Oversized valves on 8v head

Discussion in '8-valve' started by jc.., Oct 22, 2003.

  1. jimbo Forum Member

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    sorry bill [:D]
    I forgot to mention that they couldn't dial out the pinking with the programmable ecu so they swapped to a stage two head and it sorted every thing out. they were able to set the ecu so it was just right. And the car now drives as it should with plenty of low down 2ltr torque.
    But i wouldn't like to say wether the problem was to do with to much air flow thro the head or to do with the CC.
    like madmk4 i would like to know any relationship between valve sizes and CC.
     
  2. 8vvw New Member

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    Hi there i have done a fair amount of flow bench work on cylinder heads over the years. this is how i look at each head. Analise each part of the port, take a cross section and asses this sections flow potential, you will find that most of the port has the potential to flow far more than the fully assembled unit will flow. the biggest rescriction is the valve it spends very little time fully open where any improvements in port flow will show up, most of the time during a valves open period it is ofering the most rescriction to port flow ie just off the seat. The area to make improvements to port flow is the valve seat and valve. By all means fit bigger valves but if the seat area still flows poorly little gain will be made.
    Just because a head look good dosn't mean it will flow well.
     
  3. chrismc Forum Junkie

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    No, you didnt state that at all!!
    If thats what you mean then write it!! [:x] Dont make a snidey remark, like im some tit who doesnt know what he`s on about...

    Im not sure what ive done to get you started, & I am not preaching to anyone, just stating the facts as I see them.

    The head is being reworked to a Pack C spec by Autotechnik, who, incidentally, do all of their own head work, so it will be a proper job, & not just bigger valves to look pretty!

    If you read back through the thread, you will note that & C&R car with programmable ECU ran better with a stage 2 (40/35mm) head than a Stage 3 (42/35mm). What does this say about the gains of running bigger inlet valves??? Steve Cresswell himself told me to forget bigger inlets & concentrate on the exhaust side of things....

    For the record the Carvell mk1 IS a 2L 8v with standard internals, 304deg Schrick cam, Twin weber 45s, standard management, BTB exhaust, Quaife ATB LSD & gear set!!....circa 180BHP as per PVW group thrash AUG 02.

    I should imagine it also has a 42/35mm Pack D "race" head although this isnt listed..

    180 BHP isnt going to happen with an 1800 8v. Most "tuned" 2L 16v mk2s fall short of this figure!
    Edited by: chrismc
     
  4. jc.. Forum Member

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    Woooh....

    Tempers running high here, wish I hadn't brought it up.

    We shall see.

    I would be interested to see what porting is actually done in the throat of the exhaust outlet before it is reassembled.

    What are you paying for this job Chris?
     
  5. smithy Forum Member

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    My car produced max power at over 5600rpm on the rollers (sorry to nitpick) and thats with my own headwork :)

    I know what you mean though, the valves are smaller than vaux/ford etc and the 8v's dont rev out as well as other 8v's of the same era, but isnt that they characteristic of the 8v engine anyway? Why not go 16v if you are looking for increased top end power?
     
  6. G60Dub

    G60Dub Forum Member

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    Fair point smithy. At the risk of backtracking my jalopy (aint even remapped yet) wi std valves will pull cleanly all the way to the 6.5K limiter but I have a mild CAT cam - although it may be a seat of the pants con cause the torque curve will no doubt be a lot flatter than a longer higher cam?
    However pre headwork it never done that as it well ran out of puff at around 5.2K.
     
  7. chrismc Forum Junkie

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    I meant on a 2L smithy, sorry!! :o

    1800s tend to rev a fair bit better due to their shorter stroke & the fact that it isnt maxxing out a Stage 1 head, whereas a 2L doesnt breathe at all well in comparison. Mine made max.power at 5500rpm with all the flowing work possible, including matched manifolds, hence the only option is to go to a Stage 2 spec.

    I didnt mean to lose my rag with madmk4 but I just think there are ways of politely telling someone you disagree. Theres no need for nastiness...Its a friendly forum & I for one want to keep it that way :)
     
  8. M.C. Forum Member

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    Hi,

    Whilst this is fresh in evryones minds, I thought I'd ask a question:

    As part of my 1600 8v engine build, I was going to use 1800 size valves. I've then seen that the 1600 scholar engines used 40/35mm valves, and wondered if they go in with no problems. I know with Vauxhall small block 8 valves you are limited in size as the valves are too close. IS the head design the same for the 1.8 and 1.6?

