Post by oldsproject on Oct 28, 2012 18:28:47 GMT -5
Thought id throw this up in hopes that it will save others time and its been raining all day so I'm bored. By no means am I an expert and the one thing I know is that I'm sure this method doesn't work for everyone. However, over the coarse of several weeks of research and trial and error I finally found a system that truly helped me get me modified 425 running extremely well. Its a culmination of things from various authorities including many on here so I hope this helps.
First a little about the engine. Its a 67 425 with a 45 degree cam. Motor was bored .003, rotating assembly balanced, high volume oil pump with 7qt low profile oil pan, C heads that are not ported but converted to roller rockers, piston rings were file fit, valley tray replacing the turkey tray, double roller timing chain, jm 20-22 cam with cam bushing to reduce lateral play, performer manifold, msd ready to run ignition and edelbrock 750 carb. Throwing this out only as reference.
When tuning an engine most people work on timing which is a good place to start but as postcar, mongoose and others have pointed out, that's only one aspect of a complete tune job. Other items that need to be considered are vacuum pressure, carb settings, plug type, plug gap, initial timing, centrifugal timing and for some, vacuum timing which some may want and some may not. Another component which I'm still working on is proper crankcase ventilation but ill refrain as I dint have that worked out yet.
Based on every ones feedback and hours of research here's what I did that really helped.
1. Started with a vacuum gauge checking all connections by attaching the vacuum gun and using it to create vacuum through the pump feature. If it held for a few minutes, I was good. If not, ran down the leak and repaired. Next I fired the engine and looked at vacuum readings while idling at 800 rpms. In my case the readings were about 6hg which is really low even for my cam. A typical car operates 16-22hg according to research and a big block with a larger cam will run much less but usually not that low. Your gauge will come with instructions that will diagnose issues from worn or broken valve springs, bad rings, poor timing, etc. in my case the needle was holding steady which is what you want but was low. With the vacuum gauge in hand I started advancing the distributor. At its highest point it hit 14hg before it started jumping. I backed the distributor off until i hit a fairly steady 12.5hg reading. The engine rpms came up but it was already running better.
2. Next I pulled the plugs. I read an article about reading plugs which gave guidance on how to tell where to jet your carb, if your running hot or cold, gapped correctly, had too much retard and advance in your timing. What I found was that I was extremely rich as the base was black but the tip was a good color from tip through the bend which is what you want. Brown on the tip only is too cold and brown to the base is too hot. I did regap the plugs down to .45 from .60 based on recommendations even though I'm running an HEI. As referance if you use a 10x magnifying glass you can tell if there is predetonation issues, ect. I did havevsome spotting on the porcelin which was an early sign that i had the motor retarded too much.
3. Knowing the vacuum from step 1 allowed me to look at the charts in the ebrock manual to find a closer fit for my engine carb jetting versus guessing. Since I was running fairly rich and had low manifold vacuum, i stepped down two sizes on the metering rods but left the jets alone. I also went to a weaker rod spring based on the lower vacuum which solved my hesitation issues almost immediately. Knowing the vacuum numbers helped me dial in quickly.
4. Now that the carb was more closely dialed in and the plugs were happy (good color and burn), i started working more closely on timing. One of the biggest revelations for me was reading postcars old post about the uselessness of distributor vacuum in a muscle car. Also how vacuum advance will cause engines to run hotter as its designed to retard the system at idle. Everyone knows how notorious Olds are for running hot especially at idle so this intrigued me. The unit I use is an MSD "Ready to Run" HEI which has alot of great features. One is I can lock out the vacuum advance by removing the canister and putting in a lockout tab. This keeps it fully advanced which is the same as moving it from ported vacuum to manifold vacuum. The other feature I like is the ability to increase or decrease centrifugal advance by simply changing busing size. This allows you to match Initial timing with centrifugal to get close to your all-in timing target. Finally like all modern HEIs it has different springs to work on the advance curve. Before I pulled the distributor to make the changes, I wanted to get a feel for which changes I needed to make first in order to avoid pulling the distributor more then once. Therefore I worked on initial timing first which was really close based on the vacuum test above. It was advance to 12.0 at 800 rpms and after a bit of tinker, seemed to respond best at 14.0. Then I ran the centrifugal timing test by disconnecting and plugging the vacuum. Ran the car up to 2500 rpms, ran the timing light and got a reading of 33.0. This meant my centrifugal timing was 19.0 (centrifugal minus initial). I wanted something closer to a total timing event of 36-38 versus 33 so knew i needed to put in a smaller bushing. Each bushing makes about a 5 degree change so one step would put me at about 38 degrees at 14.0 initial which is close to where I wanted to be. Based on the above, pulled the distributor, made the changes and reinstalled. Keep in mind this is all before vacuum advance. Adding vacuum will increase timing 10-15 degrees from what I experianced.
Car fired up immediately and what a difference. I need to do a little more fine tuning but the car is running stronger and better then it ever has. I'm sure there are better ways but this sequence saved time and worked great.
For reference here's where everything ended up:
Initial timing 25.0 degrees creating 13.5 hg vacuum at 800 rpms (vacuum locked)
Total timing 50.0 degrees creating 20.5 hg vacuum at 2250 rpms (vacuum locked)
MSD "Ready to Run" used silver centrifugal stop busing, locked the vacuum and silver advance springs.
