spectre6000

The owner for the first 72K miles on my 09 was a housewife who didn't know that the car would beep to tell her to shift at red line. On the whole, the car is in excellent condition, but the ignition system was in obvious need of attention (attended to), and there was carbon built up on the plugs like I've never seen before in dozens of old, neglected, and abused cars. I did a compression test, and adjusted for altitude (I live at 7k') I got the following:

PSI front 91 89 89
PSI rear 94 96 94

Manual states minimum is 98.6 PSI. This is the only RX-8 I've ever driven, but I have to figure the characteristic linear pull of the rotary engine isn't lost compared to my old RX-7. I've noticed that power delivery is a bit on the flat side until it hits 3-4k, then flattens out again after that. My butt dyno says I'm not getting anywhere close to the specified output pretty much anywhere along the RPM band.

All this in mind, the car still starts runs and drives hot and cold just fine. If I didn't have previous rotary experience, I'm not sure I'd notice that it was down. I'm also not sure I'd have fallen in love with rotaries quite like I have.

With compression numbers below spec as they are, how much power has been lost in addition to the performance characteristics?

kevink0000

Compression definitely contributes to loss of power especially at altitude. But, I would be looking in other areas first. Like how well did the cat survive the previous neglect? Are all solenoids and vacuum motors functioning?

That kind of stuff.
Based on what you noted in your post:

My first job would be to pull the cat and inspect front and rear substrates.

Your question is a good one though.I would love to know the answer, if it’s ever been established.

TeamRX8

iTrader: (25)

that’s common for RX8 sparkplugs as well if they have substantial miles on them

next thing to check is to make sure all the intake valves are rotating freely and functioning properly

if I had to guess; ~150 whp, but that assumes everything else is operating correctly.
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spectre6000

I'm pretty sure everything else is in good shape. I went over the top end pretty thoroughly. I figure with the power dropping off where and how it is, I'm losing gasses past the side seals down low where the rotors are moving slowly and up high where the pressures are high. ~150hp is in the ballpark of my own educated guess. My butt dyno says I'm down about 100hp at the worst of it, but it's hard to say without a direct analog.

kevink0000

It’s probably worth it to do some 3rd gear timed 60-80 or 60-90 mph runs— a better guesstimate.
My vote is to still check the cat first and rule it out as a factor. They don’t last lost long at all with bad ignition on piston engines, moreso this car.They melt at the rear first.

Loki

iTrader: (1)

If you have a data logger you can use virtual dyno to get an idea of current output from a 0-60/60-90 pull. Ideally you'd have logs from long ago as baseline as well.

kevink0000

FWIW, I assume that if all the rest of the car is functional, those compression numbers would yield HP that would likely be surprisingly higher than that. I’ll put money on 160-170 when standardized.

spectre6000

I only got the car a little over a year ago, and it came in its current condition. I've been trying to beat up on it a bit to burn some carbon out, but I haven't run another compression test since doing so to see if it's done me any good. Doesn't feel like it.

Meanwhile, I might have something like a datalogger in the digital gauge set I installed last fall. I'll dig into that over the next few days, and see if I can't find a good clean place to get runs from various speeds to various other speeds.

UnknownJinX

One thing to consider: the peak power and torque figure are at specific RPMs, both of them quite high (peak torque at 5500 RPM and peak power at 8500 RPM). If the whole "rotary builds compression as it revs" thing is true, then I doubt you will feel much of that power loss at the peak figure RPM.

There is also the fact that at higher RPM, the OMP might be able to inject enough oil to compensate for the sealing loss somewhat.

The issue will be more significant at lower RPM on a daily driving level, I'd imagine. The time you are not in the power band is when I'd think the issue might manifest more.

spectre6000

At lower RPM, I'm pretty sure I'm down power because I barely have enough torque to let it run less than 2500-2750 RPM on anything more than an absolutely flat surface. I think break in procedure wants RPM below 2500 for the first however many miles, and that would not be possible. It makes the most power in the mid range, and loses steam around 5500 or so.

My RX-7 is what got me to fall in love with rotaries. That linear pull from idle to the moon was just intoxicating. If this was the first rotary I had driven, I would not be so smitten. The mid range is nice, but it's gutless down low (more than I recall an otherwise healthy rotary should be), and not really worth revving past 5500.

Given the cost, I can't imagine paying to get it on a dyno. I doubt anyone else has either. If someone is aware of a tired stock motor dyno chart though, I'd love to see how many horses got out.

SparklingFresca

I think you should find a local rx8 member near you and drive a health example. Everyone who drives mine at autox is surprised at how much power these have versus what they hear about.

spectre6000

Actually, that's a pretty excellent idea... Still a butt dyno result, but it's my butt dyno vs my butt dyno, so actually comparable.

kevink0000

FWIW, I am as convinced as I can be, that there is something else that needs attention with your car. Your compression is low, but not so low it would cause the problems you describe.

My s1 feels slow and tired at 7000+ ft and it has mid-high 8s compression. So does my wife's NA BMW. That's a factor.

But, any RX8 low compression or otherwise, sea level or mountains, should "feel" better at high rpm, you should want to stay up there, as it will feel like the engine "likes" it.

Guys have won races (not street) with RX8s (and 7s) against piston cars, with lower compression than yours.

I really think you would benefit by looking further.

UnknownJinX

Based on the highlighted part, you should check under the hood and see if someone has swapped a Mazdaspeed3 engine(MZR 2.3T) under the hood, just to be sure.

But seriously, RX-8's are definitely not supposed to make power this way. I'd check to see if your cat is starting to plug up(due to the failing ignition before) as Kevin mentioned. Maybe some of the valves that optimize high RPM powers are jammed by carbon. That's a possibility as well given what you described about the previous owner.

You should feel it smoothly pull to the redline and only maybe start to taper off after maybe 8500 RPM, but even then it's pretty subtle.

I think I should add that dyno charts are generally not very reliable as a source of how much absolute amount of WHP you will make. Dyno is generally a tool for you to see the general shape of the torque/power curve and the effect a mod/mods have on a car.

Brettus

iTrader: (2)

Just went through this with a car brought to me recently with similar compression numbers ( 96,94,91 ........ 89,94,88) complaining of low power.
These are the things I checked:
1/Check correct operation of all three sdais valves (SSV,VDI,APV)
2/Checked for vacuum leaks on the vac. actuators for ssv and vdi. Just checking operation doesn't tell you if vacuum leak allows them to close under WOT.
3/Check SSV still does self checking after several minutes after engine turned off( this determines if there is a leak in vacuum chamber)
4/HEI coil test per Teamrx8 coil test thread
5/Checked plugs
6/Checked for blocked cat (Disconnect front joint and dropped for visual inspection)
7/Datalogged and checked AFRs (should have done this first)

In this case the problem turned out to be a blocked fuel filter causing very lean AFR at high rpm .
Fixed this and it went great ... the difference between this car and one with excellent compression was barely noticeable on the road. Virtual dyno showed it to be within 10whp.

IMO .... 90% of all RX8s on the road today are low on power due to one of the issues above . Low compression is NOT as detrimental as most believe.