porting a head for a stock 4.0 ????????????

[obxtuf]

the heater control valve busted on my 93 and the engine overheated causing the head to crack so ive removed the head and have gotten a used one from the local junk yard for 21 bucks. I dropped it off at a napa machine shop to have it cleaned and checked out. looked around the forums for this but couldnt find much. if my engine is stock would it be worth porting the exhaust ports and polishing both intake & exhaust. I have all the tools to do this and the head has to be put on so why not. what do you guys know ? thanks.

 

[5-90]

"Polishing" and "port-matching" a head are within the reach of the garage hobbyist - "porting" is not.

Most heads will benefit from cleaning up the "sand-cast" surface in the port runners, and some work can also be done to "port-match" the intake ports to the gasket and the intake manifold, resulting in a smoother flow line into the intake runner.

However, if you're really going to go all-out to modify a head to improve airflow, a flowbench is mandatory. No gettin' around that. You can't port a head by sight alone - unless you've already done a few score of them, and know where to clean up and where to leave alone.

Also, the average hobbyist is just going to "hog out" the port runner, which would eliminate the venturi effect at the "knee" of the port, actually reducing airflow through the cylinder head.

Proper port work is done incrementally, verifying at a flowbench at (numerous!) stages throughout the work, until a plateau is reached where there are no more gains - after that, you'll actually be reducing flow efficiency!

For a good writeup on port polishing, Google <Standard Abrasives> alongside <Port Polishing> - they've got a kit that works well, and a good writeup on how to do it. I may have a copy somewhere, but Google would probably find it faster...

5-90

 

[seanyb505]

How much hp/tq would having the head professionally ported and polished give, assuming the engine is maxed on external bolt ons?

 

[5-90]

Hard tellin'. Depends on too much...

Pertinent variables -
Fuel tuning and delivery potential
Final airflow of ported/polished head
Movement of engine redline due to balance of reciprocating assembly
Exhaust airflow potential
Exhaust phasing at header collector (does each pulse assist the next? Impede it?)
Throttle body and intake manifold airflow capacity
Kerb idle timing and timing advance curve
Ignition capacity (most distributorless ignition systems have hotter potential spark than most central/distributor-based ignition systems, due to increased coil saturation)

Quite a lot goes into an engine. I wouldn't really even want to guess without more empirical data, but I don't like to guess to start with...

 

[Slo-Sho]

I would recommend atleast gasket matching the head and intake manifold. The exhaust manifold flange could also use some help. Make sure to leave a step or smaller opening on the exhaust ports than the exhaust manifold to provide an "anti-reversion" step as you don't want the exhaust gasses to try to flow back at high rpm's. The valve guide boss' and bowl area is a major deal breaker in the flow department and could also use help from some smoothing down. Being a pushrod motor and not having a crossflow cylinder head is a kind of a dampener to begin with so it needs all the help it can get.

If you don't have a carbide burr with a die grinder you will be in for some hard work.

 

[Dr. Dyno]

the heater control valve busted on my 93 and the engine overheated causing the head to crack so ive removed the head and have gotten a used one from the local junk yard for 21 bucks. I dropped it off at a napa machine shop to have it cleaned and checked out. looked around the forums for this but couldnt find much. if my engine is stock would it be worth porting the exhaust ports and polishing both intake & exhaust. I have all the tools to do this and the head has to be put on so why not. what do you guys know ? thanks.

I've home ported two 4.0 heads successfully and it's not that difficult if you have the right tools and follow the Standard Abrasives head porting guide. I did and here's my write-up:

http://www.jeep4.0performance.4mg.com/head.html

 

[SCW]

I have a 4.0 head sitting on my bench waiting to do this very thing and here is the conclusion I've come to. Take it for what you paid-

The 4.0 is not really a high-rev engine and I've never had mine over 4200rpm, so I've decided not to spend much time and energy porting/polishing the intake. In order to maintain as much low-end as possible the intake velocity should be kept high, as should exhaust velocity. Opening up the intake helps it breath at higher RPM but makes for slower gas velocity as low RPM, so I'll clean it up a bit and match the gaskets, but no more. Comustion bowl can be polished to a mirror finish and all sharp turns and edges softened as you will see on Dino site. I also want higher compression (see below) so I will try to polish without removing much material.

The exhaust can be cleaned up quite a bit and in fact brought to a mirror finish if you want. It seems common to leave the exhaust port smaller than the gasket to help prevent backflow into the cylinder, but that seems like hooey to me.

Also, the stock 4.0 is only something like 8.25-8.5:1 compression. I'll shave my head about .0030 and bump the compression up for better performance. Premium gas only costs $0.20/gallon more which add up to less than $4/fillup. Not a big cost in the large picture of things.

