Compression Testing RX8
 

Hey guys, I just bought an '04 RX8. I had a hunch when I bought it that it had low compression because it struggled to hot-start. So I did a compression test on it. I'm no mechanic but I did it the same way you'd test a normal piston/cylinder engine (taking out a spark plug and threading in the fitting on a compression guage). After taking the fuse for the fuel pump out, I turned it over and the rear rotor was peaking at 60PSI (about 4.1 BAR).
My question is, is this an accurate way to compression test a rotary engine and I need an engine rebuild? Or is there a special way to compression test rotaries? Thanks.

there is a specific rotary comp tester. you will get three #s for each rotor and cranking speed

Any idea where I could get one of those?

Thank ya much!

no problem. where are you from? i know a few people that have testers. btw welcome and good luck. please read the stickies on this forum and new owners threads

I'm up in Minneapolis, MN. I'd prefer not to buy a tester if I can avoid it so if you know anyone around here, that'd be great. And thanks for the welcome! I'll definitely be digging into the new owner threads.

sorry i dont know anyone in your area with one. ask in the regional forum for your area. someone may have one or point you to a rotary shop. the other option is a mazda dealer

No worries, I'll check it out. Thanks for the help.

Morries Mazda in Mtka had a Mazda compression tester. I did not know of anyone in the Twin Cities area that had a rotary compression tester when I lived there.

Good to know, I'll keep Morries in the books.

Buy the tester. It's the same cost as 2.5-3 compression tests. Over the life of the car, it will be a very small number. Also, you can be the hero of other rotary heads in your area and barter the use of your tester for other goods/services (beer, pizza, 1st born child, etc).

Seriously, buy the tester.

(Alternately, you can use my DIY to build your own but I haven't had time/resources to validate/calibrate the output so YMMV).

I know it'd be a good investment, I'll consider it. I've been craving pizza for a while anyway...

Sorry to hear that, about the pizza. it will depend how good your business sense is as well. After you do the compression test, decide whether or not whether you are opting for a new engine. Or, just replace the engine now. trust your instincts.

Dont know i returned the new battery. Since my optima redtop was still on a 3year warranty so waiting on news from the company. loll

Might aswell get a new one for free even if ppl say there not good now

Think you replied to the wrong thread m8.

To the OP: There is a somewhat ghetto way wherein you remove the schrader valve from your tester and video record the gauge as it cranks. Then you can look at each pulse and either figure out the RPM from that (remembering the rotors do not spin at the same RPM as the e-shaft) or use an OBDII reader to monitor cranking RPM at the time of the test. Then you normalize your readings to 250rpm via a chart, and off you go.

Right now, it's looking like I'll be doing a rebuild on it. From what I can tell, it'll be a little cheaper since I'll be doing it myself and I'll have the confidence in knowing it's all solid as opposed to putting a used motor in it.. I wanted to dig into this car and learn the ins and outs anyway.