04SilveradoMykk
10-14-2009, 04:48 PM
I've been getting EMails on carb & ignition tuning, so I wanted to make a guide I could refer people to. I'm strictly talking about performance aftermarket square bore carbs.
Here it is:
Mykk's Carb & Ignition tuning guide
Picking the right size carb for your engine. (I'm using my own SBC for examples)
357cu/2 = 178.5
6500rpm/1728=3.76
178.5 x 3.76 =671.16 cfm assuming 100% volumetric efficiency (VE).
Things like Camshaft, Heads, Intake manifold, exhaust, forced induction, even ignition timing all affect VE%. A factory mid-80's smog era 350 would be running around the 71% VE range, performance factory small blocks of the 60's era would be in the 83-85% range. A built & blueprinted engine with well thought out components working together could reach as high as 95% VE, a supercharged engine can put out 105%-115%. For the sake of my build we'll use 87% for our formula
671.16 x 0.87 =584 (rounded) cfm the engine will need to spin 6500rpm, in this case I would choose a advertised 600cfm carb, because most carb manufacturers under estimate the cfm output. (I'm actually using a 650cfm demon now, we'll get into that later)
But, luckily for us lazy dumb people there are several online calcs that will do the math for you!
http://www.4secondsflat.com/Carb_CFM_Calculator.html
Most engine builders will tell you that for a street driven application pick a carb that flows slightly lower than what your engine will handle. The idea is this will prevent from having stumbles, flat spots and just generally over carb your engine.
I have a friend in the town that dropped in a stock L31 5.7L 350ci into his 1967-72 short bed 1/2 ton, pretty clean little truck (my favorite era of GM's) He used a Vortec intake for square bore carb, and put on a Holley 750cfm carb. Granted, his truck is pretty quick and he claims his engine is more responsive and quicker with the 750cfm than it ever was with a 600cfm.
My idea as to why is because the 750cfm is adding more air/fuel ratio at a lower throttle than the 600cfm could possibly deliver, however if he ever puts his foot all the way into it I'm sure he get's one hell of a stumble that the engine won't be able to recover unless he pulls his foot out of the throttle, and in racing that will cause a slower ET.
Ok, we've determined what size carb we should use on the engine, what kind of carb should we use? Brand/manufacturer is highly personal tase, I'm just referring to manual secondaries vs vacuum secondaries.
As a general rule of thumb, manual secondaries is for manual trans, lighter vehicles. Vacuum secondaries is for automatic transmission equipped and heavier applications.
If you live at any significant elevation above sea level (like me, in Prescott Az @ 5300ft elevation) You will want to immediately re-jet (or rods too) the carb for the elevation, Every carb manufacturer has a different approach to this, I believe on my Demon carb it's 1 jet size lean for every 1200ft, but this will change in a later step.
I'm going to move on to ignition timing, and than we'll come back to tuning the carb. Most diagnosed fueling problems can be resolved in the ignition system.
Setting up your ignition timing curve can be time consuming and frustrating but a well dialed in distributor can make or break the way your performance build reacts on the street.
Some engine builders believe in setting your max time and it doesn't matter where the idle timing lands, this might be fine for an all out race motor. But anything that see's the street, stop lights & cruising will need to be concerned with the entire timing curve.
To set the maximum ignition timing, the vacuum advance must be unplugged and the port on the carb capped. So your dist is relying on mechanical advance only. Different engines like different max timing, it's mostly in the cylinder head design. My Vortec heads should only use a max of 34 degrees, occasionally I sneak up to 36-38 to see what happens. Older head designs can take 38-42 degrees max. Ideally the max advance should be set at the point right before detonation.
I've discovered my engine likes to have that max timing of 34 degrees all in by 3200-3400rpm, so that means the entire mechanical advance is maxed out in the time it takes to go from idle-3200rpm,
Setting the idle ignition timing, called initial timing. Big camshafts require more initial timing in order to idle, my Comp XR276-10 idles good with 13 degrees initial timing. I like my idle a little low, around 650-750rpm a) because I like hearing the lope of the cam b) because I'm not fighting the engine with the brake pedal while stopped at a light.
