how much exhaust port step on header?

[speedy]

ive tried to do a search on this but i cant really find much. i was wondering how much reverse step (pipe bigger than port) i can have on a street motor before it looses power. ive heard a bit of a step can be good but how far can it be taken? i have a round header plate on a square exhaust port, it has up to a 3-4mm step. thanks

[Ed-vancedEngines]

Hi,
I may get some flak about this;

Take everything you learned in magazines about port matching being necessary for exhaust ports and throw it out the window. Keep the info about port matching intakes though.

In my opinion the header primary tube can be any size that works for the application as long as it is the same size or larger than the port exit from the heads. I personally like the header flanges and primary tube to be larger with a mis-match. That gives it sort of a reversion dam to resist flowing backwards back into the chamber. I do not like for a restriction to be present though that has the exhaust port to be larger than the header plates and primary tubes.

I have seen too many guys riun heads by taking a good flowing exhaust port that was professionally done and because their header was bigger and believing the magazine article bible stuff opened up the exhaust to fir the headeer and lost horsepower and lost the flow the exhaust ports wre making before.

Bigger is nice at times, but biger is not always better. Too many times bigger is just too big.

In my opinion, the engine does not care that you are dumping from square exhaust ports into a round header. It might even like it.

I have a race motor in a customer's car that the exhaust port is a maximum of 1 15/16 at the exit in the largest area. It is not a round port or a square port. That head was ported by one of the best professional head guys there is and because of the engine horsepower we are dumping it into a 2 1/2 header tube. It is a extreme nitrous engine. If it were not nitrous but was an all motor engine, we would still be using a 2 3/8 pipe which is still a bunch larger than the port.

Try to get headers that are right for your size engine and power range and forget all the magazine trash about having to match the exhuast ports. Just don't be smaller than the ports of the exhaust with the header.

Also if this is a street engine or mild or medium competition engine, who cares if it is exactly a tuned header or not. Buy the header you can afford even if the tubes are not 100% equal.

Now guys shoot me with both barrels from your guns, if you disagree with my advice to this man. lol.

Ed

[#84Dave]

Ed....... no shots fired. Normally aspirated, no power adders, Blair says, with data to back him, use 1.1-1.2 area ratio on the exhaust. With the pipe ID being the larger. The step will help impede gas/particle reversion but have little/no effect on the desired negative pressure tuned wave back at the chamber. Might not have that much affect on your low rpm 'streeter', but your race engine should like it!

Dave

[speedy]

thanks guys.

[KennyM]

Some years ago I did some extensive exhaust reversion work on a single cylinder alky engine and found power right at the exhaust port using a venturi plate facing outward.
This all came about by pitot probing the exhaust port with the engine on a flow bench.
The exhaust valve was installed and blocked open the normal cam lift amount.
The port face showed inactivity and even a vacuum at the 1 to 4 o'clock positions.
I drew the port opening on paper grided it off and applied numbers representing activity.
Later made plates out of tin as an interference meduim that were offset on the port face as the activity showed, then ran the engine on the dyno.
The results was a broader power band (not just a little but a lot) with no loss at the top end and a gain in the low to mid ranges.
When theses things are done, the carburation has to also be complimentary to get the max benifit from these tricks as well as the rest of the exhaust tuning. (single pipe diameter and length) in this case.
This is also senaitive to valve overlap.
The indications were that more overlap could be used for top end without losing the bottom within reason.
Usually these tricks work better on a poor port and not as well on a good port.
I think there is lots more to be learned about this kind of function.
Wonder if there is power to be found on a restrictorr plate motor?
It worked on that setup as well for me.
On a restrictor plate motor; nothing like seeing the power go almost straight up, flatten off, hang, then begin to drop fast when the airflow limit was reached at the rpm limit.
Was a lot of fun playing with these things and seeing the results on the track against others that didnot have such tricks and the mayhem it sometimes caused when they could not see or find anything.

