Head Shaving Question

[Flexicoker]

Hello,

 

I've got a F54/P79, bone stock, but megasquirted in my 240Z. The head gasket gave way and I'm taking the opportunity to replace valve stem seals and lap the valves. If I have the time, I plan to do some mild port cleanup.

 

What I'm curious about, is why can't I shave the head .080 like is common practice, but just correct my cam timing using the adjustments provided with the stock cam sprocket? My math says shaving .080" will only retard cam timing 2 degrees. Taking into account the stretch of the chain, jumping to the next 4 degrees advanced position will probably be just right. This would avoid new valves and cam tower shims. Does the tensioner not have enough travel to accommodate the additional slack? Also, when shaving this much are there any piston/valve clearance issues with a stock cam, and stock flattop pistons? I realize the only way to do this right is to measure the piston/valve clearance. But this is my only form of transportation, so whatever I do has to be quick cheap, and reliable. If there are any doubts of this working, I will just put it back together in stock form. The only chance I have to make it any faster is when something breaks and I can do something "while I'm at it" =)

 

Thanks!

 

-Eric

[Xnke]

You would have to find a way to take up the timing chain slack...the amount of slack is enough that it would eat away the tensioner in very short order.

[ozconnection]

Hello,

 

I've got a F54/P79, bone stock, but megasquirted in my 240Z. The head gasket gave way and I'm taking the opportunity to replace valve stem seals and lap the valves. If I have the time, I plan to do some mild port cleanup.

 

What I'm curious about, is why can't I shave the head .080 like is common practice, but just correct my cam timing using the adjustments provided with the stock cam sprocket? My math says shaving .080" will only retard cam timing 2 degrees. Taking into account the stretch of the chain, jumping to the next 4 degrees advanced position will probably be just right. This would avoid new valves and cam tower shims. Does the tensioner not have enough travel to accommodate the additional slack? Also, when shaving this much are there any piston/valve clearance issues with a stock cam, and stock flattop pistons? I realize the only way to do this right is to measure the piston/valve clearance. But this is my only form of transportation, so whatever I do has to be quick cheap, and reliable. If there are any doubts of this working, I will just put it back together in stock form. The only chance I have to make it any faster is when something breaks and I can do something "while I'm at it" =)

 

Thanks!

 

-Eric

No shortcuts should be taken here. The mods you describe work when ALL engineering aspects have been addressed. Deviation from the beaten path is risky and potentially very costly for you. Change the HG and plan your next engine now. Eventually what you have in there will wear out (hopefully not break) and you can slot in a properly thought out and well engineered L Series! Just my opinion mate, good luck with whatever you decide to do.

[Derek]

Hi Eric

 

Shameless plug here. If you decide to do the .080 conversion I sell the cam tower shims.

 

 

 

 

Derek

 

 

[SHADY280]

no problems with a stock cam or even a .5 lift cam with flat tops. i have this setup. you really need the shims and lash pads. when you raise the towers, it angles the rockers down, so you need to invest in those too. doing this really yeilds a decent amount of extra power and i highly recommend it, but do it once and do it right.

[Flexicoker]

Shady: The goal was to omit the cam tower spacers entirely, leaving the valvetrain geometry completely stock, and correct the cam timing using the adjustments on the stock sprocket.

 

It seems the timing chain slack was the wink link in my plan. Since there's no simple way to take care of that (kameari idler gear setup is not cheap), I'll just put it back together stock. Oh well!

 

Thanks for your input.

[Tony D]

What massive gain do you expect for all this work and use of now non-standard parts?

 

Realistically?

 

Think about that aspect long and hard. I suspect the gains will be minimal in the real world, and neglible in real performance quantification.

 

And the increased CR....

 

What was that you said, you have a blown head gasket?

[JMortensen]

I would do the mod if you have the parts, and just use Derek's cam tower shims. It seems people are reluctant to use the cam tower shims, but they're usually the same people that think that the cam towers are so sensitive that if you take them out and put them back in they'll never be right. IMO, it's really not that big a deal to run shims. I wouldn't let that stop me if you have all the other stuff to do it.

