PhoKing714

iTrader: (21)

I spoke to my tuner and have been told contradictory information with rx7club's information in regards to tuning with water injection. I need to gain a better understanding of water injection.

I will be reducing a lot of information to make it simpler to understand. Based on my understanding of what I've read on this forum regarding to the subject. Please keep in mind that I will on refer to water injection only. I only want to run WI, please do not recommend to run methanol or race gas.

Rotary engine produces a lot more heat than a piston engine. As a result, a higher chance of detonation due to higher heat content inside compression chamber, leading to the difficulty of running higher boost. One of way of overcoming this obstacle is by running water injection. WI works to the rotary engine's benefit by cooling the intake air temperature, thus lowering heat content, allowing more boost. WI will reduce power, but can be made up by running more boost.

So let's take my car into the scenario. My current relevant mods are below:

94 FD
Stock port
Borg Warner s362 turbo, t4 divided, .91 a/r.
Intake, FMIC, HKS twin power, injector and fuel pump upgrade, basically all supporting mods, plus AEM WI kit (forgot exact model). WI is setup to kick in at 8 psi and cut off beyond that.

MY GOAL FOR THE CAR IS A SAFE/RELIABLE TUNE ABOVE ALL ELSE.

Please answer the following questions to the best of your knowledge in order for me gauge if my understanding of water injection is correct or not.

1. If my car did not have WI, what is the max safe boost that I can run?
2. With WI, what is the approximate safe level of boost? How many more psi can be added by having WI in place.
3. What is the approximate psi and hp I can achieve safely with this turbo and WI working together?

Please answer my questions and correct any of my assumptions above if they're incorrect. Thanks

Howard Coleman

iTrader: (6)

congrats on a very well constructed post.

"cooling the intake air temperature"

perhaps it is a small point but water provides little cooling to the intake air charge. water works it's wonders in the combustion chamber where it delivers over 8000 BTUs per gallon V gasoline at 900 and methanol at 3400.

another point re the rotary V piston is the rotary does not benefit from every other TDC event being a cooling cycle... (2 cycle V 4 cycle)

i like your turbo...

it will do over 70 pounds of air (26 psi) BUT unlike most turbos in this size it carries a 54 trim and should, therefore, spool advantageously which should fit your objectives.

just a general note for others reading this post...

the what boost question must be related to a specific turbo. it is really about flow, and more specifically, about the number of oxygen molecules being pushed into the engine. the point being 15 psi from a Disco Potato and 15 psi from a GT42 are miles apart. you probably know that...

so the what can i safely make should actually refer to torque. or hp works too.

unfortunately it is not an easy question to answer because it involves many variables.

i see some emphasis in your post

"SAFE/RELIABLE TUNE ABOVE ALL ELSE."

physics is physics. here's how a number of companies have answered your question. each has virtually umlimited resources and i can assure you they pushed the envelope as far as they thought they could and stopped short of creating carnage. each of the powerplants has realtime automatic ignition retard on knock (which we don't have) are turbo'd intercooled and are 4 cycle so benefit from significant internal cooling. (which we don't)

brand new twin turbo BMW M5 1.776 rear wheel hp per cubic inch
Nissan GT R 1.776 (that's not a typo, they both stopped at the exact same number)
911 Turbo 1.831
McLaren MP4 2.16 (F1 engineering rises to the top)
Corvette ZR1 (supercharged) 1.44

average 1.796

rotary hp at 1.796 = 285 rwhp

OE rwhp stock 216

FWIW, anything above 300 rwhp, given our lack of ignition/knock management and lack of a cooling TDC cycle is a venture.

and your turbo will make above 500 rwhp... that's 3.14 hp/cu inch.

it doesn't necessarily predict disaster as long as you recognise the amount of heat and pressure created in the combustion chamber.... and manage it.

of course water is a key and the only reason it is not on the aforementioned cars is the company has correctly made the decision that few would fill the tank and some might consider it weird which would be a marketing negative.

there will be people who will post they have been making 400 without water blah blah blah... all i say is good luck going forward. excess gasoline to gain cooling is a poor substitute. it has only modest cooling capabilities and autoignites at 450 F. it also combines w the premix (you'd better be premixing) to glue carbon to the engine internals. BTW water cleans the carbon bigtime.

as to boost levels...

your turbo will make 64 pounds of air at 15 psi. that is potentially (good tune etc) 480 rwhp.

72 pounds (max for your turbo at 26 psi) is 542.

howard

arghx

iTrader: (3)

That's the general idea. The motion of the rotor and the shape of the combustion chamber makes the area near the trailing plug most prone to detonation.

You could push it up to maybe 18psi on that turbo--and by that I mean, if it were my car and I wanted to see how hard I could push it, I'd take it up to about 18 pounds on pump fuel only. But I tune my own cars and I can tweak things as I go. You're taking it to someone else and they have to leave a wider safety margin. This is assuming you have 93 octane available, and the tuner will have to run significantly less timing and at least a slightly richer mixture.

Realistically 15 if you are taking it somewhere and paying for a day of tuning.
"Sky is the limit" is probably too strong, but I'd say you could run up to 25 pounds on 93 octane pump gas with that turbo and a sound WI system. But everything has to be working right. You can't have any kind of mechanical failure of the WI system and expect to survive long. You need to spec out the WI system and the tuner is still going to have to be easy with the timing...

Howard is right about the lack of knock control, which is something I've been hitting on a lot recently.

One piece of advice I can give you is that if you can afford it, take the car back to the tuner for a "check up" when the seasons change. So if you tune it in the cold months, take it back when it's hot. If you tune it in the hot months, take it back when it's cold. The season changes are often when bad things happen. It's impossible to fully account for this in a single tuning session, so that's why I suggest you go back at least once even if everything "seems ok." It would be an opportunity to monitor temperature, AFR, knock sensor readings, hot starts, etc.

You'd have to play around on the dyno to see at what boost level you hit a restriction. I suspect between 22 and 25psi would be the max it can hold to redline, and that's heating up your exhaust temps a lot from the backpressure. Obviously you're getting to the point where you run the setup to its ragged edge.