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Rb25/30 Trouble Shooting

Simonster

By Simonster,
in Engines & Forced Induction

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Simonster Proficient

Simonster

Posted
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Posting here for a friend, here's the specs

25 head, light port, HKS 265 9mm both inlet /ex, Jun valve springs, JE flat tops pistons, compression 8.3:1

Bottom end, RB30, ACL bearings, RB30 rods shot-peened linished balanced, crank drilled grub screwed, etc

Rigoli plenum, 1000cc Rochestor injectors, 044 pump, sard reg,

6 into 1 ex manifold , external gate turbo smart, GT3040 turbo, 17psi spring

SM4, CDI, NGK8's, tuned my Matt Spry. Car has been well and truly run in.

The car with standard cams made 330 rwhp at 17psi , now with 265 cams, making 325 rwhp??

We are all at a bit of a loss at the low numbers, we expected decent numbers from this set-up. Any suggestions are welcome.

Thanks

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R31Nismoid Grand Master

R31Nismoid

Posted
Posted

Why is the compression low?

hmm... 17psi, should be seeing a solid 270-280rwkw being i made that on a factoy head, cams, plenum etc etc... and that was 3 years ago on a 2.5ltr, so on a 3ltr, 17psi you'd almost be expecting a round 300.

What does the tuner have to say about it all?

What happens when you put more boost in?

some fairly serious gear there considering its a small turbo :)

paulr33 Grand Master

paulr33

Posted
Posted

id be looking at the dyno graph, plot power and boost

it might give away an inlet restriction, or exhaust restriction

particulary the manifold might be making it choke, so no power gains regardless of how much boost you dial in

Simonster Proficient

Simonster

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The turbo is the original 3040 from many years ago, when it came out it was supposed to be rated at 600hp, so there shouldn't be a problem there. I have another 35/40 that we can fit if need be but we don't think its the turbo.

The Matt doesn't know why the power is no there. My thoughts are ( i wasn't there when it was tuned)

1. the car isn't holding the required timing when under load...yes i know it an SM4 it has all the gear to make all the numbers- its just not. (i have an SM2- so there is no hatred of Autronic)

2. compression is down on one of the cylinders- that said it is yet to have a compression check to see if that's the case

The car owner thinks its not really making 17psi. If it was making those numbers on 13-14psi there'd be no real problem, so we are fitting other boost gauges to triple check this.

The cams were slowly being dialed in and were producing gains in the right direction, just not the numbers

Yes its got some serious gear, the compression is "low" as he is fine running big boost and lower static compression. Keep in mind it's only very recently (last 2 year or so ) that its become the norm to have 9.0:1 on turbo engines producing big numbers. My built RB25 run 8.5:1 and i think it's pretty responsive 8.8 isn't that far behind it.

Its a DE head and when the engine was originally built some 5 years ago, the selection of piston that exist today weren't available.

Thanks for the suggestions, any others?

Simonster Proficient

Simonster

Posted
  • Author
Posted (edited)

I meant that he wasn't confident that the car was actually getting 17psi even though that's what it was saying on the SM4 software. It doesn't run any boost controller on the wastegate, just the spring and its a 14psi jobbie.

The map sensor is located in the plenum yes.

Matt Spry is the tuner, so yes the AFR's are fine

Edited by Simonster
Simonster Proficient

Simonster

Posted
  • Author
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No boost controller, off the gate... 14psi spring

How on earth is it giving a reading of 17psi then?

It'll be running 14psi, or a bit less

good question hence 2 boost gauges! We dont' think its really running 17psi...maybe the map sensor in the SM4 is faulty?

Cubes Grand Master

Cubes

Posted
Posted

Check the boost its running.

A mate with a 1bar actuator went from ~1bar down to 12psi when cams were fitted. He made basically the same power but at a lower boost.

Whats the stock gauge saying? Over +7 and its running more than 1bar.

I've just dropped in a set of 265 9mm lift cams and boost does not crack 15psi (going by the stock gauge it appears to be around 13psi) I have yet to check what boost it is running as its in for a tune next week but z32 afm volts are up from 4.78v to 4.93v and it definitely clears its throat at 5k where I jump off it in fear of det and lean afr's. :D

Simon I presume?? ;) what are the cam centerlines. Mine are fairly sedate so it idles without a lope (110lca inlet 115lca exhaust 112.5lsa) Push in 5degree's advance and it lopes sounds awesome but is an instant defect, picks up mid range, drops top end 'slightly' and kills economy. I've found at the std LCA's its returning the same economy as it did with the std cams.

Throw a dyno graph up. Would like to see the curve as another I know of on the same dyno with 282duration cams gt45 etc is doing the a similar thing looks like its making good power then just falls over and goes flat up top.

Simonster Proficient

Simonster

Posted
  • Author
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Thanks for the replies, and yes its Simon...pretty imaginative eh!

The boost gauge he has reads around 12psi when it's apparently supposed to be 17 according to the SM4. I've given him another boost gauge to use as a comparison. I"m also going to check what boost the computer is supposed to be seeing with my laptop tomorrow while he drives.

I think they would in 4 degrees of advance on inlet only, power was going in the right direction. Ex is still set to 0.

You've got me Cubes, what do you mean by cam centrelines? this engine has a great lumpy idle

With the standard cams it made around 320 with an indicated 19 psi, it's still a similar figure but now indicating an alleged 17psi

A dyno graph was the first thing i asked for as well, he has one but only a power curve with boost, i wa more interseted in the torque curve- i'll get him to post it up.

Cubes Grand Master

Cubes

Posted
Posted

Ignore the other boost gauge hook it up to the dyno.

A lot of the jap rb25 cams run with a 115 inlet and 115 exhaust when using vct. Vct then drops the inlet to 111 over 5000rpm which reduces overlap and increases top end.

Without VCT then tend to start off with 110/115.

The exhaust is fairly good around the 115-118 mark depending on setup.

Simonster Proficient

Simonster

Posted
  • Author
Posted
Ignore the other boost gauge hook it up to the dyno.

A lot of the jap rb25 cams run with a 115 inlet and 115 exhaust when using vct. Vct then drops the inlet to 111 over 5000rpm which reduces overlap and increases top end.

Without VCT then tend to start off with 110/115.

The exhaust is fairly good around the 115-118 mark depending on setup.

Its not a VCT head. Thanks, we'll check overlap

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