MODIFICATIONS for Nissan P-79 heads(back)

PORTING/POLISHING

P79 intake port. Look closer and you can see the 3-angle seat (33k)

The first thing to remember is that this head is the culmination of 11 years of design work by Nissan. It's flow is very balanced, valve-stem height is exact and the combustion chamber's swirl-design has been optimized for performance. In other words, don't screw with it too much. If you are a gung-ho racer then bore it out, but for high-performance street use not much is needed. The "diamond" exhaust ports have 'glo' liners which heat up red-hot to reduce emissions, but I wouldn't lose sleep over worrying if they cause reduced flow. Put a header on it and concentrate on maximizing intake flow.

• Despite balanced flow characteristics, if you look in a valve's bowl after removing the valve you will see rough casting marks and ridges. Using medium emory paper and cutting oil (WD40) or a burr tool, spend an evening smoothing out all these imperfections. Most importantly, smooth the transition from the intake port to the underside of the valve seat. On mine I could feel a sharp rim on the seat's "lower" short side radius which I greatly smoothed out. This will improve breathing at high revs without losing low end torque. Be careful not to nick the actual valve/seat surface or grind into a water jacket. A smooth port/seat transition is more important in the intake than the exhaust..
• I also mildly polished the combustion chambers with fine emory paper. But don't go overboard, remember that every bit of metal removed here will lower the compression ratio. I also slightly sanded the port runners. But current wisdom is to keep intake runners slightly rough to create turbulence, keeping the gas in suspension.
• I smoothed (chamfered) the bowl-side edges of the spark-plug hole threads. At high-temperatures the threads can glow and cause pre-ignition. I doubt if this would ever happen on the street, but I did it anyway.

VALVE LAPPING vs SEAT GRINDING

• All P heads use hardened valve seats and will not need a valve job unless it's really in bad shape( which means it's probably warped anyway). And if you haven't noticed after setting valve clearances, a Z motor's valve timing is extremely critical to it's performance. So be very careful not to disturb the valve heights and flow by cutting new seats and sinking the valves into the head. The solution is Valve Lapping which is an excellent way of restoring valve sealing without machine work. I lapped my junkyard P-79 with 80K on it, added new valve seals and I now have 173+ psi in all cylinders. So much for an"old fashioned" fix. But be wary of shops that want to cut you "3-angle" racing valve seats. All Z heads came standard with 3-angle seats, the Japanese just never used that term which is why you rarely see it in print. "Radiused" seats are popular with the NASCAR crowd, but they are high maintenance and don't seal well.

This is my "machine shop"

RAISING COMPRESSSION/SHAVING

This mod was recommended to me by the Z Doctor in Roanoke, VA. who has close to 20 years in modding and SCCA racing Datsun motors of all types. It's one of those Smokey Yunick type shops with exotic, in-progress projects lying around. My favorites are a 2000 Roadster converted to fuel injection and a bullet-proof Rotomaster turbo 240Z running 26psi of boost, which was timed at 0-100 in 9 seconds flat (I drove it and believe it).

THE MOD FOR RAISED COMPRESSION

• Shave the head .100"
• Install valves from a '75-78 280Z motor. They are .100" longer than '81-83 valves.
• Shim up all springs with .100" bottom spacers.
• Shim up the cam towers by .100" using tower spacers

I haven't done this, but it lets you raise compression to about 10.5, but maintains valve-train geometry and keeps the timing accurate. You can also use the stock springs, lash pads and apparantly the cam.. Rob at the Z Doctor has a P-79 head/motor shaved by 1.25" that is running about 12:1 compression on 93 octane with light pinging. He maintains that the swirl design is very efficient (like Jaguar"s Fireball heads in the1980's) and that the P-79 will run high compression without detonation problems. Having tried every block/head combination possible, they swear by this mod stating it "gives you serious horsepower". BTW, they consider the N42/N47 motors an unwise choice for high-performance L-28 use. They dislike the low compression and feel that any flow advantages that earlier heads may have had at one time are now irrelevant due to sunken valve seats, worn cams, warping,etc. But don't be upset if you're using an N series L28, these guys live Z's 24 hours a day and have very strong opinions. But if the choice is between a 1975 design or a 1981, I'll take the '81.

