Head gasket replacing

[Jimbo2000]

hey

how much work is involved in chucking in a metal head gasket, is it a real mission and do you have to pull the whole engine apart or is it quite simple?

i'm guessing its not that easy or everyone would have it done as gaskets only cost a few hundred bucks

[Steve]

Dont have to pull the whole engine apart, just remove radiator and disconnect just about everything attached to the head/plenum/turbo.

I seriously suggest if you dont realise how much work would be involved by lifting the hood and having a look, it may be worth getting someone else to do it. I was quoted $300-500 to have a head gasket changed (me supply gasket). Ended up getting cams and plenum done at the same time so it cost a bit more.

[46n2]

I'm nearly certain that if you go for a metal head gasket, you'll also need to get the head & block's touching surfaces machined (this makes for a perfect fit). You don't need to be this finniky (spelling?) if using a standard head gasket (about 135 from Nissan).

IMHO you'd only need to go to a metal gasket if you were running a lot of boost.

Please feel free to disagree if anyone feels I'm talking thru my arse (ie- pls correct me if I'm wrong).

Cheers,

Michael

[Sydneykid]

Hi Jimbo2000, why would you want to change your standard head gasket?

[Steve]

Benefit of head gasket (and dropping CR)- 40deg day, 1.7bar, 3deg more ignition than my tuner (tunes a 9sec GTR) usually runs with similar setup (minus the gasket, and he got paranoid, it wasnt the car that was showing problems), 12:1 air fuel ratios, 309rwkw and only 18 on the power fc knock sensor, on the road - on standard 98ron. I am happy with the results. Since fitting a better cat back, power has increased (seat of the pants) and knock has gone down further. I did cams at the same time, and the supposed waste of money trust plenum, but off boost and throttle response is definately improved.

Ok there are other ways to skin a cat, but it works for me.

The HKS head gasket has metal stoppers around the pistons and a rubberised coatingl over the rest of the gasket so its not necessary to prep the head and block any more than if fitting a stock gasket. Just put it on and bolt up the head.

[B-Man]

Sorry for OT - but interested further in the 'supposed waste of money' Trust Plenum - I just got mine put one (haven't got the car back yet) but interested in further details on response and power (seat of pants)

[Steve]

That was a bit tongue in cheek, as sydneykid has said repeatedly, the results from his testing of the plenum (dyno, road or both????) have always been negative, ie, the stock setup performed better.

I am very, very happy with my trust plenum, money well spent IMO

[B-Man]

Oh - good !

I get the feeling that I will be happy with mine....

OK, back on topic - I have a standard head gasket (RB25DET) if anyone wants it ??? $100

[Sydneykid]

Hi B man, Steve and I have a friendly disagreement over aftermarket plenums and head gaskets. I can quote my experience which support compression ratios at 9 to 1 (or more) and standard inlet plenums at 450 bhp, which is the working max I recommend for internally standard RB25's.

I just think there are better ways to spend your money.

Some examples....

I reckon I get better value out of spending $1200 on a pair of camshafts compared to a plenum.

I reckon I get better value out of a $800 port and polish compared to spending $800 on a head gasket and the labour to change it.

Maybe it comes down to how I measure "better value". I am looking for an increase in average power over the rpm range I use, not just an increase at one single rpm point.

Hope that helps

[DoughBoy]

If you are taking your head off to port and polish don't you need to replace the gasket anyway? Or did you have a great new way to do this without affecting any engine gaskets?

[T0nyGTSt]

Yeah but Sydneykid... ya gotta admit that for "full on in effekt bling bling", the Greddy plenum looks sweet on any Skyline....

I question the effacy esp. on a GT-R.

Oh yeah I paid for full port and polish but stuck with the OEM gasket.

T.

[B-Man]

Sorry to Hi jack a thread on Head Gaskets again:

Hi Sydneykid, maybe I should put my build in perspective :

Some examples....

I reckon I get better value out of spending $1200 on a pair of camshafts compared to a plenum.

Done that !

I reckon I get better value out of a $800 port and polish compared to spending $800 on a head gasket and the labour to change it.

Done that !

Additionally, with my setup pre Trust plenum, i had a custom front facing plenum (on stock extrusion honed runners) which was bench flow tested as OK, BUT when we put it on the car on 18+ PSI boost, leaked (bad tig welding), and physically breathed (ie you could see the plenum expand and contract on boost and off boost whilst on the dyno). (ie not strongly built)

My options at this stage were to :

a) Revert to standard plenum, which would have invovled expense in re-routing intercooler piping & TB.

