News

Inside Plug/Outside Plug

Discussion in 'Vintage (thru 365 GTC4)' started by Maximillian575GTC, Feb 2, 2009.

  1. Maximillian575GTC

    Sep 28, 2006
    115
    NAS/NCE/YYZ
    Full Name:
    Max
    I saw a recent post by Jeff Kennedy asking this question in an unrelated thread and thought it could be of use to a few.

    What is the difference between and inside plug and outside plug 250 engine?

    Feel free to expound on 'feel', 'response', 'power', technical achievement or whatever else you can provide... Including directions to where I could find out more... (I love discovering Ferrari books so point away...)
     
  2. To remove this ad click here.

  3. billnoon

    billnoon Formula 3
    BANNED

    Aug 22, 2003
    1,176
    La Jolla, California
    Full Name:
    Bill Noon
    Hi Max,

    It is nearly exactly as the name implies.

    Early Tipo 128 and older Ferrari engines had their plugs on the inside of the engines "V" or in the same valley as the intake manifolds and carburetors.

    It gives a tidy appearance and these engines and the earlier examples like them going all the way back to the Tipo 125s rev like crazy but have gas flow issues at the higher rpm range.

    The late 128 series of engines used revised cylinder heads which saw the the plugs moved from inside the "V" and placed them between the exhaust ports on the outside of the "V." Gas flow was improved at the higher RPM limits and Ferrari stayed with the design from that point on.

    Some of the ultra-rare and exotic racing engines of the mid to late 1950s also had both inside and outside plug engines at the same time. These twin-plug set ups where found on the Tipo 410Sport, 290MM, 290S, 315S, 335S and a few other "one-offs" as well.

    Cheers,

    Bill
     
  4. Ed Niles

    Ed Niles Formula 3
    Honorary

    Sep 7, 2004
    2,486
    West Hills, CA
    Full Name:
    Edwin K. Niles
    As far as I am concerned, the major improvement of the O/S plug engine was the increase in the number of head studs, rendering it less likely to blow head gaskets.
     
  5. BIGHORN

    BIGHORN In Memoriam

    Sep 18, 2006
    733
    FLORIDA/NEW MEXICO
    Full Name:
    JOHN F KELLY
    +1. When the Factory went to 4 studs a cylinder from 3 it was a huge improvement made possible with the switch from hairpin to coil valve springs
     
  6. f308jack

    f308jack F1 Rookie

    Jun 7, 2007
    4,267
    Cape Town, South Afr
    Full Name:
    Jack Verschuur
    Just to be correct, it was me who asked the question and Jeff Kennedy who answered some of it.

    It never occurred to me that the hairpin to coilspring and 3 to 4 stud modifications co-incided with the change of the plug location.
     
  7. To remove this ad click here.

  8. Randy Forbes

    Randy Forbes Formula Junior

    Jul 14, 2006
    741
    Sarasota, FL
    Full Name:
    Sports Cars Plus,LLC
    Beyond all the good technical reasons (number of head studs and valve spring type) I think it was Phil Hill that said it made changing hot plugs easier too.

    I believe this to be at a time when one (1) set of plugs were used to warm an engine up, and a colder set used for sustained high RPM use.

    But hey, I've been wrong before...
     
  9. Jeff Kennedy

    Jeff Kennedy F1 Veteran
    Silver Subscribed Owner

    Oct 16, 2007
    5,339
    Edwardsville, IL
    Full Name:
    Jeff Kennedy
    As reference look to the Jess Pourret book on the 250 GT berlinettas. He does a wonderful job on tracing the developments of the 128 engines. It will show the cylinder heads as it changes from the inside plug 128D and before to the 128F.

    From the other thread on this subject - Ed: Did you ever notice an inherent throttle responsiveness difference with the inside and outside plug engines?

    Jeff
     
  10. loflyer47

    loflyer47 Formula Junior

    Sep 28, 2004
    328
    Phila. area, Pa.
    Full Name:
    Ben
    Also, with the moving of the plugs from the inside to the outside allowed for more room above the head to allow for valve seals which were not present on the earlier inside pug engines (just not enough room). It wasn't until the plugs were moved to the outside of the "V" and valve seals were installed that the eternal "smoking" at start up issue was finally resolved. Not that this was the only reason for the change..
     
