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View Full Version : When people say "275whp is safe", consider this



ChrisD
01-14-2005, 07:02 PM
The whole "275rwhp limit" rule that the mr2 board follows is quite misleading IMHO.

Using a BSFC of 0.60 (turbo'd cars .6-.65, so this is a conservative estimate)
15% drivetrain loss. 275rwhp = 323bhp
4 injectors
max duty cycle 0.90
if you do this 275whp at 17psi, for example, rail pressure = 60 psi
Using RC's calculations, this is NOT POSSIBLE. You need 482cc/min injectors

Unless you use a duty cycle of 100%. Safe? Hell no. But lets see. Well lets use .99 since 100% is maybe not all that likely to occur.

Using the same calculations

Injectors required: 438cc/min! Woohoo. So now it is "possible" to reach that level on the stock injectors, that much I will agree on. It is my belief, based on peoples results I have found on the net, that the stock ECU will indeed allow the injectors to fire at 100%.

Is it safe?

No!

At 100% duty cycle, the injectors are very prone to failure, it becomes extremely difficult to maintain a consistent A/F (or amount of fuel injected) stable across all injectors and cylinders.

From my testing, the AFM is already wide open (~0V) already by 235whp (could be sooner). This is why people report running ongodly rich when they test on the dyno. Basically, you're already firing at 100%, but at least you have the safety barrier of being so rich in case some of the injectors aren't exactly in sinc. Once you get into the "safe" A/F of maybe 11.5-12:1, those little inconsistencies could lead to a grenaded engine fairly easily...maybe not right away...might take until one really cool morning or something... So this is why people report 275whp as being the max. The injectors have finally reached their limit at that point. Based on the BSFC I chose, that follows exactly as expected.

So basically I'm saying that if you seriously want to make 275whp on the stock injectors and FPR, you are taking your engine's life into your own hands.

Until recently, some of RickyB's EMS's didn't even allow duty cycles greater than 80-85%. Want to take a guess why?

Lastly, I'll do the calculation on what fuel pressure you must run (with an adjustable FPR) in order to make 275whp on the stock injectors, at a duty cycle of 90% (still not optimal, but much safer than 100%).

Rail pressure must be 72psi at 17psi of boost to make 323bhp, base fuel pressure = 55psi
Rail pressure must be 72psi at 19psi of boost to make 323bhp, base fuel pressure = 53psi

Now, at what rail pressure does it start to become unsafe? Some argue 70psi, some say 75psi...thats right on the limit...do you want to go there?

Anyways, thats my 2c.

Chris

Lagos
01-14-2005, 07:10 PM
my opinion on power upgrades is to take a clue from toyota. if you want more power then upgrade to 550 injectors and a ct20 or ct27 turbo.

emicen
01-14-2005, 07:58 PM
Easiest way to pop and engine is to up the boost without the ancillary and necessity mods to go with that power. Taking my CS with a CT26 blower above 1bar even with its Walbro and adjustable FPR isnt happening. The set of 540cc injectors I have sitting in the garage dropped in to the rail and I might sing a different tune though.

presure2
01-14-2005, 08:18 PM
good stuff chris, good stuff. :bigthumbu

ChrisD
01-14-2005, 09:31 PM
bah, RickyB claims BSFC is 0.54.

Now, since it is pretty well known that on the stock fuel you start to lean out at 275whp, only two possibilities remain. Using the calculations from RC's site: http://www.rceng.com/technical.htm

a) The BSFC is 0.6 and the stock ECU allows 100%
or
b) The BSFC is 0.54 and the stock ECU allows a max of 90% (440cc/min injectors required to make 275whp)

With any luck, I should be able to obtain an injector duty cycle monitor soon, and I will determine what is happening. Regardless, I would much prefer to do my own testing. If the stock ECU allows >90%, than the BSFC must be >.54.

Luni
01-16-2005, 06:57 PM
Preach on brothaman.

Good stuff Chris. Keep it coming.

You still need to get that datalog of that SMT6 as well.

Sean
01-16-2005, 11:50 PM
Your right Chris, math doesnt lie, but neither do wide bands. Theres so many real world elements that come into play, why risk your engine based on a few equations. I think there great guidelines, and give a great general rule of thumb for modding your engine.

