carb vs tbi on propane
carb vs tbi on propane
Hey guys wondering if I could get a bit of help here, I'm building a new 454 motor and want to run on propane for the most part but still have the option of running gas as well. I have this setup on my truck now and have it tuned pretty well for propane but if I switch to gas it goes a little out of wack, even when back on propane (high idle, low idle, stumbling, etc) I think the change over messes with the computer a bit. So I'm thinking of switching to a carb. I know the fuel mileage will drop when on gas but what about propane? Does a carb drop the mileage vs the tbi for propane?
Re: carb vs tbi on propane
An interesting question. During the decade of TBI use we found that the big block engines were more affected then the small blocks. This was probably due to the fuel trim changing in the OEM computer while on propane, just as you indicated. It also happened on small blocks but being a lighter duty engine they did not show the same symptoms as the big blocks when returning to gas. One important question here is are you using an O2 simulator when operating on propane? An O2 simulator would mimic the gasoline requirement for a switching O2 reading and maintain the fuel trims in a reasonably normal range.
Personally I prefer the TBI over the carb. TBI systems were more reliable then carburetors since they did not require a fuel bowl. To operate on propane on older engines the gasoline carburetor had to be run completely dry. During the propane operation it was quite common for the gaskets in the carburetor to dry out and shrink. This frequently resulted in minor fuel leaks until the gaskets swelled when returning to gas. The potential for a fire under the hood always existed. We were always careful to remind our customers they should operate their vehicle on gasoline at least once a week for ten to 15 minutes to deter this drying out and shrinkage.
The other part of your question. Which is better for economy? There are several answers to this but the most basic is this. A properly tuned and maintained carb will achieve slightly better economy then a TBI system. This is because the TBI system is controlled to operate at a fuel air ratio of 14.7 to at all areas other then WOT and during relatively rapid acceleration. At WOT these systems in open loop can reach ratio's as high as 10 or 11 to 1. Carburetors were typically adjusted to ratios of 13.5:1 at idle. Cruise and light loads would be set leaner at 15.5 to 16:1 and WOT is very similar to the TBI. An owner with reasonable driving manners and very good maintenance habits could potentially get better economy on a carb vs TBI on an identical vehicle.
That being said, the TBI usually achieved better economy over the long run due to the computer control as it constantly corrected mixtures compensating for erratic driving habits, changing weather conditions and mediocre maintenance habits.
The other issue is a TBI has higher airflow capability then a carb. Reinstalling a carb creates two venturii's to deal with. One in the carb and one in the mixer. A TBI still has a venturii but it is larger then the typical carburetor reducing the restriction while operating on propane. Your main issue here is what you stated originally. How to keep the fuel trims in a range that will allow switchovers to gasoline without the stumble or hesitation? This can be accomplished with electronics but these big blocks do love to gobble fuel so they need time to adjust. I have found that switching them over while cruising and giving the fuel trims a few minutes to recalibrate keeps the engine from stumbling and hesitating when finally stopping.
One other area to check is the EGR if you are still using it. GM vehicles are notorious for having a leaking EGR vacuum solenoid which cause the EGR to be slightly open at idle when it shouldn't be. This is easily checked by just removing the vacuum hose at the EGR and plugging it temporarily while a vehicle is idling poorly. If vacuum is holding the EGR open the idle will smooth out almost immediately. A less common issue with EGR's is a deposit build up or diaphragm failure causing a vacuum leak.
Personally I prefer the TBI over the carb. TBI systems were more reliable then carburetors since they did not require a fuel bowl. To operate on propane on older engines the gasoline carburetor had to be run completely dry. During the propane operation it was quite common for the gaskets in the carburetor to dry out and shrink. This frequently resulted in minor fuel leaks until the gaskets swelled when returning to gas. The potential for a fire under the hood always existed. We were always careful to remind our customers they should operate their vehicle on gasoline at least once a week for ten to 15 minutes to deter this drying out and shrinkage.
The other part of your question. Which is better for economy? There are several answers to this but the most basic is this. A properly tuned and maintained carb will achieve slightly better economy then a TBI system. This is because the TBI system is controlled to operate at a fuel air ratio of 14.7 to at all areas other then WOT and during relatively rapid acceleration. At WOT these systems in open loop can reach ratio's as high as 10 or 11 to 1. Carburetors were typically adjusted to ratios of 13.5:1 at idle. Cruise and light loads would be set leaner at 15.5 to 16:1 and WOT is very similar to the TBI. An owner with reasonable driving manners and very good maintenance habits could potentially get better economy on a carb vs TBI on an identical vehicle.
That being said, the TBI usually achieved better economy over the long run due to the computer control as it constantly corrected mixtures compensating for erratic driving habits, changing weather conditions and mediocre maintenance habits.
