Forfatter Emne: corvette 454 1971  (Læst 6519 gange)

Offline gitcar

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corvette 454 1971
« Dato: Søndag den 29. Marts kl.14:17 »
Hej Jeg roder i øjeblikket lidt med en 454 som har udsætter ved høje omdrejninger (kæppen i bund) 3500-4500, lige før opgearing til næste gear.
jeg mistænker tændings advanceringen,
Den står 12 grader i fortænding, uden vacum ved 2500( mekanisk) omdr 26 grader, med vacum fuld advancering 34 grader?
Mit spørgsmål til panelet, er det ikke i den lave ende.

Carb er q-jet, nylig monteret for at komme tilbage til originalt spec. med ny bensinpumpe.
Den har været kørt med edelbrok og holly carb, samme resultat med udsætter/ dyk. derfor min mistanke på tænding. Advancering /
platinhop
Nogen som har prøvet det samme?

Mvh Gitcar :-\
Chrysler 300 non letter 1966

Offline Stroker66

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« Svar #1 Dato: Søndag den 29. Marts kl.15:10 »
Tænding skal altid måles uden waccum, og 10-12 gr. i tomgang er ikke helt forkert. Mekanisk avancering (armene under rotoren) er typisk på 20-22 grader og er fuld avanceret ved 3000-3200 omdr. Bøsninger/fjedre i den mekaniske avancering kan være slidte eller sidder fast så sørg for at de er i orden. Standart single-point (enkelt platinsæt) kan godt være besværligt at få til at fungere i høje omdrejninger, som du selv er inde på. Jeg ville nok købe en form for elektronisk tænding og bygge i fordeleren, man kan få forskellige sæt. En original HEI-tænding ville være ideelt men jeg mener at corvetten har mekanisk omdr.tæller og det har HEI-`en ikke udtag til. Den fylder også mere end den originale, og plads er der ikke for meget af i et vette-motorrum :)

Offline gitcar

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« Svar #2 Dato: Søndag den 29. Marts kl.15:51 »
Hej tak for respons
Læner også til at prøve noget platin løst, for at udelukke
fordelerens svingklodser virker lidt for løse til mig!

Har også en lille tanke om at flå ventil dækslene af for at måle knast løft, og chekke ventil fjedre

Hyg :)
Chrysler 300 non letter 1966

Offline COUT

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« Svar #3 Dato: Søndag den 29. Marts kl.15:57 »
Platintænding burde ikke have problemer ved kun 3500-4500 rpm. Har du tjekket platinernes tilstand/kamvinkel/tændspole osv?

Corvetterne blev leveret med hei tænding fra omkring 1975, så ingen problem med plads. Der fås en hei tænding med mekanisk udtag, hvis det er det man vil have. Bare undgå de helt billige kina modeller...

Offline gitcar

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« Svar #4 Dato: Søndag den 29. Marts kl.16:04 »
Hej
kamvikel står 29 grader, nye platiner og kondensator. ny tændspole med formodstand.nyt fordelerdæksel og ledninger.
 Har trukket en ny 12 volt ledning fra ign ved sikrinsholder til tændspole, for at udelukke den gamle restricted wire som jeg kun kunne måle 7 volt i tomgang.

tænker om det kan være man skulle se, om man stadig kan købe oktan 99 i frederikssund. Og se om det ændre noget

Hyg
Chrysler 300 non letter 1966

Offline Marx3

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« Svar #5 Dato: Søndag den 29. Marts kl.16:54 »
Din tænding lyder til stå meget lavt. Chevy Big block kan godt lide en ret høj total ( altså fuld avancering ved ~3000 rpm UDEN vacuum. 38-40 grader før top er ikke atypisk. Det kan måske blive svært at få tændingen helt derop med den originale fordeler, fordi tændingen i tomgang så bliver ret høj. Du kan nok finde et godt kompromis, men tændingen skal helt sikkert en del højere op. I øvrigt; hvis tændingen holder op med at avancere allerede ved 2500 rpm, så ville jeg prøve nogle strammere avanceringsfjedre.
En anden alarmklokke går igang his mig når du skriver at det er en "ny" Qjet. Hvordan ny? Nyindkøbt ny eller en gammel, men renoveret karburator? (De nye Qjets man kan købe på eBay, summit osv er ofte rigtig flotte, men direkte elendige: skruet sammen elendigt og shinet op så de ser nye ud )
« Senest Redigeret: Søndag den 29. Marts kl.17:17 af Marx3 »
'79 Caprice unmarked vehicle clone