 

[jeepinwi]

I've also ported 2 4.0 heads with excellent results. The second head I did was on my 4.0 for about 80 miles and I could feel the difference from the non-ported head.

IMO, if you go to all the effort of porting, you might as well get a valve job done to have a completely freshened head. It cost me $150 to get a valve job, head surface checked for flatness, milled to flatness, hot tanked, and reassembled with new valve seals. I consider that money well spent and that head is working great on my stroker.

 

[Dr. Dyno]

The 4.0 is not really a high-rev engine and I've never had mine over 4200rpm, so I've decided not to spend much time and energy porting/polishing the intake.

The exhaust can be cleaned up quite a bit and in fact brought to a mirror finish if you want. It seems common to leave the exhaust port smaller than the gasket to help prevent backflow into the cylinder, but that seems like hooey to me.

Also, the stock 4.0 is only something like 8.25-8.5:1 compression. I'll shave my head about .0030 and bump the compression up for better performance. Premium gas only costs $0.20/gallon more which add up to less than $4/fillup. Not a big cost in the large picture of things.

1. While the 4.0 isn't a high revving screamer, you can still make it breathe very well all the way to 5250rpm so why deprive yourself of that last 1000rpm of fun?
2. The stock 4.0 has a CR of 8.8:1. If you shave 0.020" from the head and use the Mopar/Victor head gasket, you can bump it up to 9.2:1. Just remember to use 0.030" thick shims for the rocker arm bridges to correct the lifter preload.
3. The exhaust ports should remain smaller than the gasket to prevent reversion of exhaust gases back into the cylinders.

 

[SCW]

1. While the 4.0 isn't a high revving screamer, you can still make it breathe very well all the way to 5250rpm so why deprive yourself of that last 1000rpm of fun?
2. The stock 4.0 has a CR of 8.8:1. If you shave 0.020" from the head and use the Mopar/Victor head gasket, you can bump it up to 9.2:1. Just remember to use 0.030" thick shims for the rocker arm bridges to correct the lifter preload.
3. The exhaust ports should remain smaller than the gasket to prevent reversion of exhaust gases back into the cylinders.

1- You and a few others like to really wrap up the 4.0 but it just doesn't seem to be the way I drive. I would rather have maxed low-end, but some day I might really wrap mine up. I can't believe the poop it's got at 3500RPM, lots of fun.

2-Is the MOPAR gasket the thinner one? I figured on staying with the stock gasket simply because of the strength, I'd prefer to mill the head an extra .010 and use a strong gasket for reliability.

3- I don't really see how exhaust gas can get sucked back in, nor do I understand the logic of using the "shelf" between the head and header to prevent it. Only gasses along the edges will be prevented from flowing back, and those gasses have the higher shear stress anyway, making them flow slower and with more restriction anyway. If you have a good header with long-ish primaries such as the Clifford or even Pacesetter, the exhaust pulse from opposing cylinders won't have much if any effect on gasses re-entering the cumbustion chamber. In fact they should create a suction to help pull gasses through the exhaust system.

Also, the exhaust valve should be shut before the piston starts moving back down, so how is it sucking anything back into the cylinder? Worst case senario would be the head is larger than the gasket or header blocking some of the flow, but I think (don't know, just think) that matching the exhaust port to the gasket should not have a detrimental effect.

Any thoughts?

 

[MudDawg]

Exhaust gas reversion is generally only a problem in engines that run lots of duration and overlap....good example is a nitrous motor that runs huge exhaust lobe centers. There is nothing wrong with giving a stock head a little going over, if you are careful....in its self with an otherwise stock motor, you won't gain a whole bunch...but 50 to 60 bux for a sand roll kit and a few hours of labor is cheap for a few extra ponies.

 

[Flash]

1- You and a few others like to really wrap up the 4.0 but it just doesn't seem to be the way I drive. I would rather have maxed low-end, but some day I might really wrap mine up. I can't believe the poop it's got at 3500RPM, lots of fun.

2-Is the MOPAR gasket the thinner one? I figured on staying with the stock gasket simply because of the strength, I'd prefer to mill the head an extra .010 and use a strong gasket for reliability.

3- I don't really see how exhaust gas can get sucked back in, nor do I understand the logic of using the "shelf" between the head and header to prevent it. Only gasses along the edges will be prevented from flowing back, and those gasses have the higher shear stress anyway, making them flow slower and with more restriction anyway. If you have a good header with long-ish primaries such as the Clifford or even Pacesetter, the exhaust pulse from opposing cylinders won't have much if any effect on gasses re-entering the cumbustion chamber. In fact they should create a suction to help pull gasses through the exhaust system.

Also, the exhaust valve should be shut before the piston starts moving back down, so how is it sucking anything back into the cylinder? Worst case senario would be the head is larger than the gasket or header blocking some of the flow, but I think (don't know, just think) that matching the exhaust port to the gasket should not have a detrimental effect.

