Same Power Different Boost

[Brezza]

hey guys,

a little bit of back ground about the car, i've been chasing 400kw out of my -5 E85 powered GTR

had issues with not making the numbers on higher boosts. so i've been making some small changes to see what the problem was. firstly i changed the exhaust from a 3in cat back to 4in and it made a 10kw difference. so then i change the cams from std to tomei type B's and this is what has happened:

i had type B tomei poncams installed. with no cam gears. so cam timing is zero.

i dynoe'd the car on std cams on 25psi and it ran 360rwkw

put the cams in and had the tuner play around with the tune to get it right on the low boost (20psi) and it made 360rwkw. i thought WIN!! same power, more response and 5psi less boost. then once we were happy with the 20 psi pulling consistently between 360-365rwkw we went to high boost setting of 25psi.

and a whopping big 365rwkw. it varied between 365-370rwkw on 25psi

so in reality 5 extra psi made no difference. the low boost is great, holds boost well, and pulls right to redline.

but 25psi is bad, the boost waves around a little and it seems like it just wont flow? the tuner seems to suspect that its got something to do with the std manifolds not flowwing enough.

its a big job to change over the maifolds, i was thinking if i did that i'd change the dumps to 3in too.

i've also had the system smoke tested and pressure tested to rule out any boost leaks,

just to give a recap of mods here they are:

forged pistons

std rods

R33 GTR crank

260 degree 9.15mm lift cams

-5 turbo's

mines dump pipes (look to be 60 or 65mm)

2.75 in front pipe into 4in cat back

std manifolds

silicone intercooler hoses

ARC 70mm intercooler

K&N air filter in STD airbox

1000cc injectors (siemens)

nismo AFM's

nistune ECU

twin in tank pumps

E85

e boost street boost controller

we've tried opening up the airbox, we jammed a screwdriver to open it up and that made no difference

the ECU and tuning ware responds to changes so it isn't the software, or electronics

he said that it doesnt look like the hoses are sucking closed because you can see the power just cut off when it does that and it doesnt do that. the power just follows the same curve doesnt matter whether its on 20psi or 25psi

the intake temps weren't rising too greatly, so the intercooler is doing its job.

the tuner tends to think that it's more of a mechanical thing like there being back pressure in the manifolds or dumps & turbo's which isnt allowing the air to be pushed out the engine and is just raising the boost because there is resistence in pushing boost into the motor?

please help, if anyone has any idea's as to what it could be or experience of similar results?

[Guilt-Toy]

post me a pic of the dyno graph.. power vs afr, also a boost one if possible.

[The Mafia]

post me a pic of the dyno graph.. power vs afr, also a boost one if possible.

what he said lol.

If you're going to get answers, then he's your man.

I'll help as much as I can too.

Sounds like you have really opened up the breathing in one spot to create a restriction in another. Sine the restriction is in another spot, you'll get different symptoms.

Personally I think that when there is a restriction in the head (Cams, valves, general flow) then you just have an entire up or down on the power.

If you have restrictions on exhaust manifolds, turbos, etc, you start to get weird symptoms, like unstable boost, interupted flow, etc. Its hard to explain in a few lines.

[Brezza]

i'll see if i can get the tuner to send me some of the dyno graph runs.

the AFR's seemed to be okay, sitting around 14.4:1 on cruise then it dropped down to 12-11:1 on full throttle.

[TiTAN]

are you also saying it has a 2.75" front pipe to the cat? what size cat? and then 4" from the cat back?

you really should be looking at afront pipe with 3.5 - 4" after the merge and 2.75 - 3" from each dump.

[Moodles2]

Were the standard manifolds port matched to the turbos? Usually it's recommended to keep the lip (for response) with -7's and -9's but to remove that lip for 350kw+

[Brezza]

there is no cat on the car

i think the front pipe is 2.75 from each turbo then merges to 4in and its 4in from there all the way back.

the manifolds weren't port matched.

the manifolds havent been modified in any way.

[The Mafia]

Just thinking about this today - You have removed a large restriction in the head by adding larger better breathing cams, and moved the restriction elsewhere, or to the next restriction.

Just remember, there is less restriction allowing more flow, which means more power with less boost. Make sense? Just because the boost is lower, it doesn't mean there isnt more gas passing from Air filter, to exhaust tip.

