Boost and Piston Damage

[Steve]

Peewee, decreasing the hole that the air has to move through would necesitate the increase in air pressure to move the same amount of air in = hotter air charge = more prone to detonation etc.

On the exhaust side, well the increase in power from the change to an aftermarket exhaust speaks enough there.

The practice of increasing port size and improving flow characteristics is a well known one that is proven, port reduction will restrict flow - all other things being equal.

The least resistance to getting the air in and out, the better, no?

I really cannot see how restricting air flow can increase power potential?

[Guest Peewee]

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[Guest Peewee]

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[Steve]

Have you heard the word patronising?

I finished school 20 odd years ago, and yes I have rebuilt carbies, so yes I know what a venturi is, and what the venturi principle is as I also studied physics.

And the pressure drop is only present IN the ventur not before or after, what is your point. Then again please dont reply.

If you dont want to contribute why the hell are you posting, the only thing I have seen so far from you is a desire to blow wind up your own arse.

Comments like " i have played with more gtr's and gtst's than i reckon many of you have even seen so y when i lend my opinion do you all get shitty I own a skyline i work on skylines all day ..."

IMHO, anybody that needs to make a comment like that without knowing the company you are in, please.

Ok, so in keeping with your above, you are a skyline mechanic - please tell me where you work, as if I ever need any major work done, I dont want to visit you - you'll tune my car so that you blow holes in the pistons, then you'll try and give me homework when I ask a question.

Then,and this takes the cake, after refferring to the vast superiority of a F1 engine to a shitty old RB25, you comment that the whole problem with the RB series ie the bore is too small? WTF Where did you come from and please go back

Another quote"..have you ever spent any time on a flowbench ..."

I am asking a question, if you have had comparisons for an RB engine before and after you have filled up the ports to make them smaller great - I would have loved to hear about it. Or even if you could have answered my question with pure scientifice theory that would have been fantastic. I have seen acutal results on a flow bench of an actual head that was ported - and do you know what, they removed material, ie made the bloody thing bigger not smaller.

If you have so much wonderful experience,and can do it so much better than the japanese, perhaps you should consider a consulting job with Nissan. I am sure that they would love you to come along and sort the problems they are having with such a crappy donk as the RB series. The whole world could benefit, becuase arguably there arent alot of better engines around than the japanese develop.

Another one "..in exhausts the slower the air moves the hotter it gets explain that cause i can if u like but you brought up the exhaust thing you prove it!..." I dont need to prove anything, and in fact I am not sure you are speaking the same language - I didnt acutally bring that up at all. Do you acutally read what people post or just pick out the small words so you dont get confused?

Just my 0.02c

[Xeron]

The RB dosn't have too small a bore, it has too short a stroke

Anyone happen to know the stock rod length on the rb engines anyway?

[Guest Peewee]

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[GTS-t VSPEC]

Peewee,

I'm with Steve, I don't believe your contributing anything useful to this thread, and would appreciate it if your comments had some thought behind them. Sweeping generalisation's like you make are pointless, and you only answer questions with more questions, leading me to believe you know absolutely nothing.

If you wish to contribute useful information then please do so, but no more rubbish.

See'ya:burnout:

[KAOS]

This was all very interesting. Starting to stray too much though guys. I really want to hear more from Sydney Kid and Warpspeed, but if you could keep explaining your ideas peewee as I am interested in those too. I'm not a mechanic or even close, but what I understand and I think everyone here will agree, is that there are a lot of different ways to approach these and all engines.

Peewee, people come here to research and a lot of us don't really have time to do the little assignments, even though they may be good. If you could just explain stuff it would be great.

I am currently in the process of fitting an rb25 head to a 3.0 L bottom end and and will be running a Trust T78.

I have been told to use smaller tubing on the exhaust manifold to increase air speed/pressure to help spool the turbo quicker.

I think this is along the lines of what you were saying peewee.

Warpspeed and SydneyKid, what do you think.

Thanks

[Steve]

Yes things are starting to stray a bit, time to move on. Apologies to all.

[Steve]

Originally posted by Xeron

The RB dosn't have too small a bore, it has too short a stroke

Anyone happen to know the stock rod length on the rb engines anyway?

