F54/P79 Swap Project

[jonbill]

I would suggest as little as possible to clean the head up. I’ve heard .030 inches is about all you can cut without affecting the timing. Maybe someone else knows the exact number. .010 to .015 is where I would try to stop.
Guy

Any amount you change the cam height affects the timing. Slimming the head a small amount I think advances the timing a small amount. This can be adjusted for with the 3 dowel jokes on the sprocket, or an adjustable sprocket.
The limit of this is the amount of chain Slack the tensioner can take up which may be somewhere around. 75mm for the standard tensioner.
Cam tower shims aren't the only remedy for it though - there are alternate tensioners you can get (from Kameari for example) which can handle more Slack in the chain.

[siteunseen]

46 minutes ago, Zed Head said:

I think that number is in the "Rebuild" book.  Mine is boxed up right now otherwise I'd find the page.

The MSA page says 7 thousandths, but they don't give a reference for the number.

Page 91...

 

Edited June 26, 2019 by siteunseen

[siteunseen]

15 hours ago, Captain Obvious said:

Thanks for the input everyone. All that sounds like fun, but in the cause of expediency, I'm really hoping to leave the crank and pistons in for now.

Diseazd, Question for you while you're here... I remember talking to you in Atlanta (over drinks and pool) about that .080 cut off the P79 head. So how far can I cut the head before I need to shim the cam towers? Do you know how much change can the chain tensioner account for before needing to mitigate with shims?

 

I'm sure you've read this before but you know me...flog that dead horse! 

http://www.datsunzgarage.us/p79/

[Captain Obvious]

The FSM says that each different alignment hole in the cam sprocket adjusts the cam timing forward by four degrees. To make sure I knew what was going on with that I checked it out using the cam gear.

Here's a quick sketch I whipped up using the cam gear off my P79 head. I traced the holes and sprocket teeth in the three different positions, and guess what... They are two (cam) degrees apart. That's perfect because two cam degrees is four crank degrees. Note that the direction is backwards because I could get a better registration with the cam gear flipped over, but the delta between the different positions is what's important. Here's what I whipped up:

It would appear from the calculations, that you could take .080 off the head and advance cam the gear to it's next setting and pretty much be exactly back to where you started.

Probably showing my inexperience, but assuming the tensioner can account for the difference, why can't you just do that instead of different valves and a whole bunch of shims under the springs and cam towers? Seems like the only thing you would need to do is move the cam gear.

Edited June 26, 2019 by Captain Obvious
Included a note about flipping cam gear over for sketch

[Captain Obvious]

3 hours ago, jonbill said:

 Slimming the head a small amount I think advances the timing a small amount.

I don't think slimming the head results in advanced timing. I believe removing material from the head results in retarded valve timing. I think about it this way... The distance between the crank and cam got smaller, but the length of chain between the two did not. So the cam will trail the crank by that additional length.

Did you mean slimming or shimming?  

[Diseazd]

2 hours ago, siteunseen said:

I'm sure you've read this before but you know me...flog that dead horse! 

http://www.datsunzgarage.us/p79/

Great illustration Site.....and I spent hours in Roanoke with the Z Doctor. He built all of my P79 heads....he’s the one that originally came up with the set up. It tricks a P79/P90 into stock chain configuration with a 10:1 c/r. As far as advancing the cam sprocket 4 degrees to make up for an .080 cut without shims Capt., I think if that worked, it would have been done that way. I think Jeff G figured out some offset bushing for the cam sprocket that eliminated shims for his race car. I miss Doc!

[Reptoid Overlords]

I'm having a machine shop do what is put forth by the Z doctor for my P79 head. I will be leaving the F54 stock except for new seals/water pump and oil pump etc. And of course headers and more exhaust.

For around $400-500 it seems like a good easy way to get a bit more out of an otherwise stock L28 and remain as reliable(leaving the cam stock for original EFI computer). At least for me, as someone who doesn't have much experience building engines.

Sent from my N9130 using Tapatalk

[madkaw]

I would think .080 head thickness reduction would be difficult at best to maintain proper valves timing without shims. First you would be  slotting your chain guides to slide them over enough to make up for the serious chain slack. I wouldn’t trust doing it without shims unless I could degree the cam. The cool part of that p79 trick with the taller valves is I believe you don’t have to mess with lash pad adjustments ( Guy can verify ) if you keep the stock cam or a slightly modified cam. 

I currently run my E88 with .050 cut off with no shims. I did degree my Isky cam . I am actually running 8 degrees advance on the cam .

[Diseazd]

Right Madkaw......I used Stage III Isky Cams in mine. .160 to .180 lash pads for that set up.

[jonbill]

I don't think slimming the head results in advanced timing. I believe removing material from the head results in retarded valve timing. I think about it this way... The distance between the crank and cam got smaller, but the length of chain between the two did not. So the cam will trail the crank by that additional length.
Did you mean slimming or shimming?   [emoji185]

I meant skimming [emoji4] yeah, I wasn't 100% sure whether it retarded or advanced. Having thought again I agree with you, it'll retard it.

[Captain Obvious]

Thanks guys. I did some searching around online and the issue with the .080 cut seems to be more about the chain tension, rather than the valve timing. The valve timing can be adjusted using an adjustable pulley or the stock holes as I described, but with that much of a cam drop, the tensioner can't deal with that much slack.

There are other options to take up the slack, but I haven't gotten to the bottom of the details yet.

But the bottom line seems to be: Yes... You can relatively easily mitigate the cam timing, but you can't easily mitigate the chain slack.

[Diseazd]

I love your attention to detail Capt.  Did you go to engineering school or just have the knack? That coming from me, who never even reads the instructions! ?