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#3130805 - 09/20/13 02:55 PM Intake Runner Surface
84zmyfavorite Offline


Registered: 07/14/10
Posts: 671
Loc: Wenatchee, W.A.
I Thought I had finished Porting and Yes Polishing the 92 Jeep 4.0 7120 cylinder head that Im putting on my 2000 Jeep Cherokees 4.0 reBuild, when a couple of AutoTech teachers told me the intake runners are too smooth, they should be roughened up, so the fuel will atomize with the air correctly. How "Rough" or irregular should a cast iron intake runner and bowl be for correct atomization of fuel/air?


Edited by 84zmyfavorite (09/20/13 02:55 PM)

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#3130848 - 09/20/13 03:52 PM Re: Intake Runner Surface [Re: 84zmyfavorite]
95busa Offline


Registered: 10/15/05
Posts: 1076
Loc: Tennessee
Not at all. It is port injected is it not?
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#3130864 - 09/20/13 04:34 PM Re: Intake Runner Surface [Re: 84zmyfavorite]
Kuato Offline


Registered: 10/23/05
Posts: 3156
Loc: Northeastern MT
Hmmm I have always been told that smoother is better, less drag on the air going in.

The injectors should be doing the atomizing, not the side walls of the intake runners!
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#3130882 - 09/20/13 05:02 PM Re: Intake Runner Surface [Re: 84zmyfavorite]
dparm Offline


Registered: 04/19/10
Posts: 12541
Loc: Chicago, IL
You do want some tumbling to aid in atomization, yes. It largely depends on the design of the plenum and runners, however.
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#3130896 - 09/20/13 05:41 PM Re: Intake Runner Surface [Re: dparm]
OVERKILL Offline


Registered: 04/28/08
Posts: 26520
Loc: Ontario, Canada
Originally Posted By: dparm
You do want some tumbling to aid in atomization, yes. It largely depends on the design of the plenum and runners, however.


Yes, however it is far less relevant to a SEFI engine than it is to a carbureted one smile
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#3130967 - 09/20/13 07:14 PM Re: Intake Runner Surface [Re: 84zmyfavorite]
Oldmoparguy1 Offline


Registered: 01/21/05
Posts: 4068
Loc: Charlotte, NC
Smooth is better in general for high rpm full throttle operation. For every other regime, a lightly pebbled surface is better. The closer you get to the valve the more the surface finish matters. The inside of the curve where the runner turns toward the valve is a critical area. An interesting trick is to lightly groove the high area across the runner with something like 16-20 grooves per inch a few thousandths per inch deep, just for the curved area. Looks kind of like a dull almost stripped bolt. BTDT...

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#3131220 - 09/21/13 03:02 AM Re: Intake Runner Surface [Re: 84zmyfavorite]
Shannow Offline


Registered: 12/12/02
Posts: 26869
Loc: a prison island
A little rough in port fuel injected is good for the air...see golf balls.

As to carbs, this is the first time anywhere in the world, these pics will appear.

First three photos are of fuel being injected ahead of the throttle plate in a rectangular section inlet manifold, at varying inlet manifold actual (corrected for density) velocities.

Throttle at around 45 degrees



Open throttle


All pics at stoichiometric, and show that with a car or TBI, "atomisation" isn't that real, a lot of the fuel was still fuel.

Throttle body did a lot of "mixing", that was pretty much negated only inches downstream with over 10% of the fuel settling to the manifold floor.

Problem with it on the floor was that it's running slower than the airflow, about 10-15% of the air velocity at WOT



and at lower manifold densities, the fuel goes even slower



Steady state, same amount of fuel makes it down the hole as air, but whip the throttle open, and the fuel on the floor is picked up, and runs into the engine, and there's a momentary lean pause while the fuel film catches back up with the air...8"/second is fairly slow...thus need for accelerator pumps.

A turbulent boundary layer along the floor would have made an interesting extension to the work.

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#3131302 - 09/21/13 06:09 AM Re: Intake Runner Surface [Re: 84zmyfavorite]
SteveSRT8 Offline


Registered: 10/10/08
Posts: 15175
Loc: Sunny Florida
Originally Posted By: 84zmyfavorite
I Thought I had finished Porting and Yes Polishing the 92 Jeep 4.0 7120 cylinder head that Im putting on my 2000 Jeep Cherokees 4.0 reBuild, when a couple of AutoTech teachers told me the intake runners are too smooth, they should be roughened up, so the fuel will atomize with the air correctly. How "Rough" or irregular should a cast iron intake runner and bowl be for correct atomization of fuel/air?


