performance cam problems: flipping shims

Discussion in 'Technical Q&A' started by snj5, Oct 6, 2007.

  1. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    San Antonio
    Full Name:
    Russ Turner
    This is a cross-post from the 308 setion, but may have a general applicibility to all cars with the Ferrari style shim over bucket cam foower, so I'd thought I would also post it here for info and comment

    The 3.0qv and 3.2 heads are identical except for intake cam lift - the 308 having .303 and 3.2 liter having a lift of .348, both having durations around 224 which sets the power curve. At the lift of .348, the stock head is max flowing as I found out: When we put hotter intake cams of .367 lift and 235 there was no improvement on the dyno output of the car.

    As many know, we are now putting my engine back together after porting the heads. As part of the exercise, we sent Lori at Webcam a practice qv head and my 367/235 cams to see if we could improve the lobe. What I have learned is that there is a point where the effect of very high lift can act to flip the lash shim off of the bucket cam follower. After she looked at the current 367 lift cam she said that this was the most she would recommend with the stock Ferrari buckets and shims, and that even backing off a bit might be better for long term wear characteristics and reducing the possibility of throwing a shim from to aggressive a cam profile. Of course, there is the somewhat expensive answer of going to a 'shim-under' bucket system, but this could entail a fairly extensive re-design of the the valve system.

    In all the small measured tweaks I have done to my engine, long-term reliabiliy and ease of maintenence has always won over going for the highest possible dyno number. With shim under buckets, the cams would have to come out at every valve clearance setting to adjust the lash, but I could run much, much more aggressive cams. Right now, the appeal of bulletproof reliability, straightforward repair and service again seems to be the compromise versus all out power.

    Even with stock 328 cams, the computer modeling predicts a pretty good result after the port and polish, and I am also going up to 10.5:1 static compression with slightly larger 44 mm Webers. My wild ass guess wold be the engine will hopefuly come in at 100 - 104 hp/liter without going to the shim-under buckets.

    I'd be very interested if anyone has personal experience with much higher lift cams with rspect to reliability and premature wear of valve train components.

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  3. fatbillybob

    fatbillybob Two Time F1 World Champ
    Consultant Owner

    Aug 10, 2002
    IMO it is easier and cleaner and faster to remove cams to adjust tappets. Also, they rarely need adjusting in the 7500mile interval and only 1-2 shims need to be changed at majors in my experiance. Bucket over shim may not be a hard conversion. FIAT/maserati/ALFA/Ferrari all very incestuous. My old Maserati used buket over shim for example and you may get lucky and find an off the shelf solution that will not break the bank. Good Luck.
  4. staatsof

    staatsof Seven Time F1 World Champ
    Silver Subscribed

    Mar 13, 2005
    Full Name:
    Two of my Maserati's and the Lambo all use the shim on stem under the bucket and I have to tell you I at times I do wish I could change the shims
    the way you describe but two of my Maserati's have very aggressive cams since their turbo so I guess that might not have worked out anyway.
    My older Maserati uses a disc style shim but it still sits under the cup in the top of the spring retainer.

    Look here:


    #15 here

    Cosworth and other's shims fit the other 2 Maserati's part #15. So I bet you could convert as something is bound to fit. What's the valve stem size?

    Bob S.
  5. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    San Antonio
    Full Name:
    Russ Turner
    #4 snj5, Oct 10, 2007
    Last edited by a moderator: Sep 7, 2017
    here's a photo from Laurie at Webcam showing the cam track that she says will tend to flip a shim. Of course, I'm not really sure what I am looking at here... this is with a 0.368/235 cam
    Image Unavailable, Please Login

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