Water Injection with stock intercooler?
 

Hello from the UK,

I have a 1999 Mazda RX-7 Type RS (276hp model) that I plan to use on 10-12 track days per year. The suspension and brakes have been modified with Ohlins DFV coilovers, high performance RS brake discs, pads, stainless steel brake lines and poly bushings all around with OEM pillow balls. The engine itself is untouched except for a 3" Fujitsubo cat-back exhaust.

I am preparing to have my car tuned for 0.8-0.85 bar (11.6-12.3 psi for the Americans) with an Apexi Power FC in a quest to have around 300 brake horsepower, but more importantly a car that is a reliable configuration for 10-12 track days per year in the UK (weather never gets hotter than about 80 degrees in peak summer) whilst preserving the 99 spec turbos for as long as possible (hence why I'm choosing to not run more boost).

Those of you that know about Series 8 FDs with 99 spec twins and 276hp will remember that the stock ECU ran 0.74 bar (10.75 psi) from the factory compared to lower horsepower models from the past which ran 0.62 bar (9 psi), and some of the key changes on the later cars were sealing off the intercooler from the intake, improving intake ducting, etc.

My car's spec when I go for a tune at 0.8-0.85 bar will be:

1. 99 spec twins running sequentially
2. stock airbox with air guide (intercooler sealed off on later models)
3. stock intercooler with stock sealed duct
4. Knight Sports twin tube downpipe
5. Stock catalytic converter and pipe
6. Fujitsubo 3" cat-back exhaust
7. NGK 9's spark plugs
8. Koyo N-Flow 48mm radiator
9. Apexi Power FC

I've been thinking of also adding water injection to act as a safe guard in case the stock intercooler becomes overwhelmed on track for some reason at these power levels. My rotary specialist whom looks after my car is adamant that I do not need water injection at this power level and that on circuit at these boost levels the stock intercooler is more than up to the task because of the constant airflow and the better ducting on the Series 8 cars.

However, everywhere you read online about the stock intercooler you see a recommendation to get rid of it ASAP so prior to my discussion with him I had purchased an AEM V2 water injection kit which I was going to hook up post intercooler to the throttle body elbow to act as an element of insurance to keep intake air temps nice and cool. I only planned to inject a very small amount of water say with a 250cc/min nozzle as I'm not looking to affect the A/F too much and have my tune dependent on the water injection happening, for if the water injection were to fail for some reason I don't want to end up running lean at the wrong time and then BOOM.

I may still go ahead and install the AEM kit but in light of my tuners insistence that the stock intercooler is fine I just wanted to get your opinions about whether you agree with his viewpoint or not for my application and power levels.

I note that the RX-7 with the stock intercooler won several races in Australia in the Bathurst race series and it also won a rigorous endurance test in Japan on Best Motoring when going up against rivals such as Evos, Subarus and Skylines.

If you could watch your intake air temperatures during a 3rd gear pull(plenty of air speed through the intercooler) you would see how much it underperforms. With water injection you will actually see your temperatures drop the longer its engaged, it may even approach ambient air temperatures. This means more power(more timing), and consistancy, as well as safety.

The biggest problem you will have is flow restriction from the stock intercooler. That being said, water injection will help with just about any pump gas setup.

Yeah, I run Shell 99 V Power here which I think is equivalent to 94 octane using the US octane rating system. I'm not too fussed about flow restriction and maximum power from my mods it's just more about having not only a good tune but a reliable car that can withstand 15-20 minutes of punishment at a time.

I had an RX-8 as my previous track car and that thing was reliable as ever but temps were never anything to worry about.

As K-Tune already stated with your Power FC commander you will be able to tell if your IC is subpar. IMO if you aren't too sure then don't worry about it just yet. Get your stuff sorted out, drive it around and see the data for yourself. Then make the decision

IMO stock IC's suck donkey nuts. My close friend has a power FC with a stock IC and his AIT's where a good 20 deg C higher that mine with a PFS unit! This is a direct comparison because we were side by side on our cars on a 80F deg day.

Where will nozzle placement be?
And I disagree with your tuner...ESPECIALLY for a track car. I'd bet in those races you cited the class required retention of the stock IC. And if you asked anyone in the pits if they would have changed it if allowed, it would have been a unanimous YES. With or without WI, there is nothing adequate about the pressure drop, harder working turbos and higher IATs when decent aftermarket SMICs are available for not a lot of money.

The injection point would be in the throttle body elbow after the intercooler like this:

https://www.rx7club.com/attachments/...p-p1170592.jpg

I agree that a SMIC would be good but in the UK they are hard to find, especially ones that are properly ducted to actually make a difference to be better than stock.

I already have the WI kit ordered and on a delivery truck somewhere. I will just install it anyway as a matter of insurance and use the stock washer reservoir. I simply wanted to see what people thought about the advice I was being given.

^My nozzle is place there as well. BUT...you should know that WI there will do little for IATs. It's not a bad placement, but it's function is mainly realized inside the engine where it absorbs heat from the combustion event during phase-change to vapor and carries that heat out with the exhaust. That reduces temp spiking from hard boost and reduces load on the rest of the cooling system. It also controls knock values and carbon deposits.
If you're looking to lower IATs a Methanol/Water mix injected upstream (pre-intercooler) would probably yield better results. If you haven't already, a run through the stickys in the AI section might be worth your time.

Thanks for the advice as I didn't realise the location might not be the best for IATs. Ideally I want all of the benefits you have described as a knock suppressant and so the computer isn't pulling timing or whatever if IATs are too high.

I'll search the forums for nozzle placement to educate myself more.

Use cold weather washer fluid. It has a small amount of methanol but is mostly water. Will cool intake temps and combustion temps.

^Still needs time in contact with the intake air. And no personal experience, but some have reported residue from at least some of the washer fluids. Presumably from the detergent.

Meth is corrosive, it's common to see those effects even in diluted form. Would take a very long time to do enough significant damage.