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Looking for a Specific Distributor
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Rick60
Senior Member
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Hi Jeff, two questions.
Please explain how comes (technically speaking) that 'engine rpms will be lower for any given driving speed' (as stated on your website) by simply recurving a distributor?
Did you know that the Lucas distributor database is not correct in all its statements? I've found a lot of small errors that have crept in. I don't want to depreciate Marcel's excellent work, but you have to take it with a pinch of salt.
Please explain how comes (technically speaking) that 'engine rpms will be lower for any given driving speed' (as stated on your website) by simply recurving a distributor?
Did you know that the Lucas distributor database is not correct in all its statements? I've found a lot of small errors that have crept in. I don't want to depreciate Marcel's excellent work, but you have to take it with a pinch of salt.
JeffS
Jedi Trainee
Offline
Eric,
If your ignition timing is truly correct throughout the advance range, the engine will produce slightly more horsepower at that rpm, allowing you to cruise at a slightly lower rpm with no hp loss. Its not uncommon to gain an extra 10 hp in the advance range with a proper recurve. This would be noticed at cruising speeds below 3000 rpms, before the full mechanical advance comes in.
I have also had comments from clients who claim the ability to rev to higher rpms than they could with their old distributor. The difference of having a Pertronix system or good points alone can help with that. along with the correct amount of total timing!
Yes, I have noticed that Marcell's advance curves tend to jump around a bit. A few of them are a bit off. A few are listed at distributor degrees but engine rpms, and those types of mix-ups. It is however, a handy chart for the average guy to "see what he's got."
Jeff
If your ignition timing is truly correct throughout the advance range, the engine will produce slightly more horsepower at that rpm, allowing you to cruise at a slightly lower rpm with no hp loss. Its not uncommon to gain an extra 10 hp in the advance range with a proper recurve. This would be noticed at cruising speeds below 3000 rpms, before the full mechanical advance comes in.
I have also had comments from clients who claim the ability to rev to higher rpms than they could with their old distributor. The difference of having a Pertronix system or good points alone can help with that. along with the correct amount of total timing!
Yes, I have noticed that Marcell's advance curves tend to jump around a bit. A few of them are a bit off. A few are listed at distributor degrees but engine rpms, and those types of mix-ups. It is however, a handy chart for the average guy to "see what he's got."
Jeff
Rob_Edwards
Jedi Hopeful
Offline
Without changing the gearing, how would the road speed/rpm relationship change? I could see that you could cruise with less throttle opening (and more vacuum) at a given road speed and engine rpm, but I don't see how the engine speed could change.
Peter_Rogers
Jedi Hopeful
Offline
It cannot. the crankshaft spped ( the RPM's) are transferred via the gearbox to the differential to the road wheels. If neither the gear ratios nor the differential ratio nor the diameter of the road wheels are changed the relationship is a constant. From memory in 4th gear in a B for every 1000 rpm the engine turns it translates to around 18 mph. Thats it - its a mechanical sequence.
JeffS
Jedi Trainee
Offline
If the amount of horsepower an engine created were a constant, your theory of a "fixed" gear ratio and road speed makes perfect sense, but you are dropping an important variable. You are making an assumption that the horsepower to pound ratio is constant, which is what helps give you your 18mph per 1000 rpm figure.
Have you never driven a poorly tuned B that had to rev much higher to achieve the same speed aas a well tuned B? Or any car for that matter? Improving volumetric efficiency and properly tuning the ignition system CAN reduce the rpms at which your car will operate, especially "before the full advance comes in." The number of engine revolutions it takes to develop enough power to push your car at 30 mph is relative to the amount of power your engine will produce. If an engine produces 50 hp at 2000 rpms in second gear it will need to rev higher to hold road speed than if it produced 70 hp at 2000 rpm in second gear. This is much more noticable half way through second or third gear than it would be at 4000 rpms on the freeway where the timing is essentially fixed. I understand that gearing is a constant, but the torque and horsepower is not and it DOES affect the engine rpm, especially at lower speeds while the engine is operating in the rpm range of the advance curve.
I once had a Chevy pickup truck with a 454 (for hauling parts and tools.) I installed a "direct linkage" to the 4bbl carb rather than a progressive linkage. It dropped the engine's efficiency enough that the truck then redlined at 65 mph in the final drive gear, rather than somewhere around 100 mph (theoretically speaking.) All from changing a 1" long piece of linkage and NO transmission, gear, or tire diameter changes. /ubbthreads/images/graemlins/grouphug.gif
Jeff
Have you never driven a poorly tuned B that had to rev much higher to achieve the same speed aas a well tuned B? Or any car for that matter? Improving volumetric efficiency and properly tuning the ignition system CAN reduce the rpms at which your car will operate, especially "before the full advance comes in." The number of engine revolutions it takes to develop enough power to push your car at 30 mph is relative to the amount of power your engine will produce. If an engine produces 50 hp at 2000 rpms in second gear it will need to rev higher to hold road speed than if it produced 70 hp at 2000 rpm in second gear. This is much more noticable half way through second or third gear than it would be at 4000 rpms on the freeway where the timing is essentially fixed. I understand that gearing is a constant, but the torque and horsepower is not and it DOES affect the engine rpm, especially at lower speeds while the engine is operating in the rpm range of the advance curve.
