Well, I'm not an engineer but just from looking at a N-47/P-79 head, I don't think your theory is a bad one. My guess would be to have the liner only at the end of the exhaust port where it exits the head instead of being totally taking up the bowl area behind the seat. On the other side of the coin, having the sleeve there I figure is helping to increase velocity, making it a straight shot out of the port. Any volunteers wanting to hack up a head and find out?

Dave

 

They Create a ..............

... Venturi Effect .............

for more rapid flow with probably an undiscernible lowering of pressure.

....pressure loss assists scavenging and at the same time is making "reversion" more difficult due to increased velocity forward flow through tubes.......

 

Trying to understand the problem first?

Is reversion caused when the piston is on it's down stroke and the exhaust valve is still open thus the piston sucks the gases back again?

Or is it because a high pressure exhaust pulse from a neighbouring cylinder blows high pressure air into the manifold and this pushes into the open exhaust valve of it's neighbour when it's neighbour's piston is near the top of it's stroke and the valve is still open.

 

Someone just posted flow numbers on a clean square port vs a "lightly smoothed" round port and the round port was -better- in flow. Takes about 3 minutes per port to clean up the seat to liner area and these flow as well as any square port unless it's had RADICAL porting done to it. The stock type round port headers are much bigger than the liner hole anyway, no reason to use a sqaure port header, they might leak.

Bottom line is: Now you don't have to dream up reasons why your round port engine is OK.

 

Or you could just enlarge the flange area of your header/manifold, the port itself acts as an AR tube.

 

those liners do help on the flow reversion because they produce a conditon in the exhaust port which will yield a greater head loss if the flow revered, on the other side of this argument the flow crossection is greatly reduced so it will cause a loss of high end while giving back some of the low end.

 

To answer Philips questions...

I believe that a useable definition for reversion is...

"Any flow in the reverse direction of normal intake and exhaust flow. This can include any part of the intake manifold, cylinder-head or even the exhaust manifold and port. Any reverse flow here is reversion."

It dilutes the intake-charge with exhaust gases!!!!
I don't think this is a good thing.

Except on some very-late model cars with optional hotrod engines...They actually stuffed-in cams with enough overlap to produce some reversion.
This acted as a sort-of built EGR, to reduce NOX emissions, only without the EGR-valve.

No ****!
Now you know why a lot of the newer, high-compression "rice" doesn't have an EGR valve and yet produce almost no NOX.

(This is an exception-rule...Not the norm!
Most of us don't want no stinkin' built-in EGR !)

Ever take off an intake manifold and notice that theres some black-junk inside the runners. I'm talking about black-junk that definately WAS NOT pumped-in by the EGR-valve. The closer the runners get to the intake-valves...the more black-junk there is inside. Some engines will have more than just a little.

Although we may not be able to actually see reversion as it happens, there is indeed plenty of evidence to show it's presence.

That's what that black-junk is!
That's your evidence!

Lots of different stuff can cause this to happen, such as but not limited to...

>>>Excessive valve-overlap,especially with too-low compression.
Even though overlap is something we must have!

>>>Intake-valve timing that closes the intake valve too-late.

>>>Excessive back-pressure or restrictions in the exhaust.

>>>Not enough scavenging or poor pulse-tuning in exhaust system.

>>>Air velocity too-low in the intake manifold.
Or any part of the intake system.

>>>Any combination of the above.

There are other factors that can make reversion worse than it has to be, but it's basically impossible to completely eliminate reversion.

Even engines with essentially zero-overlap (CBR-600 engines, as example) STILL! have some reversion.

Reversion is actually one of THE EXACT! reasons that we almost always lose some torque off the very bottom of the rpm range, whenever we stuff-in any hotter cam. Sometimes, more than just a little.

Unless the O.E. cam really, REALLY! was crap already. In these cases, you could actually REDUCE! reversion by installing a slightly larger, well-designed cam.
Scary !!!

Yes, unfortunately it's true...Some engines did indeed come from the factory with cams THAT BAD!

Thankfully, this was only on a few scattered models and engines, mostly from 1973 to 1986, many makes and models.

