Tunning carburetor with wideband AEM
08-15-2011, 10:07 PM
#1
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Tunning carburetor with wideband AEM
Here is my setup
1981 Corvette (100% street car) 355 build full hyd roller engine. Compression is around 10.2:1 with fully ported vortec iron heads. + 6 speed t56 transmission. 2.72 rear end.
Cam: XR276HR-10 Duration @ 0.050": 224° / 230° Max Lift: .502" / .510"
Carburetor: Holey 670cfm street avenger (vac secondaries) . 68 pri and 72 sec jets 6.5 inch power valve. 31 size squirter.
HEI distributor: timing set at 36 deg before 3000rpm. + 16deg vaccum advance.
I already adjusted the idle AFR to 14.1. floats, accelerator pump arm and idle screws were adjusted.
What should my cruise and WOT AFR should be?
I have a vaccume gauge + complete set of Holley jets + set of secondary vacuum springs and also full kit of accelerator pump cams to play with.
Any tips/shortcuts that you can give me?
Thanks.
1981 Corvette (100% street car) 355 build full hyd roller engine. Compression is around 10.2:1 with fully ported vortec iron heads. + 6 speed t56 transmission. 2.72 rear end.
Cam: XR276HR-10 Duration @ 0.050": 224° / 230° Max Lift: .502" / .510"
Carburetor: Holey 670cfm street avenger (vac secondaries) . 68 pri and 72 sec jets 6.5 inch power valve. 31 size squirter.
HEI distributor: timing set at 36 deg before 3000rpm. + 16deg vaccum advance.
I already adjusted the idle AFR to 14.1. floats, accelerator pump arm and idle screws were adjusted.
What should my cruise and WOT AFR should be?
I have a vaccume gauge + complete set of Holley jets + set of secondary vacuum springs and also full kit of accelerator pump cams to play with.
Any tips/shortcuts that you can give me?
Thanks.
08-15-2011, 10:36 PM
#2
Holley HP EFI w/TPI
i'm only familiar with efi tuning, but generally wot was 12-13:1 and cruise was 16:1.
If you get knock at wot, add more fuel and/or less timing, if you get lean surge when cruising, add more fuel/less timing.
I'm just starting to learn SBCs, so I'm not familiar with their limitations, but i'm sure some one will chime in
If you get knock at wot, add more fuel and/or less timing, if you get lean surge when cruising, add more fuel/less timing.
I'm just starting to learn SBCs, so I'm not familiar with their limitations, but i'm sure some one will chime in
08-15-2011, 11:25 PM
#3
Race Director
Forget about 14.1 or 14.7:1 if it idles OK at 14:1 that's fine but the idle circuit plays a major roll in your cruise. If you are are cruising at 14:1 or more and it runs OK without any lean misses then you are lucky you are saving money. But I bet you will see high 13's at cruise and maybe 14's great for fuel economy but the motor might not like it.
Gen 1 small blocks tend to like it rich so instead of trying to get it to cruise at a certain number you get it to cruise where the engine likes it. Mine likes 13:1 max at cruise after that I get lean misses. WOT on the other hand you want in 12:1- 12.5 range.
The idle and cruise are basically tuned with idle mixture screws if you are cruising at ~2500RPM. The WOT throttle is a whole new ball game and if yours goes down into the 12's with a 14:1 idle you have a pretty good/lucky setup, I've been playing with this since Innovate came out with their LM-1 many years ago
Gen 1 small blocks tend to like it rich so instead of trying to get it to cruise at a certain number you get it to cruise where the engine likes it. Mine likes 13:1 max at cruise after that I get lean misses. WOT on the other hand you want in 12:1- 12.5 range.
The idle and cruise are basically tuned with idle mixture screws if you are cruising at ~2500RPM. The WOT throttle is a whole new ball game and if yours goes down into the 12's with a 14:1 idle you have a pretty good/lucky setup, I've been playing with this since Innovate came out with their LM-1 many years ago
08-16-2011, 01:45 AM
#4
Melting Slicks
I set mine up to idle at 13.8, cruise is at 14.2, I have the power enrichment (power valve) come in at 13.5 on the primaries and my wide open throttle is 12.8 to 1. I use a vacuum gauge to set the power valve so that it comes in when it needs fuel on the primaries during transition from cruise to light power. I needed a 10.5 power valve to do this. I had to go down from what one would normally use on the primary main jets and up on the power valve enrichment orifices, this required drilling them. I had to drill holes in the butterflies and also enlarge the idle air bleeds to get the idle and off idle where I wanted them.
