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The last two performance mods I want to do to my 8 are a downpipe and cold air intake. Nothing crazy, and yes, I know, I've done extensive research, and I fully understand the gains are basic, especially without a tune. First off, I don't plan on getting a tune because in 2017, apparently finding a MM preloaded Accessport is like finding an ancient relic. Or, I can take it to a shop, but I'd rather the control at my fingertips even with a more basic Cobb AP tune.
That being said, I would simply like to get an AEM Cai and downpipe - which I'm debating right now - to add to my catback and upgraded ignition system. That's all I want to do performance-wise. Now I know I will order the AEM, but I'm looking for suggestions on which midpipe I should add to my turboxs catback. Currently I have the OEM cat with only 11k miles on it. It seems fine, but to my understanding it is very restrictive due to emissions regulations and I would like to free up that extra HP.
I would prefer a high-flow catted DP because I don't want a CEL or the stench. I've come to understand that most catted dps won't display a CEL, especially on the earlier models. This leaves with only two options really (dps w/ cats), being turboxs and BHR. Now the difference is $220 which is quite a deal, vs $600. I'm sure quality vs quality, the BHR is better, but for 1/3 of the price I was almost set to order the Turboxs one tonight. Thankfully I did some more research, and just when everything seemed all good to go, I realized that the turboxs DP has terrible clearance issues. Go figure, I'm on coilovers. The last thing I want is this thing exploding when I hit a bump in the road.
This leaves me with one other realistic option (that I'm aware of) being the BHR catted dp. What do you guys think? Am I missing any other catted downpipes here? It seems like everything is / are catless racepipes out there.
Last edited by supermiatavaper; 01-23-2017 at 09:25 PM.
To free up that little extra hp, why not just spend your money on driving school? Even with a tune, you're not getting much with your current desires. And if your goal is power (regardless of how little it is), you'll get more out of driving than out of any mods...
Or just get a cobb and have mm tune it. Or get MazdaEdit and have a pro (or one of this site's tuners) tune it?
The RX-8's stock airbox is already a well-designed cold air intake. I log my intake air temp along with ambient air temp and coolant temp during every track session, and IAT is always within 1 or 2 degrees F of AAT, which is well within the tolerances of both sensors.
The stock airbox is also free-flowing. Racing Beat threw a lot of engineering at improving the airbox, and their results are so meager, they won't publish them. That means any gains were 2HP or less.
The AEM intake works fairly well, but it is mostly just a noise-maker. I have seen dynos that show 3HP gains. I have also seen dynos that show 5HP losses. The trade-offs for virtually no additional power are potential for rough idle, potential for water ingestion, and potential for particulate (sand) ingestion. The only reasons to buy the AEM are: you like the sound, you like the shiny tube, you want access to the area under the airbox for some reason.
As for the midpipe, I echo Dan's thoughts. (BTW, there is no "downpipe" on an RX-8.) Check your OEM cat. If it is fine, use it. Spend your money on a BHR ignition upgrade, which will keep your OEM cat healthy, among its other benefits.
If you must do something for whatever reason, match the tube diameter to your catback diameter. I'm not directly familiar with the TurboXS catback, but I seem to remember it being a full 3" tube. IIWY, I would just buy the matching midpipe.
I have the BHR midpipe, and it is a very high quality unit that sounds great. I bought it without a cat, since I was tracking my car then, and my focus was on reliability over power gains. In terms of power gains, I don't have any dynos of my car, but I can tell you my lap times before and after stayed exactly the same. That is significant, because...
Nearly all of the gains we are talking about are realized at or over 7000 RPM. I spend all my time above 5500 RPM, and I typically start my upshifts at 8500 RPM. So, I live where the supposed gains are. How much time will you realistically be spending over 7K RPM? Most people shift at or before 5K for daily driving, with the occasional pull to redline for fun. Your butt dyno will only "feel" anything on that occasional pull.
The only reasons to ditch the OEM cat are: it is clogged or otherwise damaged, you want to save a couple of pounds of sprung weight, you have a thing for really hot cup holders, you like the sound of the aftermarket midpipe.
If you want cheap, instant gratification, reduce friction in your drivetrain. Replace your transmission oil with something slippery like Redline MT-L (daily) or MT-90 (track). Replace your diff oil with something like Redline 75W90. Inspect your brakes to make sure everything moves freely, the calipers retract properly, and the pads are not binding in their clips. Make sure your alignment is good and on the conservative side of factory settings to reduce rolling resistance. These things will make your car noticeably faster throughout the tach range.
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Last edited by Steve Dallas; 01-24-2017 at 08:29 AM.
To prove my point about the stock airbox being a good CAI, I present this chart from an 88F degree track day. The green line is ambient air temp, and the red line is intake air temp. When I am sitting in grid (left side of the chart), both sensors are reading high. The AAT sensor is reading heat coming from my hot brakes, hot tires, and hot asphalt. The IAT sensor is reading the heat soaked airbox temp. Once I start moving, the AAT drops to actual ambient, and the IAT drops with it, as cooler air rushes into the airbox. The IAT is sometimes 1 or 2 degrees above ambient, but sensors have tolerances, and the difference can be attributed to that. In any case, 1 or 2 degrees makes absolutely no difference in volumetric efficiency.
