Re: Domed or Flat pistons...GT6 MK3
Hi,
I don't want this response to sound like I'm "fire hosing" your idea! I think it's great that you are thinking about things and considering at your options. It's also very good that you are floating ideas here to see what others who have had some experience with these cars think about them.
Yes, in order to install the domed pistons, I'm 99% certain you *will* need to convert to the early/narrow GT6 cylinder head (someone please correct me if I'm wrong!). I think it likely there will be other things needed to be changed, too, such the head gasket. Was there also some difference between these engines in terms of crankshaft/stroke? I don't know this for certain or if it's related or would be necessary.
Take another look at the VB catalog page you referenced. It shows domed pistons in the GT6 Mk I & II up to 1970 and flat pistons in the Mk III from 1971 on. This makes me think... I can't help but ask why did Triumph change the head and pistons in 1971?
This might be a partial answer you would want to consider before making any changes:
Here's what Triumph's Competition Manager in the U.S., Kas Kastner, had to say about the early/narrow GT6 cylinder head:
"If you are intending to race your car then stop now and find a late model head that has the larger valves. There is no point in wasting time and money and effort on the early type head when the later type is so much better."
There has been a lot of experimentation with these engines over the years. Looking for performance increases, you'd be wise to track down info from those folks who have tuned and developed the cars in the past, who have a pretty good idea what worked and what didn't. I know Kas tried domed pistons and even pop-ups in some of the Triumph motors, with little success. The domes seemed to just disturb the flame path and, at best, made no additional power without huge increases in compression ratio. In many cases, the domed pistons actually cost power. Kas' Triumph engine development info was regularly being shared with the factory in England and a number of changes were made during regular production that reflect his input from the racing world (which was mostly in production classes with limited modification). I suspect this change during GT6 production might be an example of this, or at least influenced by it.
You are on track with the idea of increasing compression. That will improve performance, within certain limitations. 9.5:1 CR would likely be very workable, although you might find you need to run the car on higher octane gas. 10:1 to 10.5:1 pretty much requires premium fuel. Anything higher needs "racing fuel" that is not nearly as widely available.
The key reason higher octane fuel is needed is that higher CR causes higher temps within the cylinder and that leads to more pre-ignition and detonation, which can damage an engine pretty quickly. This can be countered to a point by retarding the ignition timing. But, that's sort of counter-productive, as retarding ignition timing reduces performance.
There are other key factors effecting how much CR you can get away with, such as the design of and finish quality in the combustion chamber. This effects both gas flow and tendency toward pre-ignition. Also, a higher CR producing more horsepower means more heat inside the engine which needs to be dealt with in some way. Higher CR also is harder on moving parts such as the pistons, wrist pins, conrods, crankshaft and their bearing surfaces. All these things have to be up to the task of handling the added power, as does the quality of the seal in the cylinder.
But, overall, 9.5:1 is a reasonable and modest bump up in CR for that engine and likely very doable with minimum other modifications needed.
So, how to do it?
I think it would be far, far easier and more cost effective to just have the late/improved cylinder head you already have milled an appropriate amount to raise the compression. I suggest not getting carried away with this. Bump up to 9.5:1, and later - when you are more familiar with the car - if you feel it can easily handle a higher CR, it's easy enough to remove the head and shave it again in another modest increment, say to 9.75 or 10:1. (On the other hand, it's impossible to put metal back onto the head, if overdone! Yeah, I know, someone will say just use a thicker copper head gasket to offset over-skimming of a head. But, why do that when it can be avoided? Besides a thicker gasket can be tough to get to seal and might lead to other problems, too.)
Your machine shop should be able to help you calculate how much to mill the head, to get ot 9.5:1 CR. Once the cylinder head is skimmed, it will need more finish work. The shop should know about this, too, but it includes smoothing all the sharp angle transitions that you will find around the edges of the combustion chambers. These must be carefully radiused and blended. Of course, all this has to be done while carefully maintaining the now perfectly flat head surface.
While working on the head, you might be very wise to also take a look at the intake and exhaust ports. First, they should be matched with their respective manifolds. Next, the ports can no doubt be improved internally. Also, the valve seats can usually be narrowed and even the back of the valves better shaped to improve flow. A lot can be done to improve flow in and out of the cylinder, without even increasing the actual size of the valves.
I'm no expert on the GT6 engine, so don't have a lot of the particulars of this work. Hopefully some others here have some details and will share their knowledge.
A couple related observations: Triumph in general can usually benefit from an increase in exhaust valve size. If any valve work is being done to the head, it's something to consider. And, the GT6 in particular is under-carbureted. Going to a pair of larger SU or ZS carbs would help, but with a 6-cylinder engine it would be even better to feed it through triple carbs (or even better still, triple double-throated carbs such as Weber DCOE).
Finally, also with respect to a GT6 street car, I'd be a little cautious to not get too carried away increasing overall engine output. The GT6 gearbox is a known weak point and won't tolerate a great deal of power increase. Perhaps if the gearbox can be improved internally or replaced with something stronger, but then the rest of the power train - diff and rear axles in particular - might suffer in reliability with a lot of additional horsepower.
Hope this helps!
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