Setting base timing for performance cam

I am still gathering what I need for parts and information to swap a TBI into my carbed engine. I have an '83 305 with a mild Summit K-1102 cam (.420/.442, 262/272, 112*), stiffer springs, Edelbrock non-egr performer intake. Using an HEI distributor I have found that I need 20* base timing for best performance (no detination). At the factory 8* the engine will barely run. Summit recommends at least 4* more timing for this cam.

I know the computer controller dizzy is supposed to be set at 0*, but I wonder if that will work ok with my engine. Is it possible or even advisable to add more base timing to make up for non-stock cams?

Last question (for now :): Will 18inHg vacuum at idle work with the stock computer or am I going to have to get a custom chip or self learning computer?

I'm located south of you in Vancouver WA. I usually start with the factory TBI settings, and make adjustments from those settings. I've installed TBI conversions with a cam spec similar to the one posted above. The factory TBI settings will work, but ultimately a custom chip will deliver the performance the cam is capable of doing. I can, or other board members here can, program a custom chip for you. If you have the budget for a self learning computer, that is a good option. Most of us board members here take pride in our hard learned abilities to save on the budget and do our own chip. Sometimes it's a matter of time vs. money, the self learning computer can save time!

dave w

Dave W,

Thanks for the reply. All good to learn. My wife and I differ on what my budget is, but it is rather small at the moment :)

I will end up needing a custom chip to remove the EGR at a minimum. It's good to know a stock chip will let it run, even if not optimally. Now to actually find one... I have a 350 PROM in my computer at the moment.

One of the important parameters (fuel) of the 305 and 350 chips will have the same setting. The important thing to do is make sure the 305 has injectors from a 305 if using a 350 chip for a 305 engine. Do you happen to know what the injector numbers are in your throttle body? Post back the injector numbers if you have, then we can figure out if you have 305 injectors. Usually the first 3 numbers on the injector are "523". Maybe you can get your engine running with what you have on hand?

dave w

I will see what it there, but the throttle body and computer I bought are off a 350. The computer is a 7747. I have not opened it up to see what's inside it yet.

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

If you need 20* of base timing to run right, you have some mechanical issue.

Either the outer ring has slipped on the balancer, or you have the wrong pointer. The engine should run and idle with 0* timing, maybe not real well, but should run.

I'm trying to think of anything being mechanically out internally of the engine that could cause this, and I'm not coming up with anything off the top of my head right now.

Sounds like that was with HEI dist on carb engine... he's still gathering parts for TBI.

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Originally Posted by EagleMark

Sounds like that was with HEI dist on carb engine... he's still gathering parts for TBI.

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When's the last time you worked on ANY engine that required 20* of base timing to run. ;)

Even when I've worked on carbed engines, the most base timing I would use was around 12*, but would run fine all the way down to 4*, 0* on some.

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Originally Posted by Six_Shooter

When's the last time you worked on ANY engine that required 20* of base timing to run. ;)

Even when I've worked on carbed engines, the most base timing I would use was around 12*, but would run fine all the way down to 4*, 0* on some.

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Some HEI without vacuum only add 12 degrees through centrifugal advance and depending on weights none at idle. So 20+ is not uncommon on carb with a big cam engine.

TBI base I always use 0 because it effects injector timing, but in the cal usually 23 for a good idle... it has to drop off idle...

I've worked with a lot of HEI and have NEVER needed more than 10* base.

What vehicles only add 12* of total timing?

Aftermarket without vacuum...

Thanks for all the replies.

It is quite common to run more timing on a carbed SBC. Granted 20* initial is about maxed. I am honestly surprised that I don't get detination with no EGR. The engine is a low compression L08. The book says 8*, but the cam I installed said to add 4* which makes 12. When I ran 12 the engine did run, but ran very rich, and yes it would still run down to 0* and below, but very badly. At 12* I could not lean it out enough to run well. The plugs were covered in unburnt fuel. Thinking that my balancer had slipped I degreed the crank and verified that my marks are 100% on the money.