I once had a Chevy pickup truck with a 454 (for hauling parts and tools.) I installed a "direct linkage" to the 4bbl carb rather than a progressive linkage. It dropped the engine's efficiency enough that the truck then redlined at 65 mph in the final drive gear, rather than somewhere around 100 mph (theoretically speaking.) All from changing a 1" long piece of linkage and NO transmission, gear, or tire diameter changes. /ubbthreads/images/graemlins/grouphug.gif
Jeff
Rob_Edwards
Jedi Hopeful
Offline
[ QUOTE ]
Have you never driven a poorly tuned B that had to rev much higher to achieve the same speed aas a well tuned B? Or any car for that matter?
[/ QUOTE ]
No, I haven't. I've driven cars that had more trouble reaching speed than others, but engine speed at road speed remained fixed. It may have SOUNDED faster, but only because the engine was working harder. It was spinning no faster, though.
[ QUOTE ]
Improving volumetric efficiency and properly tuning the ignition system CAN reduce the rpms at which your car will operate, especially "before the full advance comes in." The number of engine revolutions it takes to develop enough power to push your car at 30 mph is relative to the amount of power your engine will produce. If an engine produces 50 hp at 2000 rpms in second gear it will need to rev higher to hold road speed than if it produced 70 hp at 2000 rpm in second gear.
[/ QUOTE ]
No, it won't. If it takes 50 hp to push the car through the air at some speed (say 70 mph) and the engine can only make 50 hp at 2000 rpm, then at 70 mph the engine will be at 2000 rpm, and the throttle will wide open (the engine will be maxed out, making its 50 hp). If the engine can make 70 hp at 2000, then the throttle will be partially closed, but the engine will STILL be making only 50 hp (because in this example that's all you need to maintain road speed), and the rpm will still be 2000. The mechanical linkage betwixt the crankshaft and the rear wheels has not changed, so no change in engine speed occurs.
HTH!
Have you never driven a poorly tuned B that had to rev much higher to achieve the same speed aas a well tuned B? Or any car for that matter?
[/ QUOTE ]
No, I haven't. I've driven cars that had more trouble reaching speed than others, but engine speed at road speed remained fixed. It may have SOUNDED faster, but only because the engine was working harder. It was spinning no faster, though.
[ QUOTE ]
Improving volumetric efficiency and properly tuning the ignition system CAN reduce the rpms at which your car will operate, especially "before the full advance comes in." The number of engine revolutions it takes to develop enough power to push your car at 30 mph is relative to the amount of power your engine will produce. If an engine produces 50 hp at 2000 rpms in second gear it will need to rev higher to hold road speed than if it produced 70 hp at 2000 rpm in second gear.
[/ QUOTE ]
No, it won't. If it takes 50 hp to push the car through the air at some speed (say 70 mph) and the engine can only make 50 hp at 2000 rpm, then at 70 mph the engine will be at 2000 rpm, and the throttle will wide open (the engine will be maxed out, making its 50 hp). If the engine can make 70 hp at 2000, then the throttle will be partially closed, but the engine will STILL be making only 50 hp (because in this example that's all you need to maintain road speed), and the rpm will still be 2000. The mechanical linkage betwixt the crankshaft and the rear wheels has not changed, so no change in engine speed occurs.
HTH!
kennypinkerton
Yoda
Offline
Leave horsepower out of the figure all together. If the engine can rev at different rpms and the car is not going any faster or slower (and is in the same gear), then you either have an automatic transmission, or a worn out clutch.
Peter_Rogers
Jedi Hopeful
Offline
You hit it Kenny thats the only way it can possibly happen. Otherwise you have a mechanical linkage and by definition there can be NO variation Period. End of chat.
JeffS
Jedi Trainee
Offline
Hmm... I've experienced this in several different cars. What was happening to change the cruise rpm? We're talking about cars and trucks with known good clutches and NO apparent slipping, and it happened extremely consistently! What you are saying makes perfect logical sense, but my hands-on experience tells me different.
kennypinkerton
Yoda
Offline
Funky Tach?
Peter_Rogers
Jedi Hopeful
Offline
Kenny
We are wasting our computer time. This guy has absolutely no idea of what he is talking about and he is supposed to be a Dizzy rebuilder? Don't thinl I will be palcing too many orders with him.
We are wasting our computer time. This guy has absolutely no idea of what he is talking about and he is supposed to be a Dizzy rebuilder? Don't thinl I will be palcing too many orders with him.
kennypinkerton
Yoda
Offline
I sortof disagree... I know how transmissions and drivelines work (a basic understanding anyway, as I've built a couple), but that doesn't say anything for his potential knowledge of dizzy's. I can't say as I have a full grasping of advance curves and such. But in a manual drivetrain with a good non-slipping clutch... the engine revs are tied directly to the speed of the rear wheels, and therefore the speed of the car in any given gear ratio.
