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Napz 2.4 head porting


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I'm thinking of doing some conservative pocket porting, gasket matching and exhaust runner work with a 3 angle valve job on an extra Z24 head I have for my latest 4x4 project. The engine runs good but suffers from all of the power issues associated with this engine. Since I plan to throw some rings, head gasket and timing components at this unknown mileage engine anyway why not try to unlock a little more power? I'm not trying to build a monster, just unlock a few more ponies and maybe even some economy by improving head flow. Bottom end is strong, doesn't use oil, maintains good oil pressure with no noise other than some timing chain slap on a cold start that readily disappears.

 

 Are there any areas in the head that demand extra care around water jackets or other cavities? I don't want to ruin the head by grinding through.

 

I've read a lot about cams and the interference issues due to a high lift. Comp cams offers what looks like a conservative application. Does anyone have any input?

 

http://www.compperformancegroupstores.com/store/merchant.mvc?Screen=PROD&Store_Code=CC&Product_Code=88-123-6&Category_Code=Z2024

 

Any input or personal experience is welcomed.

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Any improved compression, induction, or exhaust?

A mild cam improvement won't give much noticeable improvement with just a mild port job.

Cams, mild port job, increase in compression, better flowing intake & exhaust will give a small increase.

Doing just 1 or 2 out of those is more of a feel good improvement.

Makes you feel good about spending money because you now have a port job, and cams to tell your friends.

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Stock valve lift is 0.413" so this 0.422" is an 'improvement' of 0.009"???? Not worth the $200. Stock duration is 248 degrees but this may not be at the 0.050" lift most cam makers use for comparison. I don't know.

 

Valve springs....

As the Z24 wasn't really designed for revs over 4,500 the valve springs are not that good. Good enough.... but if expecting to take this engine up higher you will need a little more closing pressure to keep the rocker following the cam lobe.

 

Compression...

Stock Z24 compression is a dismal 8.25 so mill the head 1mm and get 8.85 compression. The pistons are dished and notched for valves and the intakes are never at full lift at TDC but probably wouldn't hurt to check the clearance. 3/4 of a compression point should increase the efficiency about 5-6% at max revs. (not much, I know, but if running more cam and better carb this becomes more.

 

Carb...

The Z24 carb is a great compromise between mileage, drive ability and  performance really doesn't come into it. The carb tends to be the weakest performance link. Get a Weber 32/36. Some say 38/38 really wakes the Z24 up, but for living with it daily it's too big and drive ability is lacking and mileage is worse. 

 

Header...

Forget a header. spend the money on the carb. The Z24 isn't going to rev high enough to gain anything. Get a 2.25" down pipe, straight pipe the cat and run a low restriction muffler.... done.

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"Stock valve lift is 0.413" so this 0.422" is an 'improvement' of 0.009"???? Not worth the $200. Stock duration is 248 degrees but this may not be at the 0.050" lift most cam makers use for comparison. I don't know."

 

Lift is negligible, I agree. Still very conservative the Comp cam 260s total duration is more significant at 260 than the OEM 248. I wonder how significant this may be in the emissions arena?

 

"Valve springs....

As the Z24 wasn't really designed for revs over 4,500 the valve springs are not that good. Good enough.... but if expecting to take this engine up higher you will need a little more closing pressure to keep the rocker following the cam lobe."

 

I rarely exceed 4000 RPM and the improvements I'm looking for are more in the 2500-4000 range but if I've got the head all apart springs are relatively cheap, why not? Do you know of any springs that you would recommend?

 

"Compression...

Stock Z24 compression is a dismal 8.25 so mill the head 1mm and get 8.85 compression. The pistons are dished and notched for valves and the intakes are never at full lift at TDC but probably wouldn't hurt to check the clearance. 3/4 of a compression point should increase the efficiency about 5-6% at max revs. (not much, I know, but if running more cam and better carb this becomes more."

 

This answers a question I had about compression gains from simply milling the head. I know this engine is bad about head gaskets. How much can you take off safely? How much will this effect valve timing, will I need to advance it?

 

 

 

"Carb...

The Z24 carb is a great compromise between mileage, drive ability and  performance really doesn't come into it. The carb tends to be the weakest performance link. Get a Weber 32/36. Some say 38/38 really wakes the Z24 up, but for living with it daily it's too big and drive ability is lacking and mileage is worse. "

 

"Header...

Forget a header. spend the money on the carb. The Z24 isn't going to rev high enough to gain anything. Get a 2.25" down pipe, straight pipe the cat and run a low restriction muffler.... done."

 

Agreed, I have read your recommendations on this in other posts. I do have the emissions worry for a couple more years however so I think I need the CAT and possibly the electronic carburetor with the AIS though I've read others in different states are passing easily withe a properly tuned 32/36. Emissions are also a a consideration with the cam due to increased overlap.

