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How much boost is really safe?


Guest bastaad525

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Guest bastaad525

I've asked this off hand in a couple other threads of mine but never got an answer...

 

This is something I dont quite understand and really want to get clear in my head once and for all. Okay... most people here seem to agree, that the stock EFI, and stock injectors, are good for up to about 250hp at the wheels, right? Now... I'm guesstimating that it would take somewhere around 13-15psi intercooled to get to that hp number. Put 2 and 2 together and that says that the stock EFI and injectors can safely handle 15psi intercooled, right?

 

Okay, but then... little discrepancies have popped up in this logic... Tyson280zx, who hangs out here and at zcar.com (if you're here Tyson please pitch in here) went and dynoed his ZX a while back, at 10psi intercooled. He found that he was running very lean on the top end... this is with new injectors. Quote from a recent Zcar.com post from Tyson, talking about a guy who is running 12psi w/o an I/C

 

"I have NEW injectors in my ZXT, and with about 1,000 miles on them, at 10psi intercooled, I was around 14:1 air fuel ratio (this was not a guess, a calibrated wideband O2 was used to measure this on the dyno).....Which is starting to get fairly lean, fairly lean = boarderline of detonation."

 

Then there are others... I remember way back when Jersey went to the track, before he had his I/C installed, when he ran the 13.3 at 10psi... he noticed that his car seemed to be leaning way out at the top end... he then bumped the psi up a little more, and sure enough it started to ping.

 

I've seen others but can't remember when or where, but it definately seems common to me that even at 10psi, intercooled or not, the stock EFI and injectors are already starting to lean out, considerably enough to be a risk for detonating. Oh and in Tysons case... when he dynoed and was way lean at 10psi... he only put 191hp to the wheels... quite a bit short of the 250hp the stock EFI is supposed to be good for.

 

Now... I really WANT to think that the stock efi is good for 250hp, because that's the # I'm shooting for and am hoping to accomplish it w/o spending major bucks on efi upgrades. As it stands I've been running 10psi non intercooled for a while and havent noticed any ping... but am really starting to wonder if I"m seriously compromising the longevity of my engine. At the same time... I gotta wonder, if the stock efi is capable of 250hp, and the 13-15psi it takes to get there, how is it that it already seems to be struggling with a much easier 10psi? Remember guys... I'm a newb when it comes to turbos... I really want to learn as much as I can here and know just what is safe and what is not... can someone please clear this up for me?

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:-P Yo... um that answer made no sense :-P

 

for anyone who's interested here is the link to the thread over at zcar... a kind of interesting thread regarding how much boost is too much:

 

http://www.zcar.com/forums/read.php?f=1&i=414383&t=414383

 

It makes sense in the context that both Yo and I made that power with a stock fuel system and the Z31 ECU. Maybe it's not the injectors that limit you, but the 280ZXT ECCS.

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Actually, HKS claimed that their intake, exhaust sytem, intercooler, and raising the boost to 13 psi was good for 245hp if memory serves. On a stock ZXT.

 

If you want to run the stock system, and get 250, get a good fuel pump and run a rising rate FPR, and there you go.

 

Its easy math, what are stock injectors 270cc? If so you have 6 which is 1620cc. Divide by 10.5 and that equals 154, which is pounds of flow. It takes 1/2 a pound of fuel per hour to make one HP so theoretically, stock injectors could support 308hp at 100% duty cycle.

 

I believe the ecu does 90% so that would equate to 277hp. Perhaps your issues are more related to fuel flow falling off, since a stock pump does not pump well with increased fuel pressure. I believe that tidbit is well documented too.

 

Accountants are tough aren't they? :lol:

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its not the stock system thats stopping anyone from 250rwhp.

my good friend pulled 260rwhp@10psi with a flattop/p90 engine@18btdc and stock injectors with a small hybrid turbo, and he keeps the dyno slips in his glovebox. this is stock system with a great intercooler, efficiently routed piping(shortest route possible on tb side) 2.5 press bent exhaust, 55mm tb, great injectors(which are in my car now), and not much else except exceptional tuning and no shortcuts. p90 is shaved and he runs close to 9-1 static cr, and he drives his car HARD. dont know how much more he does make with bumped up cr and better turbo, but i know he makes it reliably. think he tried one pull at 13psi and made close to 300rwhp, but leaned out BAD at 4400.

isnt tysons car an auto?

191 rwhp through a auto, sounds like closer to 230crankhp or so, extra driveline loss thru auto.

starion intercooler? hmmmm?

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either way you make the power, you make the power i guess.

I just like how the higher cr engine drives off boost and its transient response characteristics alot more.

Ive been at different levels of boost and hp throughout the year now,

and the limit on my setup is not the amount of boost for me, but the age of my turbo that stops me from really trying it. Ive run 15psi a couple nights in a row on a jpipe, but now im intercooled and im just as fast as that 15psi at like 12psi :shock:. Car is in drydock now and wont be doing much for a few months, I did some idle and low rpm tuning on s-afr, but besides that, she gets to chill.

