Bottom end ?

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Al rumble

Active member
Joined
Jun 19, 2014
Messages
43
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Location
Canada
My Car
1973 grande
Hey guys.

I bought my 73 Grande a year ago. Great southern cal car that has been a pleasure to work on. It was running a little rough when I got her, typical exhaust leaks and vacume leaks, but by the end of last summer I had her running pretty good other than a knocking noise at certain rpms. Over the winter I put tons of time and $ into her and am really happy with the way she is looking now. But that Noise!

I thought it may be running lean so I put on a new fuel pump and rebuilt the carb, but it's no better. If I retard the timing to 6* with a total of around 20* it sounds less knocky. So I finally had a real mechanic check it out. He had me start it and within a minute had me stop it and told me it was bottom end, " probably main bearing or rod". Ok, makes sense, he heard it right off the start before the 351c hydro lifters had a chance to pump up. Kills my thought that it was valve noise.

He suggested lifting the engine a little to get the oil pan out and try to fix it from there. The engine is 45 years old so I am more thinking of pulling it and a complete rebuild.

My question is this... Am I foolish to put in some heavy weight oil and maybe some additive and enjoy it for the summer, driving it easy or am I risking blowing the thing up and not being able to rebuild my numbers matching block.

Input ?

 
It's a gamble, for sure. If it's a main bearing you risk the crankshaft, if it's a rod bearing you risk the crankshaft and if it's a catastrophic failure the block.

I would check the compression on all cylinders, and if they're good consider your mechanic's suggestion. It may be possible, but it's difficult to do, difficult to check clearances, and difficult to replace the rear main seal. My preference would be to pull the engine, taking care of the bottom and top (including heads/valve train).

 
compression seems fine.

1 - 125

2 - 128

3 - 128

4 - 126

5 - 127

6 - 133

7 - 127

8 - 126

thats dry.....?

 
302 or 351?

If a 351 I'd say your timing is way off- should be starting around 8-12 degrees advanced and opening up to 34-36 once the mechanical advance is all in.

If it won't run right there, then I would pull and rebuild. If there is already bearing noise, then the crank will probably need turning, driving it with heavy oil won't provide more protection-actually it will provide less when cold as it won't flow freely (though it may mute the noise until it warms up)

 
compression seems fine.

1 - 125

2 - 128

3 - 128

4 - 126

5 - 127

6 - 133

7 - 127

8 - 126

thats dry.....?
thats pathetically low . . if you want to increase the power, you could start a build thread and see what others suggest, but in general, performance engines that run on premium gas are in the 160 - 170 psi area.

.



I bought my 73 Grandé a year ago. I had her running pretty good other than a knocking noise at certain rpms. So I finally had a real mechanic check it out. He had me start it and within a minute had me stop it and told me it was bottom end, " probably main bearing or rod". My question is this... Am I foolish to put in some heavy weight oil and maybe some additive and enjoy it for the summer,
yes.

 
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For a 73 motor they were 8.0 : 1 compression so those readings look pretty good to me. You want the pressures in all cylinders to be within 10% of each other so that also looks good. The bottom end noise is troublesome though. I think that I would pull the motor and check all the bearing clearances and make sure the surfaces on the crank are good. Fix the issues early so that you don't cause more damage and spending more $$$ for new parts such as a crankshaft etc. Do you know what your oil pressure is the motor is running?

 
I agree with Barnett, those compression readings are low, even though fairly consistent. At 4,300 foot elevation my 351 was making over 150. I haven't checked it now that I'm at a lower elevation, but it should be over 160 now.

Pull and rebuild.

 
.

yes, they will "run" with low compression like that and the fact that they are within a few psi of each other is an xlnt sign as was stated . . also, not all compression gauges read the same like they should . . i have seen some gauges read 20 lbs different than others so yours might actually be a little higher . . also, compression should be done with all the plugs out and the throttle propped at least 1/2 way open.

my point, which was not entirely clear, is simply what i stated . . they are low even if the engine was designed to have compression that low . . once you hit around 115, an engine might not even run or if it does it wont go anywhere very quickly.

by comparison, if your engine was built with 8.0 or 8.5 compression by the factory, a 71 boss 351 had around 11.5 from the factory . . now, there is certainly no way yu can run 11.5 on todays crummy gas unless you use E85, but you can run around 9.5 on premium.

increasing compression increases hp and tq EVERYWHERE in the rpm range.

.

