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Thread: 40IDA Hesitation/Stutter Between 3-4000RPM

  1. #11
    Senior Member Neunelfer's Avatar
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    You guys are always creating problems!

    Decisions, decisions!



    Thanks Gib!

    Maybe I'll just take a stroll down to NM and hit Flagstaff on the way. Leave the keys under the mat!
    Eric - Sandy, Utah
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  2. #12
    Too many cooks in the kitchen on this one already, so here's some more food for thought.

    First, no twin plug motor should be timed at more than 28*. It's absolutely unnecessary. If you're putting that much timing in then you have problem.

    Second, 906 cams are shit street cams. They work from 5500-8000 rpm and that's it. Trying to make them "streetable" is a fool's errand. Steve Weiner would beat you with that 906 camshaft if he was in striking distance. IF you MUST have your 906 cams then you need to install tall auxiliary venturis to get the inlets out of the way of the intake pulses. That's why the taller auxiliaries exist.

    Third, E cams will suck with that motor. Big ports need to breathe ... they need volume and velocity and, therefore, they need RPM. An E cam is finished flowing at 6500 ... you need a cam that can breathe to 8000. You need a 120/104 grind, a GE60, or even a stock S cam (all of which will slightly restore the bottom end...slightly).

    Fourth, your carb settings look questionable to me. I don't have my books here at home, but I have a 2.5L in my racecar (36mm intake ports, twin plug, moderate compression, 40IDT carbs) and, from memory, I'm running 34mm venturis, short secondaries, 55 idles, 150 or 155 mains, F3 tubes, and 147.5 airs. AFR is 12.5-13:1 from 1500rpm to 8000rpm. That motor made 200hp at the rear wheels and 185ft/lbs of torque ... with a marginal ignition system.

    In a nutshell I'd say the first thing you need to do is time the motor properly (absolute MAX of 28*...more like 24-26*). Then, come to grips with the fact that you've got a big end motor and it's going to suck under 5000rpm. Then, embrace the big end and tune for it. 36mm venturis, 60 idles, f5 tubes, 145 airs, 155 mains, and tall secondaries.

    I think.

    PS: I went back and looked and Kurt and I aren't far off in our recommendations. Also, I'm not a fan of F26 tubes ... I've got piles of them. F3, f5, and F25 are the tubes I've been playing with lately and the 3s and 5s have been the best all-rounders I've found. Remember, gas is so bad you need to fuel these motors like they're running on alcohol. The more the merrier!
    Last edited by Mr9146; 07-24-2014 at 10:14 PM.
    -Marco
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  3. #13
    Senior Member Neunelfer's Avatar
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    All right, all right...

    I don't have 906 cams. Here's the cam profile (which I believe is what you're recommending as well), which after putting 4-5,000 miles on the car, I'm aware of the error of my ways:

    http://www.webcamshafts.com/pages/au...52_000410.html

    As you can see, it "is" the 120/104 grind. I've been told it's what you would call a "mod-906" cam with longer duration similar to the mod-s cams out there. I was hoping that with the smaller ports and the longer duration I would get a more streetable version of a GT type motor. More bottom end/torque, like you're saying. That's my stupid story and I'm sticking to it. Plus, Steve likes me more than he likes you and he will not beat me with a cam shaft (I think). :P

    Here's some info from the first post regarding the ports <<It has smaller 38 mm intake ports>> so, I "think" I'm OK there with the E or Solex cams. Yes? No? I know the factory never went over 36 with the S-Cams but, I thought it would be appropriate for these cams. I'd love some more real world advice here.

    I have set the advance at 28 "exactly" and, it simply won't idle. I'm not the cam expert but, what Kurt was saying seemed to be spot on with my problem. Because of the overlap with that cam, he stated I need the advance right now to give the motor some compression to idle. Again, I'm parrotting what I "think" I heard... I'm old, I could be way wrong. I can talk about brake calipers all day long but, I seek advice here in these matters. Regarding the timing; you're not alone, Aaron also said somethings wrong if I need to advance beyond 28 but, valves have been set and everything has been checked and re-checked. For these 40IDA-3C's to idle here at 4,300 ft. with these cams, it had to be set to 32 (and have 70 idles).

    Keep in mind, I'm at 4,300 ft. so, when it comes to jet's and even venturi's, I'm a little "different" (watch it!!). I've used the Weber book and Bruce's book along with some info off of Pierce's site and came up with on spreadsheet to calculate all of that. Then I added a good dose of advice from those like the gang here. Venturi's for this motor with these cams calculate out to 36mm using Bruce's formula (Cylinder V/100 x Peak HP RPM/1000) x SQRT x 20 = 36mm. Most people I spoke with advised 34's for mainly street. 34's it is.