Bottom line: money spent trying to get colder air into the engine is wasted money.
To prove my point about the stock airbox being a good CAI, I present this chart from an 88F degree track day. The green line is ambient air temp, and the red line is intake air temp. When I am sitting in grid (left side of the chart), both sensors are reading high. The AAT sensor is reading heat coming from my hot brakes, hot tires, and hot asphalt. The IAT sensor is reading the heat soaked airbox temp. Once I start moving, the AAT drops to actual ambient, and the IAT drops with it, as cooler air rushes into the airbox. The IAT is sometimes 1 or 2 degrees above ambient, but sensors have tolerances, and the difference can be attributed to that. In any case, 1 or 2 degrees makes absolutely no difference in volumetric efficiency.
Bottom line: money spent trying to get colder air into the engine is wasted money.
Thanks Steve, a lot of good info here. I guess you're right; in my eyes I saw the max "realistic" gains and added them up - about 4-8hp from the AEM, 6-10hp from a midpipe, and a few extra from the catback. When all was said and done, that could be a potential extra 20hp. That's something I would like. These gains may be slightly exaggerated, but that's how I figured this math.
I already got new coil packs, NGK plugs, wires, changed out the tranny and diff fluid, and it shifts oh so smooth and drives very very nice. I guess since I got it at that point I figured I'd go a little further. Simply put, it's the basic formula I do to all my cars - intake, headerback exhaust, and the rest in cosmetics / suspension.
Are there any catless midpipes that don't display CELs? I may look into catless if there was one that could get around it.
I'm sure my OEM cat is fine. My car did misfire twice in it's past, as the oem plugs were fouled pretty bad. I was going up a hill and practically racing another, when I felt the loss of power and smelled a terrible burning. Clearly, the plugs contributed to the misfire. Once I replaced them and the rest of the ignition, it's been driving 100% like new and I've had zero issues. My concern is the damage that happened to the cat during these 2 misfires, and that terrible smell. Do you think that would be enough to harm an OEM cat with only 11k?
Other than that I have no high rpm power loss or any other issues. Car drives beautifully. At around 6k rpm I feel what isn't quite a hiccup in the car, but like it's opening up more. I've read that the APV opens up around there, would one be able to feel this little boost?
Anyway, thanks for the advice. It's a shame aftermarket supplies for our cars are low and that drives prices up. I'll keep doing research and see what happens.
Your "realistic" gains are very optimistic, according to dynos posted by owners.
Here are the real numbers, as well as I remember them:
1. Intake. Up to a 5HP gain if you are lucky, up to a 10HP loss if you are unlucky. Typical gain is 1 to 3HP.
2. Midpipe. Up to 5HP gain if catless. Up to 1HP gain if high-flow catted.
3. Header. Up to 3 HP gain for short tube. More for very expensive long tube header, but gain not verified that I recall.
4. Catback. Virtually no gain for streetable units.
One of the problems with all this is, you cannot simply add all the gains together and expect to see that total. An engine is a system, and a change one place can help or hurt the performance of a component in another place, and there is an ECU with is cohort of sensors in the middle trying to make sense of it all. If you go back and read old threads, you will find many instances of guys who bolted on every bolt-on there is and were very disappointed at dyno day, when they discovered their car had dropped from 200 to 180WHP after spending all that money (but their butt dyno was sure they were at 220!). If you do all the bolt-ons without tuning for them, you can reasonably expect up to an additional 8WHP or 4% gain above 7000RPM. You won't be able to feel that on a 200WHP car.
A handful of misfires will not hurt your OEM cat. It is possible to have a dead trailing coil cause fuel and oil to be dumped into the exhaust without your knowing it. If that happens for an extended period of time, it can clog the cat. Your best course of action is to pull the 3 bolts and inspect it.
Some S1 cars get away with not having a cat without throwing a CEL. Most do show a CEL. Pretty much all S2 cars throw a CEL. Our ECU, for the most part, is simply too smart to be fooled. None of the cheaters work, so don't waste your money. Something else worth mentioning is, the stench from a catless rotary is legendary. It permeates your car's interior, your house (if you have an attached garage), your clothes, your hair, everything. It kept me from enjoying the car. I lived with it for a year and finally caved. I had a very expensive HJS race cat welded into my BHR midpipe. It just wasn't worth a few HP that I couldn't feel to me to deal with that smell any longer.
The bottom line is, Mazda engineered this car to death, trying to squeeze every last HP out of it they could, and they did a very good job. There simply isn't much more to be had that is worth the money. It is far cheaper and easier to reduce weight instead.
If you really want to improve the car, focus on its weaknesses and improve those areas. The main weaknesses are ignition (BHR kit) and cooling (Koyo or other properly installed and sealed radiator). Then, focus on its strengths and improve those. This is a handling car. Put all of your money into suspension, rims, tires, and alignment, and make its handling even better.
I have no power adders on my car. All of my money has gone into reliability and handling. As a result, I have a reliable, fast car, and I have a fatter wallet.
One final programming note: intake dynos are usually run with the hood open and a large fan blowing over the front of the car. Divide published intake dynos by 2 or 3 to get to the real number.
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Last edited by Steve Dallas; 01-26-2017 at 08:34 AM.
01-23-2017, 09:21 PM