Just the basics here.
I leave it up to you guys to carry this one further down-field!

The Enemy can take many different shapes and forms.
Learn to identify the Enemy...Only then can you prevail !

.....ase mat



Post Edited (Sep 6, 8:26am)

 

A note on your notes.......

You stated....one of the reasons for reversion.........

">>>Air velocity too-low in the intake manifold.
Or any part of the intake system."

If you give that some consideration, .... a larger TB, porting, etc., on an n/a intake manifold actually promotes this lower velocity.....

Without a turbo giving it that extra "Whoosh" , you may not be doing any good
with the "Increase the dimension" factor.

Cleaning and polishing helps the intake flow, but the increase in diameter..??

 

on the other side of this argument the flow crossection is greatly reduced so it will cause a loss of high end....
--------------------------------------------------

That isn't what a flow bench showed. A square port didn't flow any more air than a linered round port.

 

Re: To answer Philips questions...

Both you guys are correct!

Remember, even though reversion may be present at other rpms, it's bad effects are usually most pronounced on the bottom of the rpm-range.
This can really complicate our efforts to engage an effective solution...
Without strangling the engine someplace else!

As we already know, many things can effect flow velocity in the intake system.
It is not limited to just the physical properties of the intake.
If we think of only the intake manifold itself, our success will be limited.

Sometimes all we can do is...

Try to make as many accurate observations as possible.
Try our best to make some sense of this acquired data.
Then try our best to make a change(deviate) for the better.
For some, this means being innovative.
Occasionally...Very innovative!!

Sometimes...We will fail! This much is certain.

But we can never win...Unless we are willing to risk failure!

So I guess the operative word is..."Try!".

IMHO.....ase mat



Post Edited (Sep 6, 8:25am)

 

See Above!

on intake port discussion.
Why hasn't anybody used the technology in the AR headers inside the INTAKE side of the equation?

 

They have. Look at the stock z EFI, the runner is 1/8 smaller than the port is.

 

Re: The other side of the coin

So, let's take for example the Euro market. As I have recently discovered, the Europeans had gotten for the 82-83 years the F-54/P-90 N/A engines and not the P-79 head! Why? Because we had to have the stainless inserts to help burn off the excess hydrocarbon, European market didn't care about that. So they, got the better flowing head, or was it just the most convient? They meaning Nissan was already producing the turbo head for the turbo cars, it was meant to flow well. There had to be a reason the ENGINEERS went back to a square port design. Now granted, I am not an engineer by any means but by reasoning, logic would tell me that the expelled charge of burned air/fuel would want to swirl. It's being swirled in the combustion chambers and that swirling effect I my opinion would continue. And to help further that swirling effect why not have a round ports design instead of a square shape? I would think the exiting charge of air would roll and tumble, slowing down. Ever poured water down a drain? What's it do? It swirls! That's probably why they use water on flowmeters, unless I am misunderstanding the flowing process of a head. Here's a comparison that I have stolen from the How to Modifiy the Nissan/Datsun OHC engine book, page-49:
All tests were done at 25" of water
L-28 N-42 head: L-28 P-79 head:
lift: % cfm % cfm
100 53.1 40.7 100 55.1 42.2
200 50.8 81.3 200 50.8 81.3
300 71.1 114.7 300 65.0 104.0
400 82.6 132.2 400 74.4 109.0
500 84.6 135.7 500 78.0 124.8
600 86.5 138.4 600 80.8 129.3
700 700 85.0 136.0

Both heads were built to GTU specs. The N-42 had a 1.450" valve while the P-79 had a 1.5" valve. One the 700 lift of the N-42 there was nothing written in the space on the book. The N-42 had obviously outflowed the P-79. These heads were built by B.C. Gerolamy, who has a great experience building the Nissan heads.

Hope fully this helps clarify any questions but will probably create more!

Dave

 

Somethings weird about those numbers to me. The intake ports are really close to being the same on those 2 heads yet they are showing a huge difference in flow.. I agree that a square port head can be -made- to flow more but in stock or close to stock form I don't think there is much if any difference.