Offline gitcar

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« Svar #6 Dato: Søndag den 29. Marts kl.17:27 »
He ja det med carb har du ret i. Han har købt den hos ecklers. OG den var ikke helt fin da den kom hjem i papkassen :-(
Vi har brugt en del tid på at få den rettet ind.
Tro helt bestemt ikke at den volder de problemer som vi kæmper med (men alt er ved at blive endevendt og Carben er ikke glemT;-)  )
Prøver at låne en fordeler uden platiner af en ven, så må vi se

Har også chekket  om tændings mærkerne havde rykket sig på balanceren. (har jeg vist nok hørt engang om en som kendte en havde set )  ;)
vender tilbage når der er nyt, tak for input
Hyg
Chrysler 300 non letter 1966

Offline RC

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« Svar #7 Dato: Søndag den 29. Marts kl.17:28 »
Indsat:
 Har trukket en ny 12 volt ledning fra ign ved sikrinsholder til tændspole, for at udelukke den gamle restricted wire som jeg kun kunne måle 7 volt i tomgang.

Hvis anlæget er bygget til restricted wire, og du giver den 12v V. smelter din tændspole snart, og inden det har den svært ved at yde fuldt ud.
Prøv nogle nye platiner, hvis de er " gamle " kan de godt se fine, men fejderen bliver slap med tiden, og de vil " svømme " ved høje omdr.


René
Chrysler Stratus 2,0 1999

Offline gitcar

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« Svar #8 Dato: Søndag den 29. Marts kl.17:39 »
Hej RC

Den nye "midlertidige 12 volt fra ign er monteret før en ny formodstand på en ny tændspole;-) Så spolen får kun 9 volt ;-)

Dette setup var for at overrule de gamle Hårde ledninger, og lidt for lave volt målinger på primær siden

Hyg
« Senest Redigeret: Søndag den 29. Marts kl.17:43 af gitcar »
Chrysler 300 non letter 1966

c3_dk

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« Svar #9 Dato: Søndag den 29. Marts kl.20:34 »
Din tænding lyder til stå meget lavt. Chevy Big block kan godt lide en ret høj total ( altså fuld avancering ved ~3000 rpm UDEN vacuum. 38-40 grader før top er ikke atypisk. Det kan måske blive svært at få tændingen helt derop med den originale fordeler, fordi tændingen i tomgang så bliver ret høj. Du kan nok finde et godt kompromis, men tændingen skal helt sikkert en del højere op. I øvrigt; hvis tændingen holder op med at avancere allerede ved 2500 rpm, så ville jeg prøve nogle strammere avanceringsfjedre.
En anden alarmklokke går igang his mig når du skriver at det er en "ny" Qjet. Hvordan ny? Nyindkøbt ny eller en gammel, men renoveret karburator? (De nye Qjets man kan købe på eBay, summit osv er ofte rigtig flotte, men direkte elendige: skruet sammen elendigt og shinet op så de ser nye ud )

100% enig.



Technical Information Bulletin Rev. T 1-2-14

How to Set Your Timing for Peak Performance


by Lars Grimsrud 

With supporting technical information and contributions provided by:

John Hinckley, Automotive Historian & Technical Writer

Duke Williams, Automotive Engineer, MSME, Redondo Beach, CA

©2014 Lars Grimsrud, John Hinckley, & Duke Williams

This article may not be published or distributed without the written permission of the authors



“90% of all “carb problems” are ignition timing problems.”

(Lars Grimsrud)


This tech paper will discuss setting the timing on a Chevy V8.  This procedure also applies to other GM V8s and most other American large-displacement V8s.   


The procedure outlined here differs from the Service Manual, and is based on my years of experience doing this work in the quickest, least painful, most economical way while keeping the level of quality high.  It is recognized that other people will have different methods of doing things, and may disagree with specific methods and procedures that I use.


CAUTION:  This procedure applies ONLY to pre-1981 non-ECM controlled vehicles. The timing method outlined in this paper CANNOT be used on any ECM (“computer”) controlled vehicle. Setting the timing per this procedure on an ECM-equipped vehicle will result in improper timing and probable engine damage. DO NOT USE this procedure on any ECM-equipped vehicle.


How to Set the Timing

When you think about it, setting the timing at idle speed makes no sense at all:  You don’t operate your car at idle, and timing changes as the rpm changes. Fact is, the timing spec at idle speed is provided as a simple way for most people to set the timing, and is not a good procedure for optimum performance.