Xeron, too short for what? I was under the impression that shorter stroke meant more rev friendly? Is that a bad thing?

Cheers

Steve

[Warpspeed]

I have all the internals of an RB26 here with me now. The RB26 conrod looks to be 120mm long measured by eye with a steel ruler. It may actually be plus or minus maybe 1mm or so, but 120mm looks about right.

The RB25 and RB26 blocks are interchangable as far as main internal dimensions go, so if you know the stroke lengths you can calculate rod ratios from that.

I was hoping Pewee was going to give us some cam specs.

Meggala has cam specs for some of the Nissan RB and SR engines on his website. Below are some of these thieved from Meggala.

R33 RB25DET

Inlet cam 240 degrees 7.8mm lift, lobe centre 120, timing 0-60ABDC

Exhaust cam 240 degrees 7.8mm lift, lobe centre 117, timing 57BBDC-3ATDC overlap 3

R34 RB25DET

Inlet cam 240 degrees 7.8mm lift, lobe centre 100, timing 20BTDC-40ABDC

Exhaust cam 240 degrees, 7.8mm lift, lobe centre 117, timing 57BBDC-3ATDC overlap 23

R32 & R33 RB26DETT

Inlet cam 240 degrees 8.58 lift, lobe centre 113, timing 7BTDC-53ABDC

Exhaust cam 236 degrees, 8.58 lift, lobe centre 125, timing 63BBDC-7BTDC overlap 0

R34 RB26DETT

Inlet cam 240 degrees 8.58 lift, lobe centre 117, timing 3BTDC-57ABDC

Exhaust cam 236 degrees, 8.58 lift, lobe centre 121, timing 59BBDC-3BTDC overlap 0

It is a fairly well known trick to retard the GTR exhaust cam about 4 degrees to increase the overlap from 0 to 4 degrees. This helps the top end breating a tiny bit.

If anyone can add anything to this please post some details. I am particularly interested in the variable inlet cam details on the RB25 if anyone has them.

[Steve]

Warpspeed, are the RB25 and RB26 condors the same length. I would make sense to me but I have never been told for certain, and am having trouble finding the specs on the net.

Also, would RB26 rods fit on an RB25 bottom end - might be cheaper than aftermarket forged for my power applications (600bhp) - as from photos the RB26 ones look alot thicker at the neck near the little end.

Also, do you know why nissan use less overlap on the RB26? Is this offset by the increased lift?

Some really nice info there, any chance of posting a link to Meggalas site and the specific page, as I couldnt find it.

Sorry for yet more questions, you've certainly been very helpful so far.

Cheers

Steve

[Guest Peewee]

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[Warpspeed]

Steve, I do not have an RB25 rod to compare it to, so cannot say for sure.

What I can tell you though, is that the RB26 crank, rods, and pistons will drop straight into an RB25 block without any other changes. This is a very good and cost effective modification.

All three components are beefier, the compression ratio drops to a more boost friendly 8.5:1, and you keep the same engine number (hint).

There seem to be a fair few wealthy people importing low mileage R34 GTRs, and as soon as they land, dropping a stroker kit into the engine.

The very slightly used parts end up in the for sale section of this and other Forums fairly regularly. You should be able to get the lot for under a grand. But there are more people waking up to this now, so you need to be very quick indeed.

The Nissan engines need to pass the strict Japanese emissions laws, and valve overlap is a fairly critical factor for idle and light throttle emmissions. If you look at just about any recent engine from anywhere, it will have zero or only very slight valve overlap.

The design philosophy between the RB25 and RB26 is very different. The RB25 is made to have reasonably good low down off boost performance. You can see this with the higher compression, and longer intake runners. Think of it as a turbo assisted engine.

On the other hand the RB26 has lower compression and very short intake runners, it is going to be a slug off boost, and it was designed that way. The turbos provide all the performance, it is a real no compromise high power turbo engine.

How much valve overlap you can get away with depends on how high the exhaust back pressure is in the exhaust manifold. This in turn depends on exhaust turbine and housing size, and wastegate opening point.