 
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This would be a good tech subj. I always thought:

A 1:1 ratio is equal to 0 PSI. 14.7 PSI is equal to a 2:1 ratio. Just multiply your ratio by 14.7 to get PSI, or divide PSI by 14.7 to get ratio. So a motor with 9:1 CR should show approx 132 psi at sea level and goes down with altitude gain. So averaging all 8 cylinders and dividing by 14.7 I got 8.5 : 1 CR.

After reading more on the subj now I see that there are Static and Dynamic pressures in this equation. Don't quite understand this so I am hoping someone can explain this.

 
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After reading more on the subj now I see that there are Static and Dynamic pressures in this equation. Don't quite understand this so I am hoping someone can explain this.
no, lol.

you can get a lot of info on this subject from google but a short, inferior explanation is this.

static compression is the same as uncorrected compression . . its just different names for the same thing.

static/uncorrected compression is a fixed amount that will never change . . if your engine is built with 9.0, it will always be 9.0 unless it is bored out or the stroke is changed or the heads or block is machined.

dynamic compression is an incredibly complicated type of compression that varies with engine rpm and is determined by camshaft design.

one of the aspects of cam design that affects dynamic compression is the time at which the intake valve closes . . the later it closes in the relation to the position of the piston, the lower the compression will be.

another thing that affects it is the overlap of the intake and exhaust valves.

here is just one version of a combination static and dynamic compression calculator . . you can play with it and just change one value and see how it affects compression.

http://www.projectpontiac.com/ppsite15/compression-ratio-calculator

.

 
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Thanks for all the input.

The compression test was done with a very accurate digital manometer, but I probably did one plug at a time and curtainly not with the throttle at half way.

When it comes to compresion ratio, does that increase with connecting rod length or piston shape.

As far as the noise goes, I think I will start by changing the oil and seeing if there is any metal. At the same time probably put in some Lucas oil and see.........?

 
.

STOP DRIVING YOUR CAR...STOP I NOW!

Your question was answered...you didn't like the answer so now you are looking for alternatives.

If you have a rod knock that is loud enough to hear, adding 1 bottle of Motor Honey or Slick 50 or STP or changing to straight 50w oil will NOT get rid of it and will NOT sufficiently protect your engine, period, end of story, HOWEVER, if you get your oil thick enough, it WILL put so much force on your oil pump drive shaft that it WILL turn it into a pretzel or simply snap it in half, at which point your engine will just keep merrily running along for around 15 seconds until it SEIZES, after which point your next post might be, "Where can I find a good used block and crank and rods?".

As Clint Eastwood says, "Are you feelin lucky?".

.


.

the rod length has a small affect on dynamic compression but not on static/uncorrected compression.

the shape of the top of the piston affects the compression . . pistons that have a raised dome have more compression than ones that are flat on the top if all other parts are equal.

 
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OP perhaps the mechanic wasn't competent or honest-maybe you can get a second opinion, but if you don't have the experience to tell for yourself-find someone you trust to listen to it. Until then the best thing you can do is let it sit.

the easiest way for me to think of dynamic compression was to simply use a different term-cylinder pressure.

As I understand it, static compression ratios are simply a ratio of maximum cylinder+combustion chamber volume and minimum cylinder+combustion chamber volume assuming the valves are simply closed if maximum volume of a given cylinder was 27 cubic inches and minimum was 3 inches you would have a 9:1 ratio

dynamic adds in the valves and their timing events and it gets more complicated. Dynamic is always lower than static- but that is just a number.

High overlap, long duration, and big lift cams reduce dynamic compression by having the valves open more, effectively making the volume of the cylinder/combustion chamber larger.

11:5:1 will run on pump gas just fine with the right camshaft for some people.

 
I did what your mechanic said to my car and it wasn't all that hard or expensive. I had a leaky main seal so I lifted the motor off the mounts dropped the pan unbolted the oil pump pickup into the pan then I could remove the pan. I replaced the main bearings and main seals front and rear put in a new oil pump and pick/up removed a dent in the oil pan and refinished the pan. then reassembled everything. I have to admit I had help from some friends through the entire process so I can't take all the credit but it is very possible and we did it all on a weekend. Oh by the way my bearings were in beautiful condition but I had it apart so I went ahead and changed them. All that work for what was just a leaky piece of rope and a dent in the oil pan that was restricting oil from the pick-up. Oh well no more leak and great oil pressure.

 
Mrmach 1

Was yours a 351?

I think I will jack her up this weekend and see how much room I can make

 
Have you checked oil pressure at idle and 2000 RPM with a a real gauge? In the early post you said it makes a knocking sound but only at certain RPMs. What RPM? Under load or static? Chuck

 
I spoke to an engine shop today. Looks like I will pull it out and let them break it down and give me some options.

 
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