Chevys (and most other performance V8 engines) perform best when the total timing (full centrifugal advance plus the initial timing setting with vacuum advance hose disconnected) is all in by 2,500 – 2,800 rpm and is set to about 36 degrees.  If you have an adjustable timing light, this is very easy to check.  If you don’t, you need to scribe a 36-degree mark on your harmonic balancer.  Here’s how:


Measure the circumference of your harmonic balancer using a sewing tape measure (or other flexible tape measure). Get it as accurate as you can.  Take this measurement and divide by 10.  The number you get is the distance to 36 degrees.  Measure this distance CLOCKWISE from your existing harmonic balancer timing mark as viewed from the front of the engine and place a clear mark on the balancer.


Remove your distributor cap and rotor.  If you have a points-style distributor with the stock, factory, heavy springs in place, remove one of the springs. Disconnect the vacuum advance by simply pulling the hose off the vacuum advance control unit on the distributor – there is no need to plug the hose, as the vacuum leak will have no effect on timing. Install the rotor and cap.  Loosen the distributor hold-down clamp bolt just enough so that the distributor can by turned, yet leave it snug enough that the distributor will hold its position.


Start the engine.  If you’re using an adjustable timing light, set the light to 36 degrees advanced.  Now rev the engine while observing the timing marks with the light.  You will notice that the stock line on the balancer will move up towards the timing plate as rpm increases.  Continue to increase rpm until the line does not move any further (centrifugal advance is “pegged out”).  Once the timing is “pegged out,” the line on the balancer should line up with the “0” mark on the timing tab.  Rotate the distributor as needed to achieve this. 


If you’re using a non-adjustable light, perform the same process, but align your new 36-degree mark with “0” mark on the timing tab.


Shut it down.


Pop the cap and rotor and re-install the spring, if you removed it.  Snug down the distributor hold-down clamp. Put everything back together, but leave the vacuum disconnected.  Start it up.  For future reference, make a note of the timing setting at idle.  This is your new curb idle timing spec.  Now give the engine a few quick rev’s past 3,000 rpm and verify that the full timing (36 degrees) is coming in.  If it’s not, you need to change to a softer set of springs until you get full 36-degree advance before 3000 rpm.  (NOTE: A stock set of springs will often not allow full centrifugal advance to come in before redline rpm.  If you have heavy stock springs installed, don’t rev the engine beyond its limits to try to force full advance in.)    I

suggest obtaining Mr. Gasket kit part number 928G and use the gold springs (do not use the weights in the kit – throw them away)  – these springs will get your total timing all in by 2500-2800 rpm, providing very good throttle response and power. On MSD distributors you can use one of the black and one of the silver springs in the kit if you widen out the spring hook ends to get the curve to come in as the specified rpm range.


Now, hook up the vacuum hose. Re-set your idle speed and idle mixtures if necessary to lower the idle speed.  Now do a road test.


The 36-degree 2500 rpm advance curve is optimum for performance, but may require premium fuel.  Lug the car around, and punch the throttle at low rpm while listening for detonation (“engine knock”).  If you’re getting any audible knock, you MUST retard the timing. Retard the timing in 2-degree increments until engine knock stops.  Engine knock will seriously damage engine components if not corrected.  If you get no knock, you may see slightly improved performance at 38 degrees total timing, especially on larger bore engines.  This is also particularly true if you’re running at high altitude.


If you have no engine knock under acceleration, but the car “chugs” or “jerks” at cruising speed (light throttle application), you are getting too much vacuum advance on top of the mechanical advance.  You may need to change out the vacuum advance diaphragm with a unit producing no more than 12 degrees of vacuum advance.  See my paper on “Vacuum Advance Control Units Facts and Specs” for more info on this.   


Your timing is now set for best performance.  Make note of the new setting, and use this for your future tune-up work.


Lars’s Suggested Timing Specs for GM V8 Performance Applications:


* 36 degrees total timing (vacuum advance hose disconnected), all “in”  by 2500-3000 rpm

* 16-18 degrees initial timing at idle (vacuum advance hose disconnected). Note that it may not be possible to achieve the 16 to 18-degree initial spec with the 36-degree total without modifying the distributor advance stop system.  It is more important to achieve the 36 total than to hit an exact 16 to 18 initial.  However, if your initial timing is very low (below 10 degrees) or very high (above 24 degrees) with the 36 total, it is important that you repair or modify your distributor in order to achieve correct engine performance.