  11. John Vardanian

    John Vardanian F1 Rookie

    Jul 1, 2004
    2,804
    San Francisco Area
    Full Name:
    John Vardanian
    Most importantly, moving the plugs out of the way eliminated the Siamese I-ports and allowed individual ones.

    Four studs, coil springs, higher compression, improved breathing all made for a more powerful and torquier motor; but the former is a featherweight champ and it's hard to beat its bubbly disposition.

    john
     
  12. To remove this ad click here.

  13. f308jack

    f308jack F1 Rookie

    Jun 7, 2007
    4,267
    Cape Town, South Afr
    Full Name:
    Jack Verschuur
    John,

    That statement empted me to inquire about this subject in the other thread, but I mistakedly attributed it to Bill instead of you.

    I think it'll be rather difficult to make any statements about the character of either engine, let alone a comparison, without knowing its' state of tune, but given the main characteristics one would assume that the 128F was an improvement over the D engine, maybe with a flatter torque curve in comparable state of tune, which would explain the to you 'bubbly' character of the 128D?

    Which cars did you compare to arrive at the conclusion?
     
  14. Ed_Long

    Ed_Long Formula Junior

    Nov 11, 2003
    678
    Salem, Oregon USA
    Full Name:
    Ed Long
    #11 Ed_Long, Feb 3, 2009
    Last edited by a moderator: Sep 7, 2017
    There were a couple of years in which our venerable 250 pf coupe with the outside plugs shared the garage with an inside plug Boano. There was no comparison between the performance of the two cars. The Pf was very gentile, smooth, with gradual power curve and easy to drive. The Boano was a beast. You press down the go peddle on the Pf and it would take off, but not like the Boano which would blast off. You had to be quick with your left foot to get to the clutch and engage second gear.
    Image Unavailable, Please Login
     
  15. Jeff Kennedy

    Jeff Kennedy F1 Veteran
    Silver Subscribed Owner

    Oct 16, 2007
    5,339
    Edwardsville, IL
    Full Name:
    Jeff Kennedy
    John,

    I thought the second series 166 went to individual ports (not siamesed). That would mean that it would have remained an inside plug engine.

    Jeff
     
  16. Julio Batista

    Julio Batista Formula 3

    Dec 22, 2005
    2,397
    Very interesting thread...

    I used to own an outside plug GTE, and I am currently getting my new inside plug PF in shape. All people concerned tell me that the inside plug engine sounds better and is more fun to drive. My mechanic prefers them. What I am reading here seems to confirm this, although I still fail to understand why the inside plug "revs better". I hope the opinions keep coming!

    Cheers,

    Julio
     
  17. kare

    kare F1 Rookie
    Consultant

    Nov 11, 2003
    3,297
    I find the mouse trap spring set-up a clever design with long evolution on its back, meanwhile early coil spring set-ups were heavy and clumsy simply because the springs required the use of longer valves. Changing to coil springs might very well have been a set back, that took a while to solve. Best wishes, Kare
     
  18. John Vardanian

    John Vardanian F1 Rookie

    Jul 1, 2004
    2,804
    San Francisco Area
    Full Name:
    John Vardanian
    To answer the question, I haven’t driven many Ferraris. My opinion on OP vs. IP comes from driving my own two cars. Like Ed Long, I have gotten to sample like for like.

    Another disadvantage of the IP engine, one that we haven’t mentioned earlier, is the general smokiness. There is no practical way to seal the valves or the guides of the mousetrap heads. You can convert an IP from mousetrap to coil, but the results tend to be hit and miss. To convert to coil requires cutting seats. Depending on the density of a given spot in a given head, shaving the denser top layer sometimes has resulted in water/oil mixing.

    I don’t think porting of the 166MM SII should enter our arguments her since it’s not a production motor.

    john
     
  19. Fontana

    Fontana Karting

    Dec 30, 2006
    177
    Vermont
    Full Name:
    Peter Markowski
    Doing a coil spring conversion makes a Inside plug engine come alive. We have done a number of conversions from 166-250's what a great deal!