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JG Engine Dynamics in Alhambra CA did some dyno tests on BONE STOCK 280ZXT's in the late 80's. Without exception, the cars stopped making HP after 10psi. You could turn up the boost further, but it didn't make any more HP.

So Tysons claim of 10psi being the limit for a STOCK system is reinforced by Javier's testing done on his in-shop Clayton dyno in 89, 90, and 91.

Group Z did a dyno day there in 9X (90 something...) and most of the ZXT's were running 10psi, when they turned it up more, no more HP resulted, so "what was the point"...

That RRFPR really helps when you start going over 10psi. The ECU just isn't mapped beyond there, since the stock boost limiting relief valve usually pops off around 9-10 psi. If you have a limiting device installed, why map the ECU higher?

Tweak this, tweak that, all bets are off!

Then again, a Megasquirt...

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It sounds like you need Megasquirt.

 

(Must stop talking smack until MS is successfully installed' date='

must stop talking smack until MS is successfully installed,

must stop talking smack until MS is successfully installed)[/quote']

 

I am going to poo poo this one. Where are people running astounding times using a megasquirt?

 

I am just under a rock, or do we have people running mid 12's with them?

 

Just curious.

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I bet the issue is the stock fuel pump. That Stealth Z guy seriously documnented like no flow with small increases in fuel pressure.

 

As far as off boost response, sleeper and Yo2001 can also verify this, getting rid of the AFM and that damn door will do wonders for your response, even with 7.4 to 1 compression. It is a night and day difference.

 

heck when I first got the JWT set-up, and was like the 2nd guy in the country running with the Cobra MAF, my car was actually slower due to conservative timing, but off boost driveability, and power in the 3k to 5 range was incredible. It just wouldn't rev beyond 5k with power, but a few degree's of advance, and man, look out.

 

My car is very responsive off boost, and I have a stage V turbine wheel. Of course I have 87mm flat tops.

 

My money says its the fuel pump, not the injectors or ecu.

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Cranking up the boost without an intercooler or with an inefficient one might give a higher pressure at the intake, but it will also REDUCE mass flow by increasing the temp of the charge, and thus reduce horsepower which is why going above 10lbs made little or no difference. The critical component is mass flow, not pressure, and you can't ignore the temp and "assume" mass flow increase. The charge can/will be made more dense by fuel vaporization, which is why a RRFPR and a good pump will allow more boost by adding more fuel in this situation. Knowing this, it is hard if not impossible to answer the question of boost limit on a stock motor without knowing all the pieces attached to it.

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Guest bastaad525

Okay Tony, so it IS the EFI system, in FULLY stock form, that limits power at anythign above 10psi, due to the way the ECU is programmed? Or is it the limitation of the fuel pump as Sleeper suggests? Or is it that the T3 is just not good for anything over 10psi? You mention at the end of your post that an RRFPR really helps above 10psi... so, I'm assuming with more fuel you would again start gaining power when you turn up the boost on a stock T3 right? I also gotta say, I took a look at Tysons dyno chart... yeah he was going lean but he was going lean at high revs... they dynoed him way up to 6500rpm, even though his torque had started to fall off in ernest by about 4500rpm... honestly when I drive my car hard I can tell that it really doesn't have much to give above 5k and is just winding at that point, so I never rev it over 5500rpm anyways.

 

Now I'm back to wondering though... if I had to choose ONLY ONE method of increasing top end fuel up to the limit of the stock injectors, and this is assuming I have already upgraded the fuel pump... then which would be the better choice TO START WITH, an RRFPR or a programmable unit like the HKS? Or as someone suggested do I really need both to benefit? Now that I know the injectors CAN support the power I'm aiming for, I just need a way to get that maximum fuel to them w/o changing out the whole EFI if possible. This choice just keeps getting more confusing!

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why don't you just get 370cc injs?The stock efi is good for more that 250rwhp my car is living proof.The afm is restrictive but does work good with 370cc injs, it can be adjusted to idle right and work really good.The stock turbo is good for 18 psi after that it's not really any good fro my expirences.I'm sure i could get more out of the stock efi but i do know it does restrict my system alot if i rev the engine up it will suck the afm in twords the turbo and then when i let it off it'll pop back out.So i am getting a haltech but my car is very drivable even with 420cc injs.

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.

 

My car is very responsive off boost, and I have a stage V turbine wheel. Of course I have 87mm flat tops.

 

My money says its the fuel pump, not the injectors or ecu.

 

what is your comp ratio with the flattops Lockjaw??

my next L motor wants around 8.5, that what shes screamin.

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"I am going to poo poo this one. Where are people running astounding times using a megasquirt?

 

I am just under a rock, or do we have people running mid 12's with them?

 

Just curious."

 

Well, lockjaw, is 313mph fast enough for a megasquirt powered vehicle, or how about 619Hp at 10,000 rpm on a Turbo Busa Motor?

Take a look at:

http://www.turborick.com

Rick has successfully garnered several Land Speed records with his Megasquirt Fueled Turbo Busa Powered Streamliner.

Going 313mph+ makes 12's look way weak. :lol:

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