CAMSHAFT

The new lower Motorsport Auto front spring is at the right.(34k)

I don't have too much aftermarket cam experience, but I just installed a Crane stage 1. It uses stock valve springs and lash pads, unlike other models that need new ones. It has a dual duration of 262/272 (int/exh) with a lift of .450 and and idles at 850rpm (the stock 280 cam is .433 lift, 248 duration). Sounds like a mild, weird grind, but Crane is a reputable firm with close to15,000 grinds and I didn't quite trust the noname, regrind cams. From everything I've read, running a cam with 280+ duration on a stock motor drops cylinder pressure (and horsepower), so I'm not sure how people can use a "race" cam with just headers. Personally I believe most people over-carburate, overcam, and over-rev most motors instead of creating a flexible, free-breathing, torquey one for the street. So it was very tough making the decision between stages of cam. But I decided that on a quick street motor, sacrificing bottom-end acceleration for the small amount of time spent above 5k wasn't a wise move. The CHANGE? Well, the big 2.8 liter torque lump between 2-3k is lessened slightly now, but from 4-6.5k it pulls much harder, faster, smoother, and without that slight straining that L28 motors seems to have above 5.5k.At first I thought that it might be slower because the bottom end had changed, until I timed it. My unreliable handtimed 0-60 runs dropped immediately from an average of 7.2-7.8 seconds down to 3 consistent runs of 6.6. And 60-80 in third gear dropped from 4.8 to 3.7. YOW!!! And this is without re-synching the carbs.Makes me wonder what Mikuni 6-pak carbs would add.....

CAM TOWERS

I expect to get a lot of flak on this: but it's ok to remove your cam towers to replace a cam, really! Now before you get upset let me ask you one question:. Do you think it's a bad idea because of hearsay or have you actually tried it? Removing cam towers on a L-series motors is almost madatory when doing a valve job (try getting those springs off) or when adding shims to the tower after shaving the head. So why all the fear? Well, Chiltons (which is hardly a source) made a dire warning in their manual about not removing them because of line boring, which everyone now quotes as the gospel. However, racers and mechanics have been doing it regularly for decades with no problems. My  Z garage friends chuckle when they hear predictions of broken cams, implosions, etc. having done dozens over the years. In fact Frank Honsowetz (a Nissan Competition Engineer) recommends it in his book "How to Modify Nissan OHC engines". So I was worried when I had to take off my towers when lapping my head, but they bolted back easily with no problems. I took them off again when replacing my cam, and on install the new cam turned even easier than the stock one (!) Just make sure that you torque them to specs and the cam rotates as smoothly as before. After all, if you can bore your block, replace crankshaft bearings, and easily reinstall everything why should this be so hard? Brain surgery it's not..

EXHAUST

People get very passionate about which exhaust system they feel works best, as though every horsepower the engine develops comes from the muffler. I used to autocross a '76 and just broke in my '70 with an '83 L28, and I've used a single 2 1/4" pipe on both. Over the years, I've heard from several people who maintain that a 2 1/2" pipe on a Z causes a low rpm resonance which creates a large flat spot in low mid-range acceleration. I can't verify that, but I do know that running open headers on any car can leanout the mixture and also screws up scavenging, This is why tuned headers and exhausts exist. So I feel that for a balanced street motor, a header and 2 1/4" pipe flows well, and keeps the engine snappy .BTW, I've heard you should not use the Cherry Bomb brand of glasspacks due to poor flow. I checked them out and I agree.On a 2 1/2" model the inner diameter necks down to what looks like 1 1/4". Talk about a poor design. I currently use a Motorsport Auto 6-2, header, Dynomax glasspack, and a Monza single tip. Looks great, sound's incredible and pulls like a Tomahawk from idle. A muted rumble at low speeds, opening it up creates a brilliantly evil snarl that's not raspy or motorboat-like. The combination of cam, header and high-output ignition makes me want to keeping blasting it constantly. No tickets yet, I back off when I see the Bronze. Back to the Cosmic Garage

Thanks to GEOCITIES for this page. Check them out..