Zio

I've seen threads saying how they're is limited space under the hood for aftermarket mods. Also saying how the exhaust is too cool for a turbo. Will it be possible to add any aftermarket mods that will increase performance and will be somewhat reliable? If I replace the exhaust system I dont really want one that makes a lot of noise, but I still want the performance increase.

wakeech

ahahahaha, don't put much stock in that report about the "cool exhaust" of the RENESIS... it probably is slightly cooler than that of past rotaries, but rotaries in general have far higher exhaust temps than piston engines, not to mention the lengths mazda has gone to maintian exhaust temp for better catalytic reaction and better emissions.

if you're as serious as a heart-attack about adding performance, that "limited" space under the hood isn't a worry...

exhaust side modification would probably be the easiest, and might yeild some limited results, but Mazda isn't gonna screw around and try its best to choke this engine: it really IS only 1.3L big... pretty good performance no matter how you look at it.

cueball

Exhaust systems are not all loud depending on what kind you buy. When you start removing the cats and do some serious exhaust mods is when it gets loud. Since there are currently no aftermarket set-ups yet, we will have to wait and see what kinds we get.

babylou

Typically a freer flowing exhaust will be louder. I too hate load coffee can exhaust. However, the word to date is the RX-8 is very quiet so a bit louder exhaust may not be so much of a big deal.

If one is smart they will spend the extra bucks and get a Titanium exhaust. Maybe the power gain is 5 hp (+2%) but the weight reduction will be close to 30 lbs (-1%). Combine the two and you have a 3% increase in hp/lb. The really cool thing is the weight reduction not only helps acceleration but will improve handling, braking and fuel efficiency while reducing emmissions. In fact safety is even marginally improved when non-structural weight is reduced.

My focus for upgrading the RX-8 will be slight massaging of the engine (exhaust, low restriction intake, mild port work) coupled with an intensive weight reduction scheme. I think with a bunch of research, some hard work and and maybe $5,000 the RX-8 can gain power throughout the rev range and lose 100 lbs. Hell I got 25 lbs out of my Miata without spending a nickel. You would be suprised how many useless little brackets can be culled and how many fasteners are too long.

Quick_lude

I think you just described the MPS RX-8..

Zio

Does anyone know if this car can take boost? Is it better to leave it N/A?

HEVNSNT

There has been mentions from Mazda execs that the Renesis was designed to be capable of being boosted but technically any internal combustion engine can be. The biggest problem is emmissions on cold start. If you put the cat after the turbo it would take a long time for the cat to heat up. If you put the cat before the turbos, it would create a restriction and decrease performance.

MrWigggles

I'm not sure why everyone keeps thinking turbo for this car.

Not that isn't possible just that I think a Eaton-based supercharger would be much easier implementation.

With the fourth generation Eaton it should be low enough profile to fit under the hood and could replace the plastic part of the intake manifold imbetween the throttle body and the metal portion that has all the variable valves. A pretty sophisticated manifold but doable.

Now intercooling it would be tough so you would be limited to a non-intercooled version somewhere in the 6psi range on pump gas and make hopefully 40% more power and more importantly 40% more torque.

That might not be enough for some but I could live an RX-8 approaching 350 horse for something that would cost probably a little over $5K.

-Mr. Wigggles

Sputnik

Don't forget that an SC (regardless of the type) has a limited rpm range in which it is effective. Not only that, but things start to get ugly the further you get outside of that range. Attaching that to an engine that has such a broad rpm range isn't very effective, IMO.

Could you make it work? Sure, but it won't be as effective as it would be on a piston engine.

---jps

MrWigggles

Sputnik,

The RPM of the Eaton supercharger is dependent on the RPM of the engine (obviously) and the size of the pulley on the blower.

For a high RPM aplication, the pulley size is increased. Eaton's have been used in high RPM VTEC applications for a while now with good success. Any shortcomings are due mainly to the VTEC nature of the engine and not the high RPM of the application.

It is true that boost is going to start coming on strong say at 4500 RPM for a 9000 RPM engine versus say 3000 RPM for 6000 RPM engine, but I don't see many RX-8 owners driving down in the 3000 RPM region anyway.

My fearless prediction is the first readilly available forced induction kit for the RX-8 will be supercharger based. That's my guess.

-Mr. Wigggles

RedRotaryRocket

You know, about centrifugal superchargers...there is always the complaint about centrifugal superchargers that they don't come in until higher in the rev range...you can set them up to make boost sooner, but then they overspin at higher engine RPMs.

I've always wondered if it would be feasible to design a centrifugal supercharger with a small cvt type transmission between the engine and the compressor...that way you could get boost earlier from the supercharger, but not overspin the thing at higher engine RPM. If the cvt is electronically controlled, you could even have variable boost control with an in car boost controller, rather than having to change pulleys to get different levels of boost.

Drawbacks to this would be increased cost, size, weight and probably larger parasitic losses, but I wonder if these drawbacks could be minimized enough to make such a beast possible?