I dont think you can go wrong by monitoring your engine with a wide band. Every car is different, from the electrical system (amount of volts the pump is getting), the fuel componets conditions, I imagine VE's are never going to be exact from car to car, temperature, and elevation seem to also be factors that come into play.

For instance, today, i read about a TD06 mr2 that just put down 315 hp, on the factory fuel system at 15psi.... Thats going to toss some heads for a while. Theres always exceptions to the rules :)

Spud
01-17-2005, 03:40 PM
\For instance, today, i read about a TD06 mr2 that just put down 315 hp, on the factory fuel system at 15psi.... Thats going to toss some heads for a while. Theres always exceptions to the rules :)

ya dude I was just reading that last night.

Hey I have a question. Is the amount of fuel needed more a function of boost pressure or horse power made? Cause he's running the same boost pressure that I am, but probably making like 80whp more. Is it a combination of both? Or just horse power. Or do I not make any sense at all? haha

ChrisD
01-17-2005, 04:21 PM
Yeah that TD06 mr2...a few things with it. Apparently it used a correction factor that over-stated it's power compared to uncorrected numbers, thats one prob but not the main thing really. I saw his boost was spiking up and down because of his boost controller settings. So he was getting spikes in the HP graph, which netted him the 315whp on the biggest spike. So who knows, it might have happened so fast the wide band didn't pick it up, maybe it was not calibrated right, etc. Not to mention every dyno is going to vary in its readings. So who knows. The actual limits are going to vary as per testing conditions of course. Every other dyno out there starts to lean out at 275whp...so little anomolies wont affect my calculations. Some engines are freaks! Even Lamborghini's cars vary by ~5hp out of the factory.


Preach on brothaman.

Good stuff Chris. Keep it coming.

You still need to get that datalog of that SMT6 as well.

For sure man. Everything I do testing on I'm going to report back here. Hopefully I'll have some cool results to show using the wideband info, the SMT6, the duty cycle monitor, and the J&S monitor.

Now to further my calculations, I have ordered an injector duty cycle monitor. This way I can check out the actual duty cycle that the stock ECU allows. Using the AFM readings that I measure, I can determine the BSFC for my engine. I am going to put my money down that it comes out at 100% (and therefore a BSFC much higher than what RickyB states...no point really in debating it over there, most people aren't really open to any new ideas from "newbies" like me :hehe: ). But I won't have that until I can get my rear diff and engine mounts urethaned and back installed, and the streets get a little nicer out.



Hey I have a question. Is the amount of fuel needed more a function of boost pressure or horse power made? Cause he's running the same boost pressure that I am, but probably making like 80whp more. Is it a combination of both? Or just horse power. Or do I not make any sense at all? haha

Sure you make sense. Amount of fuel required is a function of airflow mainly, as well as RPM. The stock ECU is basically blind for the most part to boost. Just meters the air coming in. Most EMS systems measure boost, however they assume that the car is tuned assuming that a certain boost ='s a certain amount of air. (need remapping when you do more mods too) And your fuel requirements increase if you are running higher RPMS as well. Some people equate airflow to horsepower, so you can determine fuel needs from horsepower, yes, but that is dependent on a given airflow. :)

Spud
01-17-2005, 04:33 PM
but if he's running the same amount of boost that I am, isn't the air flow the same? And if so, how does he make so much more power, is it just because the bigger turbo is more efficient at that same boost level?

You're a pal Chris ;)

ChrisD
01-17-2005, 04:45 PM
but if he's running the same amount of boost that I am, isn't the air flow the same?

Nope, not at all. A big monster turbo can push out a lot more air at any given boost level than a little tiny turbo. ;)


And if so, how does he make so much more power, is it just because the bigger turbo is more efficient at that same boost level?



More efficient, yep. More dense air = greater mass of the charge air. More air = more power and need more fuel.


You're a pal Chris ;)

You always ask good questions man, I don't mind trying to answer them at all! :) :)

Conrad_Turbo
01-17-2005, 08:44 PM
The more mass of air/fuel burned, the more power an engine can put out. Pressure is the result from a restriction in airflow through the engine (from the filter all the way to the exhaust tip). If you reduce the restriction you can increase the mass flow rate without having to increasing the pressure since you increased the VE of the engine.

