Cam Grind Question

[tlorber]

L28 dished f54 n47 head in a '73 240z

I'm considering having the stock cam ground to stage 2 and putting in flat tops. My understanding is that I'll need to resurface the rockers and put in taller lash pads. Do I need different springs as well? Any other parts I'll need to replace?

Thanks in advance.

[Diseazd]

I believe .460 inches is the maximum lift that can be used with the stock springs.

[tlorber]

Thanks for the reply. I'll plan on new springs when the time comes for this project.

[lmcguffin]

You also have to change the valve stem seals as well. There's a lot of threads related to this issue.

Most spring sets for high-lift cams will require that the raised portion of the spring seat for the inner spring is machined flush with the outer spring seat, and then slightly recessed into the head. This is required to maintain the installed spring height for the particular spring load application.

Stock retainers can handle up to .160 lash pad, after that you need special retainers.

I've been doing a lot of research on this as I'm setting up an N42 head for use with the Nismo/Isky L-9 (490) cam.

.460 is the recommended max lift on stock springs/seals; having said this, the Nismo catalog has an L-7 spec (270/.475) cam that doesn't appear to require special springs/retainers. Not sure what to think of this. The stage II MSA/Schenider cam is a 274/.480 spec.

I presume that you have carbs in the 73 even though the N47 head was the fuel-injected, exhaust-liner head from the later 280s. It sounds like a lot of money to invest in what is generally seen as a lesser desirable head (not to start a riot or anything).

[Jeff G 78]

I have still never seen flow data to back up the "N47 is bad" claim. I believe that it is purely an urban legend based on the fact that it "looks" like it wouldn't flow as well.

[=Enigma=]

I read somewhere that with some mild cleanup in the bowl around the base of the valve seat on the P79 that it will flow as well as the N42 and may even perform better than an unmodified N42 due to the high quench chambers. I would think the same treatment would provide some extra performance in the N47 as well. Just hearsay I suppose but it makes a certain amount of sense to me and would be a lot cheaper than changing over flat tops and having to rework the chamber to avoid possible detonation issues. All this obviously depends on what you ultimate goal is for the motor as well as your budget and know how.

Diseazd has some experience with the P79 heads so he may be able to chime in on his experience with the exhaust lined heads.

Anyway, I guess this is really a thread on cam grinds anyway....

Edited January 9, 2010 by =Enigma=

[tlorber]

So it appears this is quite a bit more involved than just the cam?

The cost of the pistons doesn't bother me-the guy I get my 311 parts from can get a set of flattops + rings for under $70.00.

I have a friend that is a magician with porsches that I could have port/reshape the intake runners. All that being said, I'd rather not have to reinvent the wheel just to get a bit more flow.

I have roundtop SU's on the car, plan on running it on the street and through canyons-probably no auto crossing nor track-though maybe some day. I'd guess 180hp to the wheels would be more than enough.

Main constraint is that I want to be able to run it on California Premium (oxymoron?) which is 91 octane.

[Diseazd]

I consult with the Z Doctor in Roanoke for the "word" on Datsun heads. He has over 40 years experience and knows what he's doing. I'm building a number of engines right now, just because I like to experiment. The P 79 head (as well as the P90 head which is the same minus the liners and port shapes) can be shaved .080 inches to increase compression. The towers and valve springs must be shimmed .080 as well. He insists that the quench is the key to the performance advantage of the P79 and P90 heads. I recently asked him since I was building another L 28, should I use the e88 (which I have in stock) with L28 intake and exhaust valves or an unshaved P79 head and he said he would go with the P79. The E88 with the L28 valves would be similar to the N42, so I think that would indicate that quench is the advantage. Most people I talk to, don't think that the liners are an issue in the P79. Supposedly when you use a lift of over .460 inches, you must use valve springs for that application. I would be interested to know if anyone has used a Stage III with a .490 lift with stock valve springs and if so, what is the result? The Datsun engines respond good to higher than stock lift.....especially above 3,000 rpm.

[Diseazd]

http://datsunzgarage.com/ Try this site.......Lots of good info!

[lmcguffin]

My son is running a shaved P79 w/ flattop pistons in his 78 and has no problem with detonation on 91 oct. However, he says he can tell the difference when he gets back to TX and can get 93 oct. I don't recall how much the head was shaved but the resulting compression was around 9.8:1 (based on head cc results).

He also has a stage II (274/.480) cam with stock springs and retainers. Stock EFI and headers. He made a respectable run at the public drag strip (beat a chevelle during trial) however suffered an offseated rocker. I attributed the rocker incident to excessive lash (he didn't check prior) and possible instability due to the valve lift with the stock springs, retainers, and maxed-out lash pads.