Don Terrill’s Speed-Talk

This is question regarding solutions to high overlap cams. These cause a lot of reversion at lower rpm, which dilutes intake charge and reduces torque output.

I was thinking about the following in the primary exhaust pipe.



If this would work is there any way to determine at which point this venturi should be placed after the head ?
Also, how much smaller should the cone be in diameter versus primary diameter without causing too much restriction ?

http://books.google.com.au/books/about/ ... edir_esc=y

This book might be a good place to start.

Hysteric

Reminiscent of the old "Power Cone" insert for Harley exhaust pipes.

It did help build low end torque on open pipes. There were a few variations.

25 some odd years ago a header company ( maybe cyclone ?? , cant remember for sure ) had their " AR " ," Anti Reversion " series of headers with cones like that built into the header flange IIRC......made all sorts of claims....Some people claimed they worked others didn't , still others said you had to run an H pipe in the exhaust system in order to make them work.....they eventually went the way of the rotary dial phone.

No band aid needed, you just build the application for it's specific use, and you don't have to worry about issues to band aid. If your building an engine deliberately with a camshaft that is to large, knowingly, than shame on you. If you happen to have been delt a camshaft un-knowingly that yields the engine low cylinder pressures, than the choice of who you let grind or pick that profile was poor. a large problem in the industry, and factors that should be evaluated in the equation are not, and factors that shouldn't be in the equation are. If the cylinder pressures are in there correct area, and as long as you have the exhaust events timed within their respected area, you will not have a problem with reversion, and therfore no fancy reversion gadgets are needed. There is no doubt that lobe events are like trying to do a rubiks cube puzzle, and it can be pretty tricky at times. There are a million different combinations that can be had.

The right choice of valve timing events and you'll be fine. As far as headers. I have never really seen a header that was on the longer side for primary hurt and application in it's torque curve output and broadness, although many primary tubes that I have seen in the industry are just to short. Easier to build? Save some money? Need to fit into a certain box? Not quite the understanding of how an engine operates?

Here is a picture of the torque-cone inserts:



When I got my bike it came with on in the box of spare parts. Never even thought about trying it - I built a pipe to PipeMax specs which has a smaller 1st primary. Still waiting to ceramic coat it, but I have no doubt it will run better. Best I can tell is that the torque cones have been around for 25-30 years, now.

Exhaust gas flow reversion rarely occurs. What does occur is pressure wave reversion. If it is an expansion wave during the exhaust stroke then all is good. If it is a compression return wave during overlap, then all is not so good.

The problem is that all the above conditions exist in the same exhaust pipe depending on rpm and throttle position.
What would be more useful would be an exhaust valve, similar to the Rave valves used on some snowmobile engines.
The valve, or exhaust damper could then be altered to suit operating conditions.

Before worrying about reversional exhaust gas flow, it might be useful to install a pressure sensor in the pipe connected to a scope to see what is actually going on.

Fixing the wrong problem occurs almost as often as incorrectly fixing the right problem.

The more the exhaust system restriction (mufflers) the worse the effect of the exhaust gas dilution.
Air always moves from high pressure to low pressure, especially during overlap, at low rpm.
The cam that is just right for open headers and power is not usually right for a closed street type exhaust with mufflers and tail pipe.
A different cam (better suited and matched to the closed exhaust system may actually improve car performance. Often it does. (less overlap and duration, best exhaust open close events may change)
The closed exhaust system changes the scavedging tuning of the headers, You can optimize for open headers or your closed exhaust but not both, with one cam.
A side benefit is a nicer idle and low speed drivability. A "street cam" has to do a lot of things well.

Don't assume your cam and headers are matched-optimized, now.

I have to work with what I have.

Belgian1979 wrote:I have to work with what I have.

No you don't....you can come to that conclusion all by yourself, when the time comes.

Just helping you not waste time and money on voodoo stuff that will not fix that.

Stepped headers and primary tubes a little bigger than the ports may do some good on reversion pulses. The steppd areas on the outskirts of the circumference are believed to work. I "always" (on the three sets I have welded up...) make my headers primaries a bit taller than the ports, hoping the waves will be stopped/ reversed as they meet the wall.

Probably some here does have some numbers to show, to both of these "band-aids"? I have some numbers in one of my books, reliable or not.... I'll look up and see what it says, for curiousities sake..

As said, cam is the heart of the engine, and wrong for the combo will turn performance down.... If you're unhappy about it, take it to the strip and make it scream at those high RPM's......

This is commonly used. Also a larger exhaust than port.

englertracing wrote:

This is commonly used. Also a larger exhaust than port.

A change in area, in any direction fo flow, will reduce the flow of air; in both directions.
The degree of flow reduction is dependent of the area ratio of the pipe sections.

englertracing wrote:


This is commonly used. Also a larger exhaust than port.

As I said earlier.

Would you say these chambers would have a mission on the primaries only, or would they do a job if done on the secondaries (of a tri-Y) or even in the collector(s)? Imagine an O/S collector with all 4 (or 2 secondaries) having alot of space surrounding the dumping pipes.

I found no numbers on larger primaries VS port, but some numbers on stepped headers altough unvalid as primary pipe lenght was changed as well as the stepping... There are no clear trend in the test, difference VS a non- stepped are somewhat non- predictable across the RPM band. However, the largest %-wise difference was at the lowest recorded RPM (5500). This was a 9000 Opel 2 L 4. (A.G. Bell, 3. edition,2006, page 288).

englertracing wrote:


This is commonly used. Also a larger exhaust than port.

Wouldn't that be causing some kind of turbulence in the exhaust ?

I think the cone types would be more effective speeding up exhaust and reducing exhaust waves returning. Just an opinion.