    Mains should be roughly "venturi's x 0.21" so, 172 for 36's and 162 for 34's. Now take away 9% for altitude and it says 148's so, pretty much spot on with what you're running. Idles calculate out at 70 at sea level to 60 here at 4,300, again, what you're saying. I just couldn't get the 60's to work without bumping into the progression circuit and dealing with the roller coaster idle. I also had a huge hesitation off idle with the 60's. I realize this is just math but, I used it to get close and then went off the charts to make a determination on the idle jet and main jet sizing.

    Oh... stock exhaust (914-6 HE's) with a sport muffler (I know, I know).
    Eric - Sandy, Utah
    71 911
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  4. #14
    38mm ports are not small. Get that idea out of your head. 36mm are considered average. 32 are small. Your constantly referring to 38mm ports as small tells me that you did a lot of Internet forum reading before you built the motor. Practical experience dictates that small street motors (up to 2.7L) won't really perform on the bottom with anything larger than a 36mm port without butt tons of compression ... why do you think Porsche waited until the 3.0L before they went any bigger?

    120/104 cams are modified S grind. They are the best all around camshafts for hard-core street motors and mild race motors up to 3.0L displacement and 36-38mm ports.

    The bottom line is you built a small displacement motor with big ports. You will never get good bottom end unless you choke the carbs down to 34, but then you're pissing in the wind because you're not taking advantage of the BIG port. So strike the balance and go 36mm.

    I'm not up at 4000 ft elevation, but since the motor can't breathe up there you need to run waaaaaaay less fuel all the time. I still think F3 tubes are your best bet, and you should tune around that center (because that's where the tubes matter most ... in the center of the tach).

    Your car not idling is a function of poor carb setup and not camshaft/port choice. These motors will idle with 40mm ports and RSR sprint cams. You need to get control of your carburetors before you consider changing camshafts.
    -Marco
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  5. #15
    Senior Member Neunelfer's Avatar
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    Your constantly referring to 38mm ports as small tells me that you did a lot of Internet forum reading before you built the motor
    Smaller than 906 41's was the pretext of the original statement. When I referenced it again I was under the assumption they weren't too big for E or Solex grinds. And again, I was told that cam is more like a mod-906. I haven't got a clue but, I did also see that in other posts:

    http://forums.pelicanparts.com/porsc...120-104-a.html

    Here in post #3 Dave describes a similar issue:
    http://forums.pelicanparts.com/porsc...-120-104s.html

    Maybe it's the nature of the beast...

    The cams and the 38's are what was initially recommended. Oddly enough, It seems like it's exactly what you are recommending as well so; I'm trying to understand where this is going?
    They are the best all around camshafts for hard-core street motors and mild race motors up to 3.0L displacement and 36-38mm ports.
    So... I wanted to build a hardcore street motor. Let's just say I went with your recommendations.

    The bottom line is you built a small displacement motor with big ports. You will never get good bottom end unless you choke the carbs down to 34
    I'm not concerned with the bottom end. I never have been. It's actually exactly where I thought it would be. And, the carbs are already choked down to 34mm. The problem is, as stated in the title, the stutter and hesitation between 3-4k. It's right in my freeway drivability range. This is with your recommended F3's. After 4k the motor comes on like a beast.

    I think you're reading this wrong. I'll play around over the weekend and start over, beginning with 28* and report back.
    Eric - Sandy, Utah
    71 911
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    914-6/ORV
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  6. #16
    Quote Originally Posted by Neunelfer View Post
    Smaller than 906 41's was the pretext of the original statement. The only thing bigger than a 906 for street duty was a 3.2L Carrera. When I referenced it again I was under the assumption they weren't too big for E or Solex grinds. Those are midrange cams, and they wouldn't do much for you based on your current build specs And again, I was told that cam is more like a mod-906. I haven't got a clue but, I did also see that in other posts:

    http://forums.pelicanparts.com/porsc...120-104-a.html

    Here in post #3 Dave describes a similar issue:
    http://forums.pelicanparts.com/porsc...-120-104s.html

    Maybe it's the nature of the beast... It's not

    The cams and the 38's are what was initially recommended. Oddly enough, It seems like it's exactly what you are recommending as well so; I'm trying to understand where this is going? I am a huge proponent of the 120/104. Not so much BIG ports on small motors So... I wanted to build a hardcore street motor. Let's just say I went with your recommendations. We could say that, but it would be untrue. But that's not to say you're screwed; it just means you won't have great power until 4500. I wouldn't recommend 38mm ports on any street motor under 2.8. I just built a 3.2L turbo and ended up with 36mm ports, and my 2.5L dual purpose race motor has 36mm intakes as well.



