To all you engine builders out there what is a typical compression you like to see after a build.

*What are the typical numbers one needs to see after a build to determine a proper build has been done.

*Also is there a increase in compression after the break in period or is this a myth.
If in fact the compression does increase after the break in period, what are the numbers of the increased in PSI.



I have heard that a proper build should show numbers around 8.1bar=117.5psi.



thanks

Michael-FDSeoul

 

It depends on the quality of the parts used, esp whether the rotor housings and apex seals are new.

We typically see over 100 psi on all faces for an FD build with new apex seals and low mileage rotor housings.

You can tell if it's a good build just by listening to the exhaust and driving it...... almost every fresh build of ours fires right up and holds an idle without much of an issue, aside from some initial smoke. This is especially the case if the car isnt in need of an immediate tune, ie oem sized primary fuel injectors, original TB with all idle settings and TPS properly adjusted etc.

I doubt you'll see 8 bar/120ish psi right away out of a fresh build, but we never check compression on new motors. It's either right, or it's wrong

 

Surprised that you made this statement Rich, Few builders I have spoken to in the past have mentioned including Mazdatrix that after a proper build (with proper parts ie. new housing, new springs, Apex seal) seeing 8bar is not that uncommon.

Given that once the seals and springs have gotten a proper break in, I would think that factory spec numbers should be obtained if not exceeded.

I have recently talked to a few people regarding their builds and came across several different results obtained by using various different none-OEM components and wanted to be enlightened to the different results after the break in period.

 

I can get my result this weekend, I just installed my new build this past saturday and start it twice to get it moving, so far it start perfect I do have the mazda digital compression tester I am curoius myelf to see what are the numbers.

 

you'll have to list the parameters when you get around to it. In any case looking forward to the hearing the results.

 

FDSeoul that’s a really good question. From what I heard before is, if you break in the motor properly, by avoid high revs and quick spirited starts from lets say a stop, or rolling after the light turns green and getting on it, you actually allow the motor to get lose on it's own and this allows the apex seals and everything else to properly get seated in, therefore building compression steadily over time. I had an 88" Turbo-II motor that I managed to squeeze over 100K daily driven and hard driven miles, and I enjoyed every minute of it. It all comes down to routine maintenance.

 

When I started my motor for the first time last year, it ran for about 30 or so mins of idle then a few days worth of cranking over time searching for a fuel issue i had(bad injectors). after all that i had right around 120psi.

Seth

 

showing 120=8.3bar wow health indeed. It's not uncommon to see these numbers when one uses brand new parts especially new housings.
I am interested in hearing from people who use Ceramics and from those who utilize 3mm vs 2mm seals.

In any case there are certainly variations of initial compression readings and reading of compression after the breaking period of a new built block. *not by very much after the break in* but indeed a difference!!!!!

 

I was pretty happy about it

Seth

 

Note I said 'right away.'

There are many many variables, most importantly the condition of the rotor housings and apex seals.

I just re-read your initial post and I'm not really sure what you're looking for.... if you can give more specifics it may help us all to answer the question.

Keep in mind alot of what you're going to (including from me) will be anecdotal data points, not some kind of hard and fast rule.

I recently installed a brand new oem mazda engine w/medium port in my personal car, and took compression readings quite a few times. It's all in my build thread, I'll pick through it and see what I posted.

 

Interesting... nice thread Michael

 

Results will vary depending upon which apex seals and if the housings are new or used. Irons can and do make some difference in compression but isnt as noticeable as a nice set of housings.

Atkins seals and OEM seem to make great compression right off the bat. The ALS and RA seals tend to start a little lower but end up right around the same peak numbers. Ive seen comp results improve as much as 25psi from first startup thru break in. On a used housing engine I like to see results of 110+.

 

DjSeven & Goodfella---Thanks for chiming in on this thread.


The main reason why I'm inquiring about the proper compression rate on a freshly built motor is to provide a clear understanding of what is involved in a proper build and there after.
What should the consumer know and except from his builder(reputable builder that is)
Perhaps a list of "does and don't" prior to the exchange of money with their Builder.