[Tony D]

Question is, what do you gain by the CR bump. Is it worth trashing a head that can then never be decked again without more shims, longer valve stems, etc etc etc...

 

What realistic gains can be expected from this?

 

I posit a cam change from an earlier engine might benefit him more than the bump in compression would.

[Flexicoker]

I never said it was a blown head gasket... more like a coolant weeping head gasket =)

 

I would do everything at once if I could, but time and money situations force incremental improvements. Porting and increasing compression ratio by itself might not gain much now, but can set the foundation for other improvements down the road.

 

I don't have any big resistance to cam tower shims, I just can't afford them, new valves, and the time to source it all at the moment. I do have access to an earlier cam though...

[Tony D]

I never said it was a blown head gasket... more like a coolant weeping head gasket =)

 

I would do everything at once if I could, but time and money situations force incremental improvements. Porting and increasing compression ratio by itself might not gain much now, but can set the foundation for other improvements down the road.

 

I don't have any big resistance to cam tower shims, I just can't afford them, new valves, and the time to source it all at the moment. I do have access to an earlier cam though...

 

Nobody said do it all at once. What do you realistically think you will gain by bumping the compression? You will still have an L28 cam, which peaks at what...5300rpms?

 

Finding an earlier stock cam from an L24 will change the way the engine reacts, allowing more useable rpms, and I posit a more effective horsepower increase.

 

Do you really intend to port and polish a P79 head? And if you do, what will your long term plans be, do they include rpms over 7000? If so pistons are on the horizon and then you are stuck with a shaved custom head with a strange combustion chamber requiring custom cut compression increasing pistons.

 

I'd suggest you skip the planing of the head. I think there is some thought down the road this will be something other than a waste of money. But the head will come off for porting, so why do the head shaving now? Why not wait?

 

The cost of the shaving will not net much IMO. Other than a higher than stock compression ratio meaning now you can run premium gas all the time instead of regular.

 

Whereas a stock early cam that lets that L28 pull to higher RPMS will keep you on pump gas, increase your power, not require shims, valves, nothing (I mean the rocker arms are supposed tobe resurfaced, but if you get that cam from another older head...might find those matched rockers as well...

 

If this is a question of "should I do this now?" I'd say skip it until your plans solidify more. You are polishing a turd. When's the last time you heard of anybody buying a 'performance P79'?

 

OH, BTW....if money is a concern....I'd try half a tube of Aluma-Seal on the head gasket to stop the weep first. Ford would put that much in every car off the line at the factory (Volvo as well) so that is something to seriously consider if the engine is running well and has no other ills other than weeping a little coolant. I've done this one on several engines myself!

[Noddle]

Finding an earlier stock cam from an L24 will change the way the engine reacts, allowing more useable rpms, and I posit a more effective horsepower increase.

 

Tony, do you have a ID number for the L24 cam, if not, would it be a internally oiled one, or the older externally oiled one.

 

Nigel

[Tony D]

Good Point, the L24 cam is solid and not particularly applicable to the later cam tower setup without a spray bar being added. I always run internally oiled cams with a spray bar anyway...

 

I guess in that instance, my recomendation would be just "leave the head alone and Aluma-Seal it!"

 

The P/N is all over the internet. They are NLA from Nissan I believe, but they are in engines all over the place. The early cams in an L28 get rid of the 'slow to rev' complaint people blame on the longer stroke.

[JMortensen]

Nobody said do it all at once. What do you realistically think you will gain by bumping the compression? You will still have an L28 cam, which peaks at what...5300rpms?

 

Finding an earlier stock cam from an L24 will change the way the engine reacts, allowing more useable rpms, and I posit a more effective horsepower increase.

 

Do you really intend to port and polish a P79 head? And if you do, what will your long term plans be, do they include rpms over 7000? If so pistons are on the horizon and then you are stuck with a shaved custom head with a strange combustion chamber requiring custom cut compression increasing pistons.

 

I'd suggest you skip the planing of the head. I think there is some thought down the road this will be something other than a waste of money. But the head will come off for porting, so why do the head shaving now? Why not wait?

