Hi all,
have experienced excessive LPG consumption for quite a while (25L/100km or 9.4miles/gallon), was running an impco 300A mixer on a snorkel feeding into a rochester quadrajet carby into a Holden 5 Litre V8 (v. similar to 307/305? chev for those not familiar with Aussie V8s) but have since swapped to an impco 425 on a freshly build throttle plate with no improvement in economy but some improvement in throttle response. the motor has mildly ported heads 9.9:1 static compression, the performance Holden dual plane manifold (v. similar to an Edelbrock Performer) recurved dizzy and a mild cam with a powerband 2200-5200 (havent got the full specs in front of me). running mandrel bent competition style extractors (big primaries) and a good quality exhaust. also running 2800rpm Hi-Stall in a manualised 3 speed Auto (a lightweight trans similar to a Turbo Hydramatic 350) and 3.08 gears in the diff.
I cant find any vacuum leaks, the gas analyser on the dyno read around the 0.8 to 1% range for the entire rev range, its got me stuffed. static timing is 10' and total timing ive been told is 26'. Another place told me that this is nowhere near enough total timing and that total timing should be around 30-34'. would this be a reason for poor mileage? or should i investigate the hi-stall converter for slipping? the car feels doughy under foot until 2500rpm, then the power comes on hard (all 170hp of it which is also a bit lower than i was expecting).
running an impco 425, with a 4" tall 14" diameter aircleaner with a hole in the bonnet covered by a rearward facing scoop. the car weighs approx 3200 lbs.
any help would be most welcome as this thing is costing me a fortune to run, I wouldnt mind if the motor was making some brutal power figures but it doesnt so i seem to have the worst of both worlds.
thanks in advance.
excessive LPG consumption
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gravespropane
- Posts: 41
- Joined: Mon Jan 30, 2006 2:28 pm
I would try bumping the timing to where you have no more than 34-36 total(with more emphasis on the 34 end). If you have a vacuum advance on the distributor I would try hooking it to a manifold source(if its adjustable try to shoot for around ten degrees). That should help with the fuel a little, and make sure you have the right air valve assembly. The number 19 should be on the bottom of the little cone. What vaporizer do you have? If you have a model e try using the blue or black spring and not the orange.
Another thing to check is where is your main fuel adjustment at? ( the 9/16" head bolt). Make sure it isn't screwed all the way out( for your engine about half way should be good, Franz should be able to help out on this adjustment)
I think your high stall may be causing the air valve to be running above the "cruise" step on the cone. The only other suggestion I would have is if possible maybe consider getting a trans with a lockup converter. That way your car would launch really well and would be a little more highway friendly. If not the timing should help a little.
John
Another thing to check is where is your main fuel adjustment at? ( the 9/16" head bolt). Make sure it isn't screwed all the way out( for your engine about half way should be good, Franz should be able to help out on this adjustment)
I think your high stall may be causing the air valve to be running above the "cruise" step on the cone. The only other suggestion I would have is if possible maybe consider getting a trans with a lockup converter. That way your car would launch really well and would be a little more highway friendly. If not the timing should help a little.
John
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Imperial73
- Posts: 303
- Joined: Sat Jun 03, 2006 3:59 pm
- Location: Netherlands
- Contact:
Why hook up a vacuum advance to a manifold vacuum source? This would mean that you have maximum vacuum advance at idle, while it is intended for part throttle operation.gravespropane wrote:If you have a vacuum advance on the distributor I would try hooking it to a manifold source(if its adjustable try to shoot for around ten degrees).
I've been experimenting the timing on my 350 Chevrolet engine and I've got my timing set to the following right now:
I found that my fuel economy dropped off very quickly from 5 turns to 5.5 turns so fuel economy seems to be very sensitive to the amount of vacuum advance. I expect the optimum advance for my engine to be somewhere between 4.5 and 5.0 turns.
- 10° initial
- 28° total mechanical
- 43° total timing (mechanical + vacuum)
I found that my fuel economy dropped off very quickly from 5 turns to 5.5 turns so fuel economy seems to be very sensitive to the amount of vacuum advance. I expect the optimum advance for my engine to be somewhere between 4.5 and 5.0 turns.