Anyone have any comments or insight?

wakeech

why go to all that trouble and cost?? turbocharging (ie: exhaust-driven centrifugal supercharger) is far, far simpler, cheaper, lighter, etc etc...
you can still control the boost level, and have a small turbo that'd give you a small boost at low rpm, but still work really well at high rpm (with a biggish wastegate or something...)

RedRotaryRocket

Yeah, I know. I prefer turbos too :D I was just thinking of ways to improve supercharger applications...that's all. Centrifugal superchargers have the efficiency of a turbo (because the compressor is the same type) but they have more parasitic losses and don't make full boost until later in the rev range. Turbos, on the other hand, will make boost sooner than a centrifugal supercharger and have less parasitic losses, but still do not produce boost quite as fast as a roots or screw type supercharger. For racing, you can't beat a turbo, but for the street, something that produces boost sooner is arguably better. The trouble is, the superchargers that produce boost sooner than a turbo are less efficient. I was just trying to come up with a way to get centrifugal compressor efficiency with full boost at low RPM...admittedly at the expense of parasitic losses. This would not be the best setup for ultimate power, but it may be the ultimate setup for street driveability.

MrWigggles

You're refering to the centrifugal not the Eaton?

Some might not like the limitations of roots type blower than doesn't have an intercooler and thus can only safely make about 6 psi, but it is about as simple as you can get. (i.e. no oil lines, no exhaust manifold to tap into, and no intercooler, and superlight)

-Mr. Wigggles

wakeech

yeah Wigggles, that's true, but there isn't any quantifiable "magic number" of psi on can crank on a blower "safely"... but you're right, it wouldn't be that high...

in any case, blowers are far, FAR more rpm sensitive than a centrifugal supercharger (which means it has a narrower band of optimal operation), and are absolutely killer inefficient at highish rpms... this is why Sputnik said that it wouldn't work very well with an rpm engine like the RENESIS, where you've got like a 7000rpm range that you'd (ideally) want it to perform in... and your thing about "most people won't go below 6k rpm" is just nuts... how many cars have you owned where once you're through first gear you take it to redline each and every time??? none?? yeah, thought so...

anyhoo, other drawbacks with the roots type are that no matter how you set it up, be it for low or high rpm boost, it's still smack in the middle of your intake system, hindering airflow a bit at low rpm (for high rpm boost operation), or a whole helluva lot at high rpm (for low rpm boost)... not to mention the contorted mass of metal and plastic the RENESIS inspiration system consists of... just hookin' up a blower to the top end of that wouldn't be "easy" without some kind of pretty serious modification, and wouldn't really help you all that much 'cause the system isn't designed to inspire pressurized air: it's optimized for (about) 1 bar atmosphere (sea levelish...)

Sputnik

Sure, you can change some things with gearing (including pulleys), but at that point, you're robbing high-end boost for low-end torque, or vice versa. And don't forget, there is a big difference in boost levels "coming on" at 3k, and boost levels at 6k. It's not like you come "on cam" at that point, it only gradually builds up. On your example with the 9k engine coming on at 4500, it will be barely noticeable at 4500, and take longer to get to significant boost levels.

My point is, even on a 6k rpm engine, you have a trade-off of low-end or high-end power enhancement with an SC. You don't get both. On a 9k rpm engine, that trade-off limitation of an SC is "exagerated", and the benefit of an rpm independent turbo is much more of a factor.

Could an SC be the first available? Sure. There are many people who buy such kits, focused on "peak HP", and oblivious to technical setups. There are many companies that will put out mods that don't even increase power (did anyone see the article in SCC where they temporarily installed a couple of performance exhaust setups on a 350Z, and dyno'ed them?). People will still buy them.

---jps

P.S. BTW, you can install a roots-type and a lysholm-type SC with a traditional IC (see factory MKI MR2 SCs, Jackson Racing Miata SC kits, and Kennebell Miata Lysholm SC kits). And with the complicated intake plumbing of the RX8, I'd almost expect that kind of setup would be easier than the method of building an intake manifold specifically for the SC (see Jackson Racing Civic SC kits).

MrWigggles

Guys,

I'm talking comparative characteristics.

Any turbo is going to have more lag and less low RPM response. This won't show up at the drag strip where you spool-up the turbo before getting the green light but the turbo lag will show up for autocrossing and any time you want to pass someone. In short I'll take a typical 6 lbs of boost eaton system over a 6 lbs turbo system. The eaton system would probably give you better autocrossing performance increases which is what I care about the most. I drove my friend's '95 RX-7 autocrossing and it was a bit of a challenge keeping the turbo spooled. Now the 6 lbs turbo system would undoubtedly win on the strip.

Getting back to horsepower Eaton superchargers aren't about peak horsepower numbers because quite frankly they aren't very good. Eaton superchargers give a broad power band that starts lower in RPM than any other form of forced induction. It is the turbo systems that tip the dyno charts and give the big peak horsepower claims.

