RX-8 friend

There are two ways to look at an exhaust system. One is "get rid of all the exhaust gasses with a low restriction pipe". The other is to use "acoustic tuning" to help with this.
As I read it, the renesis has port overlap. 2 stroke engines have port timing just like the rotary, though the exact timing is different. Early 2 strokes (and lots of current ones) didn't use expansion chambers, and their timing was different. The backpressure you are talking about is in the expansion chamber only (the "stinger" or exhaust bleeder tip is designed to vent the chamber just enough that as little acousic energy is lost as possible but all the exhaust gasses escape so as not to build up back pressure - that acoustic energy is the shock wave that makes the thing work). The exhaust port doesn't see backpressure (unless you are calling the reflected wave backpressure - it isn't), it actually sees a slight vacuum pulled by the acoustic wave. The effect would work just fine for a rotary engine as the exhaust would all be "sucked out" by the tuned system. The RPM range that this happens at is controlled by several factors (see "Two Stroke Tuners Handbook" by Gordon Jennings). Input pipe length, chamber input funnel length, chamber end funnel length, chamber volume, etc.

You're right about perfect design being required for peak power, but there were lots of "street" two stroke M/Cs with expansion chambers that worked quite well over fairly wide RPM ranges (my RZ-350 for example - 40 HP from 350 cc and my DT-200 - 30 HP from 200 cc).

If you want to object due to the two rotors complicating matters, there I'll agree. I'm not sure there would be room for two expansion chambers under the car. Also, if you wanted this to be a "street" system, I'm not sure how you would treat the cat.

As I said before, it's not whether it's 2 stroke or not, it's how the venting is done. Piston or rotor porting lends itself to tuned exhausts. The current exhaust on the RX-8 has a little bit of "acoustic tuning" already. This is probably why so little improvement has been made so far. All I'm saying is it's time to try this different method.

Gord96BRG

Actually, the OEM 18" wheels are pretty porky - somewhere around 21 or 22 lbs. My 17" winter wheels (17.8 lbs ea) with winter tires are 6 lbs each lighter than an OEM 18" wheel/tire!

Regards,
Gordon

rotarygod

The Renesis unlike all previous rotaries has absolutely zero port overlap. In fact it has 64 degrees of dwell. Even on rotaries with port overlap, we do not have very much overlap in relation to that of a 2 stroke engine. On a 2 stroke piston engine there is a period of time where both the intake and exhaust ports are open in their entirety at the same time. This is only true on a peripheral port rotary and even then the rotor does not have to sweep back across the exhaust port to close. Even a peripheral port engine does not have the long duration of a 2 stroke engine in regards to overlap. As with exhaust gas velocity, acoustic energy also has high and low pressure waves. We have 2 different pressure waves in an exhaust or intake system not one and we must optimise both simultaneously to achieve the greatest results. Some of these waves do get sent back to the 2 stroke engine to slow down escaping gasses but are primarily focused at stopping the incoming fresh air mixture from passing through. This is where the acoustics come in. To say that a 2 stroke exhaust would work well on a rotary would also be saying that it could work evenly as well on a piston engine. Every piston engine in production has overlap and high performance race cars have lots of overlap. None of them uses a 2 stroke exhaust and they spend lots of time and effort on exhaust design. They have lots of overlap and don't benefit from it. The Renesis is an even worse candidate due to zero overlap so it will not benefit from it either. A properly collected 4 stroke exhaust system does scavenge a chamber clean. They already have the potential to achieve efficiencies of greater than 100% at certain rpm's. The problem with the Renesis is that the 2 center ports are siamesed. You will not get any benefit from any system until the center port is treated differently than the other 2 ports. This is why conventional headers will not do much. The expansion in a 4 stroke exhaust is best done in the collector. As an example, 2 pipes that are 2" in diameter are joined at a 2 1/4" collector that expands to a 3" pipe in only a few inches. This setup can actually promote scavenging through better mixture of each pipe with the other. If you are not using any collector you are now tuning strickly according to organ pipe resonance theory. Since we don't have a 2 stroke engine and have absolutely nothing in common with a 2 stroke engine other than lack of valves (this is not a qualification of similarity), I will stand by my words. 2 stroke exhaust technology will not work on the Renesis.

