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post #1 of (permalink) Old 12-05-2004, 10:09 PM Thread Starter
 
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Low cyl. compression = Low int. vacuum?

Well, in addition to the leakage around the Pony carb throttle shafts that we have discussed on another thread:

I decided to check the compression on the new Sumo II engine and discovered that the compression on it is lower than on the tired old 180,000 mile engine I replaced with it.

Here is what I got:
#1 = 140 PSI
#2 = 132 PSI
#3 = 132 PSI
#4 = 127 PSI

That is with about 100 miles of break-in on the new engine, doing it the way AAPA Dave recommended. Now I realize that optimal compression won't be achieved until the engine is completely broken in, but those numbers really worry me.

Now, my question is: How much, (if any) does this low compression ratio contribute to the low intake manifold vaccum (~10-12") that I've been chasing for the last week?

Is there any point in even messing with the vacuum problem before I can get a decent compression out of the cylinders?

I'm going to head back down and check the torque on the cyl. head bolts and see if that will help, but I wanted to get this question posted first.

Thanks for any and all input.
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post #2 of (permalink) Old 12-05-2004, 10:26 PM
 
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Re: Low cyl. compression = Low int. vacuum?

First of all, make sure that your butterflies are fully open along with your choke flap when you do your compression check. The engine must have NO breathing restrictions placed on it for this check. Make sure your valve adjustment is dead on also. If that doesn't help, then it could be valves not seating or rings. You can check rings by squirting a little oil in the cylinder while performing the compression check. Good luck and [img]images/graemlins/40BEER.gif[/img]
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post #3 of (permalink) Old 12-05-2004, 10:40 PM Thread Starter
 
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Re: Low cyl. compression = Low int. vacuum?

Thanks Shawn (and thanks for the beer toast too. I MAY just indulge in a couple myself).

Yeah, the carb was wide open. Valves are set correctly, at least they were four days ago, when I double checked them for about the tenth time. Hadn't tried the oil in the cylinders yet; planned on doing that after I double-check the head bolt torque.

Do you reckon that the compression is contributing to my low intake manifold vacuum? I know there is a relationship between them, but not how significant.
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post #4 of (permalink) Old 12-05-2004, 11:22 PM
 
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Re: Low cyl. compression = Low int. vacuum?

It definitely will!Suck, squeeze, bang, blow, mess up any one of these and it affects the other. If'n your head bolt torque is off, then you would have leaks, use fluids, etc.. Rings or valves, that is the question! Let me know which it is, I'm a gearhead by nature, trade and hobby. I dream about fixing things. I know I need to get a life, but I have one and it is perfectly tuned!
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post #5 of (permalink) Old 12-06-2004, 12:43 AM Thread Starter
 
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Re: Low cyl. compression = Low int. vacuum?

OK, Tightened head bolts (they were only 30-35 ft lbs) to 52 Ft Lbs per the instructions that came with the engine which I was supposed to do after 500 miles break in.---

That got me about more 3 PSI per cylinder.

Squirt Oil into cylinders, that got me another 2-3 PSI on each cylinder.

So, I'm close to 150 PSI on #1 and around 135-145 on the others.

Guess it is mostly the valves then???

Man, you'd think that a $1500 rebuild job would be better than this.

If I CAREFULLY pull the cylinder head for a visual inspection inside, will I for sure have to replace the head gasket? I hate to waste another 45 bucks on a dang gasket.

I'm going to head back down and adjust the valve lash AGAIN, to see if that will help.

I WISH I were a gearhead.... Yeah, 30 years ago, I'd a done this just for fun, and as a learning experience, but an old guy like me needs the comfort of a good book and a rocking chair in front of a fireplace, not weeks of nights in a cold garage getting sore and greasy [img]images/graemlins/wink.gif[/img]
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post #6 of (permalink) Old 12-06-2004, 03:28 AM Thread Starter
 
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Re: Low cyl. compression = Low int. vacuum?

I posted this in my other Pony vacuum thread, and am cross posting here, since it may be significantly important to both threads:

I didn't get around to working on the Pony tonight due to working on the cylinder compression mess, but I was re-reading an OLD (like 60s or 70s) engine test kit instruction booklet that said that you can "expect a drop in vacuum of one division per each 1,000 feet above sea level".... (Edited: the newer vacuum gage booklet I have, nor the FSM mention any difference in vacuum, compression or timing due to altitude...... but then the "computer" is supposed to adjust automatically for altitude, so I can see why they would not bother to mention it.)

Well, if that is correct, and "one division" means one inch of vacuum, then even with the leak around the secondary throttle shaft, I'm within specs, or at least really close, because I'm at 7,000 feet altitude and pulling 11-13 inches of vacuum at ~800 RPM depending on where I set the timing and idle mixture.

Now, I'm wondering if cylinder compression also measures less at higher altitudes? hmmm, a quick web search does indicate that cylinder compression does decrease with altitude, although I haven't been able to find a specific formula or table....... Is 7,000 feet enough to drop compression by 40-50 PSI or so?

I suppose I could test this out myself by driving Suzuk out to Los Angeles or to Death Valley, but if you guys can confirm this theory for me you'll save me gas money [img]images/graemlins/grin.gif[/img]

(Edited... now that I think about it, if the altitude does actually drop the vacuum measurement by ~25% it would make some bit of sense if compression also dropped by a similar percentage.)

And I also just saw on a Saab mechanics website a recommendation that on older Saabs, you should increase timing advance about 1 degree for each thousand feet, up to a maximum of 4 degrees extra..... Does that apply to normally aspirated, non ecm/non-emissions Suzukis too? Darn sure performs better when I have timing set at 14 degrees BTDC.
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post #7 of (permalink) Old 12-06-2004, 03:40 PM
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Re: Low cyl. compression = Low int. vacuum?

Durn toot'n altitude affects compression and vacuum.
Remember, somwhere around 10,000 feet it's suggested pilots use oxygen because the air is so thin there. 7000' is 70% of the way there. Vacuum is the differential between presures - the "outside" pressure is lower, so it'll read lower - always.

Same with compression -- there isn't as much air in the cylinders to squeeze, and the psi reading is also the differential between outside to inside pressures.

Adding a few degrees advance compensates for the lower charge in the cylinder - the lower charge takes longer to burn to full cylinder pressure -- akin to spacing BBQ charcoals farther apart.

The important thing is the compression's variation between cylinders. They should all be about the same - after it's fully broken in. It may be too early to get excited.

If it's got any type of misfire - ignition, vacuum leaks etc, the affected cylinder(s) leave a little gas in them, washing the oil off the rings, causing a compression reading to be low on those.

I woulden't worry about it till the engine's got more miles on it. Just get all ignition and vacuum leak issues out of the way -- I'd bet she'll be fine.

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