4a block, compression numbers

[oldae82]

I have a couple of questions about the 4a block.

Do the 4ac and the 4age have the same block and internals? i.e is the only difference the cylinder head?

Compression: I have an ae82 with the 4age. I checked compression on it and came up with #1 at 90#, #2 at 50#, #3 at 125#, and #4 at 135. the engine sounds great, will rev to redline in the first three gears (haven't tried for redline in 4th and 5th), doesn't burn oil, gives me about 32mpg all around, and yesterday took me down the road at 5500 in fifth gear, which should be about 100 mph. How is this possible?

In my quest for a rust-free FX16, I have bought an '88 FX. I have two FX16s which I'm going to part out to make this one a twin cam. The FX came with no power steering. I assume this means it's a 'slower' low-ratio steering. If so, is it worth putting the power steering in it to get a 'faster' high ratio steering, or leave it off to gain a couple of hp?

[yoshimitsuspeed]

I don't think there are any significant differences but there may be some small differences between F and G blocks like certain bungs not being drilled or tapped or possibly even some not being there at all. This is more likely more era specific as the block designs did change some as time progressed and they added things like the GZE and 20v knock sensors, the SP external drain and things like that.

If the car runs good and feels good I would do the test again. Maybe you didn't get a good seal or something.

[oldae82]

If I hadn't done the test I'd never have wondered. I've done the test a few times. I'm inclined to put the tester away and not think about it.

Thanks for the info on the 4a block.

[jondee86]

Compression: I have an ae82 with the 4age. I checked compression on it and came
up with #1 at 90#, #2 at 50#, #3 at 125#, and #4 at 135. the engine sounds great, will
rev to redline in the first three gears (haven't tried for redline in 4th and 5th), doesn't
burn oil, gives me about 32mpg all around, and yesterday took me down the road at
5500 in fifth gear, which should be about 100 mph. How is this possible?

This is actually an interesting question And while I don't have a proven answer, I do
have some theories. The engine is obviously well worn, as even the higher testing two
cylinders are around the factory service limit. So compression rings have lost a bit of
tension which leads to low test pressures. However, since the engine is not using oil,
the oil control rings must still be doing a good job.

As to why the engine still runs good, I believe it is probably to do with the way that the
compression rings work. Combustion pressure forces the rings down against the land,
and also outwards onto the cylinder walls. Consequently the running engine may well be
operating with normal (or very near to normal) ring function. Obviously, testing cylinder
pressure without combustion at 300rpm or so, will not achieve the "dynamic" ring sealing
that allows the engine to run sweetly.

To quote one of my favorite sayings...

"How often have I said to you that when you have eliminated the impossible, whatever
remains, however improbable, must be the truth?".

Cheers... jondee86

[oldeskewltoy]

I have a couple of questions about the 4a block.

1)Do the 4ac and the 4age have the same block and internals? i.e is the only difference the cylinder head?

Compression: I have an ae82 with the 4age. I checked compression on it and came up with #1 at 90#, #2 at 50#, #3 at 125#, and #4 at 135. the engine sounds great, will rev to redline in the first three gears (haven't tried for redline in 4th and 5th), doesn't burn oil, gives me about 32mpg all around, and yesterday took me down the road at 5500 in fifth gear, which should be about 100 mph. 2) How is this possible?

In my quest for a rust-free FX16, I have bought an '88 FX. I have two FX16s which I'm going to part out to make this one a twin cam. The FX came with no power steering. I assume this means it's a 'slower' low-ratio steering. If so, is it worth putting the power steering in it to get a 'faster' high ratio steering, or leave it off to gain a couple of hp?

1) Early 4AC and 4AG blocks are very similar 3 rib castings. The pistons, rods and crank are all different.

2) Jondee best describes this... but beware often times at this wear level just a little missed maintenance could spell a rapid demise, so keep an eye on oil level, and don't skip services

3) never assume - verify. But as a hypothetical... I doubt you'd feel the loss, but sometimes non-power units offer better "feel".

[oldae82]

Another quote that might apply: "When ignorance is bliss, 'tis folly to be wise".

the reason I'm asking whether the 4ac and the 4age are the same except for the cylinder heads is that the fx with the 4ac that I've recently bought has the best compression numbers, and I have more twin cam heads. I'm wondering if I can use this block w/a twin cam head and play with the car while I build another engine. am I using a little knowledge to mask a lot of ignorance?

[jondee86]

This...



... is apparently what you will find bolted to the head of a 4AC. Not that
I have ever looked under the hood of an FX with 4AC, but I guess both the
FWD and RWD versions are similar. Points to note are...

1. It has a carburettor (4AGE has fuel injectors installed in the head).
2. Both the intake and exhaust manifolds are on the same side (4AGE has
the manifolds on opposite sides of the head).
3. While not shown in the pic, the 4AC will have a low pressure mechanical
fuel pump mounted on the side of the head. The 4AGE has a high pressure
pump mounted inside the fuel tank (and EFI rated fuel supply line).
4. To run the 4AGE EFI system you will need to install an FX16 EFI computer.