BTW: You named the book that I mentioned. I just forgot the name when I typed this up. Go check out these books on proper exhaust theory. 2 stroke theory is also mentioned in the textbooks. I do like the book from Gordon Jennings though. It is full of some fantastic info. It just doesn't apply to the Renesis.

"Scientific Design of Exhaust and Intake Systems" (265 pgs. or so)
"The Internal Combustion Engine in Theory and Practice Vol. 1: Thermodynamics, Fluid Flow, Performance" (570+ pgs.)
"The Internal Combustion Engine in Theory and Practice Vol. 2: Combustion, Fuels, Materials, Design" (500+ pgs.)

Not exactly easy reading by any means but full of more than anyone ever wanted to know.

Omicron

Wow, interesting stuff RX8-friend and Rotarygod! I'm lovin this exchange.

Red Devil

For some reason I thought 21lbs was light for an 18" wheel?

I've seen Paul Yaw has his wheels coming out soon, I wonder how much they will weigh.

sun stroke

In the thread I believe it says about 2-3 lbs lighter than stock

https://www.rx8club.com/showthread.p...ght=yaw+wheels

neit_jnf

It is very light for a STOCK wheel, amongst the best!

zoom44

besides the obvious like lighter wheels etc- get a dremel tool, file, knife and scrape/cut/ slice an ounce off of every piece of plastic/metal/fabric, that you can safely, in the car. with like a million parts in the car you could conceivably take of a half a million ounces or more.

rotarygod

Let's see here, 16 ounces per pound divided into a half million ounces is 31,250 lbs. Good luck taking that much off!

Gord96BRG

Bah. The Mazdaspeed 18" wheels (according to Rosenthal Mazda's web site) are 17 lbs, and aren't that much more expensive than the OEM wheels. I know big tractor wheels are trendy these days, but the performance of the car would have been better if they'd just put lighter 17" wheels and tires on the car from the factory. Unfortunately, that's not the style going these days. Just look at all the guys wanting 19" wheels - heavier, poorer handling. Look especially at the guys staggering the wheels with wider wheels and wider tires on the back of an RX-8 - purely for looks, and guaranteed a staggered setup will handle worse than stock.

Regards,
Gordon

canzoomer

I tend to agree. Not only that, but the RX-8 is geared pretty darned high with those 18" on it.

I have had absolutely NO problems hitting 250kmh.

With my 17" winter wheels on I find noticeably better aceleration.
According to my GTech about a quarter second better in the 1/4 mile.

Mind you, a bit more overdrive in 6th would be nice for gas economy on highway runs. I generally drive at about 140kmh on highway 2, and that puts me at around 4500rpm, which makes for crappy mileage.

Omicron

Now isn't THAT interesting! 1/4 second is significant. Hmmmm.

canzoomer

It is quite significant.

But then (assuming equal tire aspect ratios) it is about a 5% difference in tire diameter.

I just measured my stock 18" wheel and tire and my Nokian WR on the 17" and it is almost exactly an inch shorter.
24.25 x 3.141 = 76.16" circumference
25.5 x 3.141 = 80.1" circumference.
76.16/80.1 = .9508

And it does not cause another shift in the 1/4.
And noticeably more wheel spin on 1-2 and 2-3 gear shifts, meaning less rpm loss.

It all adds up.

murix

My sentiments. Technically, the justification for going to larger wheels stemmed from the need for larger brakes. That is usually not the case for most people on the street of course.

Staggered wheel sizes works on many cars. For the 3rd Gen RX-7, I think the 225/55 coming on the rear was much too small to handle the torque it made down low, especially once you start upgrading. Putting larger wheels and wider tires in the rear did a lot for taming its loose rear end. People who complain about the RX-8 must have never driven one. Same for large displacement engines with gobs of torque down low.

As for us, with our power band, I agree I do not see the benefit of a larger wheel in the rear short of appearance. When my tires go I will most likely put a 245/40 Potenza S-03 in the rear with a 225/45 S-03 in the front on the stock wheels. This is as much as I will go. If they had a performance 17" wheel option, I would probably have taken it as the choice of good tires for us right now sucks.