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darrin1999
- Posts: 18
- Joined: Sat Dec 02, 2006 5:18 am
yep id say its the stall too... youd be better off with a little more compression and a 2200 rpm stall, and try some smaller headers... a 5.0 l shouldnt need more than 1 5/8" primaries... anything more and youll lose low end torque. the stall converter is the reason your car feels "doughy" below 2500 rpm... if you want low end power put a slightly smaller cam in and the stock converter
firstly thanks for all yoru replies, its given me some food for thought, heres where i'm at with it at the moment.
I spoke to a couple of people today and this is what they all said RE ignition timing;
the gas mob that tuned it;.
says 10' static and 26' total timing is enough and to go for ported vacuum for the vac advance hook up (so no vac advance at idle). and not to try for anymore timing because its sucking in hot air from under the bonnet (car
the engine builder;
says 26' is bullshit and that they usually go for 32'-34' of total timing ie try advancing the static timing to 12'-14' to bring the total timing up to 30' total and drive it to see how it responds underfoot (12'-14' was marked on the dizzy as the static timing from the blokes that built it and they had full details on fuel, converter, cam selection, compression etc when building it) while the i should be running. I havent had the opportunity to ask him about ported vs direct manifold vacuum yet.
the distributor builder;
says 30' would be good but dont just keep advancing until it pings and then back it off a touch. and to run ported vacuum for the advance because the cam will cause erratic vacuum resulting in a possible backfire and or stalling as the dizzy responds to the fluctuating vacuum.
a technical paper "TIMING AND VACUUM ADVANCE 101" by John Hinkley:
has this to say on ported vacuum.
"Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it."
and this regarding the fluctuating vacuum signal from performance camshafts
"For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it, they don't understand it, they're on commission, and they want to sell "race car" parts."
I have only quoted relevant parts of that very informative timing article as I havent sought permission to reproduce it all but have at least credited the Author.
Thanks for your suggestions regarding the converter, this is something i'm a bit wary of as I've had a 2800 hi-stall (which i fitted with no problems) in my previous avgas sucking daily driver and had no problems getting up on the converter to 2800rpm before the brakes wouldnt hold the car and i'd have to launch it, this one however will only let me get up to 1700rpm before the brakes will no longer hold the car in stage and the launch is a little doughy.
I'm getting suspicious of the stally and when i get the time i'm thinking of going back to running a standard converter again if my cam will allow it and keeping that converter for the 355 that will go in a 2800lb car i have waiting till i'm happy with the daily.
I spoke to a couple of people today and this is what they all said RE ignition timing;
the gas mob that tuned it;.
says 10' static and 26' total timing is enough and to go for ported vacuum for the vac advance hook up (so no vac advance at idle). and not to try for anymore timing because its sucking in hot air from under the bonnet (car
the engine builder;
says 26' is bullshit and that they usually go for 32'-34' of total timing ie try advancing the static timing to 12'-14' to bring the total timing up to 30' total and drive it to see how it responds underfoot (12'-14' was marked on the dizzy as the static timing from the blokes that built it and they had full details on fuel, converter, cam selection, compression etc when building it) while the i should be running. I havent had the opportunity to ask him about ported vs direct manifold vacuum yet.
the distributor builder;
says 30' would be good but dont just keep advancing until it pings and then back it off a touch. and to run ported vacuum for the advance because the cam will cause erratic vacuum resulting in a possible backfire and or stalling as the dizzy responds to the fluctuating vacuum.
a technical paper "TIMING AND VACUUM ADVANCE 101" by John Hinkley:
has this to say on ported vacuum.
"Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it."
and this regarding the fluctuating vacuum signal from performance camshafts
"For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it, they don't understand it, they're on commission, and they want to sell "race car" parts."
I have only quoted relevant parts of that very informative timing article as I havent sought permission to reproduce it all but have at least credited the Author.
