72 Mercury Comet GT in Germany

Discussion in 'Maverick/Comet Projects' started by tody, May 9, 2009.

  1. tody

    tody Member

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    Ohne Titel.png

    so here are the dyno numbers. pretty solid with roughly 410hp and 550nm at the crank.
    the afr curve is a little whacky. the carb was jetted 70/74 from the factory. we switched to 68/76, so the afr over 3500rpm remained the same. it leaned out a little below that, but not by much.
    what is odd are the "holes" in the afr curve where it gets richer. if you look at the power curve, you can see a drop of power in this area, too. the one at 2400 rpm, you can feel while driving the car, too.
    i think i need some professional help interpreting that.
     
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  2. groberts101

    groberts101 Member

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    Dont have much time right now but i will quickly comment. Looks like the motor prefers a sweet spot right around 13-13.5 AFR. Goes fat in the middle.. cleans up.. then goes lean up top and makes topend power fall over a cliff at about 6,400.

    What type, model, and size carb.. booster style(straight, downleg, annular).. pump discharge nozzle style(straight or tube)?

    I think there is some possibility that the timing curve may not be coming in as aggressively as it could be?.. which inevitably fattens the AFR number as more goes out the pipe.

    or.. there may be a fuel pullover issue at the pump discharge nozzles in the midrange? too small a carb/venturi to booster size can cause this issue and very efficient cylinder heads can make matters even worse.

    Or.. power valve channel restrictions (PVCR) could be a bit too fat for this application?

    Do you have any ignition timing curve plots to overlay onto these afr charts? If not it would be good to use a timing light and graph the entire curve out as that would help paint a clearer picture.
     
  3. tody

    tody Member

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    the carb is a quickfuel 680 cfm with vacuum secondaries. it's completely stock apart from the jets i swapped around. i will see if i can find the complete specs on it, should have them noted down somewhere. it does go lean up top, but we tried to assess the behavior below 3000rpm at first.

    i use a msd rtr distributor set at 13° basic timing with 34° total all in at about 2900 rpm. i had thought about timing as well, but the events don't match the afr curve.
     
  4. groberts101

    groberts101 Member

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    Those heads are nice and won;t need as much ignition lead as many other older designs but I would lay cash that you are leaving much on the table with such a lazy initial number being mixed with a slightly too tight centrifugal curve. I hope you'd at least run a vac advance pot connected to full manifold vacuum source to get light throttle cruise advance where it would prefer to be.

    As a quick and dirty test.. I would lock that distributor out at 34°(may need to retard it to not piss the starter off/start it, then reset to 34 after it's running). That extra timing will eat up quite a bit more fuel before it heads out the pipe to muddle up the AFR. I betcha that power dip will disappear on you and the car will come up on the cam quicker as well. remember that it's about the AVERAGE power output.. not just peak numbers.
     
  5. tody

    tody Member

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    distributor is hooked up to ported vacuum on the carb.
    i think i have some ideas what to tinker with when i head to the dyno again.

    in the meantime, i picked up a new set of wheels. the car is currently rolling on the rev wheel classic series, a torq thrust knock off.
    i now have a set of originals, torq thrust D in 15x7. i have a spare set of falken ziex ze912 tires. a friend of mine uses them for road racing and recommended them.

    IMG_0770.jpg
     
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  6. groberts101

    groberts101 Member

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    Old school look.. I like it!

    Tody, I'm not sure who tuned your motor.. and I mean no offense if you did it yourself.. but having only 13° ignition lead at idle rpm is not near enough advance for a stock cam let alone a performance cam like you have in that engine. Running ported advance doesn't help anything until the throttle is open a bit further so you gain absolutely nothing where you actually need it most with a performance cam.

    Ideally, for any motor really, you want to achieve the highest level of manifold vacuum at idle and light cruise rpm's to increase engine efficiency and gain additional throttle response when you abruptly go deeper into the throttle. I would highly recommend that you hook that distributors vacuum pot up to full manifold vacuum source and set the idle speed back down(it will rise considerably as the vac pot begins to add advance) and then readjust the mixture screws to smoothest vacuum readings. I bet you will gain at least 2 inches of manifold vacuum at the same exact idle rpm. Aside from that you will run identically to the way you do now when the ported source comes in at higher throttle angles but the advance will become available at idle speeds too.