I added a spring kit and vacum kit to the dizzy. Experimentation led me to use 20* base. I could give it even more initial using the high vacuum method, but that gave too much at the top end. I am running about 52* total timing and the engine loves it. Like the old saying goes, give the engine what it wants, not what you think it needs.

Computer: 7747 with AFBZ 3557 prom

Injectors: GM5235206 RPD - 55 lbs/hr 350 injectors, yes?

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Originally Posted by srgould41

Computer: 7747 with AFBZ 3557 prom

Injectors: GM5235206 RPD - 55 lbs/hr 350 injectors, yes?

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I agree, 350 PROM and 350 Injectors. Send me a private message if you would like a quote for a custom chip.

dave w

Never mind. Found what I was looking for. Now to find some injectors.

What would be the part number for 305 injectors?

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Originally Posted by srgould41

What would be the part number for 305 injectors?

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5235279

http://www.gearhead-efi.com/Fuel-Inj...d-part-numbers

Interesting information on the most recent set of 350 injectors (5235206) I sent out for service.

dave w

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Originally Posted by srgould41

Now to find some injectors.

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I've got some used 305 injectors. Want to trade your 350 injectors for a set of 305 injectors?

dave w

Absolutely! I don't know the state of mine. I pulled one out last night to get the numbers off it. They are a little rusty on the bodies.

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Originally Posted by srgould41

I am running about 52* total timing and the engine loves it. Like the old saying goes, give the engine what it wants, not what you think it needs.

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You sure? Are you super rich? Something is wrong? 50* is nitro top fuel timing and their spark plugs melt before the end of 1/4 mile but magneto will spark to anything at that point...

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Originally Posted by dave w

5235279

http://www.gearhead-efi.com/Fuel-Inj...d-part-numbers

Interesting information on the most recent set of 350 injectors (5235206) I sent out for service.

dave w

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Although his machine does excellent service on TBI injectors, the flow is not all that accurate at TBI pressures. It's not designed for the low pressure.

I have no rich running trouble now. That's for sure. I even added around 2 turns out on the mixture screws. Note that my fuel economy is about the same. So I gained very noticable power for no loss in economy.

I did all the tuning after weeks of research and discussions on other forums, lots of trial and error, lots of testing and hard pulls, etc.

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Originally Posted by EagleMark

You sure? Are you super rich? Something is wrong? 50* is nitro top fuel timing and their spark plugs melt before the end of 1/4 mile but magneto will spark to anything at that point...

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Originally Posted by dave w

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

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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.

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Originally Posted by EagleMark

You sure? Are you super rich? Something is wrong? 50* is nitro top fuel timing and their spark plugs melt before the end of 1/4 mile but magneto will spark to anything at that point...

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I am going to say his dampner has either slipped or he has the wrong one for his timing cover.

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Originally Posted by EagleMark

Some HEI without vacuum only add 12 degrees through centrifugal advance and depending on weights none at idle. So 20+ is not uncommon on carb with a big cam engine.

TBI base I always use 0 because it effects injector timing, but in the cal usually 23 for a good idle... it has to drop off idle...

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I always used 6-14* on my setups, especially with a CAM. Makes it easier to start and also brings the fuel delivery in earlier to match the longer duration cams. It also makes the engine run better if the EST should ever go into limp home mode, disabling the ECM controlled advance.

My damper has not slipped. I used a piston stop to degree it. The timing marks line up correctly.

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.

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Originally Posted by Fast355

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.

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It personally makes sense to me being a carb guy. Yes, I'm old ;)

I have proven that my engine likes the spark advance to by high.

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Originally Posted by Fast355

I always used 6-14* on my setups, especially with a CAM. Makes it easier to start and also brings the fuel delivery in earlier to match the longer duration cams. It also makes the engine run better if the EST should ever go into limp home mode, disabling the ECM controlled advance.

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Originally Posted by 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.

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"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

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.

Quote:

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Originally Posted by dave w

"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

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

dave w

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Originally Posted by dave w

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

dave w

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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.

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.

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Originally Posted by Fast355

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.

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