* 12 degree vacuum advance control unit with a pull-in spec that allows the full range of vacuum advance to be pulled in at the engine’s idle manifold vacuum level.  Connect to manifold vacuum

for most applications (this will allow the engine to idle with actual timing at idle of 34 degrees). The total maximum combined timing at light throttle cruise above 3000 rpm will be 48 degrees.

How to Alter or Correct the Length of the Centrifugal Advance Curve

(GM points-type distributors 1974 and earlier)


On a performance engine with a performance cam, you want to run initial timing in the 18-degree range with the total advance still limited to about 36 degrees.  Few stock distributors will allow you to hit this spec:  With 36 total, initial timing will likely be in the 8-12 degree range. This is not enough initial timing to allow a well-cammed engine to idle properly, and you will have carb tuning issues with off-idle stumbles and hesitations.


The “length” of the centrifugal advance curve on a 1974 and earlier GM points-type distributor is determined by the range of travel of the advance stop pin in the advance stop slot. The factory installed a black rubber bushing on the pin, thus increasing its effective diameter and shortening the centrifugal advance travel. Most high-mileage distributors are missing this bushing, and the advance curve is too long.


If your advance stop bushing is missing, you need to install a new bushing.  This cannot be done with the distributor in the car:  You need to remove the distributor and disassemble it to the point of being able to remove the mainshaft. Press a new bushing onto the advance stop pin. You can obtain a brass bushing from Mr. Gasket kit part number 928G.  This bushing will fit loose, so you will need to give it a light smack with a hammer to make it a little “oval” before pressing the bushing onto the pin.  The bushing will otherwise fall off and short out your points.


If your advance curve is still too long, you will need to shorten the advance stop slot by welding a dab of metal into the end of the slot and grinding it to suit your needs.  Here’s how:


Either in your engine or on a distributor machine, determine how long your centrifugal advance curve is:  Easiest way to do this is to install a heavy set of springs to assure that the advance is at “0” at your idle speed and then removing the springs to observe the max total advance.  Let’s say that initial timing with the springs installed is 10 degrees, and removing the springs puts the timing at 35.  Your centrifugal advance curve is 25 degrees long.  Let’s say you want to run 20 degrees of initial timing with 35 total: You want your curve to be limited to 15 degrees.


Here’s how to calculate exactly how much to shorten the slot:


* Using a set of calipers, measure the travel distance of the advance stop pin/bushing in the slot. This will be the distance from the side of bushing or pin to the contact surface in the slot.  Let’s say this distance  is 0.20”.

* Now we know that 25 degrees = 0.20”.  How do we find what is equal to 1 degree? 7th grade math tells us to divide both sides of the equation by 25:

* 25/25 = 0.20/25

* Dividing this out we get that 1 degree = 0.008”

* Since we want to shorten our vacuum advance from 25 to 15 degrees, we want to shorten it by 10 degrees:

* 10 x 0.008 = 0.08”. We want the stroke to be shorter by .008”.

* Measure the total actual length of the slot without the pin in it and subtract 0.08”. This is the new target length of the slot.  Weld a dab into the slot’s end, and use a Swiss file or a ball end grinder on a high speed to shape the slot to the new calculated length.

* Put it back together and test it.



My Comments, Observations, and Opinions about Factors Affecting Optimum Timing Specs

I see a lot of opinion about what the “correct” initial and total timing should be, and I frequently get asked, “What is the best timing spec for my engine?” 


Without actual dyno testing of the engine in question, it is virtually impossible to arbitrarily determine what the “best” numbers should be for any given engine, so some generalizations and “suggested” starting point specs are made in this paper.  The 36-degree total timing spec is a generally-recognized and agreed-upon spec amongst almost all performance engine tuners and engine builders, but it can vary from one engine to the next as the “optimum” spec.  I have done a significant amount of dyno testing of a wide variety of engines, and one of the tuning variables we always “play” with is the total timing.  I have hard data obtained from dozens of dyno runs, making it possible to draw some general conclusions about “optimum” timing. Here are my observations based on years of testing:


* The 36-degree total timing number is a good mid-point number for most small block GM and Ford performance engines with factory-type heads. Most small blocks with stock head castings will take 34 to 38 degrees total timing for best power and drivability.

* A small block with good, efficient aftermarket heads with good combustion chambers will make better power with less total timing:  I usually see best power with total timing in the 32 – 34 degree range on small blocks with good heads.