Too often I find myself reading or listening to numerous/various stories of engine builds from members and friends regarding their nightmare engine build experience.

Perhaps, if the Engine Builders on this forum can enlighten us potential buyers the basic of a rebuild list and what other basic components are needed in order to ensure a great rebuild experience to achieve proper compression.


I originally asked the question regarding compression rates of a rebuilt engine and what is considered as a good to great rate of compression on a rebuilt motor.

Most of the people on the forum and myself included IMO don't care nor have the desire to check out compression once you have experienced the dreaded " I blew my engine and paid a **** load of money to have it rebuilt".

Unfortunately most people just fork over x amount of money to their builder and hope for a miracle. Not once do they consider or investigate to the itemized list of parts they are paying for and months down the line they become that new guy that is posting about their shitty build and who and how they got F**ked over.

So in order to stop this madness perhaps we can provide a compressive list of what to do and not to do when rebuilding your motor.
Where we can afford to cut corners and not.

 

Experience. A reputable builder with experience, especially when it comes to non-OEM parts, is invaluable.

 

I have atkins seals in mine

Seth

 

rotor housings without chrome flake on the edge, new apex seals, proper clearances that are not right at the limit (sloppy). also just making sure you lubricate the inside of the engine as you are putting it together. make sure the battery and starter are healthy when you try to fire it up. The rest is just making sure you are thorough with the build. make sure everything is very clean (especially coolant seal grooves). make sure oil control rings are installed properly (OEM fits the best). make sure the front bearing stack is installed properly and eccentric shaft endplay is in spec.

#1 thing to check so you'll get compression... don't drop anything into the engine, like an apex seal piece. It sounds obvious but it's a mistake that can happen easily, especially when you are installing the middle iron. If you are porting, buy a template and use that. also try to buy a junk iron and housing for practicing porting. it doesn't have to be from an FD, you are just trying to get the feel of how to use the porting tools.

 

110-120psi+ on new engines but those are with new housings. Compression can increase depending on who and what was involved on the original build. We rarely see much increase usually it's 5-10psi at most depending on initial numbers. We've had some engines put out as much as 130psi before startup but those aren't the norm. Every engine gets comp tested before long blocked


~S~

 

^^

Out of the 3 engines so far, the MOST I've seen compression improve was 7psi. Its usually 3-5psi.

 

my fd did 8.5/8.4/8.6 and 8.3/8.5/8.6
10k miles on rebuild stock port, factory seals , slight warm 250rpm.
is this any good ? mazda digital tester

 

Wow, somebody actually stated the RPM for their compression test. If the compression numbers aren't adjusted for a 250 RPM cranking speed then they're worthless for comparision.

@250 RPM
Front:
116,124,121 psi
8, 8.6, 8.3 bar


Rear:
125,117,126 psi
8.6, 8.1, 8,7 bar

Factory new engine from Malloy Mazda.
Still breaking in. 150 miles.

 

My compression tester normalizes the readings to the 250 rpm spec cranking speed at the flip of a switch

Btw, keep in mind that these sterling compression numbers can become 0 in about 0.1 seconds

 

I say more like .01 seconds.

~S~

 

Ok gent's got my compression result, it's a rebuilt with new housing and everything else is oem (new) accept for R.A Super Seals and Pineapple Racing HD water seals and most important o miles on the engine just about 1:00 minutes total running since it was put in.
The compression was done with Mazda digital compression tester.
Front Housing Result:
Rpm= 276
1=7.9 kpsi
2=8.0 kpsi
3=7.6 kpsi
Rear Housing Result:
Rpm= 263
1= 7.2 kpsi
2= 6.9 kpsi
3= 7.4 kpsi
I was too lazy to look up to psi I thinkt he book call it kpsi? maybe some can chime in.
Khris

 

Khris, those readings come out to F 103/103/99 R 99/95/102 at 250 rpm. That's on a stone cold engine too, so the readings would be lower if thoroughly warmed up as it should be.

Those are new rotor housings? Who built it? For new housings and apex seals that's not horrible for a completely fresh build, but not great either.

 

So, compression goes down as engine temperature goes up?? I would think that it would be the other way around.