Head gasket replacing

[Sydneykid]

This is a good discussion guys, I like to have other peoples opinion and experiences. The more the merrier. I am sure as hell not going to put down what you have done, you got off your butt, stopped this keyboard stuff and went out and did it. It's all good.

So I'm not going to knock the B-Man's approach, it obviously worked out well. But let me put an alternative..........

Let's say I agree with your $3,000 for an RB26 head conversion.

How much would you have got for your RB25 stuff? $1,000?

How much did the Greddy plenum cost? $1200?

You put cams in it didn't you?

How much did you spend on cams? $1300?

Could you easily buy used RB25 cams? I never can, I always have to buy new ones.

Can you easily buy used RB26 cams? I sure as hell can, bought 2 sets for $900 last week.

So $3,000 less $1,000 less $1,200 less $1300 + $450 = $50 saving if you had gone the RB26 route.

I have done these numbers heaps of times for our RB31DET's, it always works out much the same using an RB26 top end compared to an RB25 top end. When you take into account all the things involved.

I understand that everyone is not in this situation, but I use this example so, that when faced with the plenum problem, you can at least make an educated choice keeping all the interrelated items in mind.

Lastly the comment "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 have tested this theory on a 295 rwkw RB25DET on the dyno with the standard plenum. I have a laser guided pyrometer with which I can very accurately record the temperatures of the exhaust manifold primaries. There is no discernable difference in temperature between 3 & 4 and the rest. Consequently there is no discernable difference in airflow because I have used a set of matched injectors. If there was an airflow difference then I would be seeing increased individual EGT temperatures as a result of the leanness.

I have also used individual lambda sensors (we have 4 Motec F&W's) on another car (it had fittings welded into the primary pipes). This car had a Greddy plenum and it showed leanness in A/F ratios in #6 (and #5 to a lesser extent). This was despite having the slightly higher flowing injectors in those cylinders. To fix this I had to adjust the individual injector duration on the ECU, around 3% on top of the already higher flowing injectors (2% and 1% respectively).

Now this is not much (5%) but if you don't correct for it you end up with a couple of lean cylinders or you end up with 4 rich cylinders. So it's either detonation or power loss, take your choice.

Hope that prompts some more discussion

[Steve]

And the cost of modifying the manifold to the RB26 head?

[B-Man]

Let's say I agree with your $3,000 for an RB26 head conversion.

How much would you have got for your RB25 stuff? $1,000?

How much did the Greddy plenum cost? $1200?

You put cams in it didn't you?

How much did you spend on cams? $1300?

Could you easily buy used RB25 cams? I never can, I always have to buy new ones.

Can you easily buy used RB26 cams? I sure as hell can, bought 2 sets for $900 last week.

So $3,000 less $1,000 less $1,200 less $1300 + $450 = $50 saving if you had gone the RB26 route.

I don't disagree (whoops double negative)..... Hindsight is 20:20 hey Sydneykid. I didn't think that I would need the new plenum, but I did. And yes it already had the Tomei cams in it.

How much are my std RB25DET cams worth out of interest ??

How far off the mark am I with the $3000 ?? Is it more or less than you would have thought ??

Steve - which manifold do you mean ?? I think Sydneykid meant that the RB26 head came with it's original front facing plenum - so in essence the whole job lot. (Negating the need for the Greddy plenum/manifold & Tomei Cams)

The other thing is all the gear I got was new - not second hand.

Anyway, I think this thread was about headgaskets.....

[Steve]

I was under the impression that the exhaust manifold is diff RB25 to RB26???? IF this is the case, you would have to have RB25 manifold modded or get an RB26 one

[B-Man]

Good point Steve !

Sydneykid, can you confirm ? And what would be the additional cost associated ??

Cheers,

[Sydneykid]

Hi guys, I assume it's a stainless or steam pipe exhaust manifold? If so, there are 4 small bits to be welded on and drilled to line up with the RB26 studs. Maybe $100 to $200 max, its a simple job really, any exhaust shop or decent welder could do it. If it's a cast manifold, then it's not really recommended to weld the extensions on.

You could always buy (an RB26 one) and sell (the RB25 one). Used RB26 exhaust manifolds seem to be far more common than RB25 ones. So you might even make money doing it this way.

As to the question of whether $3,000 is a good price, well it really depends on timing and what's around. I bought a complete RB26 with a blown headgasket recently for $2,800, I sold the bottom end bits I didn't need for $1400, the exhaust manifolds and std turbos for $500 and the std cams for $300. So the RB26 top end to go on the RB31 bottom end cost, $2,800 - $1400 - $500 - $300 = $600. Which I though was an OK price.

I then put some big, used Jun camshafts in it, with some new Isky valve springs and a set of used Jun adj pulleys. We flowed the head, equalised the combustion chamber volumes, polished them and the ports. So that was $600 + $450 + $380 + $250 + my time = probably a bit less than $3K but not much. For a 900 bhp capable top end, I think that was OK.

But it's all a bit academic anyway, as it totally depends on timing. I might look around my sources for another 6 months until I find another RB26 for the same price with all the things I want and none of the things I don't want. I just have to make sure that I have the money to buy the bits when they do eventually come up, they sell very quickly. It's no good looking today for stuff that you need tomorrow, I bought this stuff over a few months.

The reason why I contributed to this thread was to say to people there is another way. Have a plan of where you want to be, work out the steps on how to get there. Impulse buying or buying out of immediate necessity means you spend more money and, most commonly, end up going down the wrong track.

So when B-man says "Hindsight is 20:20" you are right, but if you have a plan and stick to it then you don't need hindsight.

Hope that clarifies some more

[Craved]

i know i'm bringing up an old thread, but i was wondering about what you need to change to fit the rb26 head onto the rb25 block.. i was under the influence that the rb25 and rb26 shared the same block design.... is this correct - what exactly would need to be changed to do this swap? as runnning the rb31 in a street car causes some power to weight ratio issues i believe when it comes time to register your car! can anyone clarify?

Is the standard rb26 headflow alot better (give more power) than a standard rb25 head?

as i might need to install the front facing plenum to go with a new turbo setup, just thinking... will it be better for power to use a standard rb26 head without aftermarket cams, or to get the front facing plenum and after market cams for the rb25 head?

[Craved]

*edit*, ive done some more research and all i found was that the head bolts are bigger on rb26, and the ignition pack would be wired different.. plus the little bits like piping and throttle cables, exhaust manifold -new one required anyway..

anything major or just little stuff like this?