So if you ran higher lift camshafts, larger A/R turbo, more efficient intake and/or exhaust manifold, high flowing exhaust you will decrease the restriction which will allow you to run less boost pressure and have the same Hp output.

Spud
01-18-2005, 02:24 PM
huh, see thats what I was having such a hard time getting my brain around, that somebody could be running the same boost as me (same pressure) and yet somehow be getting more hp than I am because somehow more air is getting into the engine!! The VE thing helped me get it more too conrad. I'll be thinking about it all day at work now, haha

Thanks guys

ChrisD
02-01-2005, 09:13 PM
Just as an update to this thread, it has been found that the stock ECU will indeed allow a 100% duty cycle, which means that my suspicions were originally correct. BSFC must equal ~0.60, mathematically (give or take a hundredth).

So anyways, if you are planning to run 275whp, tread with caution.

Zeus
02-02-2005, 12:08 AM
oy its a shame i have to go to school ide really like to contribute to this thread

but in short

psi is a relative term... ie you can have a 2 liter pop bottle inflated to 10psi and a 599ml bottle to 10psi but which one has more air...


so like i say its a relative term... ve = volumetric efficiency for those that dont know... so that term basically means that they are getting the air in the motor in a more efficient mannor...

shit i gtg ill come back to this

hobbie2k
02-02-2005, 03:49 AM
Just a quick newbie question...what's BSFC? I think I have a grasp of the rest of the stuff...

XXX_Mina
02-02-2005, 04:06 AM
i'am putting my stock 550 cc injectors on ebay soon, i bought sard 800 cc injectors , to support my power goal of 500hp ... see my other post for all the shit i just bought...

mina

ChrisD
02-02-2005, 04:06 AM
BSFC stands for brake specific fuel consumption. It is basically a measure of the engines efficiency in terms of fuel use. Specifically it is fuel flow (in lbs/hr, which we convert) divided by horsepower. So it is ___(lb/hr)/hp.

All that theory boils down to this:

Injectors go to 100% duty cycle on the stock ECU. At 275whp your A/F should be moving very close to as lean as you want to run. One small variance in the injector flow, some inconsitency, a stuck injector, anything, and the A/F in one cylinder blips just a bit for a short period of time, and ... boom.

nuclearhappines
02-02-2005, 05:35 AM
yup...

i know vr4 guys running bpu cars on stock fuel systems seeing 105% IDC's ...
we had a few members lose motors because eventually an injector failed, and when the owner took the car into a higher gear and opened it up that cylinder didn't get enough fuel and died

one thing i want to toss in here is gearing. The amount of load/fuel at a specific rpm is different in different gears. you may hear about people running xx turbo at yy boost for zz years with no problems... we had that happen on the vr4 board too... alot of claims of 'dr-500' turbos and 550 cc/min injectors.

One of the guys on the board with a datalogger and a few 'extra' bolt ons redlined 4th gear once with those turbos and quickly found out that a setup that was commonly known to be a good combination of turbo and injectors was actually running out of fuel in the top of the higher gears. So you never know how big your margin of safety is, and you always want to trust the math more than the hear say, because sometimes you run into a situation that other people have never run into before.

just a heads up... you want injectors 20% larger than your goals... gives good head room. If you go alot larger than your target without a standalone then you are probably picking up too much timing advance ... which is another issue altogether.

-nuke

hobbie2k
02-03-2005, 05:00 AM
BSFC stands for brake specific fuel consumption. It is basically a measure of the engines efficiency in terms of fuel use. Specifically it is fuel flow (in lbs/hr, which we convert) divided by horsepower. So it is ___(lb/hr)/hp.

All that theory boils down to this:

Injectors go to 100% duty cycle on the stock ECU. At 275whp your A/F should be moving very close to as lean as you want to run. One small variance in the injector flow, some inconsitency, a stuck injector, anything, and the A/F in one cylinder blips just a bit for a short period of time, and ... boom.

Thanks, that made more sense now that I know what BSFC is. I understood the leaning issues with a 100% duty cycle...boom.