Ok, we've declared my engine likes to have 34 degrees max timing and will idle fine with 13 degrees. That means in my distributor I will use the 21 degree bushing that came in my kit, as well as lightweight springs for the weights to have my advance come in fast to reach that 34 degrees by 3200rpm. This can get tricky, you can mix match thick & thin springs to get your desired timing curve. I've found by just using only light springs the engine doesn't want to come back down to idle because timing is advanced at a low rpm due to centrifugal weight.
Now, just leaving it like this would be fine for the weekend warrior/race car. But in any kind of driver, cruiser or anything that you hope to get any kind of fuel mileage out of you need to have a vacuum advance hooked up. The vacuum advance on the dist adds more ignition timing on top of what's already in place by the mechanical advance. The idea is that under light load situations the manifold vacuum is high and the A/F ratio is lean. When the load on the engine is light or moderate, the timing can be advanced to improve fuel economy and throttle response due, in part, to the slower flame travel in the combustion chamber from the lean A/F under these conditions. Under light load and cruise the engine will accept a maximum timing advance of about 52 degrees. Anywhere between 48-52 is good.
You can choose different vacuum advance modules that will tailor to your needs, but first you need to decide if you are going to run the vacuum advance from the "ported" vacuum port on the carb or from the manifold vacuum port.
Manifold vacuum allows actual manifold vacuum to the distributor at all times. Ported vacuum only allows manifold vacuum when throttle is above idle. This was used as an emissions control to retard timing at idle in order to reduce emissions. Using ported vacuum can make a more reliable and predictable idle speed since the initial timing will never change based off of vacuum. Does your camshaft duration require additional initial timing in order to idle properly? Do you have the idle screw on the carb open so far your beginning to pull fuel from the transition circuits or boosters? Radical cams will often require over 24 degrees of timing advance at idle. To much initial timing can and will fight the starter motor while cranking and in turn can break teeth off the flywheel & starter. An appropriately selected vacuum advance can plugged into manifold vacuum can provide the needed extra timing at idle to allow a big cam to idle at a lower rpm while allowing the lower timing while cranking when manifold vacuum isn't present.
If you choose to run straight manifold vacuum to your advance in order to gain the additional timing advance at idle, you must select a vacuum advance module that pulls in all of the advance at a vacuum level at least 2” below the manifold vacuum present at idle. So lets say your moderately built engine pulls 15" of vacuum at idle, you'll need a vac advance that maxes out at 13". A more radical motor might only pull 10" vac at idle, choose a vac advance that's max by 8"
My engine only makes 10-12" vac at idle, it cruises around 17-20". But I'm not using manifold vacuum for the advance and I've decided on a module that begins adding timing between 5-7" vac, it's maxed out @ 11-13" and will add 8 degrees of advance max (camshaft degrees, which equates to 16 degrees at the crank) With my max mechanical timing @ 34 degrees + the 16 degrees from the vacuum advance, my engine will not ever go above 50 degrees of timing while cruising (and that's if I cruise around at 3000rpm with the mechanical advance maxed). As soon as I put my foot into it, manifold vacuum goes away, the vacuum advance retards to the mechanical advance curve.
The same thing happens under a load, manifold vacuum is low, the distrubutor is only using the mechanical advance and will provide for more power with less chance of detonation at the cost of fuel consumption.
Ok, we've got the ignition timing thing down. Back to the carburetor.
With the engine idling, fully warmed up and you're running the initial ignition timing your happy with. Set the desired idle rpm you want to use, your carb should be set up from the factory with a generalized base setting for your idle mixture screws. Turn in the idle adjustment screws until the engine rpm barely begins to fall, than turn that screw back out by 1/8 turn. Do the following to the other idle screws. Re-adjust the idle rpm screw if needed to get desired idle RPM and repeat the procedure.
To judge if you need bigger or smaller jets, Keep your eye on your spark plugs, pull one plug out every other day or once a week.
* Normal deposits: Light brown or tan colored.
* Fuel fouled spark: plug Black fluffy carbon deposits indicate an overly rich fuel mixture or possibly a weak spark. Check for such things as a stuck choke, a heavy or maladjusted carburetor float, a leaky needle valve in the carburetor, leaky injectors, low coil output or high resistance in the plug wires.