And as I mentioned before I don't understand why high RPM applications have anything to do with it. With the proper pulley sizing, the blowers own internal RPM will be the same. If you have an engine that is reving 50% higher than typical engines, you use a pulley that has a 50% greater circumference.

The limitations to Eaton supercharger systems comes when you want to make big power and want to push PSI's in 10 or more range. You need pre-coolers, intercoolers to make an Eaton system work and with that level of extra complexity a turbo sytem or centrifugal system would come in.

-Mr. Wigggles

Ps. Any forced induction system is going to need dedicated engine management system to work optimally. I wasn't trying to indicat otherwise.

wakeech

okay, let's do some easy math...

for instance, say you had a roots blower that worked best spinning at exactly 36000, m'kay?? now, you know that the blower's rotational speed is exactly propotional (assuming zero belt slippage) to the output shaft's speed...

the deal with an rpm dependant supercharger is that the wider the operating range of revs the engine makes, the narrower the optimal range of operation your blower is gonna have, and roots-type superchargers are already more narrow-banded than centrifugal ones...

so, say this roots typer supercharger made your 6psi at those spot-on 36000 rpm, alright?? so, you said that you like the low-end boost for your autocrossing...

let's set up the pulley ratio at 12:1, so that at 3000rpm we get those perfect 36000 blower-rpms, and optimum boost. at idle, which let's assume is 800 rpm, the blower would be spinning at 9600 rpm, which would be a problem, eh?? so, up the rev range we go.
at 7000k rpm, the blower is goin' at 84000rpm, and chewing up the airflow somethin' awful (the air trying to be pumped through would be mostly cavetating)... and it gets only worse... at redline, 9k, it's goin' 108000 rpm, which is (way) more than twice over optimum... this would destroy your top end performance, and negate the advantages of this high rpm motor, not to mention completely screw its drivability with the very short gears and final drive ratio the tranny's got...

so how about peak power then, eh?? well, then you'd want your optimal 36000 rpm to hit right at the redline, so you could still carry it over to fuel-cut , and make maximum power... then, you'd set the pulleys to a healthy 4:1 ratio... but, at 3000 e-shaft rpm you'd only have 12000 rpm, which isn't enough to make a lot of appreciable boost where you (i mean you specifically) want it...
so, you see the problem, eh?? that's why turbos are simply an all-around better solution for a motor like this. lag or no lag, you can have controllable and regulated boost throughout the rpm range.

MrWigggles

Wooh, wooh, slow down.

Why is your target 3000 RPM? Make it 4500 and you'll be fine. Use a 8:1 ratio instead of a 12:1.

And I never said I wanted it at 3000 RPM for the RX-8. I said for a "6000 RPM engine" I would like boost to start coming on at 3000. Sputnik understood what I was saying although we don't agree other aspects.

-Mr. Wigggles

Sputnik

The problem with the SC is that if you overspin the SC, it heats the air, and actually decreases the amount of airflow (just like you were talking about with going to 10 psi). So, to prevent overspin, you have to change the pulley system so that it runs at lower speeds (that way, it will run at the same rpms at 9k, that it will normally run at 6k, just like you said). But doing that makes performance terrible down low, and you lose the low-end grunt that the Eaton (roots) normally has. Off-throttle response at cruising speeds (you don't expect to be cruising around at 4500 rpm, do you?) will suffer just as much as turbo lag from a big turbo. You would have to downshift to get real kick from that setup, just as if you had a big turbo.

Meanwhile, a street turbo, one that would max out at like 10 psi, will be small enough and quick enough (especially if it's a BB turbo) that turbo lag will be greatly minimized. It won't be quite as responsive as an SC, but it will be quick enough that it's ability to keep optimum boost levels stable throughout the rpm range will make up for it. And that's in street driving, not just drag racing.

---jps

P00Man

"...and how many fasteners are too long."

that CANT be healthy.....
blahAHAHAH ai LOVE IT
________
Can Cocaine Help You Study

MrWigggles

Sputnik,

I actually due plan to be crusing close to 4500 RPM.

Sixth gear (overdrive!) is only 20.6MPH per /1000 RPM. Doing 80 on the highway I will be at 4000 RPM. Where as on my SC300 (which has the same manual tranny as the NA Supra), 80 MPH comes at a little less than 3000 RPM in fifth gear.

As a point of reference Corvette's six speed, in Sixth gear will make 80MPH at about 2000 RPM.

Like most I haven't driven the car yet, but Tito Ramos of Alamo Rotary test drove the car at Laguna Seca. He knew it was time to shift when he kept accidently hitting the fuel cut-off at 10K. He couldn't believe how smooth the engine was at redline.

The RX-8 was meant to be rev'ed high cruising or otherwise.

-Mr. Wigggles

Supercharger

Contact an expert:

http://www.askcb.com

For those of you who are in Texas, visit Corky Bell's shop.

Zio

Well I would be using the turbo for lower boost like from 1000-3000 RPM because I heard the power doesnt kick in until 3000+ RPM.