 

The cost of the shaving will not net much IMO. Other than a higher than stock compression ratio meaning now you can run premium gas all the time instead of regular.

 

Whereas a stock early cam that lets that L28 pull to higher RPMS will keep you on pump gas, increase your power, not require shims, valves, nothing (I mean the rocker arms are supposed tobe resurfaced, but if you get that cam from another older head...might find those matched rockers as well...

 

If this is a question of "should I do this now?" I'd say skip it until your plans solidify more. You are polishing a turd. When's the last time you heard of anybody buying a 'performance P79'?

 

OH, BTW....if money is a concern....I'd try half a tube of Aluma-Seal on the head gasket to stop the weep first. Ford would put that much in every car off the line at the factory (Volvo as well) so that is something to seriously consider if the engine is running well and has no other ills other than weeping a little coolant. I've done this one on several engines myself!

Tony, did someone steal your computer? Not following your logic at all. All of the stock cams were tiny. Swapping out an early cam isn't worth the effort IMO. I can't tell you what the difference will be before and after, but I would imagine that the compression bump would outperform the measly cam improvement. Compression bump with a decent regrind, now you're talking. I'm just surprised to hear anyone knowledgeable talk about a stock to stock cam swap as if it were a worthwhile expenditure of time and effort. They're all puny, even the lauded "C" cam.

 

Not sure what to make of the cut head/piston comment either. I believe the P90 chamber is deeper to start with, which is why they used shorter valves to keep the valve geometry the same. So cutting it isn't going to really make it radically different anyway, would certainly have less of an effect than cutting an E31 or an N42. Regardless of the head, if you're going for really high compression where the pistons pop up inside the chamber the pistons will have to be modified to fit the chamber. You get to the point where everything has to be checked and clearanced pretty fast. Maybe there is a point in there around 12:1 where you can use an off the shelf dome piston, I don't know, but I certainly wouldn't use it without checking and clearancing the piston as necessary. I seem to remember my buddy had to cut on his Venolias for his L18 build at about that compression ratio. If you're not going for really high compression, cutting the head gets around 10:1 IIRC, which is pretty good for a pump gas L series.

 

According to this calculator (who knows how good it is) you'll get a 5% increase by changing from 8.3 compression to 9.8. I ***think*** that's the bump you get from the shave .080 mod. I would fall out of my chair if you got a 5% increase in power from swapping from any stock Z cam to any stock Z cam (exclude turbo). http://www.camaroz28...s/crchange.html

Edited June 22, 2010 by JMortensen

[Tony D]

The point is that it's better to leave the damn thing alone than start going after 'incrementals'---if you really think an 080" cut is worth the effort, then so is swapping to the "C" cam, and blah blah blah...

 

You want an incremental increase from a P79, throw it in the dumpster and stick a properly dressed N42 on it.

 

Why cut a turd? To get one that doesn't smell quite as bad.

 

Sometimes absurdity does not come across well on the internet. But I am also at work and couldn't really put a lot of effort into making it really over the top.

 

Given the choices given originally, I think I said it more than once that 'leaving it alone' would be the best thing to do. And that Aluma-Seal was a better remedy for an 'all stock' engine as opposed to pulling the head for a gasket change.

 

If you are going to do it, do it right (not all at once, but for gods sake why waste time on a P79???????)

Edited June 23, 2010 by Tony D

[JMortensen]

I am going to disagree again and say that the P79 is fine unless you're doing a serious all out build. If you are, then yeah, junk it and start with something that doesn't have liners. But for the vast majority of weekend racers, they can shave it and do a mild port job and not run into the limitations of the head. I'm sure that the P79 would have been fine on my motor. I had very little time into the ports near the manifolds, and much more down in the bowls and in the chambers themselves. And it would have run on pump gas with the lower compression. Hindsight is 20/20 I guess...

 

EDIT- I do like the Aluma-Seal suggestion. It works well IME. Only person I've seen with a problem was a friend who used it several times a week to seal a bad radiator on a diesel rabbit for several months. He eventually blew the headgasket, and although it's tough to be certain, the several lbs of Aluma-Seal that went into it might have had something to do with it...