The other issue is a TBI has higher airflow capability then a carb. Reinstalling a carb creates two venturii's to deal with. One in the carb and one in the mixer. A TBI still has a venturii but it is larger then the typical carburetor reducing the restriction while operating on propane. Your main issue here is what you stated originally. How to keep the fuel trims in a range that will allow switchovers to gasoline without the stumble or hesitation? This can be accomplished with electronics but these big blocks do love to gobble fuel so they need time to adjust. I have found that switching them over while cruising and giving the fuel trims a few minutes to recalibrate keeps the engine from stumbling and hesitating when finally stopping.
One other area to check is the EGR if you are still using it. GM vehicles are notorious for having a leaking EGR vacuum solenoid which cause the EGR to be slightly open at idle when it shouldn't be. This is easily checked by just removing the vacuum hose at the EGR and plugging it temporarily while a vehicle is idling poorly. If vacuum is holding the EGR open the idle will smooth out almost immediately. A less common issue with EGR's is a deposit build up or diaphragm failure causing a vacuum leak.
Re: carb vs tbi on propane
Well that's some awesome info and is was made easy to understand. I've never had a o2 simulator, never even heard of one actually.
Does sound like the carb may be the better option. My typical drive is 15 km and that's with a few lights along the way, so i assume the carb wouldn't have time to calibrate.
If I am going on a longer haul it may only be once or twice a month and only from spring to fall.
Does sound like the carb may be the better option. My typical drive is 15 km and that's with a few lights along the way, so i assume the carb wouldn't have time to calibrate.
If I am going on a longer haul it may only be once or twice a month and only from spring to fall.
Re: carb vs tbi on propane
Just for argument sakes, if I were to unplug the 02 sensor for propane would that make a difference? I can get it running back to normal on gas eventually it just takes some messing around, it's when I go back to propane it acts up but if I disconnected the o2 on propane Mabey it wouldn't throw things off seeing as how it doesn't really use the functions on lpg. Just my thoughts.
Thanks
Thanks
Re: carb vs tbi on propane
There are three or four popular or well known ways to keep a fuel trim from going out of range. I will put them in order of the best but not necessarily the recommended way to do it.
1. Use an oxygen sensor simulator. This is a small electronic box that will send a varying voltage to the OEM ECU lying to the OEM ECU that it has control of the fuel mixtures, even though it does not. The main benefit is it is inexpensive to do, easy to install and no programming is required. The only concern is the year of the ECU you have as some were capable of recognizing a problem of the frequency of the simulator was too constant.
2. Use an electronic fuel control system on propane which will keep the fuel mixtures reasonably close to the OEM range.
3. Determine the amount of time it takes to reset the fuel trims to 0% when power is lost to the ECU. Early model ECU's could have any memory wiped out with a brief loss of power. Some companies actually used the center position of the switch to shut off power to the ECU momentarily while switching fuels. Several methods were developed to accomplish this, however be aware some countries made this modification illegal. This was because both the memory and any stored trouble codes were erased. To test for this just operate on propane for a while and then pull over at a convenient location and momentarily disconnect the battery. Reconnect and start on gasoline and see if the engine runs properly.
4. Make the ECU stay in open loop during the propane operation. This may be possible by using a relay to cause an open circuit through the O2. Some companies did this and had success with it but it is only found out through trial and error. This was used on some newer dodge models with success. I believe a better method would be to use a resistor in the circuit that would freeze the output voltage at .45 volts. The ECU would be tricked into thinking the O2 had failed while on propane and cause the ECU to open. This should not impact performance or economy on propane as the TBI models of GM vehicles controlled timing properly even if the ECU went into open loop.
All that being said I am a firm believer that the best option is the use of the electronic fuel control. Depending on your location the common devices to accomplish this are the Autotronics 8789-6M along with the Autotronics 5952 or equivalent. The 8789-6M does three things.
1. Generates a false O2 signal.
2. Generates a false knock signal for the knock sensor test when starting a cold or hot engine. Without this feature a code 43 will be set. This test takes the place of the factory diagnostic test that momentarily advances the timing until the knock sensor see's a knock confirming it is working. Due to the limited advance of the timing curve from the factory it does not reach a point where it can cause propane with its high octane rating to knock.
3. Momentarily shuts off the false O2 signal when the EGR operation is commanded by the OEM ECU so it sees a momentary lean condition. Without this feature a code 44 will be set.
The 5952 is used to control mixtures along with the 8789-6M functions. It is great for controlling fuel mixtures long term but if you are using the 8789-6M it actually has no bearing on the fuel trims. I have seen many conversions that used the 5952 without the 8789-6M beginning around 1992 or 3 model year. GM increased it's maximum timing advance in this and newer models which allowed the normal function test to reach a point where propane would knock confirming the knock sensor was working. On these models a simulator was not required if the 5952 controller was used.