Rick60
Senior Member
Offline
[ QUOTE ]
... The number of engine revolutions it takes to develop enough power to push your car at 30 mph is relative to the amount of power your engine will produce. If an engine produces 50 hp at 2000 rpms in second gear it will need to rev higher to hold road speed than if it produced 70 hp at 2000 rpm in second gear. This is much more noticable half way through second or third gear than it would be at 4000 rpms on the freeway where the timing is essentially fixed. ...
[/ QUOTE ]
Jeff, first it is irrelevant which gear is selected for the "rpm/hp view". Engines produce hp at certain rpms, not related to the gears. Or does a running engine with no gearbox attached produce no hp?
Secondly, a car drives a constant speed @2,000rpm (without changing gears), regardless whether it produces 50hp or 70hp or whatsoever hp. I think you mean a proper tuned engine with a recurved dizzy CAN produce more hp. I'd agree on that.
There are even more errors on the Lucas distributor data page. Marcel did not only mix up dizzy and crank degrees, he's mixed up rotating direction on nearly every second dizzy. I found this page about 2 years ago and everything seemed to be fine. Then he made "amendments" and "extensions" to the database and many things got scrambled. I emailed Marcel and he told me that he will get things right whenever he's got enough time to do it. In my opinion you can't use the database right now because you can't be sure of a single distributor's data. Nevertheless, I appreciate Marcel's knowledge and effort he's put in his site.
... The number of engine revolutions it takes to develop enough power to push your car at 30 mph is relative to the amount of power your engine will produce. If an engine produces 50 hp at 2000 rpms in second gear it will need to rev higher to hold road speed than if it produced 70 hp at 2000 rpm in second gear. This is much more noticable half way through second or third gear than it would be at 4000 rpms on the freeway where the timing is essentially fixed. ...
[/ QUOTE ]
Jeff, first it is irrelevant which gear is selected for the "rpm/hp view". Engines produce hp at certain rpms, not related to the gears. Or does a running engine with no gearbox attached produce no hp?
Secondly, a car drives a constant speed @2,000rpm (without changing gears), regardless whether it produces 50hp or 70hp or whatsoever hp. I think you mean a proper tuned engine with a recurved dizzy CAN produce more hp. I'd agree on that.
There are even more errors on the Lucas distributor data page. Marcel did not only mix up dizzy and crank degrees, he's mixed up rotating direction on nearly every second dizzy. I found this page about 2 years ago and everything seemed to be fine. Then he made "amendments" and "extensions" to the database and many things got scrambled. I emailed Marcel and he told me that he will get things right whenever he's got enough time to do it. In my opinion you can't use the database right now because you can't be sure of a single distributor's data. Nevertheless, I appreciate Marcel's knowledge and effort he's put in his site.
Nunyas
Yoda
Offline
The only thing that can change your Speed vs RPM ralation on a MGB is a change in gear at the transmission. Changing tire sizes and changing your differential gear will not change your indicated speed at a given RPM. The reason it won't change your indicated speed in relation to the RPMs of the engine is because the speedometer is taking it's speed readings off the transmission out put. So, any change in gear ratios AFTER the transmission will result in ACTUAL speed errors, but the relation between the Tach and Speedo will remain the same.
It's been my experience that my Tach says 3000 RPMs at 50MPH, no matter what size tires I have on the car. The tires on the car when I bought it were 185/70, and replaced with 185/65. Naturally, at any given indicated speed, I'm actually traveling a little slower with the smaller tires, but the Tach/Speedo ratio has remained the same.
The only way to change the Tach/Speedo indication ratio is to change out gears in the gear box, or change the gear used in the Speedo cable.
It's been my experience that my Tach says 3000 RPMs at 50MPH, no matter what size tires I have on the car. The tires on the car when I bought it were 185/70, and replaced with 185/65. Naturally, at any given indicated speed, I'm actually traveling a little slower with the smaller tires, but the Tach/Speedo ratio has remained the same.
The only way to change the Tach/Speedo indication ratio is to change out gears in the gear box, or change the gear used in the Speedo cable.
Offline
[ QUOTE ]
I've taken my beating. Enough said.
[/ QUOTE ]
In Jeff's defense, I can say from personal experience that he is very knowledgeable about distributors, has a very good stock of obsolete parts, & knows how to use the equipment.
I don't know what caused the previous misunderstanding, but he does know how to make distributors work well & at a reasonable cost. He supplied me with an very obsolete DM2 type distributor body, upgraded with the later DM25 advance mechanism, advance curve set to my requested specs, & absolutely no slop in the mating parts.
D
I've taken my beating. Enough said.
[/ QUOTE ]
In Jeff's defense, I can say from personal experience that he is very knowledgeable about distributors, has a very good stock of obsolete parts, & knows how to use the equipment.
I don't know what caused the previous misunderstanding, but he does know how to make distributors work well & at a reasonable cost. He supplied me with an very obsolete DM2 type distributor body, upgraded with the later DM25 advance mechanism, advance curve set to my requested specs, & absolutely no slop in the mating parts.
D