All in all a great deal of work for little gain if you have a 4AC powered car to
run around in until you are ready to swap all the electrics and mechanicals
from the FX16 into the FX.

Cheers... jondee86

[oldae82]

I've expressed myself poorly. I have two complete fx16s here waiting to be dismembered. One is the unit with the mystery compression numbers. In addition, I have a complete twin cam head w/intake and exhaust siting in storage. I know I have to change everything- wiring, computer, install a manual transmission to replace the automatic, replace the rear suspension members, tie in the strut braces, etc. The current 4ac has the best compression numbers (165) so if I can put a twin cam head on that block I'll do so. Otherwise I'll have to take one of the tired 4age engines and put it in. I'd like to end up with a turbo engine, but money-wise, it's going to take a while. I'd like to maybe get the 'new' fx sorted out in the mean time.

[jondee86]

It appears that there may be a problem with the 4AC pistons, so this is probably
the one thing that you will need to clarify...
http://forums.club4ag.com/zerothread?id=33698
http://forums.club4ag.com/zerothread?id=17274

Otherwise, you should be able to take everything you need from one of the donor
cars. You will have to do this in any event, so providing piston clearance is not a
problem, you should be able to do exactly as you propose.

Cheers... jondee86

[oldae82]

thanks Jondee. That's EXACTLY the info I was looking for. Not worth the effort of swapping to the 4ac block. I'm sorry you had to lead me by the hand.

[NH-hillbilly]

there is also the possibility that you have a few buggered valves that aren't seating entirely causing the low compression numbers, right?

[totta crolla]

How the test is performed can have a big effect on the results.

[NH-hillbilly]

usually when a valve/valves are the issue you'll end up with a heavy misfire, but you never know. Just food for thought I guess. Do a a quick leakdown test of that cylinder using a pressure regulator and shop air and let us know.

this thread has peaked my curiosity.

[oldae82]

Basically, before the compression test I assumed I had a fine running engine. There were, and are, no symptoms to indicate otherwise. I took the test with the ignition disconnected, start injector disconnected, WOT and all the plugs out. The numbers were so far off I took my newly-purchased compression gauge back to NAPA and insisted on a replacement. A bit of oil down the affected cylinders barely budged the numbers. The only variance from book method is that the engine was cold, but I don't think that would affect the readings cylinder to cylinder. If I hadn't been so stunned by the low readings I'd have paid more attention to the fact that there were two cylinders reading significantly higher, and that the gauge, if it was faulty, should be consistently faulty.

[totta crolla]

Valve clearances ?
Might be worth doing the test with a warm engine, people will quote chapter and verse on how it doesn't matter but engines are not used cold and you are quoting your engines performance with it warm.

[NH-hillbilly]

if you did the test "wet" and the readings stayed the same, compression isn't leaking past the rings my man.

you have some unhappy valves.

this thread has piqued my curiosity because of what I have previously stated.....that when valves aren't happy, it usually shows up in the form of a miss. But your engine isn't mis-firing.

do a quick and dirty leakdown test. not a percentage leakdown over a period of time test, just a "nothing fancy" test.

if your curious as to what I mean by quick and dirty, don't hesitate to ask.

if it ends up that air is rushing past some valves, "LASH" on said valves needs to be checked. If lash checks out, a vale job is on its way. If lash is tight, you have time to remedy the problem before you burn some valves up without having to remove the cylinder head : )

nawimean?

[oldae82]

Don't now what a 'quick and dirty' leak down test is. I could check valve lash. Be a good idea, anyway.

[NH-hillbilly]

so using a pressure regulator and the hose from your compression tester, bleed roughly 10 psi into a healthy cylinder and slowly rotate the engine with a large ratchet off the crank pulley.

remove all 4 plugs to make rotating the engine effortless.

it needs to be quite (no radio, air compressor off etc)

you'll hear air rushing past the exaust valves into the tailpipe when engine is on its exhaust stroke. you'll hear air rushing into the intake when its on the intake stroke. as you near TDC compression, things quiet down and you'll feel the ratchet start to fight you. Lets say the engine had a seriously bent valve, or a hole in piston... (just for example) it would never build pressure in that cylinder and the ratchet would never fight back and things would never get quiet....there would be a constant "wooshing"

so now that you know what a healthy cylinder sounds and feels like, do the same thing on the cylinder that is giving you a 50psi reading.

please let us know how things go.

p.s. a hole in the piston, or broken ring lands, or a "gouged" cylinder wall would result in air "wooshing" into the crankcase which could be observed with your ear by removing the oil fill cap off the valve cover. none of that **** is your issue though cause you have none of the accompanying symptoms.

[NH-hillbilly]

so....where we at?

[oldae82]

So where we're at is the middle of winter and no garage. I tried to upload a picture of three amorphous mounds of snow that is my ae82 collection. I'll try the leak down test and check valve lash when we get a break in the weather on a day I'm not working. I sincerely hope that's soon. forecast is for several inches of snow here tomorrow.