Valve clearances and effects on compression?

[70XIN]

Hi guys,

I have a 20V 7AGE. It's fairly basic, 10.6:1 comp, and was built only a few thousand k's ago. I am embarrassed to admit, the ONLY thing i didn't check were my valve clearances (until now).

I did a compression test the other day, and i was pretty sad that i got ~150psi across all four. Adding a good teaspoon of oil to the test made less than 5psi difference. When i rebuilt the motor, i DID notice a bit of pitting on my valves and valve seats - so i just lapped them, and thought they looked fine after that.

So i have just checked all my valve clearances tonight, and they are really tight

Stock 4AGE BT 20V should be

Intake 0.19mm - 0.29mm
Exhaust 0.39mm - 0.49mm

MY valve clearances ranged from:

Intake 0.10mm - 0.15mm (most of them were 0.127mm)
Exhaust 0.15mm - 0.20mm

I have small TODA 264/9mm cams, and i don't think their overlap(?) would be enough to have much effect on a compression test, but keep this in consideration.

Would my valve clearances be my problem? Or should i be taking off my head and pulling everything down to check? (I am going to upgrade my cams before i do a valve adjustment, so i'd like to get opinions before i do anything else )

Thanks, Seamus

***Compression test and valve clearances were done with the engine stone cold.

[Celica RA45]

need to do a hot test with all plugs out and throttle fully open and only 10 turn overs for comp check
also check with toda as in what shim clearences you need

[love ke70]

yes, a tight valve clearances keeps the valve open too long and you lose compression
as was said, contact TODA for specs, or search their site, they will usually have their own valve clearances

[70XIN]

I have asked TODA aus, and they say the clearances are 0.20mm for intake, and 0.25mm for exhaust. This is for current cams though, my TODA-R's will be pretty old and might want something different.

RA45 - Unfortunately i can't do a hot test, i have permanently removed various pieces from the car.

Because i did the compression test & valve test dead cold, and i have at least SOME valve clearance, wouldn't my valves still be sealing (at least when cold)? As the cam lobes aren't *quite* touching the buckets when fully closed. (I'm trying to think this through logically, but it isn't working for me).

[love ke70]

yes, they will be closing, but how long is the the intake valve behing held open costing compression?
theres no point setting your valve clearances unless the motor is hot anyway, so maybe just wait til you can measure them hot?
gaps open up when hot as a rule, or atleast the hot gap is wider than the cold gap in the service manual...

[70XIN]

Hmmm, i've never heard of setting the valve clearances hot. The FSM (and cam manufacturers) all only give cold measurements, as it's too hard to regulate the temperature and expansion of metals of a hot head.

[love ke70]

mmm, all pushrod motors have a hot setting, and should be set as such, but i think you might be right, i do recall always doing my twin cam ones cold, in that case, open those buggers up a bit

if you need shims, if you search "cheap shims" or something like that on here, there was a thread with a guy selling new ones for a good price, will be an old thread but would be worth the search...

cheers, andy

[70XIN]

thanks man

[Rodger]

Compression ratio is different from compression pressure. The two inter-relate when the engine is running. However one has little effect on the other at cranking speeds.

Compression pressure is normally measured at cranking speeds and is more affected by the cam timing, lift and duraton. The later the inlet valve closes the more air that is pumped back out the intake reducing the pressure reading on the compression stroke.

The fact you have small clearances means the inlet valve is staying off its seat just that bit longer but at least they are closing. An indication they are not closing is no pressure reading. Perhaps the intake cam is a little late in its timing this may increase the readings but may not benifit the actual engine tune in the long run?

Reset the clearances but worry little about what the actual pressure reading is, as it could well still be less than what Toyota quotes due to the way your cams work. You look for a consistant value across all four cylinders and 150 may well be a good number for that cam.

[70XIN]

Thanks alot man

[Suavemechanic]

At mechanic shcool the first step in a compression test was warm the motor, and do the valve adjustment
dont worry too much that you cant warm the motor
150 is pretty good , motor will run fine on that

cheers