The main problem. It is getting harder and harder to find the Autotronic 8789-6M since manufacturing stopped over 10 years ago. The 5952 or a similar device is still available.
Hope this information helps.
1. Use an oxygen sensor simulator. This is a small electronic box that will send a varying voltage to the OEM ECU lying to the OEM ECU that it has control of the fuel mixtures, even though it does not. The main benefit is it is inexpensive to do, easy to install and no programming is required. The only concern is the year of the ECU you have as some were capable of recognizing a problem of the frequency of the simulator was too constant.
2. Use an electronic fuel control system on propane which will keep the fuel mixtures reasonably close to the OEM range.
3. Determine the amount of time it takes to reset the fuel trims to 0% when power is lost to the ECU. Early model ECU's could have any memory wiped out with a brief loss of power. Some companies actually used the center position of the switch to shut off power to the ECU momentarily while switching fuels. Several methods were developed to accomplish this, however be aware some countries made this modification illegal. This was because both the memory and any stored trouble codes were erased. To test for this just operate on propane for a while and then pull over at a convenient location and momentarily disconnect the battery. Reconnect and start on gasoline and see if the engine runs properly.
4. Make the ECU stay in open loop during the propane operation. This may be possible by using a relay to cause an open circuit through the O2. Some companies did this and had success with it but it is only found out through trial and error. This was used on some newer dodge models with success. I believe a better method would be to use a resistor in the circuit that would freeze the output voltage at .45 volts. The ECU would be tricked into thinking the O2 had failed while on propane and cause the ECU to open. This should not impact performance or economy on propane as the TBI models of GM vehicles controlled timing properly even if the ECU went into open loop.
All that being said I am a firm believer that the best option is the use of the electronic fuel control. Depending on your location the common devices to accomplish this are the Autotronics 8789-6M along with the Autotronics 5952 or equivalent. The 8789-6M does three things.
1. Generates a false O2 signal.
2. Generates a false knock signal for the knock sensor test when starting a cold or hot engine. Without this feature a code 43 will be set. This test takes the place of the factory diagnostic test that momentarily advances the timing until the knock sensor see's a knock confirming it is working. Due to the limited advance of the timing curve from the factory it does not reach a point where it can cause propane with its high octane rating to knock.
3. Momentarily shuts off the false O2 signal when the EGR operation is commanded by the OEM ECU so it sees a momentary lean condition. Without this feature a code 44 will be set.
The 5952 is used to control mixtures along with the 8789-6M functions. It is great for controlling fuel mixtures long term but if you are using the 8789-6M it actually has no bearing on the fuel trims. I have seen many conversions that used the 5952 without the 8789-6M beginning around 1992 or 3 model year. GM increased it's maximum timing advance in this and newer models which allowed the normal function test to reach a point where propane would knock confirming the knock sensor was working. On these models a simulator was not required if the 5952 controller was used.
The main problem. It is getting harder and harder to find the Autotronic 8789-6M since manufacturing stopped over 10 years ago. The 5952 or a similar device is still available.
Hope this information helps.
Re: carb vs tbi on propane
This has been an awesome bit of info, wish I had Ask this question months ago for my current setup.
But I'm thinking I'm still gonna go with the carb just cause it will soot my application better in the long run.
I'm building up a 468 for 1988 chevy truck with a air gap manifold, mild cam, with ported peanut port heads, and a hei ignition. Im trying to get about 400hp with 500lbs of torgue on gas. I came across a good hotrod mag build with this exact combo and it seemed to work pretty good.
For the most part i will run propane just for day to day use.
Simply due to the fact I'll be using the air gap manifold, which from what I've heard that tbi doesn't Jive well with the air gap manifold and ide have to deal with chips or reprograming as well.
So as long as the mileage isn't that far off between the 2 on propane.
But thanks, ive learned a lot from this. If I have to change it back in the future, cause I don't like the set up Ive got I'll keep a lot of that in mind, I think the o2 simulator or running while in open loop could work pretty good for my application
But I'm thinking I'm still gonna go with the carb just cause it will soot my application better in the long run.
I'm building up a 468 for 1988 chevy truck with a air gap manifold, mild cam, with ported peanut port heads, and a hei ignition. Im trying to get about 400hp with 500lbs of torgue on gas. I came across a good hotrod mag build with this exact combo and it seemed to work pretty good.
For the most part i will run propane just for day to day use.
Simply due to the fact I'll be using the air gap manifold, which from what I've heard that tbi doesn't Jive well with the air gap manifold and ide have to deal with chips or reprograming as well.
So as long as the mileage isn't that far off between the 2 on propane.
But thanks, ive learned a lot from this. If I have to change it back in the future, cause I don't like the set up Ive got I'll keep a lot of that in mind, I think the o2 simulator or running while in open loop could work pretty good for my application