Can I "Bench Test" Compression?

[Eagle7]

'86 ITS RX-7 w/ '90 engine. 90 PSI peak and regular on #2 rotor, but irregular and 60 PSI peak on #1. I've not had it out of the garage, so I don't know how bad this will be, but I guess It's time to go to a spare engine.

I've got two spares (both '88s), but don't know much about either one. Can I do a compression test on a spare engine before I install it? Seems like I could mate a tranny to it, stick on a starter motor and test, although I can't get it warm first. Comments?


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Marty Doane
ITS RX7
CenDiv WMR

[ddewhurst]

Marty, did you do this compression test with a rotary compression tester or a normal piston compression tester ?

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Have Fun ;)
David Dewhurst
CenDiv Milwaukee Region
Spec-7 #14

[ddewhurst]

Marty, I am not trying to be an expert I am just trying to provide some information. If you have a Mazda workshop manual there is a very detailed procedure for checking compression. & the procedure is different for the 12A or the 13B. The engine needs to be at running temp along with other details.
If this motor has not been run for some time it could be possible that the apex seals are gumed or carboned up & having an impact on compression. You might try some automatic tranmission fluid down the carb so that the rotors are soaked, let it sit for a couple days, start it up & this would free up the apex seals if they were the culpert. The thing will smoke out everything in the area. If the engine has FI I guess you would need to pour some ATF into the leading plug hole while the engine is rotating by hand with the eccentric shaft. What ever your using to poure the ATF into the plug hole be sure it's not sticking in so far as to have the apex seals hit the item.

The following is copied information.

How do i know if my engine's apex seals are
ok?
With the engine assembled, a compression check is run. To do it "by the book", a special compression tester is required, because a normal tester for boingers is designed to measure only one combustion chamber at a time. Since three rotary combustion chambers share a spark plug, a normal tester will only show the highest reading of the three. The special testers are expensive tools normally only found at dealers or shops that specialize in rotary repair.

Using the special Mazda compression tester at sea level, if the readings are below 6.0 kg/cm2 (85 psi) and the starting system is functioning correctly (cranking speed of 240 RPM), there is a sealing problem. The compression specification drops roughly 4% per 1,000 feet above sea level.

There are a couple alternatives that are a relatively good guide to compression seal serviceability. The first is to simply remove the trailing spark plugs, disable the ignition, and crank the engine over. Listen for uniformity and strength of the hissing pulses escaping from the spark plug holes. If all the pulses don't sound the same, which is a steady and even rythm, or if they are weak, and the starter is doing its job correctly, there is a sealing problem.

The other alternate method is a normal compression tester with its valve disabled or held open. To do this, you must watch the tester while cranking the engine. The readings will be lower than those from the special tester. What you are looking for is relative uniformity between chambers, and minimum variation between the rotors. Using a normal compression tester with valve removed/held open will typically result in readings about 20% under spec.

If you check compression on an engine that wasn't run less than 15-30 minutes ago, you can be badly mislead. Compression must be measured as specified in the shop manual to be properly indicative of engine condition.

Oil will boost compression and allow starting of a so-called flooded rotary, but you can't count on any particular relationship between normal compression readings and those from a flooded engine with oil added.

What should my engine's compression be?

The basics on this topic are covered in your shop manual and the previous section, but both assume a stock engine. Porting results in lower than stock compression test readings. This is an unavoidable consequence that mirrors use of radical camshaft profiles in boingers.

Most engines end the intake cycle, beginning the compression cycle, after top dead center. The later this point is, the lower will be the test readings. Such porting and camshafts are designed to shift power up to higher RPM to raise total power. At elevated RPM, intake inertia overcomes the late closing to effectively raise the dynamic compression. Test readings of 90 psi are typical of a good running ported engine, while readings around 110 psi are more typical with stock ports. Likewise, minimum acceptable readings for a ported engine are less than the shop manual specification.



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Have Fun ;)
David Dewhurst
CenDiv Milwaukee Region
Spec-7 #14

[Eagle7]

David, sorry I missed you at Blackhawk. Maybe we can meet at Gingerman in June (if I get this thing wrung out by then). Thanks for all the info. I used ATF to get the thing going initially, but I haven't tried soaking it for a couple days. Sounds worth a shot.

I did the compression test on a warm engine on the trailing plug holes with WOT with with the EGI fuse removed with a boinger compression tester with the valve removed (whew, out of breath). #2 showed a very regular pattern, all peaking at 90. #1 was very weak on two of the three pulses, with the good one reaching 60.

I'm assuming with a reading this bad that it's not worth trying to run this engine.

I was hoping I could check the compression of the spare engines without going to all the trouble of installing them. I'd hate to go through this all over again.

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Marty Doane
ITS RX7
CenDiv WMR

[paulydee]

What David mentioned pretty much says it all. It sound like you have a weak apex seal, if not cracked or damaged. The lower pulse strength (60) may be a result of a damaged rotor housing as a result of the apex seal damage.

The only way to check a uninstalled rotary engine is with a leak down tester. It is not as acurate as a compression tester, but it will tell you if you have a blown seal or bad side seal.

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Paul D'Angelo
73 ITS CENDIV
Indy Region
www.iridiumracing.com

[Eagle7]

Originally posted by paulydee:
The only way to check a uninstalled rotary engine is with a leak down tester. It is not as acurate as a compression tester, but it will tell you if you have a blown seal or bad side seal.


That makes sense. I'll look into getting one. Thanks.



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Marty Doane
ITS RX7
CenDiv WMR

[ddewhurst]

Marty, along with the leak down suggested by Paul did you try the sound thing ?

***There are a couple alternatives that are a relatively good guide to compression seal serviceability. The first is to simply remove the trailing spark plugs, disable the ignition, and crank the engine over. Listen for uniformity and strength of the hissing pulses escaping from the spark plug holes. If all the pulses don't sound the same, which is a steady and even rythm, or if they are weak, and the starter is doing its job correctly, there is a sealing problem.***

My only date at Gingerman will be July 13th & 14th.

See ya then http://Forums.ImprovedTouring.com/it/wink.gif
David