(OTHER) dynoed the fb, big letdown
05-13-06, 08:51 PM
#76
Old [Sch|F]ool
Originally Posted by trochoid
I understand manifold vacuum too well. When I hooked up the vacuum gauge to the rear intake port on the bp, at idle, the needle vibrated so bad it was just a blur. This was due to the overlap, and the overlap made worse by still having part of the stock exhaust system. I recently completed a full header back 3" system and the needle has settled down a bit.
I'll have to put it back together now and find a sidedraft or downdraft to pop onto it.
Since I'm running the Mikuni, I'll have to check if it even has a port I can use. I have the vacuum advance unhooked.
I think I found a ported advance nipple on aussiesmg's Mikuni, just "above" one of the throttle plates on the topside of the carb. Only a vacuum gauge can tell you for sure.
05-13-06, 08:52 PM
#77
Thanks for the compliment. Somedays I can be pretty boneheaded stupid too. There are many on the forum that are very well versed and highly intelligent, you just have to read between the lines occasionally.
I suspect Robert is right with the smaller chokes, it should increase intake velocity, unless Purple82 disagrees with me, then I would listen to him. LOL.
I suspect Robert is right with the smaller chokes, it should increase intake velocity, unless Purple82 disagrees with me, then I would listen to him. LOL.
05-13-06, 08:54 PM
#79
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ive concluded that my car just needs mass tuning, or the fact that its got 175k on it and its an original motor??? i dont know, its strong as hell.
05-13-06, 09:01 PM
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From everything I've read in regards to traditional piston motors, it's more beneficial to run manifold vacuum. Is it different for rotors?
Kev
From a GM Engineer in regards to HEI ignitions.....
As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.
TIMING AND VACUUM ADVANCE 101
The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.
The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).
When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.
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.
If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.
What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.
Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.
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.
Kev
From a GM Engineer in regards to HEI ignitions.....
As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.
TIMING AND VACUUM ADVANCE 101
The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.
The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).
When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.
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.
If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.
What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.
Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.
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.
05-13-06, 09:13 PM
#81
Originally Posted by peejay
Err.. wow. Thinking more and more that my plenum manifold was a bad idea. The needle was more or less rock steady at idle, forget the exact numbers but it was something like 4" at 1500, 6" at 1800. Maybe that's why it didn't brap at idle, just rumbled I'll have to put it back together now and find a sidedraft or downdraft to pop onto it.
I dunno how much good it'll do you. On the PP as soon as you opened the throttle vacuum died away so ported vacuum was next to useless. There might have been some vacuum at just-barely-open-enough-to-uncover-the-port but there was no way to cruise at that low a load, had to be at least 1/4 throttle to cruise. I *did* hook it up to manifold vacuum for a while (engines with huge cams seem to tolerate manifold vacuum for the advance) and it didn't seem to help or hurt anything, other than the idle hanging up at 2100 and having to pop the clutch slightly at a light to bring it back down to 1500.
I think I found a ported advance nipple on aussiesmg's Mikuni, just "above" one of the throttle plates on the topside of the carb. Only a vacuum gauge can tell you for sure.
I'll have to put it back together now and find a sidedraft or downdraft to pop onto it.I dunno how much good it'll do you. On the PP as soon as you opened the throttle vacuum died away so ported vacuum was next to useless. There might have been some vacuum at just-barely-open-enough-to-uncover-the-port but there was no way to cruise at that low a load, had to be at least 1/4 throttle to cruise. I *did* hook it up to manifold vacuum for a while (engines with huge cams seem to tolerate manifold vacuum for the advance) and it didn't seem to help or hurt anything, other than the idle hanging up at 2100 and having to pop the clutch slightly at a light to bring it back down to 1500.
I think I found a ported advance nipple on aussiesmg's Mikuni, just "above" one of the throttle plates on the topside of the carb. Only a vacuum gauge can tell you for sure.
When I was first setting up the engine, with the restrictive exhaust, idle at 800 and the fuel pressure a little too high, I could literally see the fuel charge come out of one air horn and get sucked into the other.
You may be right on the vac source on the Mikuni, I'll dig out my manual.