RX-8 friend

Just pony up and get the magnesium Mazdaspeed wheels. They only cost as much as a Hyundai.

JimW

Pardon my lack of knowledge but, will installing smaller diameter wheels on a car designed for larger ones alter the computer, odometer ect? Will this also diminish traction and ride quality. I do like the overall time improvement though, it's almost like installing a stage 1.

rabinabo

I think people earlier were talking about wheel diameter (minus the rubber). You can use a different size tire to maintain basically the same total diameter (including rubber), so it won't affect the odometer. The smaller wheels will have a larger profile tires, so more of the diameter is rubber. More rubber means they absorb more shock, so ride comfort should be improved, maybe with a loss of a little handling. Lighter wheels will reduce unsprung weight (not supported by suspension), which makes the car much quicker than an equivalent amount of sprung weight (supported by suspension) would.

canzoomer

I am talking about the same profile tires on smaller wheels.
My car runs on winter wheels and tires whcih drop it 1"
The effective ratio is 5% lower.
So, when it shopws 100MPH I am going 95.
So what? Less speedingtickets maybe.
Ride? No worse. Remember the base RX-8 as shipped in Japan comes with 16" wheels.
Handling? As long as the contact patchis the same size. Might take wider tires to accomlish that.
As far as traction goes, depends on which case you desire. smaller contact patch may allow more wheelspin, useful for 1/4 mile times.
But will have negative effect on cornering.
MAybe. More a matter of tire stickiness and road conditions.

Omicron

Doesn't the tire sidewall being taller, resulting in more flex, affect handling?

JimW

Sorry! I forgot about this thread. Thank you for the help guys! If your very serious about draging, this is a great way to do it. I would still keep the 18s for daily driving and make the switch for the strip, some extra sticky rubber or drag tires would help as well, but I have to keep in mind the guys that were killing their transmissions on low psi stock tires! Omicron: your right, like rabinabo said, the thicker rubber acts like a shock giving you a more compliant ride but more body roll in corners.

Omicron

Yup, about what I figured. Sure wish I could drive one with the 17s on it for a few days, as I'm seriously considering getting aftermarket 17" wheels, and using THEM for my daily nice-road driving. Figure I'll use the 18" factory wheels for snow tires. LOL, is that a switch , or what?

JimW

I guess, I wouldn't know anything about snow, as I've only seen it once! I'm a native!I think there was another thread about the smaller diameter helping to achieve more gear in the lower rpm range, but you would have to shift more often which sounds like more fun to begin with. Canzoomer stated that the only difference using the smaller wheels and overall diameter is you are going about 5mph slower than what reads on the speedo, so I guess it wont screw with the computer to much!

Pk14

I was doing a search on the whole stock wheel issue - there are a couple of different posts stating the weight of the stock 18"x8" wheel (21 and 26 lbs). If it is the later, that is fairly heavy unsprung weight - there are a few good looking options in the 15.5-17 lb range that can have a noticable improvement in acceleration and handling. Cut 40 lbs of unsprung weight is ~ a 0.1 sec 0-60 improvement. 2-piece aluminum brake rotor hats on the wheel hub vice iron can save 10 lbs at each wheel as well. Now you are up to 80 lbs of unsprung weight removed. 1 lb of unsprung weight is equivelent to 10 lbs of weight anywhere else on the vehicle. 800 lbs off of a 3000 lb car...

CjsfastSS

Well I'm new to this community and don't know if you found mods other than Intakes and Exhaust, but I've been able to find pullies already, light weight flywheel, turbo kit in the works, a 20b 3 rotorary engine swap, and a few other things. Here's the web pages I've found.

http://www.srmotorsports.com/RX8-Parts.html
http://www.yawpower.com/
http://www.acostamotorsports.com/rx8parts.htm

I hope some of these websites help you guys out. I haven't even picked up my 8 yet, going to pick it up wed the 18th, but trust me I've been looking around already for performance mods, and body kits and the such. So hopefully ya'll can use these web pages and get some of the parts ya'll want for your cars.

Omicron

Nice contribution.... good use of the search too. Pretty unusual for someone new to the forum, so you're definitely starting out on the right foot. :D

All of these items have been discussed in one form or another here too, just keep looking and you'll find multiple threads about them.