What's good compression for 4A's?

**Drifty Midship** · 06-02-2006, 07:38 PM

Got my hands on a compression tester on the weekend and tested both my 4A's.
Though both have different C/R's so not really sure what a good reading is.

Tested:
86kw AFM Black/Red top from Aussie 1989 AW11 (280k) = 230-250psi
AFM 4AGZE from a Jap 86 AW11 (125k) = 150-160psi

Sorry, not exactly sure on the C/R's for each engine so can post that info.
So basically since I can't compare the 2 against each other so not sure whats good and whats bad.

Cheers

**BigWorm** · 06-02-2006, 08:34 PM

The main thing you are looking for is a maximum of 15% difference between different cylinders, the actual pressure is really only a secondary concern.
Although for what it's worth, both my old flogged N/A 4age and my rebuilt lower comp. & low Km 4AGTE both had around 185psi, with no greater than 4psi difference between cylinders.

C/R should affect the results, so N/A should usually be higher than F/I figures, but 230+ sounds pretty damn high to me!

**Rolla Boy** · 06-02-2006, 08:36 PM

Enjoy...

**wiso** · 07-02-2006, 12:41 AM

this also covers the gze readings

230psi on the NA does sound very high on my old NA I had shaved the head and block which obviosly raised the compression and that was reading 195psi across the cylinders

**takai** · 07-02-2006, 08:26 AM

Yeah, my 11:1CR engine is only around 190psi across the cylinders.

**wiso** · 07-02-2006, 09:57 AM

now the next question you should be asking is what could be causing your NA to have such high compression

**Drifty Midship** · 07-02-2006, 10:11 AM

One thing I did not do that is mentioned in the instruction above was have a fully opend throttle when cranking.
Will have another go this weekend... also paying more attention to the differences between cyls.
Some handy info tho thanks!!!

**TooF** · 07-02-2006, 10:20 AM

yeah soemthing sounds amiss... a mate of mine tested his excel an read 280 or soemthing crazy. natually thinking the guage was bung.. new guage and it came up the same !?!?!

but yeah my gze spins 155 across each and the na is between 160-170 on each,

**Rolla Boy** · 07-02-2006, 05:33 PM

How many kms on the car???

Has the head been shaved or is the head gasket thinner than normal??? This will cause higher compression...

**takai** · 08-02-2006, 09:00 AM

Yeah, but not stupidly high comp like he is showing there. Race engines (like mine) which do have shaved heads, thin head gaskets and different pistons are still only 180-190psi

**wiso** · 08-02-2006, 10:47 AM

that makes a little differance, but not a great amount, i did this the first time then i re read the instructions, and i think it only varied by about 5psi

Originally Posted by Drifty Midship

One thing I did not do that is mentioned in the instruction above was have a fully opend throttle when cranking.

**andy** · 08-02-2006, 12:07 PM

anyone have stats for silver tops?
would be good to record it here for future.

**sillycar chick** · 08-02-2006, 02:01 PM

Check out the thread I started recently for my engine
http://www.toymods.net/forums/showthread.php?t=2038
I know they are 2 different engines, but the concepts are all the same. It covers a lot of issues regarding compression ratios, pressures, the relationships between the two and the relationships between compression pressure and valve timing. I have learned a LOT working on my engine and playing around with compression and also from everyones help on this site. Hopefully it will help you wrap your mind around the concepts behind why your compression is so high!!