Thanks for your suggestions regarding the converter, this is something i'm a bit wary of as I've had a 2800 hi-stall (which i fitted with no problems) in my previous avgas sucking daily driver and had no problems getting up on the converter to 2800rpm before the brakes wouldnt hold the car and i'd have to launch it, this one however will only let me get up to 1700rpm before the brakes will no longer hold the car in stage and the launch is a little doughy.
I'm getting suspicious of the stally and when i get the time i'm thinking of going back to running a standard converter again if my cam will allow it and keeping that converter for the 355 that will go in a 2800lb car i have waiting till i'm happy with the daily.
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gravespropane
- Posts: 41
- Joined: Mon Jan 30, 2006 2:28 pm
In my experience I have had better luck with manifold vacuum than port. Everyone has their own opinion on this. I suggest trying both and see what works best for you.
Another thing you might want to try is setting the timing with a vacuum gauge. Advance the timing until you get the best vacuum at the lowest possible idle. Retard the timing until you get a 1/2" drop in mercury. This is where some of my hot rod stuff likes to run. Make sure you don't exceed 34 total and no more than 14 initial. If your distributor is curved right you should be ok. This does not work for every engine but it might help you get a little more out of the fuel.
I have a couple of friends here that have overdrives with lockup converters that make the cars much more streetable. In fact one friend drives his car 130 miles to the dragstrip. Puts on slicks when he gets there. ( I have seen the car run 11's with a small block). After flogging it he then puts the street tires back on and drives it home. With the lockup he gets 20+ miles per gallon on the highway.
Good luck and keep us posted
John
Another thing you might want to try is setting the timing with a vacuum gauge. Advance the timing until you get the best vacuum at the lowest possible idle. Retard the timing until you get a 1/2" drop in mercury. This is where some of my hot rod stuff likes to run. Make sure you don't exceed 34 total and no more than 14 initial. If your distributor is curved right you should be ok. This does not work for every engine but it might help you get a little more out of the fuel.
I have a couple of friends here that have overdrives with lockup converters that make the cars much more streetable. In fact one friend drives his car 130 miles to the dragstrip. Puts on slicks when he gets there. ( I have seen the car run 11's with a small block). After flogging it he then puts the street tires back on and drives it home. With the lockup he gets 20+ miles per gallon on the highway.
Good luck and keep us posted
John
sorted,
went to a different dyno place (had to pay for another tune which sucked but at least its right now) they ran up more advance and leaned off the mixture a bit. runs like a dream now, starts easier, really howls and gets up and go like it should now when I stick the foot into it. i ran it interstate recently and returned 19L/100km on the highway with a lot of driving between 100 & 130kmh and smooth. she's slightly less forgiving in the city but a lot more responsive under foot now so i dont have to stick the boot into it to get what wold be considered normal acceleration.
i think the problem is that the first dyno place i went to sell their own gas carburetion, good as it is its too expensive for my daily driver. they tuned it and said "this is the best you'll get out of that induction setup, you really need OUR setup on that motor to get the most out of it" meanwhile i noted no improvement in ecomony or power.
the second tuner confirmed for me what the distributor builder said and also gave me a print out of the power/torque curves,
I'm finally getting enjoyment out of that motor.
went to a different dyno place (had to pay for another tune which sucked but at least its right now) they ran up more advance and leaned off the mixture a bit. runs like a dream now, starts easier, really howls and gets up and go like it should now when I stick the foot into it. i ran it interstate recently and returned 19L/100km on the highway with a lot of driving between 100 & 130kmh and smooth. she's slightly less forgiving in the city but a lot more responsive under foot now so i dont have to stick the boot into it to get what wold be considered normal acceleration.
i think the problem is that the first dyno place i went to sell their own gas carburetion, good as it is its too expensive for my daily driver. they tuned it and said "this is the best you'll get out of that induction setup, you really need OUR setup on that motor to get the most out of it" meanwhile i noted no improvement in ecomony or power.
the second tuner confirmed for me what the distributor builder said and also gave me a print out of the power/torque curves,
I'm finally getting enjoyment out of that motor.