    Comparing a late inside plug engine to a early outside plug engine.... I like the inside plug engine, seems more responsive, smoother and more fun.
     
  20. Eagle1

    Eagle1 Rookie
    BANNED

    Feb 13, 2009
    48
    Wisconsin
    Full Name:
    Mark Poulter
    We have done many of these conversions as well. The rule is to never machine down into the original valve perch deeper than the casting depth of the head. This way the integrity of the cooling passages will be preserved. We machine our own valve guides and retainers and have enjoyed many successful conversions.

    I.P. or O.P.? I've found the I.P.s are a bit more responsive.
     
  21. John Vardanian

    John Vardanian F1 Rookie

    Jul 1, 2004
    2,804
    San Francisco Area
    Full Name:
    John Vardanian
    Hi Mark,

    This subject of good quality mousetrap springs has come up and gone away a few times. But, I just wonder, (and I'm not questioning the many sucessful conversions,) there is a whole vintage motorbike world out there that survives on these types of moustraps. Has anyone really dug deep into the cottage industries that sustain the vintage motorbike hobby? Could these guys be of help to us?

    I wonder how the Brits, for instance, go about redoing these heads. (Andrew T, are you reading this???)

    john
     
  22. Eagle1

    Eagle1 Rookie
    BANNED

    Feb 13, 2009
    48
    Wisconsin
    Full Name:
    Mark Poulter
    John,

    That is an interesting point.... I've heard of this platform being used in bikes, but never really pursued it much. Do you remember what kind of bikes used these springs?

    I will say the double bee hive configuration exponentially reduces the chances of something catastrophic happening ("grenading") in the event of a spring failure/mishap.

    It is hard to forecast the life expectancy of these mouse trap springs I'm not sure what condition is worst... sitting for 20+ years or being driven every day..... (the age old question ...ehhh?)

    Have a good day,

    Mark
     
  23. John Vardanian

    John Vardanian F1 Rookie

    Jul 1, 2004
    2,804
    San Francisco Area
    Full Name:
    John Vardanian
    Hi Mark,

    I haven't put much research in it, but the time will come. Personally, I'd just as soon test the old springs and reinstall, rather than cut the heads.

    john
     
  24. t walgamuth

    t walgamuth Formula Junior

    Mar 13, 2005
    850
    I think I am with John on this. The longer valves would be heavier and limit the ability to rev, providing the spring rates are equal. Heavier valves would require heavier springs to allow the same rpm.

    Another difference in the in vs out engines is the oil filtering arrangement. When I disassembled my inside plug engine I was shocked to find that the huge filter was only filtering a small portion of the oil flow and most was simply going through that large strainer.

    No wonder folks say they are worn out at 50K miles.
     
  25. 246tasman

    246tasman Formula 3

    Jun 21, 2007
    1,429
    UK
    Full Name:
    Will Tomkins
    #22 246tasman, Feb 26, 2009
    Last edited: Feb 26, 2009
     
  26. t walgamuth

    t walgamuth Formula Junior

    Mar 13, 2005
    850
    #23 t walgamuth, Feb 26, 2009
    Last edited: Feb 26, 2009
    To install the coils are the seats for the springs milled into the head far? Are the cups on the top of the springs recessed for the top of the coil? The tops could not be recessed much I wouldn't think. I just remember that there was no room at the top ofthe valve guides to install a proper seal, so I was wondering where there would be room for the spring.

    If the valves are the same length, thats impressive! they are very small things and its easy to see why the engines didn't mind revving up.

    Of course on reflection the valves must be the same length unless you have different rockers, or change the location of the cam, neither of which is at all feasible.
     
  27. 246tasman

    246tasman Formula 3

    Jun 21, 2007
    1,429
    UK
    Full Name:
    Will Tomkins

    Only a touch of milling in the head, and we have room to fit valve guide oil seals.
     
  28. henk3

    henk3 Rookie

    Feb 14, 2011
    46
    Amsterdam
    Full Name:
    Henk de Vries
    For many years now I have owned and driven (hard) a GTE, and I am just getting a 250GT inside plug. Will report on the differences in driving and feel here.
     

Share This Page