Edited March 7, 2012 by The Mafia

[The Mafia]

oh crap, I just read your post, lol.

Sounds like your exhaust is up to the task. Wonder what else could be the restriction? It could indeed be the manifolds.

[bigmikespec]

Were the standard manifolds port matched to the turbos? Usually it's recommended to keep the lip (for response) with -7's and -9's but to remove that lip for 350kw+

That would have to be one of the dumbest things I have heard... recommended by who, a bunch of internet users, crazy!

[The Mafia]

That would have to be one of the dumbest things I have heard... recommended by who, a bunch of internet users, crazy!

I was thinking the same thing when I read that... ahahaha

[mico]

y do people do that then ?

[bigmikespec]

Why do people do sit ups thinking they will lose weight around their waist?

Sorry, off topic.

[mico]

valve springs ?

[Brezza]

Just thinking about this today - You have removed a large restriction in the head by adding larger better breathing cams, and moved the restriction elsewhere, or to the next restriction.

Just remember, there is less restriction allowing more flow, which means more power with less boost. Make sense? Just because the boost is lower, it doesn't mean there is more gas passing from Air filter, to exhaust tip.

so what you're suggesting is that because the cams allow more flow through the head = less pressure/resistence in the plennum for the same volume of air?

so that might explain why its making 360kw on 20psi (with cams) instead of 25psi (without cams)? same volume of air passing through the motor, just less resistence in the plennum?

and if the intake isnt sucking closed or causing a restriction before the turbo, then the restriction might be after the turbo? maybe intercooler? (but its a 70mm ARC cooler so it should flow pretty well) and there were no signs of intake temps spiking higher on 25psi.

there shouldnt be a restriction in the head, because its obviously allowing more flow, because its pushing 360kw @ 20psi whereas before it needed 25psi to get 360kw.

-5's should in theory be fine to flow 25psi with everything else allowing them to flow that much?

so the flow is either getting choked at the manifolds pre turbo? or its getting choked after turbo (dumps)

unless anyone else can shine any light as to why i'm not getting any real gain from an added 5psi?

the extra 5psi wouldnt be enough to cause valve float would it? and would valve float cause the symtoms i have?

[The Mafia]

I'm in my iPhone now but will explain the best I can.

The head was never a major restriction, but when you think about it, everything is a restriction.

At this point though, your head was the biggest restriction, and now that it flows better, you don't need to ram (boost) more air pressure into it. It just flows better.

So, the air is flowing through the head with less resistance now, hence why you can make the same power on less boost. The air is passing through the head easier and more of it.

Anyway, at 25psi, there is even more volume of air trying to get through now. But, since the head is allowing it, something else has to be holding it up.

Lol this is always hard to explain.

Find out what max of the exhaust manifolds is rumored to be, same with the intake manifold, same with your intercooler, and find the now new weakest link.

Edited March 7, 2012 by The Mafia

[Guilt-Toy]

I am starting to think that the issue is the turbo's.

Don't matter if it was ok on 25psi before, fact is that your turbo's were out of puff before on 25psi, and when you free'd up the engine a little the turbo's are out of puff on 20psi and still making the same power because its flowing its maximum amount of air.

remember boost is your mortal enemy, the less boost you run the better.

Don't compare your -5 results to other peoples, different cars, different engines, different elevaion, different dyno's... etc etc so please don't tell me (ohhh but it should be making X amount of HP) this is tuning not maths.

An engine is a big air pump, the more air that goes in and out the more power you will make, so you had your turbos running at its max air flow and instead of upgrading them you just made it easier for them to flow the same amount of air at a lesser boost pressure, which is a mod done well if you ask me!

If you want to move forward in power then upgrade those turbo's. Grab a big single T51r or something and then start pumping some serious air into the engine or leave it how it is and be happy with what you got.

[GTR_JOEY]

slightly off subject but those 1000cc must be getting close to maxing out on E85 at those power levels

[Jap_star]

Could just be me. But I'd be pretty happy with 360 at the wheels from a relatively 'simple' build. Short of messing round with port matching manifolds or spending heaps on porting the head . Spend the money else where and enjoy the car brezza

[800KD]

I think it has something to do with valve springs and valve float... Std springs???? How old r they??