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MG MGF Technical - Cams and cam timing: some rhetorical questions

Hi all,

just a couple of questions. If one assumed that one had a completely standard head, what benefit, if any, is there in getting the standard cams re-timed? Would there be any power gains through re-timing the cams using vernier gears?

Second question concerns the 135Ps 1.8 MPi engine's cams. Anyone know the specs of these, and how they compare with other cams, both standard and after-market?

Okay, pretty mad questions, but there is method to them! ;o)
Rob Bell

"a rhetorical question is one asked solely to produce an effect
(especially to make an assertion) rather than to elicit a reply"

I guess you don't want an answer then? (rhetorical)
Will Munns

looked that up on www.dictionary.com did you Will? ROFL!

I was more asking a question, to which the answer would not necessarily be acted upon ;o)
Rob Bell

theoretical

Will
(why can't I post this?)
Will Munns

Theoretic , Theoretical , a. [Gr. ?: cf. L. theoreticus, F. th['e]or['e]tique.] Pertaining to theory; depending on, or confined to, theory or speculation; speculative; terminating in theory or speculation: not practical; as, theoretical learning; theoretic sciences. --, adv.
Will Munns

ah, it doesn't like the *&^&* giberish put into dictionarys, thus stripped (full version on Dictionary.com ;-)

Will
Will Munns

LOL - okay Will, yeah, theoretical might have been a more appropriate term (although hypothetical would be much better), although I think I know the answer to the first question but wanted confirmation (which would be more-or-less appropriate use of the term 'rhetorical question').

SO... to restate:

1. If one assumed that one had a completely standard head, what benefit, if any, is there in getting the standard cams re-timed? Would there be any power gains through re-timing the cams using vernier gears?

2. Regarding the 135Ps 1.8 MPi engine's cams. Anyone know the specs of these, and how they compare with other cams, both standard and after-market?
Rob Bell

Just a guess, but maybe the effect would be the same as getting the ignition timing re-mapped?

:)
Smiley

That'd be my thinking too Smiley - re-timing the cams would have a modest effect - perhaps ±5%
Rob Bell

It's less likely to have an affect than re-timing aftermarket cams because the sprockets are manufactured to give the correct lobe centres for the standard cams, after market cams are often cut for different lobe centres and will therefore *always* be wrong on the standard pulleys. That said there is sufficient innacuracy in the sum of the standard parts to make retiming worthwhile, the gains you get will be according to how far out the timing was to start with. I'd say an estimate of 5% wouldn't be far out.

The 135PS engines cams are standard.

Dave
Dave Andrews

Thanks for that Dave.

Stunned to discover that the 135 uses 120PS engine's cams! Perhaps erroneously, I thought that the profiles had been altered... So the 135 gets its power from the revised induction system and exhaust back box?
Rob Bell

A "hot" cam will always give some pwer gain at the expense of flexibility. This is why the VVC is so clever, a docile cam at low revs and lots of overlap for extra power at high revs.


Brian

Bri,

Close to the truth, but it depends where you are starting from in terms of duration and overlap, clearly a cam with no duration and no lift will give no power at all so there is a sweet spot beyond which low RPM flexibility is lost, on the K that is at around 260-264 degrees, so there is plenty of scope for cams which improve torque everywhere usable without dropping any low speed torque provided the cams are properly timed.

The only problem with the VVC is that it has extendable duration, but meagre lift and also chronically slow valve acceleration which cuts the lift at TDC and therefore *effective* overlap, that's why the torque although flat isnt that good.

Dave
Dave andrews

Zoom..... that was a lot of stuff going over my head.. :0)
tim woolcott

@Tim,
don't worry. Check out everything on this silly site and you will be able to tell Dave *what's going on*
;)
http://www.kfz-tech.de/index1.html
Only German language :(

http://www.kfz-tech.de/Ventilueberschneidung.htm
This is what he said about *overlap*, I think.
Dieter K.

In my teenage motoring days I read a book by John Sprintzel about tuning A series BMC engines.

He started by breathing on his mother's A30. A skimmed, airflowed head, bigger carb and hot cam made the car go a lot faster but with no torque below 4,000rpm. Not a lot of good for shopping.

I'm sure it would be only too easy to do that to a K series.


Brian

I'm sure that working with a K-series head is not so far removed from an A-series in the principles used Brian :o) Differences occur in the details - not least that the K-series head has far better port designs...

>>The only problem with the VVC is that it has extendable duration, but meagre lift and also chronically slow valve acceleration which cuts the lift at TDC and therefore *effective* overlap, that's why the torque although flat isnt that good.<<

I had heard that Piper had been working on a VVC cam with greater lift (?) Why has this now apparently been abandoned Dave?

Dieter, will look at those sites later - will take a while to understand them once Altavista has 'monkeyed' with them to get them into English! LOL
Rob Bell

Sorry, not been able to post for 24 hours..

The VVC cam redesign was abandoned because of fears of the VVC mechs failing under increased load, although I have a number of VVC exhaust cams with a better profile on them for anyone who wishes to upgrade, these have 12 degrees more duration and .7mm extra lift, in conjunction with a ported head and 53mm TB they make quite a difference.

Bri,

The differences between the A Series and K series couldn’t be more marked, The ‘A’ series is a small bore, long stroke 2 valve engine with limited valve area, siamesed ports, horizontal port entry and carotid chamber shape. The K by comparison is light years ahead with semi downdraught ports, loads of valve area by comparison, neat compact chamber design with fast burn times and massively better volumetric efficiency. I spent 10 years building A series engines and know their strengths and weaknesses quite well.

Gererally 2 valve engines have such poor volumetric efficiency that it is necessary to rev the engine high to make any serious horsepower, to do this requires a great deal more cam than that required on a 4 valve engine hence the drop off in low/mid range torque . A 4 valve engine requires a lot less cam and is able to make more torque per litre than a 2 valve and sustain this through a much wider RPM band.

Consider this, the 649 cam, widely used on the A series has 300 degrees of duration and 95 degrees of overlap, even the lowly formula junior 544 has 290 degrees and 80 degrees of overlap, the sprint and supersprint profiles are even more extreme. Even a 649 equipped engine is lucky to push over 100BHP per litre, by contrast the K series engine can produce over 135BHP per litre on 1227 cams, these have 280 degrees of duration and only 68 degrees of overlap, because of these relatively mild settings the bottom end torque of the engine is not so radically affected.

Sure its possible to overcam the K series, one or two well known tuners do this because its cheaper to bolt in a pair of cams than to do work on the rest of the engine which may be more labour intensive and require a lot more time and skill. My point is that you can go a lot further than the stock cams without compromising the engines performance.

Dave
Dave Andrews

This thread was discussed between 04/02/2003 and 06/02/2003

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