[exhausted]

Hi,
I may get some flak about this;
Ed

It is very common for me to build a header for my customers that has a tube diameter that has an area reduction from the port. Even in Cup, the area of the tube was as much as 10-11% smaller than the port. I currently build some headers for some well known stock and Super stock cars and do the same thing. Even production engines have exhaust ports that are too big, not to mention head porters. The transition from the port to the pipe is critical to being able to make maximum power. I am not saying that reversion damns can not help, I would say that they have their place elsewhere in the system. A properly designed exhaust header really has very little need for a reversion dam in primary pipes. The velocity of the exhaust gases are the highest closer to the valve. Every rule of flow that you use on the intake side is just as important on the exhaust side. The reality is that most headers and their builders use a larger tube size off the head because it is the easy and cheap way around the flow issues at that point, and people assume that the increase comes from the larger pipe and the step. It is not the case.

[exhausted]

Some years ago I did some extensive exhaust reversion work on a single cylinder alky engine and found power right at the exhaust port using a venturi plate facing outward.

The effects you have seen are true. The problem is that when you put a bunch of cylinders together into the same exhaust system, the flow and pressure waves can not be controlled as cleanly and effectively as a single cylinder. I continue to find that anything I can do to "minimize" the magnitude of the pressure waves in a system, the more power I can make. Pressure wave tuning works great on single cylinder enignes, but from there it all goes out the window and the pressure waves hurt us more than help.

[jmarkaudio]

For most apps I think Ed has the right idea. Custom headers have there place with racing that requires the most HP at all costs. Make sure the header port does not block the heads port.

[exhausted]

For most apps I think Ed has the right idea. Custom headers have there place with racing that requires the most HP at all costs. Make sure the header port does not block the heads port.

Every rule of flow dynamics are applicable. You are absolutely correct. any mismatch or negative step will hurt flow and therefor power.

[nitro2]

Some years ago I did some extensive exhaust reversion work on a single cylinder alky engine and found power right at the exhaust port using a venturi plate facing outward.

The effects you have seen are true. The problem is that when you put a bunch of cylinders together into the same exhaust system, the flow and pressure waves can not be controlled as cleanly and effectively as a single cylinder. I continue to find that anything I can do to "minimize" the magnitude of the pressure waves in a system, the more power I can make. Pressure wave tuning works great on single cylinder enignes, but from there it all goes out the window and the pressure waves hurt us more than help.

Generally what we have seen is that maximizing the initial suction wave at valve overlap is a good thing, but for some multi-cylinder engines the effects of resonance and the effects of other cylinders can interfere with the "planned" suction at overlap negating (or worse) the benefits of the "planned" suction. Sometimes the best solution is to minimize the waves. In any event unless you can "see" the waves (via pressures) its a tough thing to conquer. Even when you can see the waves, you still have to figure out what you are going to do (physically) to get things where you want them.

Maximizing exhaust flow for nitrous, nitro, supercharged engines is very important but often not given an appropriate amount of attention. Exhaust ports in general are more or less adequate for NA gas engines. Once the engine has a power adder flow must increase. There are numerous tricks that can be used to get flow increases. Oversized headers are one way, though there are other ways that will flow even more.

Clint Gray
TFX Engine Technology Inc.
(Combustion/Intake/Exhaust Pressure Analyzers)
www.tfxengine.com

[jmarkaudio]

How about your engine specs and current header size?

[exhausted]

Maximizing exhaust flow for nitrous, nitro, supercharged engines is very important but often not given an appropriate amount of attention. Exhaust ports in general are more or less adequate for NA gas engines. Once the engine has a power adder flow must increase. There are numerous tricks that can be used to get flow increases. Oversized headers are one way, though there are other ways that will flow even more.

I built a set of headers for Harold Martin and his Pro-Mod. 700 something cu.in., FI and Nitrous. 421 header 2.375-2.5" @about 18" and a 2.625 secondary and the collector choke was 3.25". He still runs them. We tried some larger 4into1's that other people are running and he slowed down.
Many people are suprised how big headers do not have to be.

[Erland Cox]

Hello Calvin! How long was the secondary pipe and the collector? Do you have any pictures? Erland.

[exhausted]

how do you put up pictures?

[exhausted]

They were about as short as possible. Including collector about 10" oal.
Collectors at that size are rather long. Will post some pictures on my mail server when I have time to get up on that learning curve.