Re: p series not always the best...
> First off your right about the P90A.
> Anything with hydraulic valve adjustments
> isn't the best for high performance high rpm
> motors.
I have to disagree here. Done correctly, they most certainly can. I use it on mine and reguarly drive up to and over 7k,(rev limiter at 7300). But for the average joe, it is much simpler and eaiser to use the P-90. But if you are refering to race type motors, yes, what you say is correct.
> There is one important difference you
> mentioned but didn't really look at.
> Combustion chamber shape. The P-series
> unshrouded the valves and spark plug for
> better flow and combustion. A person can put
> 46mm intake and 38mm exhaust valves into a
> P-series head with minimum mods, try this in
> a E31 or E88 and you'll run into problems
> (trust me I know, thats the valves I'm
> running in my E88).
Yes, it is much harder to put the larger valves in the early heads and see the benefit of them without some involved chamber work to unshroud them. The P series head solves some of that by the deeper chamber, they had more room to play with. The valves you are talking about putting in the early heads are huge for that head, 4mm and 5mm respectivly bigger. That is no small change. So I wouldn't say minimum of mods. Even on the later heads, that still requires redrilling the seat, etc... And I would say is only noticably effective if you are running high rpms.
> To make up for the larger combustion
> chamber the '73 240's have a raised dome
> piston (the high performance cast pistons
> Motorsports sells is the stock '73 240Z
> piston), the 260Z went back to the early
> pre'73 style piston because of the increased
> displaced it didn't need the increased
> compression height make up for the larger
> combustion chamber.
Be careful about calling it a dome of any kind, that implies a raised portion of the top. What you are refering to is a piston whose pin height is lower, which puts more of the piston up in the cylinder, reducing the final volume.
> I read your article on heads at your web
> page and I have a couple questions.
> The N47 Maxima head you mention. That looked
> like the head of choice to me, small
> combustion chamber (smaller than the E31 in
> fact) with a with a P-series design. Are the
> valves really small or what?
One of the valves,(would have to check old records), was the later larger size,(later Z heads), and the other was smaller. It was not identical to the P-series. It was a smaller version of it. But more pronounced than the N-42, etc... It still has the exhaust liners, so that limits you, but it offers excellent potential. I can give you the e-mail of the guy who found them and is working on a project budget L28 with that head.
>I would almost have to say that someone modified the head
> that you looked at (if all the Maxima heads
> that you've seen, and I assume that would be
> 3 or more, are all the same design then I
> stand corrected), its a lot more common then
> people think.
It is common like you say, but the design is more sublte than the p series, so some people may not recognize it. again, my friend has a lot more knowledge on it. it was stock, not modified. we were surprised that it has the n47 casting on it, not some other number. so it is possible that there are some Z style n47 heads on maximas. but it was a smaller motor, so the n47 from a z would make the compression low,(it was a L24). lots of posibilities.
>Anyone that knows OHC motors
> knows to increase compression you either
> change pistons (most likely to a dome top)
> or more perferably have the head welded up
> and reshaped. Dome top pistons (large domes
> that is) decrease the amount of HP a motor
> can make at high rpm (the dome interfers
> with the air flow in and out of the head
> reduces the swirl in the combustion chamber
> and all sorts of nasty stuff).
pistons are an expensive way to get hp. so is welding up the well thought out combustion chamber of nissans. you really need someone who REALLY knows what they are doing when you start modifying the combustion chamber. the eaisest and cheapest way to gain hp/ or compression ratio is by shaving the head. it usually produces a better chamber by reducing some of the excess space, emphasizing the better parts, etc... You just need be careful about the slack in the timing chain.
-Bob Hanvey