RB20 Rebuild questions.

[TokyoTaxi]

Hi,

I need to know if anyone has had any bad experiences ( performance wise ) after rebuilding an RB20DET especially with regards to forged pistons or having head work done on an RB20DET ?

I've just had my engine rebuilt with 40 thou oversize forgies and port / polish work on the head and now with more boost, fuel etc. the car's making less power on the dyno than it did before the rebuild. Nothing else has been changed.

I'm stumped. :confused:

[Fast31]

this might sound silly but did they double check the cam timing. I have seen this done before where it was one tooth out and caused a massive power loss

[TokyoTaxi]

As far as I know it was checked while the car was on the dyno and all was well.

I think I might triple check it myself because it and the crank angle sensor are the only things that I can think of at the moment that might be the problem ( outside of the rebuild ).

[Roy]

What was done in the way of head work? Also was the compression ratio changed with the forged pistons?

Its possible that the thing wont make the same amount of power depending on what was done with regards to headwork/std cams, drop in compression, change in weight of reciprocating mass etc etc.

Same Dyno???

[Cubes]

Exactly what did you have done to the motor?

Blueprinted?

Balanced?

Type of headwork?

Bearing type used?

[TokyoTaxi]

Roy & Joel,

Yes it was the same dyno and there was nothing really done apart from the head was ported ( mainly above the valve area, polished up somewhat, and the seats were cut to a larger size.

Other than that the only things that were done were the balancing of the bottom end and a set of Arias forged pistons ( 40 thou oversized ).

I asked them about the possibility of compression ratio drop and I pretty much got a blank expression and I did tell them 3 times throughout the build that I did not want monster power. Just wanted more torque and a bit more power than standard but I think they've gone overboard. I've got a horrid gut feeling that another head will be required to fix this problem because I don't have the money for cams, bigger turbo, more dyno time etc, etc.

[V8skylineMAN]

u said more boost........is it possible that ur turbo is out of its efficiency range? i dunno, just a thought

[TokyoTaxi]

That's a distinct possibility in my opinion but that's what I'm either trying to prove or disprove so I know if I can either place the blame back on the engine builder and get him to fix it ( since I specifically told him/them that I wanted a build that would make the most of what I had but would not require more $'s in the future to make the build work ). I have put it to the builder and he swears that it should all work fine. Guess again.............

That's why I'm looking for someone who's built an RB20DET in much the same way as me and is running a GT2530, injectors, and a programmed ECU and see if any similar problems occured.

So frustrating...........

[Roy]

Could very well be a drop in compression ratio thats doing it. Perhaps get the details of the Purchase order of the mechanic and chase Rocket / Arias with the purchase order no in hand and get the details of the piston that was supplied.

Then do the calcs of what the compression ratio should now be.

[T0nyGTSt]

Also you might want to be absolutely sure what the new CR is.

I'd imagine this is a cylinder volume test (???) not too hard to do.

T.

[TokyoTaxi]

Can a drop in compression ration make that much difference ??????

The engine builders are going to do a difference check between stock pistons a nd the ones I got to see what the difference will be.

I've never done a cylinder volume test before ( from memory ). How's it done ?

Assuming that the engine actually has a lower CR, what would be the fix ?

A bigger turbo ?

[Cubes]

Its fairly easy to get a rough idea where the comp ratio is at.

Pop down to the library pull out a gregory's or similiar Car manual, anything a ford falcon.

Have a look at what the psi should be then check the compression ratio. It may say something like 155psi 9:1cr.

Then do the math, test your compression. You may get 140psi.

So... 9/155psi = 0.0581

Then 140 * 0.0581 = 8.1CR.

Thats just an example. I have no idea if 155psi = 9:1.

I did that when I first got my R32 and it came up to exactly 8.5:1.

I also used a EL Ford Falcon manual to get the initial figures from.

I'm not sure if its possible but can the block be decked while the bits that go round and round are installed?!? Shave the head?!? Thinner Head gasket??

Also for every .5of a CR you drop you will need to run an extra ~2.9psi to make the same power.

[TokyoTaxi]

Joel,

Thanks for that info. Will try to figure it out. Got this horrid feeling that there's going to be a huge brawl in getting this all sorted

I don't know about the other points you proposed but I guess I'm going to have to look at those options now.

Gee's it's not like I can even get a bigger turbo and run more boost since there's a standard head gasket fitted.

[RedDrifter]

Rb20 when tested should be 140psi from memory.

Get a compression test and see what happens. Good luck mate.

[TokyoTaxi]

Thanks for that RedDrifter.

So if I'm correct, a stock RB20DET should have a compression ratio of 8.5:1 and 140psi on a compression test. With that in mind I'll do a comp test on mine and do the formular.

In the event that my comp ratio is down and I should choose to go a bigger turbo, does anyone have an opinion on what I should use ?? ( Considering the head work and 40 thou oversized forgies )

[rb20-calais]

just run more boost with the 2530, like 2bar

Evan

[RedDrifter]

When I do my rebuild I would like to get a GT2835. Heard they are a really good turbo.

So yeah 2835 would be good I guess.

[Minorci]

I've just been quoted 5,600 for a complete rebuild, bit more if i wanna chuck the tomei forgies in.

[RedDrifter]

Good thing I am going to do most off the stuff myself.

I intend going with tomei 2.2 pistons, tomei 260duration 8.5mm lift cams and tomei valve springs for reliablity up in the revs. Oh yeah 550cc injectors and GT2835.

I have every intention taking a good 12 months to complete this project. That way I won't notice the cost so much. As for the bearings etc and gaskets will go genuine nissan.

[Roy]

FAT32, just rip the head off and machine the face a little to bump up the compression, IF thats the problem.

Oh and you are making less power... what ECU are you running, if its std, and the same tune as before then that could very well be the source of the power loss, you have changed the breathing characteristics of the engine