Edited June 23, 2010 by JMortensen

[Zmanco]

If the real issue is weeping out the side of the HG, then maybe you should try a different gasket. I had this issue consistently with 2 different heads on an N42 block. The block was checked for flatness before I rebuilt it. And at various times both heads were checked and shaved .0005" IIRC as a clean up. Yet it still wept on the passenger side during track days. I've always run Felpro HG and some say that's just the way those are. If the factory HG was easier/cheaper to get, I'd try that.

 

BTW, I have a P90 that I shaved and I'll suggest something in the middle that picks up on some of what both Tony and Jon say. The cost to shave and shim the head was in hind sight, not worth it. But it's rather cheap and easy to put a reground cam in while you have the head off. I used a Delta Cams regrind of a stock L28 cam (don't remember which one) that makes great power from 3500 to 6500 and still idles great. Plus, with the lower compression you can be sure to run full advance without detonation, something that's iffy with a CR in the 10:1 range, especially on hot days.

 

Put your money and time into cleaning up the P79, add a reground (Delta or Isky), and slap that on and I think you'll find yourself with a potent and reliable motor Try a factory HG while you're at it as they seem to be less susceptible to weep with a straight head and block.

Edited June 23, 2010 by Zmanco

[Flexicoker]

The original reason for asking about the shaving is because the head was already off, and already headed to the machine shop to get cut (there was some slight erosion from where the head gasket was leaking) So if cutting more would be helpful, and realistic without buying more parts, then I'd do it. Since its not though, its just getting cleaned up and going back on.

 

I do have an early 'A' cam, rocker arms, cam towers, and spray bar. If its any better, I can put it back together with that cam just as easy as I can put it back together with the stock cam. It might be marginally less worn as well.

 

When I build an engine in the future, it will most likely not use any parts from this one. But since that might be a year or two away, the goal is just increased smiles per mile until I can afford to do it the way I really want to do it =)

[Tony D]

"BTW, I have a P90 that I shaved and I'll suggest something in the middle that picks up on some of what both Tony and Jon say. The cost to shave and shim the head was in hind sight, not worth it. But it's rather cheap and easy to put a reground cam in while you have the head off. I used a Delta Cams regrind of a stock L28 cam (don't remember which one) that makes great power from 3500 to 6500 and still idles great. Plus, with the lower compression you can be sure to run full advance without detonation, something that's iffy with a CR in the 10:1 range, especially on hot days."

 

This is more along the lines of what I was getting at: the CR increase will give you nothing but problems running regular gas, in addition to rendering a head useless for any other use.

 

You didn't say the head was ALREADY OFF and NEEDED PLANING. In that case: shave AS LITTLE as possible. If you didn't port the head ($$$$$) it's coming off AGAIN anyway to have that done, so if you shave it now and anything happens between now and then, you're in the market for a new head...

 

The "A" cam is better than the later cams. Notwithstanding JM's comment on them all being 'dinky' or whatever, they do have timing which promotes higher rpms than a pedestrian L28. Drive an original L24 and an original L28 powered anything and realize the 'zing' above 4000 just isn't there in the late models. The key is in the cam.

 

And speaking of 'incrementals'---you will change to a solid cam and spraybar ANYWAY so why not do THAT instead... and like suggested have that early cam reground to something that will be slightly better than an early cam.

 

That will pay LOADS more dividends than ruining the head by shaving it 080.

 

Which, I think I said in the beginning...

[Zmanco]

I'll add that including shipping and resurfacing a set of rockers, the entire Delta Cams upgrade was around $150 IIRC. If you choose a grind that keeps the lift near stock (less than .450" IIRC) then you can keep the stock springs and valve seals too. I can't think of an upgrade with more bang for the buck than that.

 

Just remembered you said you need the car as a daily driver so I imagine you couldn't send your main cam to them. Perhaps they have one you can swap with yours as a core, or you can pick one up on hybridz cheap? I'd call them before you get too far along keeping it stock.