mmcilroy

Well, rebuilt my engine over the winter and so far I can't get it to run right. Pretty sure I'm looking for a vacuum leak but who knows. All that aside, by measuring my compression is there any way to determine my compression ratio. If not, does anyone know what stock compression measurement would be for a stock, 1972 350 with stock pistons and stock 7a heads?

Just want to know much higher my compression is than stock. If my math was right I'm much higher than I was shooting for!

don71

Did you use a calculater like this one?

http://www.csgnetwork.com/compcalc.html

Did you give it accurate information from personally measuring your parts? I'm asking because it could be actually lower than you expect.

To answer your question, Yes you can get an idea on your compression by using a compression guage.

If your guage reads around 135psi to 145, your about 8:1 and up. If your getting readings of 160psi or more your pretty close to 10:1. If thats the case, tell us a little more about your build, and don't leave out the cam shaft info.

don71

By using that calculater, and figures for typical "stock" components, I come up with:

7.3:1

7.6:1

The main variance here is the head gasket used. The only way to get it up signifigantly higher (with stock pistons) is to zero deck the block and use a thin head gasket with heavily machined head surface.

mmcilroy

I'll put all the information together and post it. During my search for a mysterious vacuum leak I decided to test my compression to see if I had a valve to tight or sticking and I came up with numbers from 170 to 188, arghhhh, what have I done!!

I'll post all the details tomorrow, thanks for the reply. I guess the math was right based on the compression reading.... Rats.

don71

Ok, well then, thats pretty high. You got me thinking on that one

Hmm....

Sticking,tight intake valves...yea that might explain it.

mmcilroy

Here's what I have in front of me
Block - 1972 - 350 Bored .030
Heads - 7a Heads, measured in at 64cc
Sealed Power L2321F Pistons (.076 x 2.44 dish, 5.8cc's?)
http://www.flatlanderracing.com/trwolds.html
Cam - Compcams XE268H with matching lifters, roller rockers, springs and push rods
http://www.compperformancegroupstore...DSCAMHydFlatXE
Stock Connecting rods - Stroke not changed from stock
Stock crank, no machining was required

Can't find spec on head gasket or deck clearance, I have it somewhere.

Well I can now add "possibly higher than expected compression" to the list of things to worry about. Well I can live with more expensive gas I think but this vacuum leak is going to drive me nuts. crappy idle and only about, at best 7-8" of vacuum at 900 RPM in neutral, hunts all over the place. In drive it drops to 5".

I'll post those other numbers tomorrow.

Thanks don71

don71

So far, were at 9.8:1 with negative deck height of -.015, typical .

With a deck height of zero it goes to 10.2.

Yes your assumtion is correct, given the numbers so far.

88 coupe

Compression height is 1.612"

So, if stroke = 3.385", rod length = 6" and deck height = 9.33".

Piston to deck = .0255".

Norm

cutlassefi

I consistantly have 175-180 with a Hyd Roller, 222/230 at .050 on a 112 in at 110. Measured comp ratio is about 9.7:1.

You need to check all the mating surfaces. Did you degree the cam? I had to advance my cam 9 degrees to get the 2 degrees retard I wanted. In other words it was 11 degrees off from what the cam card recommended installing it at.

mmcilroy

For some reason it looks like I used a set of head gaskets that were given to me. Can't recall why I would go through all that and not invest in some head gaskets??? Well I'm still trying to track down the specifications of the head gasket. It was from a Mr.gasket 7140 rebuild kit.

The cam was degreed, not by me but by someone with a brain!

Can't remember what it was set at but here's a picture of us getting set up.

mmcilroy

Cleaning up the original cast iron intake to put on. Can't find anything to correct the low vacuum problem at idle. Must be a leak somewhere in or around the underside of the manifold. Guess you get what you pay for, a used manifold that looked like it only had a few hours on it, maybe there was a reason for that. I should be getting some info on the gasket tomorrow from their tech people. Hopefully it has a larger diameter bore than I was putting in that calculator last night.

Warhead

You sure about the 64cc's? You have to cut .030"= to get the 7a's down to that.

Get a pair of needle nose pliers, start pinching lines while it runs.
Could be that you have a bad advance unit, or brake diaphram.
Also could be a number of other things. We be only guessing, since we are not over the fenders.
Good compression, though.

Jim

mmcilroy

Warhead (great name by the way)

We measured the heads the same way as the picture attached. Was glad that I had 64cc heads! Also confirmed it by reading the tech page http://tech.classicoldsmobile.com/43.shtml

If my measurement was incorrect and the tech page is wrong then maybe my heads were machined at one point and that would cause my intake not to seal properly??? But my compression calculations were made based on a 64cc chamber. I'll have to double check and see where the stock 7A heads should come in at. (Still waiting for a reply from Mr. Gasket about my head gasket specifications).

As for trouble shooting what I think is either a vacuum leak causing the idle to suck (no pun intended) or that maybe the low vacuum was a result of a bad carb (the chicken and the egg dilemma). Here's what I have tried so far. Note, brake booster, distributor (HEI) and all vacuum lines were replaced during the rebuild.