* Wet spark plug: A wet spark plug means the plug has not been firing. If not due to engine flooding, the problem may be a bad ignition cable (excessive resistance, shorted or arcing). But wet fouling can also be caused by dirt or moisture on the outside of the plug that provides a conductive path to ground, or by an internal crack in the ceramic insulator that shorts the plug to ground.
* Oil fouled spark plug: Heavy black deposits with an oily appearance. These are the result of oil entering in the combustion chamber, probably past worn valve guides, guide seals or rings. Switching to a hotter plug may help prolong plug life somewhat, but no spark plug will survive long under such conditions. The only permanent cure to this condition is to fix the oil consumption problem.
* Glazed spark plug: Yellowish melted appearing deposits on the insulator tip that result from high temperature operation. The engine may be running too hot (check for cooling problems), the EGR valve may be inoperative and/or the heat range of the plug may be too hot for the application. Switching to a cooler plug may be necessary if no other problems are found.
* Damaged plug: If the electrodes have been smashed flat or broken, somebody put the wrong plug in the engine. A plug that protrudes too far into the combustion chamber may hit the piston or a valve. Always follow the plug manufacturers application recommendations when selecting replacement plugs to prevent this kind of problem.
* Overheating If the spark plug: insulator is blistered, white and free from deposits, something is making the plug run too hot. If the heat range is not too hot for the application, check for cooling problems, incorrect ignition timing or a lean fuel mixture.
* Melted electrode: A symptom of severe preignition. The spark plug has been running too hot for a long time (see overheating above). This can be very damaging and may burn a hole through the top of a piston!
* Detonation: If the insulator is split or chipped, detonation (spark knock) may be occurring in the engine. over advanced ignition timing, excessive compression due to accumulated deposits in the combustion chamber, or engine overheating.
There are more specific ways to read a plug, but this should get you going in the right direction for.
Once you're confident you've got your carb & ignition dialed in. For those select few who decided to go with a vacuum secondaries type carb. Don't forget to pick up a secondaries spring kit for your carb and play around with lighter springs! Usually the spring in the carb out of the box is too conservative and won't ever fully open up at any rev.
Vacuum secondaries do not function off of manifold vacuum, like the misleading name implies. They function off of the air passing by a small port inside the venturis. The faster the airflow through the venturis the more the secondaries open.
Pics:
Idle RPM adjusting screw:
Speed Demon/Holley:
http://i40.photobucket.com/albums/e227/mykk/carb%20tuning/IdleScrew.jpg
Edelbrock/Carter:
http://i40.photobucket.com/albums/e227/mykk/carb%20tuning/edelidlescrew.jpg
Idle Fuel screws: Speed Demon
There are four idle fuel screws on the speed demon, one at each corner, two on each side. Some carbs only have two screws located on the primaries fuel metering block
http://i40.photobucket.com/albums/e227/mykk/carb%20tuning/Fuelscrew.jpg
The Edelbrock carb has two idle fuel screws located directly on the front of the carb.
http://i40.photobucket.com/albums/e227/mykk/carb%20tuning/edelfuelscrew.jpg
Try to keep all the idle fuel screws turned out equally.
The vacuum ports for Manifold vacuum and Ported vacuum:
The edelbrock's ported vacuum is the higher up port on the left. The other two, one bigger port and smaller port are both manifold vacuum. The bigger port is for the PCV hose and the smaller port can be used for ignition vacuum advance
http://i40.photobucket.com/albums/e227/mykk/carb%20tuning/vacports.jpg
On the Speed demon, the forward most vacuum port is ported vacuum, the one directly behind it is manifold vacuum (tough to see, but my manifold vacuum port is plugged under the throttle position sensor on my carb)
http://i40.photobucket.com/albums/e227/mykk/carb%20tuning/demonvacports.jpg
The vacuum spring pod on the Speed Demon has an easy access plug that allows the changes of springs a snap.
http://i40.photobucket.com/albums/e227/mykk/carb%20tuning/SecPod.jpg
http://i40.photobucket.com/albums/e227/mykk/carb%20tuning/vacspring.jpg
Don't be afraid to change up or play with your carb & ignition, as long as you approach what your changing with an educated direction.