Fit a pressure gauge to your exhaust manifold before the turbo, and be in for a real surprise. Stockie engines might typically see twice boost pressure in the exhaust manifold. Winding up the boost actually makes this even worse.

If you fit a high overlap cam to an engine like that, exhaust reversion is going to kill power.

BUT if you fit a bigger turbo with a bigger exhaust turbine there will be less back pressure, unfortunately boost threshold rises at the same time. You can run more valve overlap with a bigger turbo. So little turbo, little cam. Big turbo big cam. You need to get the combination right.

You can get to the stage where the turbo is so big there may be no boost at all below the extreme top end. But the exhaust back pressure may be lower than boost pressure at that point. You can really put a massive overlap cam into something like that, and the top end power would be unreal. But it would be just about undrivable.

There might be no power at all up to a certain high RPM point, then it will just wheelspin up to the rev limiter in any gear. Sounds like fun, but you would not want to drive to work in it every day.

[Steve]

Thanks Warpspeed, so when going up in turbo size, as I will be very shortly, would it be fair to say I should get the car going with the turbo first, look at where boost is being made before sizing up the cams., or is it possible to calculate when looking at the turbine A/R ratio? I have heard it is a good idea to look at the head flow characteristics first too (which will be done during the port etc).

Thanks again

Steve

(more bloody quesitons)

[EnricoPalazzo]

Pewee: How can u say that cosworth engines are the closest to the money that uve seen?

Yes they build F1 engines, and they are probably one of the worst enigines in the series. Constantly blowing up, and underpowered compared to the rest of the cars.

Sorry to get off topic.

My mind is overflowing try to comprehend all this info. Very good guys.

[Warpspeed]

Steve, as far as I know there is no real way to calculate the various combinations of turbine size, back pressure, and camshaft specs.

The only help I can give, and its not much, is to say that for every extra pound per square inch of pressure you can get rid of from the exhaust manifold, you will increase engine output by roughly one percent at the same boost level.

Low exhaust back pressure is also a very good way of fighting the detonation demon. Let the heat escape out of the combustion chamber and down the pipe. A constipated engine will detonate far sooner than a free flowing engine, all else being the same.

When you fit a larger turbo, and keep the boost the same, power will increase. This is due to the reduction in total exhaust back pressure. A larger more efficient compressor may have lower discharge temperatures, but if you run an intercooler, the effect on the engine will be absolutely minimal. (at identical boost level)

Exhaust back pressure is made up of the drop across the exhaust turbine, plus the drop across the whole exhaust pipe length. We all know what a larger exhaust pipe can do for a turbo engine. But a larger exhaust turbine works the same way, but the effect can be even more dramatic.

To figure out what is going on, you need to take some pressure readings. As for camshaft specs, it all depends on what you want and who you talk to. I cannot really give definite advice.

But having said that, a longer duration exhaust cam will free up the engine considerably without losing any low end torque. It will make the engine feel more revvy. If you only change one cam, do the exhaust first. The only downside is that a longer exhaust duration shortens the power stroke, and lengthens the exhaust stroke (obviously).

The effect is that it reduces pumping losses at full power, but reduces engine thermal efficiency at small throttle openings. The reason manufacturers do not use long duration exhaust cams is because it lowers light throttle cruise fuel economy slightly. This may be a big thing for them, but maybe not for you.

A longer duration inlet cam shifts the power curve upward. If you keep fitting a series of longer duration inlet cams you will find that at first there is a slight increase in top end, and a slight decrease in bottom end. As you go more radical, the drop of at the bottom becomes very pronounced, with only a slight or zero top end increase.

For a small turbo street engine, using the stock inlet cam, with about ten or fifteen degrees more duration on the exhaust side. this will liven things up more than you might expect, at half the cost.

With a mid size turbo you might want to increase both cams the same, which is what most people do. Same with a monster turbo, two monster cams to go with it.

Everyone has their own ideas about all of this, so I suggest you talk to as many people as possible. But someone elses idea of a good cam may turn out to be less than wonderful.

[Sydneykid]

Great thread guys, I am really enjoying the read and the debate. I hope I can add something useful.