    Cheers,

    MC
     
  9. smithy Forum Member

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    ChrisMC - I thought that after you posted! [:D]

    1600 and 1800 heads are the same, bar the digifant 1800 head which uses 2mm bigger inlet valves ;)
     
  10. scottmk1 Forum Member

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    what engine code on the 1600, if its an eg, im sure that the valves cant be made any bigger
     
  11. M.C. Forum Member

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    I managed to get the head off on Saturday, and the gap between the valves is tight. I didnt check the engine code, didnt see the last reply till now, but the car is a 1982 1600 if that helps. The gap between the valves is tight, only 3.5mm. The inlet valve is 38mm and the exhaust 31mm. If I wanted 40 and 35 I would be looking at a 0.5mm gap. Both valves run parallel, so shouldnt touch, but I know that valves do vibrate. If this head cant have those valve sizes, what head can I source that will? I want to keep the K-jet system.
     
  12. jc.. Forum Member

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    I spent Friday PM at my local engineers.
    Apart from inadvertantly being shown a ClubGTi members project, he also showed me a head he built for a sprint car customer.
    The valves were both oversized and were so tight that the new valve seats had to be machined into one another!
    The gap between them was probably less than 1mm.
    The port job was viscous! left the valve guides so short he was replacing them after four or five meetings.

    He was also running 84mm pistons in a 1.8 block and standard short throw 1.8 crank to give 1915cc and a CR of 12.5:1

    Not much space in a 1.8 head for oversized valves but they fit
     
  13. concept Forum Member

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    hello jc,

    i'm picking up my tuned motor now from alex at engine services! those guys are good!

    if you saw my head you'd see where he opened up the combustion chamber around the valve seats to prevent shrouding. we have done a lot of work on this motor but have opted to keep the standard valve sizes at the moment even with 2057cc. i think the c.r. is at 11:1 but i'll find out for sure when i see him.

    once this is all set up i may consider bigger ex valves if it needs more top end, but in the mean time i cant wait to get building!
     
  14. jc.. Forum Member

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    doesn't look like you will need them!!
    I am very interested to see what its like when finished
     
  15. concept Forum Member

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    i've got all bits home now so will be putting parts back together! c.r. is 11:1, its all looking good. will keep progress posted and show some pics tomorrow i think!
     
  16. MotorcyclesFish Forum Member

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    I don't know if this has been mentioned much, but i would say that rather than absolute size of valves as a limitation to performance, that the ratio of inlet:exhaust valve cross sectional area should be considered. As with any gas flow situation, smooth flow is as important as mass flow rate, and turbulence would be reduced by an appropriate ratio of valve areas - especially during overlap phases. I don't think you can argue the benefits of any one size, as on engines like these, torque is as important as peak power, and torque is not always aided by a huge free-flow system - rather it depends on a certain amount of backpressure. The ford-vw cross-section comparison is interesting, but iirc, they use totally different combustion chamber designs, resulting in different gasflow dynamics, resulting in a different ideal flow setup dictated by the valves.
     
  17. chrismc Forum Junkie

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    I guess I was a bit wide of the mark with my VW to other manufacturers valve size comparisons....

    Just seemed that the VW sizes were particularly small, even in comparison to engines of a lower cc

    Hopefully shall have a decent idea of what difference the Big Valve Head makes to the breathing on the 2L within a week!
     
  18. set_aui New Member

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    So, I have my car back with big valves 42.7mm,34.9mm and the Schrick 276 cam with all Schrick valve gear. No other mods yet. Idle a bit lumpy but OK. Torque seems to be the same as it was standard up until about 5000-5500 then she goes like an absolute rocket up until redline (and beyond I expect). I really need to up the maximum engine speed. I will post some pictures of the head work soon.
     
  19. jc.. Forum Member

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    set aui...what did you pay for 42.7/34.9 valve job?
    Who did it and what problems did you have with seat overlap? Is the combustion chamber the same?

    Sorry lots of questions..
     
  20. 8vvw New Member

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    hi there i agree with motorcylesfish that bigger is not always better but i disagree with his thoughts on backpresure on a four stroke engine any back presure saps power and torque, the reason you may lose torque when fitting large bore manifolds is due to the reduction in gas speed.
     

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