If you have any more ideas, or would like to post up what you have done, please do.
Cheers ~Mykk
Here it is:
Mykk's Carb & Ignition tuning guide
Picking the right size carb for your engine. (I'm using my own SBC for examples)
357cu/2 = 178.5
6500rpm/1728=3.76
178.5 x 3.76 =671.16 cfm assuming 100% volumetric efficiency (VE).
Things like Camshaft, Heads, Intake manifold, exhaust, forced induction, even ignition timing all affect VE%. A factory mid-80's smog era 350 would be running around the 71% VE range, performance factory small blocks of the 60's era would be in the 83-85% range. A built & blueprinted engine with well thought out components working together could reach as high as 95% VE, a supercharged engine can put out 105%-115%. For the sake of my build we'll use 87% for our formula
671.16 x 0.87 =584 (rounded) cfm the engine will need to spin 6500rpm, in this case I would choose a advertised 600cfm carb, because most carb manufacturers under estimate the cfm output. (I'm actually using a 650cfm demon now, we'll get into that later)
But, luckily for us lazy dumb people there are several online calcs that will do the math for you!
http://www.4secondsflat.com/Carb_CFM_Calculator.html
Most engine builders will tell you that for a street driven application pick a carb that flows slightly lower than what your engine will handle. The idea is this will prevent from having stumbles, flat spots and just generally over carb your engine.
I have a friend in the town that dropped in a stock L31 5.7L 350ci into his 1967-72 short bed 1/2 ton, pretty clean little truck (my favorite era of GM's) He used a Vortec intake for square bore carb, and put on a Holley 750cfm carb. Granted, his truck is pretty quick and he claims his engine is more responsive and quicker with the 750cfm than it ever was with a 600cfm.
My idea as to why is because the 750cfm is adding more air/fuel ratio at a lower throttle than the 600cfm could possibly deliver, however if he ever puts his foot all the way into it I'm sure he get's one hell of a stumble that the engine won't be able to recover unless he pulls his foot out of the throttle, and in racing that will cause a slower ET.
Ok, we've determined what size carb we should use on the engine, what kind of carb should we use? Brand/manufacturer is highly personal tase, I'm just referring to manual secondaries vs vacuum secondaries.
As a general rule of thumb, manual secondaries is for manual trans, lighter vehicles. Vacuum secondaries is for automatic transmission equipped and heavier applications.
If you live at any significant elevation above sea level (like me, in Prescott Az @ 5300ft elevation) You will want to immediately re-jet (or rods too) the carb for the elevation, Every carb manufacturer has a different approach to this, I believe on my Demon carb it's 1 jet size lean for every 1200ft, but this will change in a later step.
I'm going to move on to ignition timing, and than we'll come back to tuning the carb. Most diagnosed fueling problems can be resolved in the ignition system.
Setting up your ignition timing curve can be time consuming and frustrating but a well dialed in distributor can make or break the way your performance build reacts on the street.
Some engine builders believe in setting your max time and it doesn't matter where the idle timing lands, this might be fine for an all out race motor. But anything that see's the street, stop lights & cruising will need to be concerned with the entire timing curve.
To set the maximum ignition timing, the vacuum advance must be unplugged and the port on the carb capped. So your dist is relying on mechanical advance only. Different engines like different max timing, it's mostly in the cylinder head design. My Vortec heads should only use a max of 34 degrees, occasionally I sneak up to 36-38 to see what happens. Older head designs can take 38-42 degrees max. Ideally the max advance should be set at the point right before detonation.
I've discovered my engine likes to have that max timing of 34 degrees all in by 3200-3400rpm, so that means the entire mechanical advance is maxed out in the time it takes to go from idle-3200rpm,
Setting the idle ignition timing, called initial timing. Big camshafts require more initial timing in order to idle, my Comp XR276-10 idles good with 13 degrees initial timing. I like my idle a little low, around 650-750rpm a) because I like hearing the lope of the cam b) because I'm not fighting the engine with the brake pedal while stopped at a light.