 

 

The idea is to possibly increase fuel mileage so no 38/38. I thought about the 32/36 and have read all about it. Worked on them a lot back in the day and I know they are decent carburetors.  My Hitachi is working well and it will pass emissions. I think the AIS, carburetor and the cat are the largest factors in its ability to do that.

 

Do you have any thought on this Hitachi progressive linkage mod? 

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I much more prefer the vacuum secondary. It only comes in as needed for more power, so is much better for mileage. The progressive is on whenever you step on it, and at low speeds can bog from not enough air flowing through the venturi to draw in enough gas. You would have to learn how to drive it and step into it at the right times. I've driven a Weber 510 and too much throttle too soon and there's a loud WAAAAAAA sound but little increase in speed. Eventually it catches up and goes. The trick was to step into it part throttle and then jam it. Probably overall was faster than the vacuum secondary but from a mileage perspective you would be using more gas. 

 

My '70 Dart had an AVS Carter carb. I adjusted the secondary to come in sooner and  it was just too much carb for the lower revs. If you floored the 4bbl at 35 it went pretty good in 4th gear and at 45 you could hear the change in engine sound and power as the secondary came in. Setting it earlier just made it moan louder and it lacked power. It would crawl to 45 and then start to go. I reset to factory tip in and all was well.

 

I think you should try this trick and see for yourself if better. I'm sure the mileage won't be.

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If rebuilding, a set of KA24E pistons are dished only 2cc and will put you in the low 9s for compression without milling a fat chunk off the head.

 

Any significant opening of the port cross section can increase over all max flow, but can reduce lower rpm velocity so you often move the power band up the rpm range.

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If rebuilding, a set of KA24E pistons are dished only 2cc and will put you in the low 9s for compression without milling a fat chunk off the head.

 

Any significant opening of the port cross section can increase over all max flow, but can reduce lower rpm velocity so you often move the power band up the rpm range.

 

Are these the pistons that I've been reading are free floating and will need valve reliefs cut in them before installing in the Z-24? I was thinking about them but balked a little with the machine work and having to check clearance with clay, the whole assemble, disassemble, reassemble... Don't know if I'm up for that with this engine. That said, I worry about taking too much mass out of a head on an engine that is know for its blown head gaskets. How much can be safely milled?

 

I do not wish to build a head that will not work below 2500 RPM. I would be happy with a conservative enhancement of the existing power band. I'm thinking about slightly opening and blending the valve pockets into the runners while smoothing out the short side radius on the exhaust side. I plan to polish the exhaust side runners and open them to match the OEM gasket. I plan to open the intake side slightly, matching the gasket while working a venturi into the intake side runner. I'll leave the intake runners a little rough without polishing to maintain turbulence. Maybe I'll polish and cc the chambers as well as knock down any sharp edges that might create hot spots. I have done this before with domestic V-8's and been very happy with the results. Maybe the single biggest improvement to the engine I've made without replacing parts. I have a 455 Olds that I built in a 66 C-10 that I used to race. A real torque monster. The exhaust radius is a real problem with the Olds head.  I've been reading that the Z 24 suffers in the same way and that the exhaust radius is a bottleneck. That's what got me to thinking about working on this extra head I have. Maybe applying what I've learned from the Olds head would net me some improvement in the Nissan. I have time tools and some ability but not an endless supply of cash. All that said, I haven't worked on a Z 24 head in this respect. I have no experience improving performance in 4 banger outisde of a few bolt ons we've all read about. That is why I'm asking for pointers before I jump in.

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Both valves are tilted and the port is low on the head with a sharp bend onto the valve seat, not just the exhaust. I would concentrate more on the intake which relies in atmospheric pressure 14.7 PSI to push air into the cylinder. Exhaust is under high pressure and want's out NOW. It is less affected by a bend and gains less when the radius is opened. In any case porting benefits high rev breathing more than low RPM. At low speeds restrictions are less noticeable.

 

 

 

 

I do not wish to build a head that will not work below 2500 RPM. I would be happy with a conservative enhancement of the existing power band. I'm thinking about slightly opening and blending the valve pockets into the runners while smoothing out the short side radius on the exhaust side. I plan to polish the exhaust side runners and open them to match the OEM gasket. I plan to open the intake side slightly, matching the gasket while working a venturi into the intake side runner. I'll leave the intake runners a little rough without polishing to maintain turbulence. Maybe I'll polish and cc the chambers as well as knock down any sharp edges that might create hot spots. 

 

 

Absolutely port and polish, there's no harm done doing this. It's a hemi head so no hot spots. I noticed both ports under the seat were badly machined on the Z heads I have worked with. Big burs and casting ridges and terrible transition to the port sides of the seats. So there's lots to do.

 

As for the head gasket blowing this is to do with the block not the heads. Maybe the bolts don't go down into the block far enough or the metallurgy is wonky... something.The cure for this is to re-torque the head bolts every tune up or once a year. Loosen only one bolt and re-torque it to 60 ft lbs then move to the next. This way only one bolt is loose and you don't have to follow any sequence. Do this only on a cold engine.    

