Setting base timing for performance cam

[Fast355]

350 injectors / stock 350 chip on a 305 would be bad! 350 injectors / stock 305 chip on a 305 would be bad! 305 injectors / stock 350 chip / stock 305 chip, would be OK, but not great because of the cam specs.

dave w

Having done this swap before, 305 injectors on this 305 would be BAD. Mine ran best on the stock 350 chip and 350 injectors. Had to correct the BPW in the chip and rework the VE table to make it run best. Also had to give it about 10* initial at the distributor.

I had a Mellings MTC1 204/214 @ .050, .420/.442" lift, 112 LSA, 107 ICL in my 1983 G20s stock 305 with a 92 350 TBI intake, TBI, exhaust manifolds and exhaust system.

The stock 305 injectors and stock chip are so lean even at 16 psi (the highest I could get the stock pump to output) it was too lean to be driveable.

[srgould41]

My cam has the exact same specs as yours. It's even out of the same year and vehicle ;)

David, any thoughts on his comment about the injectors and timing?

My Holly Blue pump is modded for 18 PSI, but I have not modded the TBI regulator.

Having done this swap before, 305 injectors on this 305 would be BAD. Mine ran best on the stock 350 chip and 350 injectors. Had to correct the BPW in the chip and rework the VE table to make it run best. Also had to give it about 10* initial at the distributor.

I had a Mellings MTC1 204/214 @ .050, .420/.442" lift, 112 LSA, 107 ICL in my 1983 G20s stock 305 with a 92 350 TBI intake, TBI, exhaust manifolds and exhaust system.

The stock 305 injectors and stock chip are so lean even at 16 psi (the highest I could get the stock pump to output) it was too lean to be driveable.

[dave w]

My cam has the exact same specs as yours. It's even out of the same year and vehicle ;)

David, any thoughts on his comment about the injectors and timing?

My Holly Blue pump is modded for 18 PSI, but I have not modded the TBI regulator.

"Had to correct the BPW in the chip and rework the VE table to make it run best."

That's the key to tuning! There are slight tuning differences (VE ~ BPW) between engines, even when the engines are identical.

I posted a Base Pulse Width (BPW) calculator here: http://www.gearhead-efi.com/Fuel-Inj...BPW-Calculator

The BPW calculator is usually accurate within +/- 2 BPW for the 1227747 for a mild performace cam, and usually dead on calculation with a stock cam.

The only accurate way to tune is with data logs and wide band O2. Anything else is an estimate of what the engine needs. The BPW calculator would support my suggestion that a 305 engine with 305 injectors programmed with the stock BPW would be a good estimate to start with. The BPW calculator would support my suggestion that a 305 with 350 injectors programmed with the stock BPW would be rich.

Without seeing what the actual corrections are for a specific engine, I would expect the BPW calculator to be "Close Enough" as a starting point for most mild cam upgrades. I've tuned engines that required small fuel pressure adjustments, and small BPW adjustments, after using the BPW calculator, but I had data to show how much change was needed.

dave w

[srgould41]

After thinking it over and over again, I agree that starting with a 305 chip and injectors is the way to start. What I end up doing later on will depend on just how it runs with the first chip. It's not a race car so I don't need every pony I can eek from it, but it does need to run well, especially at lower RPM's.

"Had to correct the BPW in the chip and rework the VE table to make it run best."

That's the key to tuning! There are slight tuning differences (VE ~ BPW) between engines, even when the engines are identical.

I posted a Base Pulse Width (BPW) calculator here: http://www.gearhead-efi.com/Fuel-Inj...BPW-Calculator

The BPW calculator is usually accurate within +/- 2 BPW for the 1227747 for a mild performace cam, and usually dead on calculation with a stock cam.

The only accurate way to tune is with data logs and wide band O2. Anything else is an estimate of what the engine needs. The BPW calculator would support my suggestion that a 305 engine with 305 injectors programmed with the stock BPW would be a good estimate to start with. The BPW calculator would support my suggestion that a 305 with 350 injectors programmed with the stock BPW would be rich.

Without seeing what the actual corrections are for a specific engine, I would expect the BPW calculator to be "Close Enough" as a starting point for most mild cam upgrades. I've tuned engines that required small fuel pressure adjustments, and small BPW adjustments, after using the BPW calculator, but I had data to show how much change was needed.

dave w

[dave w]

A few screen shots to show the "BPW Math" 305 vs 350.
Including a actual factory programmed BPW.

dave w

[Fast355]

A few screen shots to show the "BPW Math" 305 vs 350.
Including a actual factory programmed BPW.

dave w

A stock 80s 305 with that cam, a pair of long tube headers, good exhaust and good tuning makes around 240-260 hp at the flywheel, dyno'd 191 RWHP/268 RWTQ on a Mustang dyno. The stock 305 injectors will not support that amount of HP without drastically raising the pressure. The older smog heads also do not burn fuel as efficiently as the TBI heads, which further increases the fuel requirement.

The reason I suggested the 350 injectors is because they are closer to the fuel requirement of the engine in question.

I was running this engine setup when RBob introduced the EBL Classic. His out of the box LG4 calibration for his Elcamino ran my "rv" cammed 312 very well with only slight tweaks.

My prior calibration was the 92 299 ecm. AWLD and 350 injectors were far more driveable than the stock 305 calibration. The 350 calibration did not have enough TPS AE either and wanted to lean pop on quick throttle transitions. With the 305 injectors it ran so lean that the headers would glow.

This exact setup is actually what pushed me into TBI tuning to start with.

[srgould41]

I'm running rams horns since they are the only thing that fits. My truck came with an inline 6. Narrow frame rails.

Of note... Summit techs told me my 15InHg idle vacuum was not caused by the cam. Since I had no external vacuum leak they suspected an internal leak. Last weekend I pulled the intake and replaced with with Oring gaskets. Last night was the first chance I had to time it. I set it to 19* base and limited vacuum to 14* (53 total). Today I had a chance to drive it. Drives nice and has 19InHg at idle. I took it for a drive and had a long steep hill climb on the way back. It now pings bad at low RPM. It never did that before. So I'm going to have to start dialing back the timing and work on my vacuum can to limit it. Just until I am ready for the TBI.

A stock 80s 305 with that cam, a pair of long tube headers, good exhaust and good tuning makes around 240-260 hp at the flywheel, dyno'd 191 RWHP/268 RWTQ on a Mustang dyno. The stock 305 injectors will not support that amount of HP without drastically raising the pressure. The older smog heads also do not burn fuel as efficiently as the TBI heads, which further increases the fuel requirement.

The reason I suggested the 350 injectors is because they are closer to the fuel requirement of the engine in question.

I was running this engine setup when RBob introduced the EBL Classic. His out of the box LG4 calibration for his Elcamino ran my "rv" cammed 312 very well with only slight tweaks.

My prior calibration was the 92 299 ecm. AWLD and 350 injectors were far more driveable than the stock 305 calibration. The 350 calibration did not have enough TPS AE either and wanted to lean pop on quick throttle transitions. With the 305 injectors it ran so lean that the headers would glow.

This exact setup is actually what pushed me into TBI tuning to start with.