Ok, we've declared my engine likes to have 34 degrees max timing and will idle fine with 13 degrees. That means in my distributor I will use the 21 degree bushing that came in my kit, as well as lightweight springs for the weights to have my advance come in fast to reach that 34 degrees by 3200rpm. This can get tricky, you can mix match thick & thin springs to get your desired timing curve. I've found by just using only light springs the engine doesn't want to come back down to idle because timing is advanced at a low rpm due to centrifugal weight.
Now, just leaving it like this would be fine for the weekend warrior/race car. But in any kind of driver, cruiser or anything that you hope to get any kind of fuel mileage out of you need to have a vacuum advance hooked up. The vacuum advance on the dist adds more ignition timing on top of what's already in place by the mechanical advance. The idea is that under light load situations the manifold vacuum is high and the A/F ratio is lean. When the load on the engine is light or moderate, the timing can be advanced to improve fuel economy and throttle response due, in part, to the slower flame travel in the combustion chamber from the lean A/F under these conditions. Under light load and cruise the engine will accept a maximum timing advance of about 52 degrees. Anywhere between 48-52 is good.
You can choose different vacuum advance modules that will tailor to your needs, but first you need to decide if you are going to run the vacuum advance from the "ported" vacuum port on the carb or from the manifold vacuum port.
Manifold vacuum allows actual manifold vacuum to the distributor at all times. Ported vacuum only allows manifold vacuum when throttle is above idle. This was used as an emissions control to retard timing at idle in order to reduce emissions. Using ported vacuum can make a more reliable and predictable idle speed since the initial timing will never change based off of vacuum. Does your camshaft duration require additional initial timing in order to idle properly? Do you have the idle screw on the carb open so far your beginning to pull fuel from the transition circuits or boosters? Radical cams will often require over 24 degrees of timing advance at idle. To much initial timing can and will fight the starter motor while cranking and in turn can break teeth off the flywheel & starter. An appropriately selected vacuum advance can plugged into manifold vacuum can provide the needed extra timing at idle to allow a big cam to idle at a lower rpm while allowing the lower timing while cranking when manifold vacuum isn't present.
If you choose to run straight manifold vacuum to your advance in order to gain the additional timing advance at idle, you must select a vacuum advance module that pulls in all of the advance at a vacuum level at least 2” below the manifold vacuum present at idle. So lets say your moderately built engine pulls 15" of vacuum at idle, you'll need a vac advance that maxes out at 13". A more radical motor might only pull 10" vac at idle, choose a vac advance that's max by 8"
My engine only makes 10-12" vac at idle, it cruises around 17-20". But I'm not using manifold vacuum for the advance and I've decided on a module that begins adding timing between 5-7" vac, it's maxed out @ 11-13" and will add 8 degrees of advance max (camshaft degrees, which equates to 16 degrees at the crank) With my max mechanical timing @ 34 degrees + the 16 degrees from the vacuum advance, my engine will not ever go above 50 degrees of timing while cruising (and that's if I cruise around at 3000rpm with the mechanical advance maxed). As soon as I put my foot into it, manifold vacuum goes away, the vacuum advance retards to the mechanical advance curve.
The same thing happens under a load, manifold vacuum is low, the distrubutor is only using the mechanical advance and will provide for more power with less chance of detonation at the cost of fuel consumption.
Ok, we've got the ignition timing thing down. Back to the carburetor.
With the engine idling, fully warmed up and you're running the initial ignition timing your happy with. Set the desired idle rpm you want to use, your carb should be set up from the factory with a generalized base setting for your idle mixture screws. Turn in the idle adjustment screws until the engine rpm barely begins to fall, than turn that screw back out by 1/8 turn. Do the following to the other idle screws. Re-adjust the idle rpm screw if needed to get desired idle RPM and repeat the procedure.
To judge if you need bigger or smaller jets, Keep your eye on your spark plugs, pull one plug out every other day or once a week.
* Normal deposits: Light brown or tan colored.
* Fuel fouled spark: plug Black fluffy carbon deposits indicate an overly rich fuel mixture or possibly a weak spark. Check for such things as a stuck choke, a heavy or maladjusted carburetor float, a leaky needle valve in the carburetor, leaky injectors, low coil output or high resistance in the plug wires.