I use a vacuum advance and made a limiter on it to adjust how much total timing it could add. It is also adjustable for how much vacuum it takes to pull it in. With the distributor set up like this you can run leaner cruise mixtures and get great gas mileage. I have got 22 miles per gallon on the interstate cruising at 80 to 85 MPH, which is not bad for what my engine is.
If you don't run a vacuum advance you will have lean miss problems as Motorhead has described along with extra heat. Not having the power valve come in when you need it will also cause lean part throttle problems. You can't go by idle vacuum to set up your power valve or vacuum advance, you need to watch your vacuum and AFR gauges as you drive to see when the transition needs to occur. It takes time to tune like this but it is well worth it. You will get better mileage and have less carbon build up in your engine.
I use a vacuum advance and made a limiter on it to adjust how much total timing it could add. It is also adjustable for how much vacuum it takes to pull it in. With the distributor set up like this you can run leaner cruise mixtures and get great gas mileage. I have got 22 miles per gallon on the interstate cruising at 80 to 85 MPH, which is not bad for what my engine is.
If you don't run a vacuum advance you will have lean miss problems as Motorhead has described along with extra heat. Not having the power valve come in when you need it will also cause lean part throttle problems. You can't go by idle vacuum to set up your power valve or vacuum advance, you need to watch your vacuum and AFR gauges as you drive to see when the transition needs to occur. It takes time to tune like this but it is well worth it. You will get better mileage and have less carbon build up in your engine.
Last edited by v2racing; 08-16-2011 at 03:17 AM.
08-16-2011, 10:43 AM
#5
Melting Slicks
I set mine up to idle at 13.8, cruise is at 14.2, I have the power enrichment (power valve) come in at 13.5 on the primaries and my wide open throttle is 12.8 to 1. I use a vacuum gauge to set the power valve so that it comes in when it needs fuel on the primaries during transition from cruise to light power. I needed a 10.5 power valve to do this. I had to go down from what one would normally use on the primary main jets and up on the power valve enrichment orifices, this required drilling them. I had to drill holes in the butterflies and also enlarge the idle air bleeds to get the idle and off idle where I wanted them.
I use a vacuum advance and made a limiter on it to adjust how much total timing it could add. It is also adjustable for how much vacuum it takes to pull it in. With the distributor set up like this you can run leaner cruise mixtures and get great gas mileage. I have got 22 miles per gallon on the interstate cruising at 80 to 85 MPH, which is not bad for what my engine is.
If you don't run a vacuum advance you will have lean miss problems as Motorhead has described along with extra heat. Not having the power valve come in when you need it will also cause lean part throttle problems. You can't go by idle vacuum to set up your power valve or vacuum advance, you need to watch your vacuum and AFR gauges as you drive to see when the transition needs to occur. It takes time to tune like this but it is well worth it. You will get better mileage and have less carbon build up in your engine.
I use a vacuum advance and made a limiter on it to adjust how much total timing it could add. It is also adjustable for how much vacuum it takes to pull it in. With the distributor set up like this you can run leaner cruise mixtures and get great gas mileage. I have got 22 miles per gallon on the interstate cruising at 80 to 85 MPH, which is not bad for what my engine is.
If you don't run a vacuum advance you will have lean miss problems as Motorhead has described along with extra heat. Not having the power valve come in when you need it will also cause lean part throttle problems. You can't go by idle vacuum to set up your power valve or vacuum advance, you need to watch your vacuum and AFR gauges as you drive to see when the transition needs to occur. It takes time to tune like this but it is well worth it. You will get better mileage and have less carbon build up in your engine.
I do like to run 14.5 to 15 for cruise, however, with my Quadrajet due to its better part throttle metering.
08-16-2011, 11:24 AM
#6
Race Director
Don't tune towards a number. Tune it as lean as it will go while still running good. Every engine is different and where one engine will run happily at 15:1, another will miss so bad you think it's missing some pistons.
My 454 requires high 12's at idle and no load up to 2000 rpm. But under light/moderate load I can lean it to 15:1. My theory is that the large plenum single plane intake allows cylinders to rob fuel from each other at low port velocities.
The only thing I tune to a number is WOT where mid-high 12's seem to be a safe bet for most any naturally aspirated engine.
My 454 requires high 12's at idle and no load up to 2000 rpm. But under light/moderate load I can lean it to 15:1. My theory is that the large plenum single plane intake allows cylinders to rob fuel from each other at low port velocities.
The only thing I tune to a number is WOT where mid-high 12's seem to be a safe bet for most any naturally aspirated engine.
