chinx

hey guys,

forgive me for lack of knowledge, but i found a s5 tii w/ a rebuilt engine but it has low compression. um...what does this mean? is low compression typical w/ rebuilt motors? how does this affect power, etc.? how does one remedy this?

WAYNE88N/A

if they're rebuilt with used housings, yeh...what are you calling "low"? need numbers...

chinx

lemme find out...

DragonRx7

Don't mean to hijack but I have a similiar situation. I got a rebuild and the Front has L=75, T=75 and the Rear has L=55, T=65.

Is that really too bad? They were used housings and it's streetported, too. I just want to get it running.

Roy James

anything below 80 is low. you did have the other spark plug in while testing right?

Digi7ech

iTrader: (1)

also you take the reading from Leading/lower right?

80psi is the general limit on engines.

Rebuilds when BRAND new will have low compression because they have not make a good seal against the used rotor housings. If the housings were brand new then you woudl get much more compression quicker.

My engine has run for about 4 hours and has already started to build good compression.

The first time I tried it. It would non stop flood without a hint to starting up.

Now it is almost ready to start by starter.

If it has over 2k on the new engine and it's still low compression then it's blown or something is very wrong.

DragonRx7

Ok. The test was done at a shop so I'm not exactly sure of their procedure but I'm pretty sure that the other plugs were in correctly. And yeah, it's a pretty fresh rebuild(never really "started/ran") and it had used housings.

vvhitewabbit

Put some atf in it to help build comp, Wayne helped me out and worked like a charm. Now the beast starts everytime without hassle and i have only 8 hours of run time on used housings/rebuilt motor

DragonRx7

How much and where? I've read about it before and I understand the plug holes...but could I get anything more detailed?

OverDriven

Low compression means you have a blown engine UNLESS the rebuild has VERY low miles on it. It will get better compression after it breaks in. I would say that if the engine has 1000 miles on it and is still holding the same low compression, then the engine is gone and you will need to have it rebuilt properly. BTW some people say that an engine is only bad if it is lower than 70, while others say that anything below 90 is bad. Mine is at about 105 psi, and I would consider that pretty good. Dragon...if those numbers are right, then you definatly need a rebuild.

-Joe

MazdaRx7Racer4Life

Like overdriven said, you have to drive it about 750-1000 miles before the engine gets its optimum compression. Things are still gettig broken in so it needs to build up the compression.

DragonRx7

Ok. It was 75 all around the front(+/- 1 or 2) and the rear read 65,65,55. How's that?

MazdaRx7Racer4Life

And wgat? It doesn't start or floods all the time? Mine is a fresh rebuild with 700 miles on it and it floods everytime its hot, but this might be because of the FCON, im not sure.

DragonRx7

It'll crank but not start.

OverDriven

That engine is blown...it needs a rebuild

-Joe

DragonRx7

That was the rebuild.
Or are you saying the rebuild needs a rebuild?

boosted1205

iTrader: (9)

You said you found a TII with a rebuilt engine. Do you personally know it was rebuilt or the person just said so? If you have even compression on all 3 sides, it could just be needed to be broken in. A cracked apex seal would show very uneven numbers such as 95-0-95 etc.
It's not uncommon to take 2000 miles to increase compression. A lot depends on what was used for a rebuild such as new housings, new apex seals etc.
A low compression motor (fresh) will be a bitch to start when hot. Install a fuel cut off switch (very easy) to unflood it.
If you are 100% sure it is rebuilt, take it easy on the motor until about 1000 miles. The compression will increase.

scathcart

The FSM states to use the Trailing spark plugs, and not the leading, for compression testing.
Makes sense to me, as the smaller hole would act in the same way as the restrictor pill does in the line to the pressure sensor, by limiting instant pressure "spikes" while still alowing the same total pressure to be measured. The leading spark plugs could give an erroneously higher number than actual compression numbers because of this.
I always test via the trailing plug holes.

Digi7ech

iTrader: (1)

Thx for the tip. I never knew which one your suppose to use.

Husemoller

The engine should have low compression for awhile. go to rotaryresurrection.com and follow the unflooding procedures to get it started or the rebuild starting procedure i think he has on the site. dont worry about it right now untill you have a nice chunk of miles on it. I have heard of people getting full compression at like 10k miles and such. but around 1k miles i would think you would be seeing some decent numbers.

My dads truck just put 50k miles on it and you can tell the power differences from when we first bought it. i know its not a rotary but still...over time it should make better compression. so just get her started and let her idle for awhile and dont let the rpms go over 2500-3000

WAYNE88N/A

Scathcart- Are you sure it's not just because the trailing plugs are easier to get to? Your explanation sounds kinda funky, lol...I've used both positions just for ***** and grins, and they both read the same...besides, the I.D. of most pressure gage sensing ports is much smaller than the trailing plug orifice at the rotor face...

scathcart

That's the point of what I stated. It does have a smaller I.D, so it would act as a restriction to reduce pressure spikes.

Its just a guess... it could very well be just for easier access purposes, but then, why would they explicitely state that? Why not just state, "Remove the easiest spark plug".