BLSTIC

what does school holidays have to do with anything?

It all depends on whether I get a car that can easily take an RB. If I get something like a VL or a cefiro, we will probably find out. But I don't like my chances of finding one in good nick for next to nothing.

8.7:1 is not that low. Factory rb25det is 9:1. See the difference? It isn't much. The 8:1 figure was for comparison purposes, (ie if I started with an rb25det). 8.7:1 also more compression than what japanese (8.5:1) and australian (8:1) Legacy RS's use. I wouldn't call it rediculously low. One of the questions was about the quench pads. I asked you whether they have them or not, and noted concerns about the combustion chamber shape. I know if they use them it's not ideal to pull the pistons away from them, but if they don't use them it doesn't have nearly the same effect. I have done the calculations. here. Just so you know, the difference in stroke is 2mm, the major displacement change (RB20-RB25) comes from the bore change. I am moving the pistons down 1mm from where they previously sat. Nowhere near as bad as the spacer plates they used to use in the 80's on 202's and so on, and it's only a little worse than using a thicker head gasket (as some people do to drop the compression). Another question - Do the pistons in an RB30E come all the way to the top of the bore (or very close)?

After some research, I discovered the existence of the RB24S, a combination of RB25 block, RB20 crankshaft, RB30 SOHC head, a carby, and unknown rods and pistons. Which leads me to believe that the RB20 crank can fit into an RB25 without dramas. I want to do this for two reasons, the stroke-rod length ratio improves slightly, and it drops compression significantly. I won't go into the maths (epic long calculation) but if the compression was 10:1 to start with (I think that's what the n/a R33 RB25's have) it drops to 8.77:1. If I start with 9:1 (the turbo version) it drops to 8:1. The only issue that might prohibit big boost is the combustion chamber shape. So three questions: 1. Does the RB25 combustion chamber use squish (also known as quench) in it's design, or is it closer to a hemi style of head? 2. Has anyone done this before? 3. Are there any reasons (such as oil pump drive, or similar) why this wouldn't work? Yes I'm aware that it's ideal to use lower compression pistons (or a genuine DET engine), but for a budget bitsa approach it makes sense, especially when the combined cost of an RB25DE and an RB20 crank is far less than the cost of an RB25DET. And I only give away 70cc for the lower compression. And even if it's not ideal, it's probably still way better than an RB20DET. Thoughts? Ben

You forget, lifter noise doesn't change with noise. I had a similar knock detector picking up engine noise experience... I had a Legacy GT with rooted rings and bearings. After ~2000km the oil would get horrible, and the engine would make enough noise to trigger the knock sensor, retarding the timing, heating the exhaust valves, causing knock, retarding the timing, heating more, etc. But the point was the knock sensor came back with a signal with worn big-ends. [/pointless post]

Opening the valve will drop boost (less spring pressure = more air gets past = wastegate opens sooner), but half a turn didn't do much at all on my GT. There are three normal noises that a turbo car has over an otherwise identical car without one. Turbo whistle - Sounds like someone trying to hiss. Gets louder/higher pitched with boost (and torque). You can hear this noise free-revving occasionally, in which case the hiss follow throttle position, but delayed and a lot quiter than it is while you are driving. If an air raid siren replaces the hiss generally turbo bearings are f.u.b.a.r. Wastegate noise - Sounds like someone doing a really long "shhhhhhhhhhhhhhh" (with the standard exhaust). With a big exhaust it sounds similar, except when it does it the car gets slightly louder/harsher. With a screamer pipe it sounds like you are running without anything attached to the exhaust manifold, it's brain numbingly loud. You will only hear this while in gear and under load, and generally not in normal driving. If you hear this while free revving you are a) Lying. b)are *really* giving your car death in neutral, or c) Have a turbo 4 sizes too small for your engine. Blow-off valve - Sounds like someone opening a screw top beer (even in standard cars, but it's quiter). Disconnecting the hose from the blow-off valve back to the pre-turbo intake will get that w*nk factor noise little old ladies hate so much. You will hear this a little bit when free revving, a lot when backing off at high loads, and every time you change gears (if yours is externally venting. If it is 'plumb back' you generally won't hear it in normal driving). Now does the accelleration stop building (ie you get pushed back in your seat at a pretty constant rate (but less than what you want)) until you need to change gears, or does it actively drop off (ie a ghost puts the brakes on a bit in the top end of the rev band, and the 'push' drops off noticeably)? *edited: Punctuation*

Want to know how much? Disconnect your wastegate lines and go for a very careful blat. No boost controller, however many lights, buzzers, displays, and functions it has, can get better than that regarding lag. Just be very careful that you don't over-boost, as I'm assuming you no longer have a boost cut. With no functioning wastegates, your right foot can be stronger than your engine (except for GTR700, but that's too cool to mention again in this conversation). If you still have lag issues without the wastegates connected, you have other issues. -Ben

You will find that the turbotech boost controllers are pretty good at holding spikes. Unlike bleed valves where the method of boost control is to restrict airflow along the hose and then bleed off to reduce wastegate pressure (and raise boost), the turbotech valves are actually a pressure reducing valve (an adjustable break pressure check valve, with a small bleed), and increases in boost pressure get to the wastegate much faster (there is no fixed restriction in the system, because it opens up quickly as boost exceeds the set point). Also because of the 'pressure reducing valve' design, the wastegate doesn't actually see any pressure at all until 3-4psi below the max boost level. Ie with the valve open fully (assuming the wastegate has a 4psi spring in it, as most cars with electronic control do) the wastgate will see boost from 0psi. With the valve set to make 8psi, the valve will stay blocked until ~4psi. With the valve set to 12psi, it stays blocked until 8psi. So it's also pretty good for boost response. The only 'wastegate opening early' problems you will have with the turbotech fitted will be at boost levels higher than the stock skyline ceramic turbo can live at (as in, some wastegates can blow open from exhaust pressure alone. At 20psi or so...) No I don't sell them, I have just bought a couple and they worked well. The only gripe is that boost rises slightly with load (as in peak boost in 1st was 12psi, 2nd was 13, 3rd onwards was 14). But every pneumatic boost controller I have come across does that, regardless of design.