    I'm not concerned with the bottom end. I never have been. Good. But there was talk about "torque" and such and that motor isn't designed to make any. So I wanted to clear the way for information relating to what your motor is actually capable of. It's actually exactly where I thought it would be. And, the carbs are already choked down to 34mm. The problem is, as stated in the title, the stutter and hesitation between 3-4k. It's right in my freeway drivability range. This is with your recommended F3's. After 4k the motor comes on like a beast. I am advocating for 36mm venturis. You're shooting yourself in the foot with those ports, cams, and vents. The larger Venturi will flow more air at higher rpm which will change your mixture and partially address your stutter issue. The 34 gets the air moving sooner, but they can't keep up with the volume of the 38 port. The F3s should provide plenty of midrange fuel, so a change in tubes is not needed. Your exhaust is just right, too, which aids in cylinder filling.

    I think you're reading this wrong. I'll play around over the weekend and start over, beginning with 28* and report back.
    I added comments in bold.

    Do you have access to an AFR meter? It would be good to know where you stand currently, but my gut says your venturis are wrong, and your air correctors and mains are too big. (The altitude thing gives me pause)
    -Marco
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  7. #17
    Senior Member Neunelfer's Avatar
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    Initially I was a bit timid to go over 36mm. The cams made me do it.

    I'm putting my 3.0 MFI motor back together for the 911 and you'll be happy to know I stuck with 36mm ports there.

    I have some 36mm venturis coming. I'm trying everything.

    I have some antiquated AFR meters here... Old SnapOn stuff. I can source something better.

    The real puzzle is the timing though. I swear it's a bear when set to 28* it just doesn't want to work with me at all. Set it to single plug specs and everything seems to fall into place.
    Eric - Sandy, Utah
    71 911
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    914-6/ORV
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  8. #18
    Quote Originally Posted by Neunelfer View Post
    Initially I was a bit timid to go over 36mm. The cams made me do it.

    I'm putting my 3.0 MFI motor back together for the 911 and you'll be happy to know I stuck with 36mm ports there.

    I have some 36mm venturis coming. I'm trying everything.

    I have some antiquated AFR meters here... Old SnapOn stuff. I can source something better.

    The real puzzle is the timing though. I swear it's a bear when set to 28* it just doesn't want to work with me at all. Set it to single plug specs and everything seems to fall into place.
    Yes, I'm happy about the 36mm ports in the 3.0L. I had a 3.0L mfi motor with 36 ports and 11.5:1 compression ... one bad motor scooter let me tell you.

    Good luck with the carbs.
    -Marco
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  9. #19
    Quote Originally Posted by Mr9146 View Post

    Second, 906 cams are shit street cams. They work from 5500-8000 rpm and that's it. Trying to make them "streetable" is a fool's errand. Steve Weiner would beat you with that 906 camshaft if he was in striking distance.
    Yessir,...he certainly would do just that.

    906 cams, while not great, work OK in a 3.5 litre or larger engine where their narrow lobe centers and long duration do not exact such a penalty. That said, there are FAR better alternatives now.

    Call me about jetting. I could write a book about this here, but my old hands won't go the distance anymore.
    Steve Weiner
    Rennsport Systems
    Portland Oregon
    503.244.0990
    E-mail: porsche@rennsportsystems.com
    http://www.rennsportsystems.com

  10. #20
    Try F26s, easily available and were used for the IDA(S) equipped 911S engines prior to the introduction of IDS carbs which did use F3s. They have a first hole just above fuel level that acts as a "early activation" to fuel delivery right in your speed range. Normally holes in the E-tubes act as mixture leaning features as a function of fuel delivery but when located slightly above fuel level they actually blow across the fuel right at the earliest portion of main circuit activation and help fill the lean hole you describe. They will require your main jet size to be increased by about 10 over what you have now but the leaning effect of the holes in the rest of the E-tube keeps fuel mixture from going too rich.

    Easy to try and reject if they don't help. If they do help then you have a direction to follow-up with.

    Get an AFR to help diagnose but seat-of pants is good to solve the big issues.
    Paul Abbott
    Early S Member #18
    Weber service specialist
    www.PerformanceOriented.com
    info@PerformanceOriented.com
    530.520.5816

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