Carrying on from Warpseed's most recent post, the RB26 rod ratio (120 / 73.7 = 1.63) is relatively low, but seems to be fairly common for RB engines. From memory RB30 rods are around 140 mm which gives 1.65 and RB20's around 115 mm which gives 1.67. This may also help to explain the relative strength of the RB20 despite its lighter weight components.

KAOS asked about T78'S, I have seen 2 exhaust housings and turbines on T78's, from memory the smaller one is designated 29D and the larger 33D. I have never seen it done but I believe you can also fit the 34D from the larger T88 as well. If you want better response, the 29D is the way to go, you will loose a bit of top end power though.

I am not a real fan of Trust turbos with their lack of water cooling and plain bearings. Made by Mitsubishi, their routes are in their trucks (Canters and such like). That said they are very effective at high boost levels and don't seem to suffer much from their non state of the art design.

On that subject, there is a lot of time spent on the forum over boost levels, but the current generation of turbos make this somewhat irrelevant. The real thing that matters is airflow ie; I can (and will) now choose a turbo that provides 45 lbs per minute of airflow at 1 bar compared to another turbo that has 45 lbs per minute of airflow at 1.5 bar. Both will give me 450 bhp, but the 1 bar one will have lower air outlet temperatures, lower shaft stress, come onto boost quicker and build to full boost earlier.

As Warpseed said, the lower exhaust back pressure is a real factor in making maximum power. So this has to be factored into the process of selecting the right turbo for your particular application.

So going back to the T78, for a road car (sometimes track) I believe that for the same power output, there are more driveable turbos available. For a drag only or dyno hero car, they are a very satisfactory product.

The move to variable valve timing is driven more by the requirement to pass the current emission standards, than by the quest for more mid range or top end power. As always it is a compromise, larger overlap and higher lift would give more power but cause more emissions. So VVT is used to close the overlap down when required to pass the emissions testing. I am looking forward to solenoid opened and closed valves, then I can simply program the software rather than change the hardware.

For what our cars do (circuit racing) we have to be very selective in our compromises. We value throttle response and maximising the power within the whole RPM range we use. Not just the single maximum horsepower value at one rpm point. We are also conscious of the acceleration potential of the engine, it's ability to increase revs, this is not the same as its static power output.

In general, these are the same attributes that a "nice" road car possesses. A dyno hero and a drag car do not, of necessity, require these things to the same degree. This is one of the reasons why you see automatics commonly used in drag cars, where the ability to hold engine rpm relatively static is important.

Back to KAOS's question, to give sensible suggestions to appropriate questions regarding turbo engines (in particular) I really need to know exactly what is expected of the engine and the car it is going into.

Hope that adds to the knowledge base.

[Warpspeed]

Excellent sydneykid.

I would just like to add to your mention of variable inlet cam timing on the RB25. As sydneykid says the variable timing is there to meet emissions, it is not there for power.

What Nissan have done is put in an inlet cam with fairly short duration. At idle the inlet cam is fully retarded to remove any troublesome valve overlap, that would degrade emission under high inlet manifold vacuum.

At some low RPM, probably around 1200RPM (not sure ?) but well above idle, the inlet cam is fully advanced. This causes the inlet valve to close early, increasing the low end torque that is available.

At a higher RPM (4,200 I believe) the inlet cam is again retarded. The later inlet valve closing increasing the top end torque and power.

The thing to realise is that the inlet cam duration is still rather short, and moving the cam around is really a band aid solution. For good cylinder filling, really more duration is what is required.

There must be something about the RB25 combination that has caused the Nissan engineers some real grief in having to go this extreme solution, to get it to pass emissions.

Whatever the problem was, they do not seem to have had it with the RB26 engine, so there is no variable inlet cam timing.

If it offered any performance advantage at all, you can bet it would have been used on the RB26 in a flash.

The only other thing I can think of to mention with RB cams, is that the exhaust cam has the tang to drive the cam position sensor. The inlet cam has no tang. So you can use an exhaust cam in either position, but an inlet cam must always be used as an inlet cam.

So if you are going to buy a whole bunch of various cams to mess about with, get yourself all exhaust cams if you can.

[Merli]

This thread sure has degenerated.