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1975 Plastic fantastic aka Corvette

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  • Is there a mechanical bypass in the throttle body that's letting more air into the engine than you need?
    There's always something new to learn.

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    • lots o gear ... I guess with teh Super T10 ratio spread it should be interesting. I'd cube the hell out of it, but that's me. I see where KMJ has Howard's 100% USA forged Chevy cranks on sale for 550.00 - 6" rod in a 350 with a modern lightweight piston should rev like crazy if you want it to.
      Flying south, with a flock of bird dogs.

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      • What thumpr cam did you put in it? It'd drive me nuts idling around at ~1,400+ rpm.
        Escaped on a technicality.

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        • Originally posted by TheSilverBuick View Post
          What thumpr cam did you put in it? It'd drive me nuts idling around at ~1,400+ rpm.
          this one
          Brand:COMP Cams
          Manufacturer's Part Number:CL12-600-4
          Part Type:Camshaft and Lifter Kits
          Product Line:COMP Cams Thumpr Hydraulic Flat Tappet Cam and Lifter Kits
          Summit Racing Part Number:CCA-CL12-600-4

          UPC:036584181224
          Cam Style:Hydraulic flat tappet
          Basic Operating RPM Range:2,000-5,800
          Intake Duration at 050 inch Lift:227
          Exhaust Duration at 050 inch Lift:241
          Duration at 050 inch Lift:227 int./241 exh.
          Advertised Intake Duration:279
          Advertised Exhaust Duration:297
          Advertised Duration:279 int./297 exh.
          Intake Valve Lift with Factory Rocker Arm Ratio:0.479 in.
          Exhaust Valve Lift with Factory Rocker Arm Ratio:0.465 in.
          Valve Lift with Factory Rocker Arm Ratio:0.479 int./0.465 exh.
          Lobe Separation (degrees):107
          Camshaft Gear Attachment:3-bolt
          Grind Number:279TH7

          John has a good idea - I'm going to mess with the IAC a bit more before I give up.... strangely, it did idle fine the last time I drove it - so that may be round one, go drive it 20 or so miles.
          Doing it all wrong since 1966

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          • Granted I don't know small blocks for crap, but I would expect that to at least idle down to 1,000. I'd be looking for either a vacuum leak that the EFI is compensating for and checking the IAC settings to see if at idle the valve is all the way closed. Or maybe in the software settings there is a target rpm setting that is fubar'd? Can you get a read out of what all the sensor are reporting to the ECU? Incase it's an issue like Tartis(sp?)'s friend's van and the temp sensor. I know how mine is programmed if the temp sensor wiring comes loose or grounds out it'll report cold, making it run rich AND idle up the car to 1,200 because it thinks the engine needs to warm up.
            TheSilverBuick
            ALMOST Spidey !
            Last edited by TheSilverBuick; December 28, 2012, 09:23 AM.
            Escaped on a technicality.

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            • Originally posted by TheSilverBuick View Post
              Granted I don't know small blocks for crap, but I would expect that to at least idle down to 1,000. I'd be looking for either a vacuum leak that the EFI is compensating for and checking the IAC settings to see if at idle the valve is all the way closed. Or maybe in the software settings there is a target rpm setting that is fubar'd? Can you get a read out of what all the sensor are reporting to the ECU? Incase it's an issue like Tartis(sp?)'s friend's van and the temp sensor. I know how mine is programmed if the temp sensor wiring comes loose or grounds out it'll report cold, making it run rich AND idle up the car to 1,200 because it thinks the engine needs to warm up.
              all good points, I've already checked for vacuum leaks, timing issues, spark issues, valve hanging issues.... I suspect the IAC because it won't idle, especially when cold..... And the biggest clue, when I was checking for vacuum leaks, I disconnected a direct port while the motor was running - and the idle speed went up
              Doing it all wrong since 1966

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              • Originally posted by SuperBuickGuy View Post
                new differential

                2.73 gears..... posi is in good shape, needs a left yoke; or simply a shim....we'll see which one I go with....