* Wet spark plug: A wet spark plug means the plug has not been firing. If not due to engine flooding, the problem may be a bad ignition cable (excessive resistance, shorted or arcing). But wet fouling can also be caused by dirt or moisture on the outside of the plug that provides a conductive path to ground, or by an internal crack in the ceramic insulator that shorts the plug to ground.
* Oil fouled spark plug: Heavy black deposits with an oily appearance. These are the result of oil entering in the combustion chamber, probably past worn valve guides, guide seals or rings. Switching to a hotter plug may help prolong plug life somewhat, but no spark plug will survive long under such conditions. The only permanent cure to this condition is to fix the oil consumption problem.
* Glazed spark plug: Yellowish melted appearing deposits on the insulator tip that result from high temperature operation. The engine may be running too hot (check for cooling problems), the EGR valve may be inoperative and/or the heat range of the plug may be too hot for the application. Switching to a cooler plug may be necessary if no other problems are found.
* Damaged plug: If the electrodes have been smashed flat or broken, somebody put the wrong plug in the engine. A plug that protrudes too far into the combustion chamber may hit the piston or a valve. Always follow the plug manufacturers application recommendations when selecting replacement plugs to prevent this kind of problem.
* Overheating If the spark plug: insulator is blistered, white and free from deposits, something is making the plug run too hot. If the heat range is not too hot for the application, check for cooling problems, incorrect ignition timing or a lean fuel mixture.
* Melted electrode: A symptom of severe preignition. The spark plug has been running too hot for a long time (see overheating above). This can be very damaging and may burn a hole through the top of a piston!
* Detonation: If the insulator is split or chipped, detonation (spark knock) may be occurring in the engine. over advanced ignition timing, excessive compression due to accumulated deposits in the combustion chamber, or engine overheating.
There are more specific ways to read a plug, but this should get you going in the right direction for.
Once you're confident you've got your carb & ignition dialed in. For those select few who decided to go with a vacuum secondaries type carb. Don't forget to pick up a secondaries spring kit for your carb and play around with lighter springs! Usually the spring in the carb out of the box is too conservative and won't ever fully open up at any rev.
Vacuum secondaries do not function off of manifold vacuum, like the misleading name implies. They function off of the air passing by a small port inside the venturis. The faster the airflow through the venturis the more the secondaries open.
Pics:
Idle RPM adjusting screw:
Speed Demon/Holley:
http://i40.photobucket.com/albums/e227/mykk/carb%20tuning/IdleScrew.jpg
Edelbrock/Carter:
http://i40.photobucket.com/albums/e227/mykk/carb%20tuning/edelidlescrew.jpg
Idle Fuel screws: Speed Demon
There are four idle fuel screws on the speed demon, one at each corner, two on each side. Some carbs only have two screws located on the primaries fuel metering block
http://i40.photobucket.com/albums/e227/mykk/carb%20tuning/Fuelscrew.jpg
The Edelbrock carb has two idle fuel screws located directly on the front of the carb.
http://i40.photobucket.com/albums/e227/mykk/carb%20tuning/edelfuelscrew.jpg
Try to keep all the idle fuel screws turned out equally.
The vacuum ports for Manifold vacuum and Ported vacuum:
The edelbrock's ported vacuum is the higher up port on the left. The other two, one bigger port and smaller port are both manifold vacuum. The bigger port is for the PCV hose and the smaller port can be used for ignition vacuum advance
http://i40.photobucket.com/albums/e227/mykk/carb%20tuning/vacports.jpg
On the Speed demon, the forward most vacuum port is ported vacuum, the one directly behind it is manifold vacuum (tough to see, but my manifold vacuum port is plugged under the throttle position sensor on my carb)
http://i40.photobucket.com/albums/e227/mykk/carb%20tuning/demonvacports.jpg
The vacuum spring pod on the Speed Demon has an easy access plug that allows the changes of springs a snap.
http://i40.photobucket.com/albums/e227/mykk/carb%20tuning/SecPod.jpg
http://i40.photobucket.com/albums/e227/mykk/carb%20tuning/vacspring.jpg
Don't be afraid to change up or play with your carb & ignition, as long as you approach what your changing with an educated direction.
If you have any more ideas, or would like to post up what you have done, please do.
Cheers ~Mykk