    Having the correct ignition lead and improved burn efficiency will also clean up the chamber/plugs and even extend oil life as an added bonus(less unburned fuel/hydrocarbon contamination). Unfortunately it will stink more from the higher NOx levels created by the higher cylinder pressures(this is what improves the engines vacuum levels in the first place) but 13° at idle speed is WAAAYYYYYYYY(yeah, a LOT) too low for a performance engine. IOW, idle and light cruise mixture density and mixture motion is much lower and has nothing to do with WOT total timing advance requirements. Bigger duration higher lift cams hurt this even more and need more ignition lead as a result of this phenomenon. Higher manifold vacuum also helps pull on the carb's booster harder during part throttle operation as well.. which helps the rpm rise faster to come up on the cam even quicker. Aside from the higher NOx readings(high performance is already subject to that issue anyways).. there is no downside.
     
  7. tody

    tody Member

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    new shoes are on...
    i'll try to follow your advice on tuning the engine as well. i have a vacuum gauge somewhere.
    IMG_0787.jpg
     
  8. tody

    tody Member

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    oh, and here's a video from that drag racing event.
    they had a mav there, dedicated drag car, running 10.6:

     
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  9. CaptainComet

    CaptainComet Large Member

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    I stumbled on the video from the event on Youtube (since I already looked at your vid for the event, cookies linked it up).... that Mav looks pretty serious.
     
  10. tody

    tody Member

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    finally put the mercury script on the spoiler. looks good. forgot to take pictures, though.
    also, i ordered a set of hedman 88300 headers. the hooker 6901 are driving me nuts. still hit the frame and will lock up the z-bar every now and then. hopefully the hedmans will work better...
     
  11. tody

    tody Member

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    it's been some time since i updated this thread.
    there's been some additions to the family - a second kid - as well as to the stable - a 1970 bronco. and i still have my 67 club wagon in the making.
    so i just drove the comet but still have to sort out some issues.

    hooker headers are still in, couldn't be bothered to install the hedmans and rework the exhaust system yet.
    will try to fab a different pivot bracket for the z-bar this weekend which hopefully works a little better.

    also i just bought a set of original shock plates. my car never came with original ones and the chassisworks mustang plates for my 9" versailles rear have the wrong shock angle and are slowly eating up the rubbers on the rear shocks.

    and i picked up a borgeson power steering unit which i might add later on.
     
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  12. mojo

    mojo "Everett"- Senior Citizen Supporting Member

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    I will be following the Borg install. I installed the kit on my car 3 yrs ago, and still having issues w/ highway speed steering sensitivity. I recently installed the shim kit to reduce pressure an hope that eliminates the issue. Below 50mph steers fine. I have all new suspension and steering components and recommended alignment specs. Today I added 2 shims, had added 1 last week, Saginaw pump came w/ 2 shims from fac. When, I finished today, was too tired to test drive.
     
  13. Stefan

    Stefan Big Cheese Administrator

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    @tody Congratulations on the addition to your family!
     
  14. tody

    tody Member

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    wow, has it nearly been a year since i updated this thread?
    and it has now been roughly NINE years since i bought the car and started this build.
    the car is still an absolute joy to drive, which i do regularly. i have tweaked some stuff, mainly ignition curve related. need to sort out some driveline vibration, will pull the transmission and check the tailshaft bushing.

    what i have done in the meantime was finish this:
    31958629_1859772334042780_4953843634580488192_n.jpg

    and my 70 bronco is in the making.

    i updated another thread as well - chip's car has resurfaced in germany, and i might jump on it.
     
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  15. tody

    tody Member

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    Again, a year has passed... And I missed the 10 year mark of my acquisition of this car. Just wanted to let all of you know that I still enjoy it very much and drive it on a regular basis, nearly all year round.

    I'm still chasing the driveline vibration from last year. To be honest, I just couldn't be bothered to tackle the task ;)
    I'll pull the T5, as it's also slightly leaking at the tailshaft. Will put in a new bushing and seal, and replace the U-joints and probably the yoke, too.
    Note to the non-European members: the vibration is only occurring in 5th gear at 80+ mph. Autobahn issues :D
     

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