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I think there is no downside to a general cleaning smoothing polishing job. I just did that to my ka24e head.

 

Like Mike said, there are some terrible transitions to valve seats leaving large lips, casting flash in the throats, and bosses that can benefit from a little shaping. Correcting all this will help. But honestly i don't know HOW MUCH help this will be. I don't know if a butt dyno would be able to tell. I hope the difference is significant.

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I think there is no downside to a general cleaning smoothing polishing job. I just did that to my ka24e head.

 

Like Mike said, there are some terrible transitions to valve seats leaving large lips, casting flash in the throats, and bosses that can benefit from a little shaping. Correcting all this will help. But honestly i don't know HOW MUCH help this will be. I don't know if a butt dyno would be able to tell. I hope the difference is significant.

 

I pulled the donor head off the engine today and tore it down. I inspected everything and the head appears to be ion great shape. It was on a running engine that smoked so I was concerned about valve guides and stem wear but they show no appreciable wear. The biggest concern I have is that there is not an overabundance of material that can be removed. After looking at the head I estimate that the casting in the valve bowls is only 5-6 mm thick. Here is a picture that shows the bowl from the gigantic water jacket as viewed from the rear freeze plug.

 

fcAcjRl.jpg

 

There is not a lot of port floor material either. The water jacket extends under the intake floor to the intake manifold as seen in this picture. Working the short side radius will be limited due to this. I'm not sure how much I'll be able to raise the roof. It would be nice to see a cross section of this head.

 

D6fvgl8.jpg

 

I live in the mountains and the butt dyno will be in the form of a couple of hills. As I said, I'm not trying to make a hot rod. It would be nice to bring this 86 4 banger a little closer to a present day stock performance level.

 

I've been thinking about throwing some head studs at this thing in favor of head bolts when I  finally swap out the finished head. I'm going to mill the head at least 1 mm as Mike suggested would more than that be crazy? Also, Mike thought valve springs might be in order. Since I've got it pulled apart now is the time. Can you recommend a proven product that won't be too much spring? I'm concerned about over doing it with a stock valve train. I don't see this engine exceeding 4500 RPM except by accident. 

 

Here are some exhaust port pics for future comparison. 

 

XManamP.jpg

 

PFwa3Cs.jpg

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The valve seals control the oil that lubes the valve stems. Always replace them.

 

 

GWO3kFW.jpg

 

When the factory ground out the port for the seat to sit in they left not only a burr but a bench. I ground the port away till the seat flowed smoothly into it. The side of both ports closest to the center of the cylinder had the most removed. On the short radius I cut 80? grit sandpaper strips 3/8" wide and fed them through the port and out the valve opening. With little trouble I seesawed the strip back and forth so the strip wore away only this sharp radius and the bottom of the valve seat. All you need watch is you don't hit the valve face.

 

 

Lj47mFO.jpg

 

 

The short radius is at the bottom. The wider curve of the short radius included removing a small amount of the seat to get it started. You can see the bronze colored seat has been blended into the aluminum port everywhere else.

.

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Since no one came forward with a valve spring recommendation and virtually nothing shows up for a lowly Z24 engine with a Google search I started digging. I went to Rock Auto, looked up a stock valve spring for the Napz in an 85 720 2.4 and clicked on the buyer's guide. According to the Rock Auto buyers guide this spring fits a lot of different Nissan engines. So what models showed promise for aftermarket performance parts? The 300zx or the 200sx are both listed, how about them? I searched performance valve springs for a 1985 300zx and a 200sx. They both came back to an Isky Part # 7005-8 performance spring through CarID. Then I searched the ISKY parts number and found them through Summit.

 

https://www.carid.com/isky-racing-cams/valve-springs-mpn-7005-8.html?singleid=84035380&url=80639190

 

https://www.summitracing.com/parts/isk-7005-8/overview/

 

It is not a double spring as the OEMs are. If the pics are correct it does have a damper spring. I can't find an OEMlbs per inch or coil bind spec in my manual to make comparisions.

 

I found these too but the metric specs confuse me compared to what I remember from the old days racing domestic stuff. At first look they don't seem to be much more than stock. They are double springs though.

 

https://thmotorsports.com/supertech-performance/supertech-valve-springs/sprh1021dsr20/i-252853.aspx

 

Thoughts?

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A 96mm stroke is limited to a 6,250 RPM red line. So shoot for this if going to modifications. The Z24 lift is only 0.413" and can't be raised much without hitting other things things. New valve springs are probably good enough. The Z valves are 10mm longer than the L20B but the springs are stiffer 115 lb vs. 108 lb, so don't go to L series for the springs. I seem to remember someone using a Ford valve spring. 135 lb. The valve retainer may have to be changed also for these to work.

 

 

The 300zx and the later S12 200sx had the same exact engine so the spring would be the same. A high performance engine does not automatically have stiffer springs. I'm guessing the VG30 engine may have lighter valves and not need high rate springs.  

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