Oppai

Hi Folks,
I recently created a thread about hot start issues and posted my compression readings.
I was able to pinpoint the cause of the hot start issues to injector issues rather than compression readings.

With that said, when i bought the car, I forgot to normalize the compression results for RPMs. Stupid mistake, as I've owned an Rx-8 before and knew this information already.

After normalizing for RPMs using the calconic.com calculator, I realized the compression results are in the yellow zone, especially for a stock motor:
85/82/82 - 83/89/84

Foxed.ca yielded:
88.5/85.7/85.7 - 88.1, 93.8, 89.0

Now that I know I'm pretty much on borrowed time with this car, how much mileage can I expect to get out of it before requiring a rebuild?
The car has 55,000 original miles. Is this an unusually accelerated compression loss?
8 5
Here are the compression results provided by the seller:



Thanks,
Joey

XanderCage

iTrader: (1)

I've read quite a few differing opinions here regarding pre-emptive rebuilds. That said, if boost levels etc are kept stock then frankly if it were me, I'd just run it and enjoy until compression drops enough to make it difficult to start. Who knows, you might get more life out of it than you expect.
Also, just to throw a potentially dumb question out there as well- are you certain that the compression tester pictured doesn't auto-compensate readings for RPM? I believe some do, so just want to make sure you're not double-compensating

Oppai

I'm not absolutely certain, but it would be kind of odd to me that it would display the rpms if that were the case.
I'll do a little research into that particular model.
Thanks,
Joey

Oppai

Just looked at the website written on this model of compression tester and the website for it has a ink directly to the foxed.ca compression test normalizer that I used for one of those calculations.
Guess that means the results displayed are unfortunately non-normalized.

Pete_89T2

iTrader: (4)

I don't think a preemptive rebuild is necessary in this case. If the motor is still starting up easily hot/cold, runs well, makes good power, why bother? With proper maintenance, it can last a long time.

As for the compression testers, I have one of these: https://www.rotarycompressiontester.com/

When the test is complete, the display will continuously scroll the actual compression figures (in metric & imperial units) at the actual test RPMs, and then give you the compensated figures, normalized to 250 RPM & sea level altitude.

Oppai

Thanks Pete. Are you able to tell if these are the normalized results or not? does it still show engine RPMs when displaying the normalized results?
I think this might not be the same tester you have, since the branding on it has a link to https://www.digitalcompressiontester.com instead of rotarycompressiontester

Pete_89T2

iTrader: (4)

You got a different tester, I'm not familiar with that make. But if it displays an RPM figure that's not 250 and the maker is giving you a link to help you calculate normalized numbers, it's a safe bet that your tester is displaying just the raw data. Generally speaking, if the actual cranking RPMs are > 250, your raw compression numbers will be a bit higher than the corrected/normalized figures, and the inverse is true too.

j9fd3s

iTrader: (3)

if you are going to modify the car, tart saving for a new engine. if you're going to keep driving as is, please do its fine.

there are two reasons, the first is applicable to every engine, the less compression it has the less well it seals, and no matter what power will be down. the rotary has a second problem, and that is leakage between chambers, basically for us this means more hot exhaust gasses in the intake stroke, which makes detonation more likely.

or if you like, the more power you're trying to make the healthier the engine should be.

EDIT: oh and the other odd thing with the rotary is that the better it runs the better the compression will be, so it needs to be running nicely and warmed up. if you tested it when it wasn't running right, and then again when it was the numbers can come way up

Oppai

Thanks I appreciate the info. This is pretty much where I thought i stood on this engine.
I currently have no plans modify the car, i'd like to keep it as bone stock as possible for the foreseeable future.

DaleClark

iTrader: (55)

The rotary compression tester Pete linked to is fantastic, I have one and it's SO simple and quality. The fact that it gives you numbers corrected to 250 RPM is awesome. Price is also really reasonable.

That said, a typical stock engine seems to last about 80,000 miles. You've still got a good amount of life left there. Just ENJOY the car for now. At some point you will either need a rebuild or you can get one of the new Mazda short blocks for the car, but I don't see the compression as a "you need to do this now or soon".

Once you get it running right some good hard runs may actually bring the compression up a little bit.

Dale

j9fd3s

iTrader: (3)

keep driving and enjoy it!

Oppai

Thanks Dale, I'll be picking one of those up whenever it is that I decide I want to start building rotaries again. I'm really enjoying the car so far!
Hope this virus is over soon so I can register the car and take it for a real drive.

Pete_89T2

iTrader: (4)

^This is absolutely true, and it even applies to a newer engine. As an example, I built a brand new 13BT for my FC three years ago. It's been intelligently modified and dynos at 350~380 RWHP, 310~330 lb-ft and it has just under 10K miles on it now. Since buying that RCT5 compression tester 2 years ago, I've been periodically testing my FC motor. One definite trend is that when I test my FC right after winter hibernation, or any other lengthy period when the car is driven infrequently, my compression numbers are lower - not below spec mind you, but notably lower than they are after I drive the snot out of it and retest.

Oppai

I've tried the italian tune up on old my rx-8 that was losing compression with no luck.
But after having heard this, I think I'll throw some marvel mystery oil in the tank and thrash it once I'm done with all the maintenance items and repairs.