Compression range
 

What is the low-end of a "good" 20b compression for the 3 housings?

I would like to know when a 20b is too low in its compression.

Compression check is as follows: T1-90 L1-88; T2-90 L2-90; T3-80 L3-75.

Are these "low", "marginal", or OK compression readings for a unmodified 20b?

those are not correctly measured. somebody is trying to BS someone. it should read somethinglike this ?.5-?.4-?.5 rotor (a) ?.6-?.6-?.6 rotor (b) ?.7-?.8-?.5 rotor (c) or something to that effect.

T1 L1 refers to trailing plug and leading plug, some idiot stuffed a piston compression guage in each hole, and then cranked the motor this would/should chause the reading to be almost identical for each rotor ie: L1 equals 5 then T1 should = 5 as they are the same rotor!

the numbers I used are arbitrary but FWIW 9.0 is a fresh rebuilt/new and quickly drops to and stabilizes at about 8.0~8.4 for a quality rebuild. this is the compression number for one rotor face, and you should get 3 per rotor x 3 rotors you should get 9 numbers total

anything below 6.0 needs a complete overhaul 7.0 is ok but watch it , and 8.0 and up is a great compression number. these are for a stock ported motor and are an average, ported will be slightly lower due to the fact that the exhaust ports open earlier and close later allowing for more overlap between the intake and exhaust ports on any stock ported rotary.

doug

6.0, 7.0... what units are you using? Bar?

Thanks Doug and Turbo 3 for responding.

The numbers were taken by a piston based person. I know each rotor has three faces and three compression readings per revolution.

One of my friends takes his home made tester and hooks it to his ocilliscope to figure the compression on each rotor. Unfortunatly the 20b is not available to me to test the compression myself, so I only have the piston guys numbers.

I know that a compression gauge(for a piston engine) will show the highest of the three compressions per rotor, so I think the readings that were taken by the piston guy are from that kind of compression gauge.

So what is the lowest (in lbs of pressure) that one could still call OK for a 20b?

he is probably using KG/cm2 thats what we use here in Japan, 6.0 would be ike 85 psi 7.0 would be like 100 psi

you can use a piston based tester to measure, you just remoe the schrader, and check to see if the pulses are consistent, but IMO anything that reads 75 or 80 PSI needs a rebuild in a bad way.

kenn

I thought the OEM specs said anything below 70 was bad and 70-90 was average, above 100 psi was great. I've used a piston compression checker but unfortunately you don't get to see the pulses if the refresh rate is really fast and only "see" the average number on the gauge. If you don't have a scope, it'll get you by quick and dirty.

Thanks for the input kenn.

if it's a j-spec, don't worry about 70 if all the pulses are consistant--the seals are probably just gummy.

Yeah 70~90 might be average, for the average joe, but when I race a car, anything less than 90 is a cause of concern, making power is all about the amount of heat and pressure you build in the engine, and if it ain't building pressure, it ain't making power. and most of the time as it looses compression you tend to push it harder trying to make up for what its missing, which results in a big pile of FUBAR when you break the engine down next time.

A lot of the guys there in the states seem to like paying excess money for their engine, they rag on it till it blows, and then pay a lot to get it rebuilt, better to have 2-3 spares, and when one starts to get marginal, pull it, and rebuild it, before it blows, much cheaper in the long run. that why I say anything below 80 psi is in dire need of a rebuild (by my standards), and not something I would run in my car.

On the question about JDM spec engines being gummed up a lot with carbon, thats true, as a lot of them get babycake driven, but believe me, when i say, that there are plenty of them that don't live to see carbon.

A stock car owned and driven in the tokyo , kawasaki, Yokohama area = high carbon due to the traffic they operate in,.

Anything from Chigasaki, or from the gifu, shizuoka area, stand by as they won't have hardley any carbon build up, the people that own them there tend to drive hard on there cars due to the wider open spaces (figurativly speaking)

kenn

you make some very good points there. everything really depends on what each person is willing to deal with. you can recycle most of the hard seals in an engine when it's rebuilt, but it's the difference between a decent build and a great build.

i put a motor for my car together with spare parts--used EVERYTHING, side seal gaps were up to .006, and it ran great. the fuel economy suffered, though. i think a loose motor doesn't drop the power all that much in the high rpm/high boost range, it's the low end and fuel economy that suffer.

for a motor as unique and expensive as a 20b, there's no reason not to go all out when you rebuild. it's too much of a hastle to take it out, tear it down and rebuild later just so you can save a few bucks now.

agreed, not to mention the risl of damaging something if its marginal.

ken

agreed, not to mention the risl of damaging something if its marginal.

I am just not willing to deal with marginal parts do to the way I drive.


ken

i dont' on customer cars, and i wouldn't on a 3 rotor, but for my personal engine in my old fc... best $300+recycled parts i ever spent :D