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Hotrodding the 10,001+ GT6 engine


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Saturday I drained the oil for an oil change, stuck my pinky in the hole as usual, but this time there's Something In There.

I don't know if it's the result of having always run on unleaded or me always driving it like a bat out of hell, but after only 25 years or so on the bottom end & 20 years on the head, it looks like the engine must be dealt with again.

I have another 10,001+ engine waiting in storage, so I'll be taking it apart & putting it together for installation.

Like the last engine, I would like to give it some performance modifications.  The problem is that now that I know more about the GT6 engines, I realize I actually know less about what to do with the 10,001+ ones.  

It seems all the reference material I was working from for performance modifications dealt only with the pre-10,001 engines.  Factory competition manuals, & apparently even correspondence with the BL/JRT competition department.

The one in now had the head ported & polished, & skimmed "by the numbers" to get a 10.25:1 compression ratio.  I really believe now that given that the US versions of the 10,001+ engines had flat pistons (like this block), I may have only increased it from the US 8.0:1 to maybe 9.25:1 or something unknown.

I've noticed that all the performance camshafts (factory & aftermarket) were developed before 10,001.

The variables with the camshafts, carbs, piston shapes, & head are so dizzying that I'd sort of like to just give up & tell Chris Witor "send me a nice sprightly head & camshaft for the 10,001+ GT6!", but with the feeble state of the dollar, £500 plus shipping cost of  those chunks of iron to the US is a bit much this close to the holidays.  So it looks like I'll have to figure out what to do with what's on hand.

Hypoid mentioned that the head change at 10,001 was fitting the 2000 head, so it seems to me that basically I'll be working along the lines of performance mods for a 2000 more than what's typically thought of as a GT6.  You folks are more familar with the 2000, they are rare over here.

At any rate, does anyone have thoughts on:

1.  Head/block:  Can the head be reasonably skimmed to achieve 10 or 10.25:1 without decking the recessed block & using flat pistons?

2.  Camshaft:  What would be a good performance road camshaft with such a skimmed 10,001+ head?  I'm a little leery now of the standard GT6 choices, since all seem to have been developed for the pre-10,001 head.

3.  Carbs:  I thought I'd found a great upgrade when noticing the Mk3 Strombergs got different needles for the higher-compression & hotter cam UK GT6's, but Hypoid blew a hole in that by revealing that was only the case pre-10,001.  Rolling road shops around here won't help much for needle selection because they won't have a colleciton of CD/CDS *or* CDSE needles to try out, so selection will have to be as best as can be done by the book.  I've often wanted CDS carbs to take advantage of the CD/CDS needle selection, but have seen few CDS carbs here (only CD or CDSE).

4.  Ignition:  Engine #2 has no distributor, so I might as well get one for it.  The 123 is attractive, but it's out since I'll not give up my mechanical tachometer.  My GT6 has inexplicably had a Lucas 22D6 instead of a Delco in it as long as I can remember.  What should go in the new engine?  What can be found over here, maybe something the TR6 used?  I notice the 2000 used the 25D6 & 43D6.  What's the bag with the 22D6?

My head hurts now.  Any thoughts appreciated.  There goes the budget for a high-entertainment exhaust manifold for a while.

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Well, if it is any consolation, I am fitting the pre-KE10000 cam 308778 which was the home market 26-65 cam (270 degree duration) fitted to the Mk2 Vitesse, GT6 Mk2 and early 3. To this I have added a 22D6 Lucas distributor (replacing the horrid Delco D204) from a 1970 Vitesse Mk2, so it is already set up for this cam. Using the 'favourable' exchange rate I have also purchased a pertronix ignitor for use with this distrubutor. I will be refitting the B5BT needles as they were as pre-KE10000 spec.
I understand that the 2000/2500 type head can be skimmed considerably to raise compression ratios with flat top pistons, but Chris Witor suggests that the block should be decked so the pistons are 10 thou above the block face to improve combustion quality as a 'priority modification'.

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(I hope this link still works!)
As rotoflex was asking about 2000's he might find it interesting
The only thing I have sourced from Mr Witor was some good quality tappets.

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jimio wrote:

Gareth, speak to GT5-r about Witors loony ideas, get him to show you his video of a GT6 lapping Goodwood with a witor inspired engine, then see if you still call him a loony!


Not quite sure how I've been linked with Chris W on this one - the engine in the video was not insipred at all by Chris - there is no way I would not run a + ve deck clearance on a high reving short stroke block - with regard to the performance of this unit it is nothing speacial - if it were footage of my actual race car then my lap times are 14 sec quicker than the one feature on the video - which for a GT6 on a track that is 2 1/4 miles long is a substanial difference per lap


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Thanks for the link, Hypoid.

Triumphs over here are almost exclusively TR's & Spitfires (with an occasional odd Stag), so experience & discussion involving the other models is pretty much nil.  The variation in manifolds for the 2 litre engine is interesting.  

Mention of the CDS carburettor with the 6J needle reinforces my interest in finding some CDS carbs sometime.

Your 22D6 is from a Vitesse!  I can't figure where mine's from, except perhaps the BL dealership long in the past put it on when I had work done there.  How they got hold of it I have no idea.

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"The one in now had the head ported & polished, & skimmed "by the numbers" to get a 10.25:1 compression ratio.  I really believe now that given that the US versions of the 10,001+ engines had flat pistons (like this block), I may have only increased it from the US 8.0:1 to maybe 9.25:1 or something unknown."

If you are in any doubt, then you haven't done it 'by the numbers' but by a crude estimate based on the height of the head.  The one and only way is to measure the volume of each chamber.  This allows you to KNOW the theoretical CR, which will differ from the actual CR by the flow characteristics of the ports but will always be higher (and so the actual is safer), AND to ensure that the chambers are the same volume across the head and so equalise the CRs.


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Thanks John.

Yes, that is exactly what happened.  The head gasket expired while the GT6 was my only car in the mid 80s, & I figured that while I had the head off I could hold out briefly & squeeze just a little more out of the head.  With little free time, I just dropped the head at a machine shop, told them how much to lop off, showed them the simple head clean-up in the factory competition manual & told them to do that, & said I'd be back in a week.

For others who might find themselves with that situation & idea, it was the wrong thing to do.  When I picked it up it looked like they did the work with a chisel, but I was out of time & had to slap it back on.  I plan to be more scrupulous with the work this time.  I've got the support car to get around in.

GT, who are TRI?  I thought the only SU's that would fit under a GT6 Mk3 bonnet were some rare-in-the-US short variants of the HS4.

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