I have never seen a PP in person, and it's been a while since I've looked at the port timing differences between a BP and PP, but iirc the PP doesn't have as much overlap, which is where the brap comes from. As loud as mine is with the new 3" exhaust, I don't think I could drive a fully flowing PP, I get enough looks as it is.
What are you running for carburation on the PP?
05-13-06, 09:29 PM
#82
Old [Sch|F]ool
Originally Posted by trochoid
I have never seen a PP in person, and it's been a while since I've looked at the port timing differences between a BP and PP, but iirc the PP doesn't have as much overlap, which is where the brap comes from. As loud as mine is with the new 3" exhaust, I don't think I could drive a fully flowing PP, I get enough looks as it is.
What are you running for carburation on the PP?
What are you running for carburation on the PP?
Intake timing was 80BTDC open - 60ATDC close. Exhaust timing was stock open to "no idea but DAMN it's high". Lots overlap but late exhaust opening and early intake closing.
05-13-06, 09:36 PM
#84
Originally Posted by trochoid
I suspect Robert is right with the smaller chokes, it should increase intake velocity, unless Purple82 disagrees with me, then I would listen to him. LOL.
05-19-06, 05:35 PM
#87
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Wow **** as i sit in aww ****
I just read all 6 pages and damn you guys are Nucking Futs!
But it all helps, not that i understand now but i will.
I am really interested because tommarrow im getting a 13b 6 port so this thread has helped.
I just read all 6 pages and damn you guys are Nucking Futs!
But it all helps, not that i understand now but i will.
I am really interested because tommarrow im getting a 13b 6 port so this thread has helped.
05-19-06, 06:22 PM
#88
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well i got a gsl-se last week so im converting it back to EFI, going to mod the efi. hope to break 140 with fuel injection. will dyno it in june.
this summer im also rebuilding the motor from the -se (blown coolant seal) going to do a 1/2 bridge and run s4 efi on it with larger injectors. should be good for 200whp. time will tell
this summer im also rebuilding the motor from the -se (blown coolant seal) going to do a 1/2 bridge and run s4 efi on it with larger injectors. should be good for 200whp. time will tell
05-19-06, 06:32 PM
#89
You might want to consider going MegaSquirt and toss the stock ecu. It won't do much good with the bp and you can tune the MS, the stock one you can't.
Hitting 200 rwhp on the 6-ports is very difficult, check the 2nd gen section, very few have got there and I don't think any of them are running the stock ecu.
Hitting 200 rwhp on the 6-ports is very difficult, check the 2nd gen section, very few have got there and I don't think any of them are running the stock ecu.
05-19-06, 09:36 PM
#90
Absolute Power is Awesome
There are a lot of factors to keep in mind when playing with fuel injected cars. One is the method used to measure the load on the engine. Aftermarket systems typically use a manifold pressure sensor in addition to air temperature to calculate engine load/volumetric efficiency. Most stock systems and some of the aftermarket ECUs use a mass airflow meter for load measurement.
While the map sensor method frees up the intake track, it needs to be re-mapped any time there's a modification to the engine's volumetric efficiency (because it's calculated as opposed to measured), like porting or exhaust modifications. The mass airflow systems actually are measuring VE and so they can "self adjust" to such engine modifications.
While the map sensor method frees up the intake track, it needs to be re-mapped any time there's a modification to the engine's volumetric efficiency (because it's calculated as opposed to measured), like porting or exhaust modifications. The mass airflow systems actually are measuring VE and so they can "self adjust" to such engine modifications.
.
05-21-06, 12:14 AM
#94
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I'm putting *about* (estimation with Rx7doctors gtech) 115-120 to the wheels on my mild street-ported 12a with a RB exhaust and OER carb. So figure about 140-150 at the flywheel. And it is definately not tuned to perfection yet. Close but not yet. And need to go to direct fire ignition too.
05-21-06, 02:39 PM
#95
Wow, there is such a wealth of information in this thread. I'm going to archive it for now, though I dont' know how to class it.
I especially liked that Vac Advance post. That was really informative for me. Makes me want to go get a vacuum gauge to make sure my vac advance is working.
Jon
I especially liked that Vac Advance post. That was really informative for me. Makes me want to go get a vacuum gauge to make sure my vac advance is working.
Jon
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