So I obviously found lots of threads on compression, but I was curious to know what you experienced engine builders would consider "acceptable" compression on a 3mm seal motor?

My motor has roughly 15k miles on it. A compression check was done today, and it came out on a Mazda checker at consistent mid-6s. I don't have all the numbers, but let's just assume they were mid-6s and consistent.

From what I've read, that is not good.

The car made around 360 whp @ 13 psi and 425 @ 20 psi. The turbo is not in good health either with some shaft play, a bent fin on the compressor, and some oil seepage. Before I decide to pay to have the turbo pulled off to rebuild it with perhaps a Q-trim wheel and some other stuff, I'd like to have some assurance that consistent but "low" compression does not mean imminent death.

Thanks for the advice.

 

Are those numbers from a fully warmed up engine? 3mm or 2mm seals, a healthy motor is still in the 8+ kg/cm^3 range. Did you reuse the rotor housings on your rebuild?

 

not quite sure what you are looking for? would i rebuild/upgrade a turbo and put it on an engine that has def seen some serious time and wear? prob not. save the money drive the car, if the engine gives out get it fixed and the turbo together. why have the car down twice? what if it grenades on the new turbo when you retune it? the real question is what does scott say as he built it right?

 

It might not lead to death, but mid 6 compression is a fairly weak motor. You could be losing a lot of power.

 

As far as I know, yes the motor was fully warmed up.

I'm not sure what was done with regard to rotor housings on the rebuild. I bought the car when the motor only had 300 miles on it and the turbo was allegedly brand new.

James, I think you're right. What you said is exactly what I don't want to do. Rebuild the turbo and have the car down for a while (albeit relatively short) only to have the motor go the first time we go to the dyno with the rebuilt turbo.

Scott says that the motor will probably be ok. He wasn't happy with the numbers, but he wasn't totally against the idea of upgrading the turbo. I told him that I would think about it tonight and give him a call tomorrow afternoon.

 

if you are considering upgrading the turbo at all i would just wait. and nobody ever gets their tubo back from rebuild or upgrade as fast as they want or expect....

 

mid 6s is fine. 3mm seals don't seal as well as 2mm so if everything is equal the compression will always be lower. You'll just have slightly less pull and bad mileage. If it's consistent on both rotors it may last longer than a high compression engine because the combustions will be less powerful if something goes wrong with the AFRs.

 

Yeah, I was actually thinking this as well. Just like a piston motor may be rebuilt for lower compression to handle more boost, shouldn't a similar principle carry over into the rotary realm? Fritz, thanks for the reply. I don't want to fool myself here, but that is much better news. I've always had shitty gas mileage (10 MPG around town, never done any highway driving), so that might help explain something.

More opinions please.

 

8.5 CR will still net higher than 6's. They will pull 100's on good equipment.

Porting will make a difference though.

I would do a compression test yourself. Make sure the pedal is depressed/warm etc.

If you get 80's-90's that fine for a port.

what seals are they?

 

why was i under the impression that 8+ compression was rare and never? i need to get my motor checked too...

 

In my mind i think of it as a grade scale.

these number are not in kg/cm^2

sometimes they convert them to a 1-10 scale

5=F
6=D
7=C
8=B
9=A
10=Perfect

8.5 kg/cm^2 =120psi = perfect

if your compression was 6.5 kg/cm^2 then that's 92.45173 psi and is good.

If you have a SP thats really good numbers. That why I say do one yourself.

 

"Aftermarket 3mm" is what I was told. I do have a streetport.

 

I think if it still starts up fine, and pulls well I wouldn't worry about it.

 

if you want a Koenigsegg, you better start saving your pennies now

the FD is an incredible money pit....some people could afford a new Z06 with all the money they have dumped into these cars, it's kind of crazy

 

what is your vacuum at what idle rpm? does the car hot start immediately? Two important questions that haven't been asked yet.

 

I've never had a hot start problem. Sometimes I have to turn the key 6 times before the starter reacts, but it will always turn over rather quickly. The only starting issues I've ever had were unexplained random times when the car was cold. Also, sometimes when I start the car, it will begin to idle very low (like 200 RPMs) then kick in after a few seconds and settle at 1,000 RPMs. Car seems to miss intermittently at idle especially after a lot of stop-and-go driving. If I wind it out through 2nd under boost, the idle quality will improve for a while and gradually degrade to a slight inconsistent miss.

Vacuum has always been "low." Depending on which device you trust, 12-13 in @ 1,000 RPM idle. I think the car is running a bit more rich at idle than it used to which I know could have an effect on vacuum. IIRC, vacuum used to be closer to 14 in @ 1,000 RPM idle.

Car pulls strong and still made 425 @ 20 psi with a turbo that needs a rebuild. I'm not even sure what specs the turbo is other than it's a "T66."

 

I agree with mad_7tist totally
Just ride it out you dont know that the turbo or engine is about to go that's just an educated guess at best
there shouldnt be any unknowns if scott built the engine and he maintains it regularly it seems. he should be able to answer all your questions and know why it has low compression and if that is acceptable etc. he's probably built more than 1 3mm engine and kept tabs on the compression I hope

for reference in my last car the pettit 3mm engine would give 12-14 vacuum @800rpm depending on the weather.

 

Run it until it lets go. 3mm seals are harder to break, and even if they aren't sealing perfectly for one reason or another (be it housing wear, seal wear, lack of premix/omp ratio, weak/flattened apex springs, binding rotor slots/milling, etc.) it doesnt mean they'll break anytime soon. Mustang owners walk around talking about what gear they're running in the rearend, and rx-7 owners walk around talking about how much compression they've got...it doesnt matter that much really, you either have it or you don't, and right now you do

 

Kevin,

Thanks for the reply. I was using one of your old posts to determine that my compression is "bad," so I appreciate that you've chimed into this thread. I think I'm going to just run the car as is. I have a supercharger kit waiting to be installed in the FC, so I really need to get that done to see how much that will end up costing before I go spending any money on the FD.

Alan

 

Let me amend my previous statement. Don't run it until it lets go, but run it until compression drops to 6.0 or below all around, which is about the lowest acceptable limit for a running engine (~85). At that point it's going to rapidly deteriorate and begin causing driveability problems, so it's best to catch it before something happens inside that'll cost more money.