                So I dinked with the car tonight, working on tuning it.... I've come to the conclusion I think I made a mistake by putting the 1.6 ratio rockers on it.... it won't idle under 1400 rpm, even with the rockers at 1/8 turn past zero lash..... ah well, I can fix this pretty easily

                good score!! not sure what i could come up with on the idle problem, but, i think you got a handle on it...
                Patrick & Tammy
                - Long Haulin' 2008, 2009, 2010, 2011, 2012, 2014...Addicting isn't it...??

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                • SBG, you idled before, therefore there is nothing wrong with your cam.. Something went "off" ..
                  I ran the "280H" cam in my flatbed.. Power band started at like 1200rpms it idled at 800-850 warmed up.. Didn't have a choke either and it would run at maybe 600 rpms till it warmed up..
                  I always wanted to know what that cam woulda done in a car.. Moved the flatbed pretty good!

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                  • Originally posted by SuperBuickGuy View Post
                    And the biggest clue, when I was checking for vacuum leaks, I disconnected a direct port while the motor was running - and the idle speed went up
                    On EFI I'm not sure that is diagnostic of anything as any way you add air to the system it's going to add fuel, thus up the rpm. It doesn't really care what the throttle blades or TPS is doing.
                    Escaped on a technicality.

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                    • I think Randal may have run without IAC for a while? I'd turn the minimum idle screw in on the TB with the IAC disconnected and see if that will let it idle lower. Busted IAC function is .. no idle. If you force it to 850-900 with the throttle blade and IAC undone, I'm thinking it's a clue.

                      Is a stepper motor IAC?
                      Flying south, with a flock of bird dogs.

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                      • My Skylark has always ran with an IAC, my Thunderbird runs without a functional one. I like the idea of unplugging the IAC valve if you can and manually set the idle.
                        Escaped on a technicality.

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                        • Originally posted by TheSilverBuick View Post
                          On EFI I'm not sure that is diagnostic of anything as any way you add air to the system it's going to add fuel, thus up the rpm. It doesn't really care what the throttle blades or TPS is doing.
                          it just occurred to me that to adjust idle air would simply mean opening the throttle blades and zeroing the tps.... the effect of it being that when it is cold, it would be less than steller for idling - but I had holleys on here before, so the net effect would be better....

                          or, put another way, I've yet to find the control for the IAC on the tuner.
                          SuperBuickGuy
                          No Life Outside BangShift.com
                          Last edited by SuperBuickGuy; December 28, 2012, 06:48 PM.
                          Doing it all wrong since 1966

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                          • I've run my '86 Mustang with the IAC unplugged. Just fiddled with the idle screw on the throttle body. Early EEC IV Fords have horrendous idle "hunting" issues.
                            I'm still learning

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                            • Originally posted by Bob Holmes View Post
                              I've run my '86 Mustang with the IAC unplugged. Just fiddled with the idle screw on the throttle body. Early EEC IV Fords have horrendous idle "hunting" issues.
                              particularly if there is any vacuum leak at all. What do you mean by horrendous? About 600 rpm variance? I know nothing of this. Haha. My 88 stopped doing that right after I put a gasket set in that covered all 8 intake ports. I've never had that problem with an MAF car. The LSC hunted but IAC was non-op, so I twisted the idle up manually. Not ideal but it kept it from stalling. I have always thought they were supposed to run without IAC and it was for cold temperatures / air conditioning / automatics?
                              Flying south, with a flock of bird dogs.

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                              • My Mustang has the IAC deleted (I made a solid gasket), and I set the idle to 600rpm. The 1200rpm idle was driving me nuts, made parking lot driving awkward, and made the car sound horrible.
                                1997 Ranger 5.0L HO, GT40 heads/tubular intake, 65mm TB, 1.7rr, B303, Tri-Y headers, dual 2.5" exhaust, Flowmaster mufflers, T5 trans, Tri-Ax shifter, CenterForce Dual Friction clutch, 8.8 Traction Lok 3.55 gears, Cobra 13" front brakes, Cobra 11.65" rear discs.
                                1997 Mustang GT
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