Any thoughts?

Hmmmm It real isn't getting sucked back in to the cylinder, as much as when the pressure of the piston slows the pressure of the ex gas it trying to go back into the sylinder.........this doesn't happen as much as the ex gasses are less completely expelled from the sylinder....witch is where a tuned header can help!

in a real long duration cam, the ex. could actually refill the cylinder(at close to T.D.C. EX stroke) causing it to travel up into the intake slightly thru the intake valve that has just started to open while the ex valve is just about to close, Making the lopy eng idle you here from a big cammed eng.

Hope that made sense!

Flash.

 

[tealcherokee]

regursion isnt the exhaust getting sucked back in it when the port in the head is bigger than the tube or weld in the exhaust manifold, so it has a sharp step down in size, the exhaust hits it, and it trys to turn around.

 

[obxtuf]

been working on porting my head all is well ill post some pics soon
as far as parts ive run into a small problem my cherokee is a 1993 and the head that i have gotten is a 1996 when ordering the valve stem seal sets there are different part numbers through advance auto for felpro valve seals for the 1993 head is ss70819 and the set for the 1996 head that im porting is ss72835. for the moment i going to order the parts for the 1996 head are there going to be any other problems with this upgrade ? like the lifters, rocker arms, ect any info helps thanks guys

 

[75SV1]

I ported, polished and put SS Hi Flow (Chevy) valves in mine. I did a 351W set of heads in the late 80's before. I did follow Dino's instructions and the Standard Abrasives. I did leave a smaller diameter in the exhaust to the header tube for the anti reversion. Some headers use to have an anti-reversion cone. I also had .010 shaved off the head and used the MOPAR HiPo gasket. I cleaned up the bowl area and polished the combustion chambers. I didn't CC the chambers though. I did polish the ports. I would say unless you know what you are doing and have a flow bench, I wouldn't do more than polish the contours of the ports. I left the intake ports a bit rough #60 tp #80 grit and polished the exhaust to #120 or better.
Tom

 

[SCW]

I ported, polished and put SS Hi Flow (Chevy) valves in mine. I did a 351W set of heads in the late 80's before. I did follow Dino's instructions and the Standard Abrasives. I did leave a smaller diameter in the exhaust to the header tube for the anti reversion. Some headers use to have an anti-reversion cone. I also had .010 shaved off the head and used the MOPAR HiPo gasket. I cleaned up the bowl area and polished the combustion chambers. I didn't CC the chambers though. I did polish the ports. I would say unless you know what you are doing and have a flow bench, I wouldn't do more than polish the contours of the ports. I left the intake ports a bit rough #60 tp #80 grit and polished the exhaust to #120 or better.
Tom

Results?

 

[BBeach]

Whats the max that you can mill the head down to? I wouldn't mind having a higher compression ratio depending on how much power I could get from it. Does anyone know what the timing is at WOT? Also, how easy is it to advance your timing or would the computer change it back to stock?

 

[Dr. Dyno]

Worst case senario would be the head is larger than the gasket or header blocking some of the flow, but I think (don't know, just think) that matching the exhaust port to the gasket should not have a detrimental effect.

Any thoughts?

There are two other compelling reasons for not going overboard when enlarging the exhaust ports. The first is that if you make them too big, this will hurt port gas velocity and you'll lose torque especially at lower rpm. The second is that you could leave the port walls too thin risking them cracking or worse, break into the water jacket altogether.
If you stick to the port dimensions that I specified for the head that I ported for my stroker, you can't go wrong.
http://www.angelfire.com/my/fan/prep.html

 

[MudDawg]

Be careful with CR in a 4.0....remember this is an old engine design....and as such lacks good combustion chamber turbulence because the design has poor quench...If you don't mind running Premium gas, you can boost the CR to the 9.5 range....It is widely reported that going to the Hesco aluminum head allows higher compression since the combustion chamber surfaces run cooler because of better thermal conductivity.

 

[Dr. Dyno]

Whats the max that you can mill the head down to? I wouldn't mind having a higher compression ratio depending on how much power I could get from it. Does anyone know what the timing is at WOT? Also, how easy is it to advance your timing or would the computer change it back to stock?

The timing is controlled by the computer so the only way to advance it is to slot the bolt holes of the CPS and move it towards the centerline of the vehicle. You might not necessarily gain more power unless you live at a much higher elevation than sea level. Maximum ignition advance is 34*BTDC on a stock computer.
If you mill 0.020" from the head and use the Mopar/Victor head gasket, the CR will increase to 9.2:1 and that's about as far as you should go if you have the stock cam to avoid detonation problems. You'll need to add 0.030" shims under the rocker arm bridges to correct lifter preload if you go ahead with the head shave.