* Big block (large-bore) engines will typically take more timing. Big blocks with inefficient factory iron heads (like 455 Pontiacs with stock heads) will typically develop best power with total timing in the 38 – 41 degree range.  Large-bore engines with good heads will typically take 36 – 38 total.

* “Optimizing” timing on the dyno from the 36-degree suggested starting point will often result in power gains in the 5 to 10 horsepower range.  You cannot feel a 5 to 10 horsepower change in the “seat of your pants” in a typical street driven vehicle weighing over 3,000 lbs., so don’t worry about trying to optimize total timing within 2 degrees without the use of a dyno.  If you’re at 36 total, and do not have detonation, you’re going to be making very good power.


Although I downplay initial timing, it is important that the initial timing hit a certain range in order for the engine to idle properly.  Initial timing has a significant effect on emissions, if that’s a concern to you.  It will also greatly affect idle quality and manifold vacuum (for proper carb metering signal) on engines with bigger cams. My generalized recommendations are:


* On mild or stock engines, initial timing (checked and set with vacuum advance disconnected) can be in the 8 to 12 degree range.  This will provide a smooth idle, low emissions, and good drivability.

* On engines with an aftermarket performance camshaft, the initial timing should be raised (i.e., the centrifugal curve should be shorter).  The 16 – 18 degree suggested spec is a good target range for any performance engine. This will produce better manifold vacuum at idle on an engine with a larger cam, thus allowing the carb to properly operate on its idle circuit (rather than into the transition circuit). 

* Engines with big solid or roller cams, and engines not utilizing vacuum advance, should be run with initial timing in the 18 – 24 degree range. 

* The engine will run cooler with more initial timing, up to a point. If you’re having overheating issues running in stop-n-go traffic, and your initial timing is in the 8-degree range, try shortening your advance curve and running 16 initial – it can make a dramatic effect on coolant temp while idling.


As for the transition from initial timing into total timing (the “quickness” of the centrifugal curve), this will depend on the weight of the vehicle, engine compression, and the quality of fuel used.  Generally, the lighter the vehicle, the quicker the curve can be. Heavier cars generally will take a slightly slower curve in order to prevent detonation under load. For light street driven vehicles, I’ll bring in the total advance in the low-2000 range. For “normal” cars (Camaro, Vette, GTO), I’ll set the curve up for about 2800 as a target.

Generally, something on the 2500-3000 range works well for most performance applications, without getting too aggressive.




Questions, Comments & Technical Assistance

If you have questions or comments regarding this article, or if you notice any errors that need to be corrected (which is quite possible since I’m writing this from memory…), please feel free to drop me an e-mail.  Also, if you need any technical assistance or advice regarding this process, or other maintenance

Offline RC

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« Svar #10 Dato: Søndag den 29. Marts kl.20:40 »
Ok, så er jeg mere rolig mht. spole.
Hvad med platiner, er klodsen der aktiveres af akslen smurt ved montering ??, hvis ikke så prøv at tjekke camvinkel nu, jeg vil næsten væde på at det er det der er galt.
Er der slør i akslen på strømfordeleren ?, det vil ændre camvinkel i takt med omdr.
Prøv at have camvinkelmåler på ved alle omdr. og se om den ændre sig.

René
« Senest Redigeret: Søndag den 29. Marts kl.20:41 af RC »
Chrysler Stratus 2,0 1999

Offline Marx3

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« Svar #11 Dato: Mandag den 30. Marts kl.17:35 »
jep, hvis kamvinkelen ikke flytter sig andet end et par grader uanset hvor mange omdrejninger du giver den, så burde det jo ikke være dér...
'79 Caprice unmarked vehicle clone

Offline lightning

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« Svar #12 Dato: Mandag den 30. Marts kl.18:00 »
er du sikker på at den får nok af leve af?

Offline gitcar

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« Svar #13 Dato: Mandag den 30. Marts kl.20:25 »
Vi har desværre ikke nået en skid idag, det forbandede arbejde tager for meget af fritiden.

Bliver nok først i påsken vi roder videre, skal nok vende tilbage

Lightning : Den har ikke ændret adfærd med 2 andre carbs, edelbrok og Holly. tvivler på at den går for magert

Tak for input til alle
Chrysler 300 non letter 1966

Offline lightning

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« Svar #14 Dato: Mandag den 30. Marts kl.21:12 »
jeg mener hvad med pumpen

har prøvet at den ikke ville skifte op godt nok i en bmw fordi den ene pumpe ikke kunne lave tryk nok