Smoother is good up to a particular point. Laminar flow is a big issue when tweaking for HP. The PROPER roughness allows a film to attach and then 'lubricate' the rest of the air, allowing slightly better flow and volume.

Wet manifolds have a lot more tricks as Shannows excellent photos show you. Dry manifolds are MUCH easier to tweak...
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#3131789 - 09/21/13 06:37 PM Re: Intake Runner Surface [Re: 84zmyfavorite]
84zmyfavorite Offline


Registered: 07/14/10
Posts: 671
Loc: Wenatchee, W.A.
Thank You All for the exellent replies. The Auto Tech Instructors I was talking to used the golf ball analogy and also talked about where the runner "Blends" with the Bowl is an important area. Since I have already pushed the limit on wall thickness, a sandblaster was recommended for uniform "pebble-ing" or roughing of surface as was a Dremel, which I do have and spent using nearly 35 hours on the porting and polishing,.. Used it today for couple of hours enlarging oil return holes in Block(they were pitiful, jagged broken off holes). The Next 4.0 head I work on I will make channels or fins in the Intake runners as suggested. I have seen fins on certain intake runners, some nearly 3/8ths in tall. I will really try to get pictures on here soon. Of course Ive said that for a while now. Thanks Again!


Edited by 84zmyfavorite (09/21/13 06:41 PM)

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#3131804 - 09/21/13 06:54 PM Re: Intake Runner Surface [Re: 84zmyfavorite]
84zmyfavorite Offline


Registered: 07/14/10
Posts: 671
Loc: Wenatchee, W.A.
The engine is a 2000 Jeep 4.0 block and 1992 cylinder head(7120) with MultiPort Injection, equal length runner Intake manifold with rectangular runners in both manifold and cylinder head. Injectors are nearly touching cylinder head when bolted up, so only time for fuel and air to mix is in cylinder head runners.


Edited by 84zmyfavorite (09/21/13 06:56 PM)

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#3131913 - 09/21/13 09:03 PM Re: Intake Runner Surface [Re: 84zmyfavorite]
DuckRyder Offline


Registered: 10/20/05
Posts: 1455
Loc: Atlanta
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#3132182 - 09/22/13 08:49 AM Re: Intake Runner Surface [Re: 84zmyfavorite]
OVERKILL Offline


Registered: 04/28/08
Posts: 26520
Loc: Ontario, Canada
Originally Posted By: 84zmyfavorite
The engine is a 2000 Jeep 4.0 block and 1992 cylinder head(7120) with MultiPort Injection, equal length runner Intake manifold with rectangular runners in both manifold and cylinder head. Injectors are nearly touching cylinder head when bolted up, so only time for fuel and air to mix is in cylinder head runners.


Sounds similar to the SEFI Ford's that fire the injector at the back of the intake valve.

Here's how my old TFS-R aligned with my ported GT40's:
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#3132570 - 09/22/13 03:33 PM Re: Intake Runner Surface [Re: 84zmyfavorite]
84zmyfavorite Offline


Registered: 07/14/10
Posts: 671
Loc: Wenatchee, W.A.
Nice, Im thinking my 99 Cherokees 4.0s manifold, gasket, cylinder head mating look like that as well. I thought That is Port Matching. where the walls and gasket are the same size dimensions, like That! I am a plumber(though New construction), I should borrow a poop Scope from a Service plumber a see what my 99s 4.0 looks like inside all bolted together. Of course Ill clean the scope first. Maby.


Edited by 84zmyfavorite (09/22/13 03:36 PM)

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#3132573 - 09/22/13 03:38 PM Re: Intake Runner Surface [Re: 84zmyfavorite]
84zmyfavorite Offline


Registered: 07/14/10
Posts: 671
Loc: Wenatchee, W.A.
Thanks DuckRyder.

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#3132609 - 09/22/13 04:13 PM Re: Intake Runner Surface [Re: 84zmyfavorite]
OVERKILL Offline


Registered: 04/28/08
Posts: 26520
Loc: Ontario, Canada
Originally Posted By: 84zmyfavorite
Nice, Im thinking my 99 Cherokees 4.0s manifold, gasket, cylinder head mating look like that as well. I thought That is Port Matching. where the walls and gasket are the same size dimensions, like That! I am a plumber(though New construction), I should borrow a poop Scope from a Service plumber a see what my 99s 4.0 looks like inside all bolted together. Of course Ill clean the scope first. Maby.


That's the idea thumbsup

My heads were port-matches to a Felpro 1250. The intake, out of the box, matches a Felpro 1250. So the alignment was very close to perfect.
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