1968 383 Compression Test & Restart

EngineerDoug · Oct 22, 2016

Hello all,

I have a stock 2BBL 383 that came with a project car. It has been sitting around for about 4 months since I bought it, and I don't know how long previous to that. I would like to get an idea of the health of this engine, as I might want to use it as a runner until I build a fresh 440 for the car.

I compression tested it and the readings are 120-135 psi. The engine turns over evenly, and will build about 40 psi oil pressure with no plugs installed. Note this is a 9.2:1 motor, per the '68 specs. My first question is, how do these readings sound to you? Also, I am wondering if I should use something to lube the cylinders before starting it. Any ideas? Of should I just go ahead and start it?

Thanks!

Cranky · Oct 22, 2016

Squirt some oil in the cylinders and see if the compression comes up. If it does, that's a good indication the rings are either really dry or are worn. If it's building oil pressure, that's a good sign and priming isn't needed but I would at least pull the valve covers and see if any oil is coming up on the rockers. If so, fire that puppy up.

Jcharger · Oct 22, 2016

I agree, but have one or two additional observations/suggestions. Before rebuilding my 383 (it also had 9.2 from factory) I did a compression check and had similar measurement for 4 bumps and leaving the plugs in. The engine ran OK. One thing I did not do was to wire the carburetor open. I suspect that wiring the carburetor open and pulling my plugs would have caused it to increase.

Also, note that the compression test will depend on cranking speed and how many times the needle is bumped (# of compression cycles).

My guess is that you are pretty close to a reading that says all cylinders are behaving similarly, so if it doesn't really spike when oil is added, then it may be that you are fine.

Cranky · Oct 22, 2016

That's the way a compression test is supposed to be done....carb WOT and no plugs.

oldbee · Oct 22, 2016

++1 for both of Cranky's posts.. Then fire it up,fingers crossedxxxxx. Should be good.Mine sat for 14+yrs.

EngineerDoug · Oct 23, 2016

oldbee said:
++1 for both of Cranky's posts.. Then fire it up,fingers crossedxxxxx. Should be good.Mine sat for 14+yrs.

Hey guys,

The one thing I did not do is hold the throttle open, so my readings are probably low. Given the compression ratio, I would think the max achievable cylinder pressure would be (9.2-1) * 14.7 psi above ambient atmosphere. Given that, I think it is time to go ahead and start it up. I have it mounted on a homebrew engine cradle - see enclosed fotos. The cradle is pretty rough, but it only something that will serve for a while. If the 383 does fire up and run well, I may consider upgrading it with a mild cam, 4BBL intake/carb, and some headers. It could power the Coronet project once the car is back together.

Thanks for the feedback.

miller · Oct 24, 2016

Somebody straighten my act up!
Top photo...you should NEVER leave the torque converter bolted to the flexplate, if trans is off. Why? Real good chance the weight of the converter will warp the flexplate, making big time vibrations.

EngineerDoug · Oct 25, 2016

miller said:
Somebody straighten my act up!
Top photo...you should NEVER leave the torque converter bolted to the flexplate, if trans is off. Why? Real good chance the weight of the converter will warp the flexplate, making big time vibrations.

Miller,

I thought about that, but the ring gear is welded to the converter. I would have to cut those welds...I can do that, but am squeamish about re-welding it if I choose to use the converter again. Is that something I would be wise to have a trans shop re-weld for me? Assuming I don't go with a new converter....

oldbee · Oct 25, 2016

Just go with a new flexplate when all done.Miller & you are both correct. How can starter crank it over w/o converter? That's a real conundrum(?)

IQ52 · Oct 25, 2016

oldbee said:
Just go with a new flexplate when all done.Miller & you are both correct. How can starter crank it over w/o converter? That's a real conundrum(?)

That's why God created flywheels. Should be one somewhere you could borrow.

If you do cut the ring gear loose, how you gunna get the starter to turn that? Hold it against the starter with gloved hands?

Cranky · Oct 25, 2016

Hold it against the starter with gloved hands....man, again, my screen was spared the spray splatter from not having a mouth full of (fill in blank) lol. I've started a lot of engines on stands and on the floor with just a hoist chain for top support and a set of headers for out riggers but I've never even considered firing one up with just the converter bolted on. I don't even know anyone that's done it that way. I guess it may be ok if you don't let it rev any higher than just an idle but even that is going to be around 7 to 800 rpm. If you've ever ran an engine lathe, you know that 800 rpm is booking it especially for something that large in diameter....PLUS, the guts (turbine which isn't small) of the converter may not be aligned very well without the engagement of the input shaft of the transmission...

miller · Oct 26, 2016

EngineerDoug said:
I thought about that, but the ring gear is welded to the converter.