B) Get another front facing plenum.

c) Fix the current plenum - which would have been relatively easy

I opted to 'go the whole hog' with the Trust plenum/intake as it was line ball on cost to me, since I didn't have the std plenum (top bit) any more.

Additionally, I was able to test std plenum and the results proved that the middle two cylinders receives a lot more air then the outer two (on each side). ie 240 cfm on 1,2,5,6 and 340 cfm on 3 & 4 - again this was on a bench. So at high boost, this was dangerous to my engine setup.

So I am hoping that my package will emulate the results that other high boost RB25DET cars have seen with the Trust plenum.

I can should be able to provide before before and after dyno readings for my setup (ie before Trust and after Trust) but like I said before, I had some issues with my old plenum.

As a side note, if you look at the design of the Trust plenum/intake, you can immediately tell that the unit has had much R&D - the lines, the curves, the way the intake flows accross all runners is just magic. There is no comparison to my old plenum.

Cheers,

[Steve]

Hmm, some more perspective,

$800 for a head gasket and change vs

$800 for a head port, what about fit and remove, new head gasket, disassemble/rebuild head? All adds up. If you can get a head removed, disassembled, port polish reassembled and fitted for $800, I dont think you are paying the same prices as the general public.

So, lets look at head gasket approx $400, vs port polish approx $700, then you have to allow for the dissasembe/reassemble of the head too.

Not really a cheaper option.

Cams, yep, worthwhile, but so is a plenum:D

[Jimbo2000]

Hi Jimbo2000, why would you want to change your standard head gasket?

simple answer sydneykid- to run higher boost.

i already found out i've got 125 compression across but i want another test done to be sure, i'm doing another few mods this year that i hope will see me to around 230-250 rwk a gasket is just an i idea i have been mucking with.

fellas, dont apologise for the hijack either keep going this is interesting stuff too

[RBVS]

is it true a rb30 gasket is exactly the same as a rb25?

[Steve]

Doesnt RB25 have a gallery for the NVCS oil supply?

[RBVS]

not sure. why does the 25 have a gallery and the rb30 not.....?

[Sydneykid]

Hi Steve we have had this discussion before. I would never put a new head gasket in an RB engine without making sure the head was flat. This requires a modicum of disassembly, a bit less than P&P admittedly.

But for the sake of this discussion, let's go along with your $400 for a headgasket versus $700 for a P&P. For the $700 I get increased performance, if I did nothing else. For the $400 I get decreased performance, again, if I did nothing else. Now I would much rather spend $700 on P&P and get a gain, than spend $400 on lowering the compression ratio and get a loss.

Now your gunna say, "but I can run more boost to make up for the loss of performance due to the lower compression ratio". But I don't believe that's true, because ......

1. Turning up the boost won't help the off boost performance

2. Turning up the boost won't help the engine build boost any faster

3. Who's to say that with a "better tune" I couldn't run the higher boost anyway

I don't want to get in to a "better tuner" argument, I can give heaps of examples from our experience. But that won't achieve anything, the best I can do is say that I have seen a Japanese 1100 ps RB engine run 9.3 to 1 compression ratio. If they can do it, so can we.

Let's move onto plenums, I have no argument with the "it looks better, so I'm doing it" reason. But anyone who tells me that they are "doing it for performance" is gunna get a healthy discussion going.

Sorry B-Man, but looking inside the Greddy plenum is not what I would call "just magic". It is full of casting dags and tooling join marks, there is not one tiny bit of hand finishing on the inside. Sure the outside looks pretty, but I don't think the engine cares 2 hoots about what it looks like. Personal experience has been that they don't flow very well and they have poor distribution. The rear cylinders (particularly number 6) suffer from pressure wave build up and get more airflow than the front cylinders. This results in leanness of the rear cylinders which can easily be seen on the dyno with primary exhaust manifold temperatures and lambda readings.

I have never ever seen this with the standard inlet manifold. Testing under vacuum on a flow bench that shows a difference of 41% (240 / 340) is simply not the same in the real world under boost. If that was realy the case Nissan would have 370cc injectors in #1, #2, #5 & #6 and 525 cc injectors in #3 & #4. Now we all know that is simply not done.

Sure upgrading plenums has a place, my rule of thumb is on an engine at the limit of the standard internals (ie; over 450 bhp). But until you start doing internal work on an RB25 then there is nothing to be gained from spending your money on changing the plenum. Just recently I have seen a couple of cars out of Japan with over 500 hp that still have the standard plenum on them, Zeni Tani for example are strong advocates for keeping the standard plenum on RB25's up to 550 hp.

