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Engine rebuild and camshaft options


siroldvolks

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Hi All

Well this year as been a very quiet year for my gt6, mainly due to engine issue taking it off the road for the worst time in the year!  Missed using it for sure.  

I'm just finishing off an engine rebuild due to a piston problem, and a rather poorly one at that on removal, see attached...top ring is fused!

Fortunately not too much damage, had the block rebored to +.030 (was .020 previously).  Crank has gotten away with just a polish and re-balance although it's been reground -.010 in the past. Short block is pretty much ready to mount back, but one thing that has been bugging me is camshaft options.

The old camshaft was pitted on a couple of lobes (311399, gt6 mk3 cam type) so I got new one from blank several months ago ready to go back in.  however in the back of my mind thinking should I put something a little less limiting.  I'm thinking along the lines of the cam profile from earlier gt6 (308778?) with a slightly longer duration - im aware it's quite a rare cam now and only seems available as a reprofile.  In real world terms, would I gain/see much difference?  I'll be honest and say it's area of mods I've never delved before, but keen to at least explore and understand more about.  But otherwise the car is stock, domed type pistons, standard strommy CD150SE's and head too.  I don't intend to change anything else, at most I would probably ever do is an aftermarket manifold if I came across a suitable one.  or is it more hassle than worth fitting another cam considering the lower CR/different head etc?  is it even possible with having domed pistons and just stuck with the standard cam for the mk3?

I am trying to keep the budget limited where possible, but considering things that are really worthwhile doing whilst I can - what about fitted duplex timing kit or is that really a peace of mind thing?

thank you for your help

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On last year's RBRR one of the other drivers commented, on passing my Mk3 at Land's End, "that's the really quick GT6". Like yours, it's a late engine with domed pistons and is on the standard camshaft with standard carbs and manifolds. In fact the only thing non-standard with the mechanicals, apart from the J-type overdrive, is the Herald 13/60 diff that some previous owner has fitted. One of these days I really must find a 3.63:1 to get more comfortable cruising.

Anyway, to your question. I think you probably can get away with the 308778 without any other mods and see some benefit. You'd probably benefit slightly more if you also got reprofiled needles in the carbs (I'm sure there's a normal setting to use, as it can't be a rare conversion). To get a properly noticeable power increase, though, is going to cost some money.

The duplex timing kit isn't worth it. The reason they fitted it to the 2.5 engine is purely because the injection pump puts extra load on the timing chain. If you're on carbs and not intending to push the revs up, then the simplex chain is fine.

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Excellent thank you, that's all is a great help !  certainly goes along with my thinking...without investing too much £££ and I'm not after major gains.  I think on balance I realise how tired the old engine was   I've replaced/done way more already that I originally anticipated, but I guess that's always the way   I've also has the head redone with new inserts, although it had double valve springs fitted which i don't think is stock for the mk3. I'll replace like for like however.
I'll give the duplex timing kit upgrade a miss too

Will give Newmans a buzz this week and see what they can do - hopefully there isn't too long lead time

cheers

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Eeek... So thats what was causing your 'tapping noise' and scoring on the bore/ piston damage.
Looks like a ring seized then freed itself up.

I wouldn't fiddle with camshafts to be honest. A good Mk3 engine with good fuelling and an accurate spark pulls really well.
My new Mk 3 had a stock set up and was greatly improved by electronic ignition (a 123) refurbed NOS carbs and an electric fuel pump.

I think once you start going down cams/needles/ manifolds you are in for a lot of pain...
Regards
Brendan

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Hi Brendan, yes it was indeed!   Pretty much piston slap on #6 as there was a noticeable rock on first inspection.

I also side you on those thoughts.  I am in such a dilemma on which route to take, I guess it's more the not knowing without trying really but I know it a slippery slope once you start....I don't have much to compare to/remember as a good setup but longed for it not running out of steam after 4k.  
Have spoken to Newman and happy to make up a 308778 cam profile which is great, and as I'm nearing early mk3 territory in terms of CR calculations and along with a bit of a needle profiling I shouldn't be far off matching setup to suit.  I've always got a new 311399 cam spec, so it's not too much of a biggy to swap that back in if I don't get along with it.

I need to make a call by the end of the weekend or else I'll never get it done to enjoy the last of the good(ish) weather!

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