to 1 5/8".
I haven't understand why using bigger and bigger pipes yet, so if anyone have a nice way to say it please do.

[Sarge]

I don't have any dyno figures or flow bench numbers; just seat-of-the-pants experience.  I agree with Shag55's post especially the part about most headers being too big for the application.  After running around with an 1 5/8" system on a 2017 for a dozen years, I recently had A-1 build me a stepped 1 1/2" to 1 5/8" header and picked up a lot of bottom end performance while still pulling strong to 6500 rpm.  Fumio suggested an 1 1/2" exhaust when he ported my heads years ago; I wish I'd listened.

[Bruce]

regarding thermo dynamics, when the exhaust gases is cooling down they take up less space, so why bigger and bigger pipe?
Is it as easy as that the gases don't got the time to change temperature more than, lets say 100 degrees Celsius?

There's no time factor when you're talking thermodynamics.  It's instant.  Consider the burn in the combustion chamber.  Some of the heat of combustion is converted into work pushing the piston down.  The more work the burning mix does, the colder it is when the exh valve opens.

Your initial statement made me think a bit.  What if the exh is expanding on it's way out from the chamber?  First into the port, then larger into a small short primary, then into a larger primary like you and Shag said.  As it expands, it's pressure reduces, creating more of a "vacuum" to pull the next exhaust charge out.  Of course, it will take someone like Johannes to figure out the step sizes and lengths.

[Jon]

Engine analyzer does a pretty good job of testing stepped headers. You might need a stepped header if you cant make the primary tubes short enough to fit you desired rpm.
The reason for the stepped header is to my knowledge to make the pulses in the exhaust turn. Pulses turn when they reaches  the "open", normally in the collector, but here it turns in the pipes. And that's why you step up.
Burns stainless is also providing a good exhaust calculation service.

[Dalland]

Thanks, very interesting.
Understanding more and more that exhaust system is much more than just nice pipes.
But I still don't know why we shouldn't port the heads out to the exhaust size, something I would do to avoid turbulence...

[Shag55]

If you do not open the port size to match the header it is best to put a radius or small chamfer to open up at the edge to reduce turbulence yet help to build a pressure barrier there which helps in reversion. This is why most keep the port smaller. When reversion acures there can be no vacume in the port. I believe the short step up in the header as this gives more time for acceleration of the gases and increases vacume. As alwas the smoother the transition the better. We are already plagued with such a short port and quick turn so better to help ease when possible. Or just make angle flow ports!

[Dalland]

Do you actually get reversion in the exhaust pipes?

[Airspeed]

Do you actually get reversion in the exhaust pipes?

Yes, a lot!
Stepping up a header size may just work so well because it hinders the ill effects of reversion for all we know.

[TexasTom]

I start this thread with a sentence I read on the STP forum.
"Larry Widmer once said something along the same lines, that if you port the exhaust out to match the header, throw away the heads, it will kill torque."

Right now I am running a 2275, with 42-37 valves self ported heads, fk-8 and 45 drla, with the BAS SS exhaust.
I am planning to buy a new exhaust from Turbo Thomas, the "python" version with the "peeshooters".
This will be with 42mm header.

Well the question is about matchporting the heads to the exhaust, why shouldn't you do it?

Dalland,
Without spending the day in the Engineering Library, I would say the reason to keep the exhaust port small is to: promote, balance and maintain the highest velocity (up to a certain point) of the exhaust flow. This is in order to absolutely reduce the possibilities of reversion back into and promote scavenging of the exhaust from, the cylinder. This high-speed flow will also allow the pulses to synchronize at the collector to even further boost these effects in the designed powerband.
So simply put, it's to maintain velocity in the exhaust tract, the same way too large of an intake tract and port will kill power, for a given size engine. It's all about efficiency and airspeed, going in AND coming out.

Make sense?
TxT

[Dalland]

Hehe kind of..
But then if I am going to by a system with 42 mm primary pipes, since money is a factor, what should i do with the port?
The valve is 37, so the seat is probably 35 mm...

[TexasTom]

I think you'll be OK with that setup ... 45 would be too big for sure.

Leave the port alone!
The most important shaping to be done on the exhaust port is to smooth the small/short radius of it's curve/turn ... it's often quite sharp and inhibits flow. You can feel it with your finger inside the port.
Additionally, I like to smooth the casting so the walls are super smooth, but do not enlarge!
The 'step' up in diameter, as the exhaust flows out of the head and into the header primary pipe, actually promotes flow too, by relieving pressure in the port ... kinda like opening the door to a hot oven(?) ... but it's still important that the 'door' isn't too big.


Cosider the two photos here ... I ripped them off from the FMF Products website.
These are head pipes they make for Off-Road and Motocross motorcycles.
The first is basically a stepped header ... as the ehaust steps up in size, the pressure is relieved a small amount and flow potential increases. Good for Off-Road performance where flow is increased but torque is maintained.
The second is more used in high horsepower Motocross machines where maximum power is wanted. As the pulse from one exhaust stroke is pushed out of the cylinder, funneled down into the port creating pressure and velocity, then expands a bit into the primary increasing flow, then expands out into the open chamber where the exhaust is literally drawn, like the hot air from the oven. The initial primary pipe is just long enough so that the charge from one cylinder of exhaust air fits before being drawn into the expanse of the large chamber.

Pretty cool technology from another venue ...
TxT

[58vw]

two strokes...its all in the pipe...

[TexasTom]

These are for 4-stroke engines!!!
 

[karl h]

since we are talking motorcycle headers: i remember a harley exhaust with anti reversion rubes, i think that means a smaller pipe protruding into the bigger header pipe a certain lenght. anyone tried this on a vw?

[Nick]

... as the ehaust steps up in size, the pressure is relieved a small amount and flow potential increases.

Hi TexasTom,very interesting ! One point is not not 100% correct: wenn the exhaust steps up in size, the velocity slows down and so the pressure raises !!!

Keep in mind that in a closed system:

The higher velocity, the lower pressure. The lower velocity, the higher pressure

Greets,

Nick

[Neil Davies]

since we are talking motorcycle headers: i remember a harley exhaust with anti reversion rubes, i think that means a smaller pipe protruding into the bigger header pipe a certain lenght. anyone tried this on a vw?

I've seen these in an old magazine from the 70's on V8s. I suspect it wouldn't work so well on a VW motor because of the sharp turns that the exhaust tubes make straight from the head.