Gotta think outside the box on that one...just like your run stand. Of course, you do not cut the ring gear off!
But, you DO need to add to your run stand, on a set up like your's. A support fixture should be made, that bolts to the rear of the block (in place of the trans), with a linement coupler for the converter's rear shaft, at least for support. Also need that shaft to be plugged...to keep the fluid in.
Understand the ring gear is needed to fire the thing off. But, as I mentioned, just the loose weight of the converter on the flex plate, can bend, or warp that puppy...just sitting as in your photo.
Just tryin' to give you a head's up.

EngineerDoug · Oct 26, 2016

I see why you guys are concerned about the converter. And shortly after I mentioned cutting the ring gear off, I realized that would not get me anywhere. I'll have to find a flywheel, preferably one that is in bad shape so I don't have to invest in something I will not use in the long run.

I don't intend to run the engine much above idle, but then again I don't want this thing flying apart, either.

Thanks for heads up - I hadn't realized the converter would be problem.

Cranky · Oct 26, 2016

EngineerDoug said:
I see why you guys are concerned about the converter. And shortly after I mentioned cutting the ring gear off, I realized that would not get me anywhere. I'll have to find a flywheel, preferably one that is in bad shape so I don't have to invest in something I will not use in the long run.

I don't intend to run the engine much above idle, but then again I don't want this thing flying apart, either.

Thanks for heads up - I hadn't realized the converter would be problem.

If you can find a good flywheel without paying more than it's really worth, you can either use it on a future project or just turn around and sell it. There's plenty of people around that still like stick cars plus, it can be used again on the start up stand. Man, I've had a startup flywheel for the past 42 years and still have it...and it's still usable in a car and it's more than paid for itself

mopar_man · Oct 29, 2016

Converter or flywheel either way but you need a bell housing to bolt the starter to or build an adapter of some sort .

EngineerDoug · Nov 2, 2016

Got a flywheel on the way - found a used one for a good price. As for the starter, I realized that would be an issue when I built the stand. I cut a scrap of 3/16" steel into the shape of a "C" and put four holes in it - two for the starter and two for the block. Works fine.

Now we will see how this thing runs.

5.7 hemi · Nov 5, 2016

So how does it run?

wagonman · Nov 7, 2016

One thing more is to put a timing chain set in it, get rid of the plastic toothed cam gear. You can thank me later.

EngineerDoug · Nov 10, 2016

Thanks for all the tips, guys. I scrounged a flywheel and bolted it up. After setting the timing (it was way off), I got her started. I had squirted some oil in the cylinders just to help things a bit - once that burned off she settled into a clean idle. A healthy rumble, you might say. My son and I looked at the cloud of blue smoke in the driveway and then agreed it was a good thing that my wife was not home....

I still need to adjust the carb, but we have good oil pressure. This thing looks like it is a runner. Note I had rebuilt the stock 2BBL carb, but at first was hard pressed to identify it. It is an old Holley, but not a modular one. I had not seen a Holley without the side fuel bowls - this one is built much like an AFB with the one piece main body.

So assuming I get the carb adjusted and it still runs fine, I have a few questions. This is a stock 2BBL 383 (1968), so I am wondering what the cam specs are @ 0.050" lift? I am giving some consideration to giving it a mild boost, with a ~650 Holley, dual plane intake, headers, and a mild cam kit (maybe 220 degrees @ 0.050"). I will probably find some used items for the first three, so I don't drop much dough into the motor. Any suggestions along these lines are welcome.

I do plan to flush the cooling system to see if there is much muck in there. As for the oil system, I see there is a lot of dry black residue under the valve covers. Not sludge, but what you might judge to be carbonized oil. Would I be well advised to try anything in the oil to clean some of this up? Or maybe just leave well enough alone?

Thanks again for the guidance.

miller · Nov 11, 2016

Only one way to deal with carbon...something at this time you probably don't want to do. It's called overhaul. Probably even an engine flush won't help...not on carbon...
I'd say do a good compression test, see where it's stands on that. It'll tell you the basic condition of rings, cylinders, valves. If it's okay, probably get a little more miles out of it.

1968 383 Compression Test & Restart

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