I'm not going to do an RB20 any more, since the RB25 head is so hard to fit. If I could edit my first post to change the topic I would. But I can't. So budget rb25 info is now what I'm after

I'm aware of that. Squish/squash has a lot to do with it as well. Simply lowering the pistons (what de-stroking does in regards to compression) will make any squish area ineffective. I haven't actually seen an RB25 head off an engine though, so whether or not it actually has a combustion chamber that relies on squish is something I don't know yet. If it has no sqish pads (for reference I am talking about the flat parts of the head that are inside the bore area. The piston comes up to within a millimeter or so of these and it creates turbulence in proportion to the size of the pad) de-stroking can provide a win. If not, it won't work as well. But does anyone know what RB*20* pistons will give the lowest compression in an RB20DET? Just acedemic interest now, because I can't use the high flowing head on the RB20 I won't bother. Still want to know though. ***EDIT: Where's my edit button for my previous post? I made a mistake and want to fix it. This: should read ***

Caution - Maths follows Well here are the numbers for lowering compression by de-stroking. (after re-calculating due to brain fade while calculating initial combustion chamber size. Starting with a 10:1 compression RB25 (as they all are I think). compression ratio = (swept volume + combustion chamber) / combustion chamber 2498cc / 6 cylinders = 416.333333333cc/cylinder. 10 = (416.333 + x) /x 10x = 416.333 + x 9x = 416.333 x = 46.259 so it appears that combustion chamber volume = (compression ratio - 1) / swept volume 416.333 * (69.7/71.7) = 404.72 416.333 - 404.72 = 11.613cc (new swept volume, but only half of that gets added to the combustion chamber) / 2 = 5.807 46.259 + 5.807 = 52.066 new CR = (404.72 + 52.066)/52.066 = 8.77:1 So a de-stroked RB25DE has lower compression than an RB25DET (while becoming an RB24). Not by much but it's cheaper getting an RB25DE and an RB20 crank than it is to get a genuine DET. I'll do it again starting with a DET as a base (9:1 compression) x = 8/416.333 = 52.042cc combustion chamber 52.042+5.807 = 57.849cc new combustion chamber new cr = (404.72 + 57.849) / 57.849 = 8:1 compression [/math] 8:1 compression is highly desirable for big boost, and can be dropped even further if there are pistons that have a lower crown height (compared to the gudgeon pin) available out of any 86mm bore RB... It might be worth comparing the factory RB30 pistons to the RB25 items.

I did a search and couldn't find it in the first two pages "rb20 block rb25 head". I can however try my hand at porting and modifying the combustion chambers of the rb20 head. Won't be as good as a stock RB25 head though. Alternatively there's a 2mm difference in stroke between the two, meaning I might be able to de-stroke an RB25 for lower compression while keeping the good head. Dropping 70cc in the process... I'm about to head home so I'll do the compression ratio calculations when I get there.

I used a turbotech boost controller (essentially a pressure reducing valve) on my Legacy GT. Cost $30 or so delivered and the boost came up fast and smooth. With a 4psi actuator (stock for most ecu controlled turbo systems) and the valve set so that I would just miss the boost cut (14.5-15psi) I would get 12psi in 1st, 14psi in 2nd and 3rd, 14.5 in 4th and 5th. On cold nights it would hit the boost cut under heavy load (ie car not accellerating much but at full throttle for a while). After fitting the large exhaust the boost rose maybe 1psi in all situations (so I had to back off the boost controller to avoid hitting the boost cut), and I was able to get 13psi in 1st while still avoiding the boost cut. I really don't see why you need better boost control than that, and it's worth spending $30 on something that has a good chance of working (and is easy to set up) before spending $300 on something that is guaranteed to do the job but takes a lot more to set up. Plus you can sell it again if you don't use it. Incedentally taking the wastegate line off saw no change to boost response below 9psi with this particular controller, so no boost controller is going to bring boost up faster to that point without an anti-lag system.

Ok I'm considering getting a skyline or cefiro. Or even a VL crumpledoor. Here in nz they come with RB20E's... So I'm assuming that there are three RB20 bottom ends available, with the pistons being my biggest concern. Based on theorising the DOHC engines will have the highest crown height (due to multiple small, low lift valves, as opposed to the SOHC's larger valves that probably open further). So given the same head n/a DOHC pistons have the highest compression and SOHC pistons have the lowest compression. Is that correct (ie will SOHC pistons should give me lower compression than any of the factory DOHC items)? And can I use the RB25 head on the RB20 bottom end without hassles? This is due to the higher flow of the RB25 head and hopefully lower compression. Assuming that the combustion chamber in the head isn't larger than the bore, which is the thing most likely to stop me. Basically the goal is to assemble the lowest compression RB20 I can screw together. Mainly because my 'tuning methods' are a little agricultural so I need a little leeway in regards to detonation. And wiping the floor with my mates while costing half as much is a pre-requisite. Any thoughts or suggestions? Thanks Ben