Ebrock Carb primary jet .410, 7537 metering rods with yellow springs.
Plugs R45S gapped at .45
Will keep working on it but this seems to have me fairly dialed in and best of all....... The car runs tremendously cooler!!!
First a little about the engine. Its a 67 425 with a 45 degree cam. Motor was bored .003, rotating assembly balanced, high volume oil pump with 7qt low profile oil pan, C heads that are not ported but converted to roller rockers, piston rings were file fit, valley tray replacing the turkey tray, double roller timing chain, jm 20-22 cam with cam bushing to reduce lateral play, performer manifold, msd ready to run ignition and edelbrock 750 carb. Throwing this out only as reference.
When tuning an engine most people work on timing which is a good place to start but as postcar, mongoose and others have pointed out, that's only one aspect of a complete tune job. Other items that need to be considered are vacuum pressure, carb settings, plug type, plug gap, initial timing, centrifugal timing and for some, vacuum timing which some may want and some may not. Another component which I'm still working on is proper crankcase ventilation but ill refrain as I dint have that worked out yet.
Based on every ones feedback and hours of research here's what I did that really helped.
1. Started with a vacuum gauge checking all connections by attaching the vacuum gun and using it to create vacuum through the pump feature. If it held for a few minutes, I was good. If not, ran down the leak and repaired. Next I fired the engine and looked at vacuum readings while idling at 800 rpms. In my case the readings were about 6hg which is really low even for my cam. A typical car operates 16-22hg according to research and a big block with a larger cam will run much less but usually not that low. Your gauge will come with instructions that will diagnose issues from worn or broken valve springs, bad rings, poor timing, etc. in my case the needle was holding steady which is what you want but was low. With the vacuum gauge in hand I started advancing the distributor. At its highest point it hit 14hg before it started jumping. I backed the distributor off until i hit a fairly steady 12.5hg reading. The engine rpms came up but it was already running better.
2. Next I pulled the plugs. I read an article about reading plugs which gave guidance on how to tell where to jet your carb, if your running hot or cold, gapped correctly, had too much retard and advance in your timing. What I found was that I was extremely rich as the base was black but the tip was a good color from tip through the bend which is what you want. Brown on the tip only is too cold and brown to the base is too hot. I did regap the plugs down to .45 from .60 based on recommendations even though I'm running an HEI. As referance if you use a 10x magnifying glass you can tell if there is predetonation issues, ect. I did havevsome spotting on the porcelin which was an early sign that i had the motor retarded too much.
3. Knowing the vacuum from step 1 allowed me to look at the charts in the ebrock manual to find a closer fit for my engine carb jetting versus guessing. Since I was running fairly rich and had low manifold vacuum, i stepped down two sizes on the metering rods but left the jets alone. I also went to a weaker rod spring based on the lower vacuum which solved my hesitation issues almost immediately. Knowing the vacuum numbers helped me dial in quickly.
4. Now that the carb was more closely dialed in and the plugs were happy (good color and burn), i started working more closely on timing. One of the biggest revelations for me was reading postcars old post about the uselessness of distributor vacuum in a muscle car. Also how vacuum advance will cause engines to run hotter as its designed to retard the system at idle. Everyone knows how notorious Olds are for running hot especially at idle so this intrigued me. The unit I use is an MSD "Ready to Run" HEI which has alot of great features. One is I can lock out the vacuum advance by removing the canister and putting in a lockout tab. This keeps it fully advanced which is the same as moving it from ported vacuum to manifold vacuum. The other feature I like is the ability to increase or decrease centrifugal advance by simply changing busing size. This allows you to match Initial timing with centrifugal to get close to your all-in timing target. Finally like all modern HEIs it has different springs to work on the advance curve. Before I pulled the distributor to make the changes, I wanted to get a feel for which changes I needed to make first in order to avoid pulling the distributor more then once. Therefore I worked on initial timing first which was really close based on the vacuum test above. It was advance to 12.0 at 800 rpms and after a bit of tinker, seemed to respond best at 14.0. Then I ran the centrifugal timing test by disconnecting and plugging the vacuum. Ran the car up to 2500 rpms, ran the timing light and got a reading of 33.0. This meant my centrifugal timing was 19.0 (centrifugal minus initial). I wanted something closer to a total timing event of 36-38 versus 33 so knew i needed to put in a smaller bushing. Each bushing makes about a 5 degree change so one step would put me at about 38 degrees at 14.0 initial which is close to where I wanted to be. Based on the above, pulled the distributor, made the changes and reinstalled. Keep in mind this is all before vacuum advance. Adding vacuum will increase timing 10-15 degrees from what I experianced.
Car fired up immediately and what a difference. I need to do a little more fine tuning but the car is running stronger and better then it ever has. I'm sure there are better ways but this sequence saved time and worked great.
For reference here's where everything ended up:
Initial timing 25.0 degrees creating 13.5 hg vacuum at 800 rpms (vacuum locked)
Total timing 50.0 degrees creating 20.5 hg vacuum at 2250 rpms (vacuum locked)
MSD "Ready to Run" used silver centrifugal stop busing, locked the vacuum and silver advance springs.
Ebrock Carb primary jet .410, 7537 metering rods with yellow springs.
Plugs R45S gapped at .45
Will keep working on it but this seems to have me fairly dialed in and best of all....... The car runs tremendously cooler!!!