Symptom(s) not sure what is causing what but I have a hunting idle that goes between 600 and 1000 RPM at about 5 second intervals. Idle drops to 500-600 rpm (at any RPM the car will usually die when put into gear) At the same time the vacuum gauge will drop to about 5" of vacuum, wide band air/fuel gauge (installed that with the hope it would provide some clues) pegs into the "rich" area. Covering the primaries with my hand will bring the idle up and seems to be (closer) to stable. Then the idle will drift back up to about a 1000 RPM, vacuum climbs to around 12" and air/fuel gage goes to about 14.5. Timing light, with vacuum advance disconnected (otherwise timing drifts with the vacuum) seems to hold steady.


Have tried the following to get it to idle smooth.

• Spray areas with starting fluid, no change in idle
• Replaced carb with three different, rebuilt carbs (only thing not replaced in carbs were the primary shaft bushings but all seemed pretty tight, spraying bushings while engine was running had no effect on idle.
• Played with rods and jets, one carb we even drilled out the idle circuit orifices per Cliff Ruggles How to Rebuild and Modify Rochester Quadrajet Carburetors recipe # 2
• Disconnected ALL vacuum lines (and capped off), transmission, distributor, vacuum canister for HVAC controls, PVC valve, brake booster and that temperature/vacuum control tree thing (can't remember what it was called) all resulting in no change in idle quality
• Tested compression to be sure a rocker was not to tight holding a valve slightly open or a valve was stuck
• Changed choke heat riser gasket and inspected EGR block off and replaced EGR block off gasket.
• Changed stock power piston spring with a lighter on thinking that maybe the idle circuit was unstable with a lower than stock vacuum, had a slight change but that was probably due to its ability to meter better with bad vacuum.
• Idle screws have no effect on idle quality, a slight change can be seen on the air/fuel gauge but not much.

I think that's about everything, tried to make only one change at a time to be able to determine the results brought me any closer to a solution. This weekend I'll be putting the stock, cast iron manifold back on. Pulling off the (used) (that might be a clue as to why it looked like it had very few hours on it) Edelbrock performer intake. Will inspect intake gaskets and sealing surfaces for any clue that there might have been a leak. Will also look for any cracks in the manifold. If the stock manifold does the trick then I should be able to dial this thing in. If not something is FUBAR ed and I'll probably try a aftermarket carb to see if anything changes. This one has me stumped, right now it no longer fun....

Stay tuned I'll post the results of the manifold change, hopefully it will happen this weekend.

Sorry for the long post, I had to vent to someone. Thanks to all of you for listening and sharing your expertise.

mmcilroy

Oh, and if anyone lives near buffalo, NY (yeh, I know, who would want to) and is looking for a challange .

mmcilroy

Well with 88 Coupe's deck height calculation ( thanks ) We have an estimated compression ratio of 9.67:1 just about on target. The head gasket was .04" compressed with a bore diameter of 4.250, which actually should bring it down a bit because the calculator did not request the bore diameter. Stay tuned for the manifold change results. If anyone has any other things I can try before I pull the manifold I would be interested in hearing them, not looking forward to changing the intake.

Warhead

Just so you know...
THAT tech page is WRONG!
STOCK 7a heads will be 68-70cc's from the factory.
I have never seen an unmachined 5,6,7, or 7a head less than that.
Yours had to be cut if they measured 64.
Just so you know.

I am thinking more than just one of the lines. A vacuum advance unit could leak, or even a power brake booster can rupture a bladder.
Start pinching lines if you think it is a vacuum leak.
A BUNCH easier than swapping out the intake.

Are you, by chance, Irish?

Good luck,
Jim

mmcilroy

Thanks Jim,

They might have been machined, I got them from the same guy I bought the engine I rebuilt from but used the extra heads because they were in much better condition than the ones that were on the block. They were clean and you could tell that they had fresh valves in them, they very well could, actually must have been cut because we did measure them. Actually I only measured one combustion chamber????????????? And my compression measurements were 15 lbs, on average lower on one side. I assumed it was because I measured one side before dinner, engine was warm and the other side later that evening when the engine was cold. I wonder how crappy an engine would run if one head was milled down and the other wasn't . Any idea if there is any external reference I could measure to see if one side is .030 shorter!!!!!!!!!!!!!!!!!! This would really suck....................

And yes I'm Irish! Born here but my parents were off the boat.

Warhead

The engine would not run crappy cause of that, just would make like 1 or 2 hp per cylinder more on one bank, compared to the other. Happens more than I'd like to think.

The only outside way to tell, is if you can accurately measure the short head bolt bosses, they should be 2.200 inches from the top of the boss to the head surface.

BTW, I'm German, but it's the Irish and Germans who are the beer drinkers!

Jim

mmcilroy

Beer

Just as good as beer, well almost is the fact that I just went out to the garage and measured my compression again. I would not have been able to sleep wondering if I could have been so foolish as to mismatch my heads. The good news is, with all measurements taken with the engine cold were:

1 - 179
3 - 181
5 - 180
7 - 175
2 - 178
4 - 181
6 - 177
8 - 183

Feel much better now that I'm back to only one problem

Intake is cleaned and painted, some gaskets tomorrow and sometime this weekend we'll find out if changing manifolds solves the problem. Also noticed that the gaskets I used on the Edelbrock were paper thin pieces of crap................

Time for a beer...........