Don't get me wrong, I am not saying you should never upgrade the plenum. What I am saying is there are better ways to spend your money. Sure if I had done everything else on an RB25DET and still had some money left then I would do the plenum. But it would be very low on my priorities.

Actually it wouldn't even make it onto my priorities, if I had that much money to spend. I would put an RB26 head and inlet system on it and benefit from the multiple throttle bodies, non hydraulic followers, no VVT, wider variety of camshafts, larger head studs, etc.

For RBVS, RB25DET's have VVT and RB20/26/30's don't. The oil gallery supplies oil under pressure to the actuator that rotates the inlet camshaft drive and changes its timing. It is easy to add the necessary hole in an RB30 head gasket though. There are a couple of other small mods that you also need to do, but they are obvious when you put the 2 side by side.

Hope that adds some more to this interesting thread.

[B-Man]

Hi Sydneykid,

What I meant by 'just magic' was the design of the plenum/inlet - not the quality of the manufacture. Infact I agreee with you whole heartedly about the casting dags. I found these myself and posted a different thread about it (to which I only received only a small response). But as you would know, these dags are easily removed (yes I can hear you say more money wasted on a plenum).... Oh by the way, when I took my standard plenum off, there were many more casting dags and manufacuring imperfections on it than the Greddy plenum.

Back to the design. It was pretty clear to me that the Greddy plenum has been designed to at least try and get even air distribution to all runners. Unlike (in my opinion) the standard one where air from the TB hits runners 3&4. At high boost that can't be a good thing (IMO).

If I take the Greddy plenum and look directly through the inlet where the TB goes, I can physically see all 6 runners. The plenum is angled back near to perfectly so that the distance between each runner (plan view) is the same - hence (I would think) a more even air distribution. Notedly, this doesn't take into consideration the time it takes for the air to get from the front of the plenum to the back of the plenum. There are no flat stops withing the plenum all intake runner to plenum joins are curved nicely - even 'looks' to have a kind of bell mouth effect at the end of each runner where the runner spills into the plenum chamber.

When look into the standard plenum, I see runners 3 and 4 only and that's it. I got my figures mixed up in the previous post it was more like 340 cfm to 3 & 4 - 260 cfm to 1 & 6 - Still a 22% differenence.

I have no-doubt that you are seeing 500HP cars with standard plemum - I would think though they are seeing more air flow to 3 & 4 than 1 & 2.

I also have no doubt that an RB26 head swap to my RB25 is a good mod. To me, this sounds like a very expensive mod too , with maybe only a few good workshops in Sydney having the experience to perform this - And I bet they would charge me for it too. ! And those without the experience but have the ability - would charge me for it too.

What would be the price of RB26 head, plenum, multi throttle boddies, gaskets, machining plus installation onto and RB25 ? I would be guessing maybe $2000+ for the head and what, a day and half to fit (12 hours) x $90 per hour - Roughly $3000 all up. (my Guestimate).

In my case - I needed a new plenum anyway.

I had $1500 for the cost of the plenum and labour to fit.

I didn't have $3000 for RB26 head upgrade.

Yes - the RB26 head upgrade would get me more than twice the performance increase than the Greddy plenum would give me.

But I have already spent way to much money on my car - so every $1500 I save, I can put into my mortgage.

Lets look at it from the other perspective - I go back to standard plenum - $200 for standard plenum, TB & TPS (I had to buy a new plenum,etc as my old one was butchered to make my crappy custom plenum). $200 worth of piping to redirect pipework from LHS to RHS. 4 hours labour incluing all the TIG welding - $400. Thats $800 cost to me -

(note the above is a guesstimate)

Lets call the difference between the $1500 and $800 - 'high heels'

Cheers,

[Steve]

Sydneykid, I agree that a P&P will give a power increase, where as a headgasket will have the opposite effect.

Just a note though, I had the head skimmed for around $80-100 (from memory) which I would have had to had done with a P&P. BUT I didnt have to pay to have the head pulled down and reassembled. Labour here varies between $60-70 an hour, so it isnt like we are talking small change - it all adds up.

I have seen as high as 9.5CR on a gtr that puts out well over 400rwkw, BUT it has forged internals, AND it doesnt run on straight pump fuel, as mine does.

So dont get me wrong, the decompression is purely for a street car that I can drive around all day on pump fuel with 1.7 bar on a hot day and dont have to worry about detonation.

It all boils down to application, I just want a car I can get in and drive without having to worry every time a hot day comes around or if I dont have enough tolulene in the boot when I want to fill up - and it doesnt go too bad either, throttle response is good, off boost is good. Very streetable.