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View Full Version : GT6 Domed or Flat pistons? GT6 MK3



Flinkly
06-19-2006, 01:52 PM
hey guys,

after taking the head off my engine, i found that my engine has new flat pistons rather than the original domed pistons. domed pistons would give me 9.5:1 compression ratio and flat gives me 9.0:1. i'd like to go to domed pistons for originality's sake, what would keeping the flat pistons give me over domed in regards to power, gas mileage, etc?

i need to get this sorted soon too, since i need to get it into the shop. thanks for the help guys.

https://www.victoriabritish.com/icatalog/sg/0062.html

Tomster
06-19-2006, 02:01 PM
Pardon my ignorance, but would you not have to change the head on the engine to accommodate domed or hemispherical ("Hemi"- Woo Woo)pistons ???

tomshobby
06-19-2006, 02:07 PM
You have me curious now too. I had one of the first GT6's in this country when it was new and it was pretty fast. I did have the head off but can not remember what the pistons were.

Andrew Mace
06-19-2006, 03:10 PM
Did we ever decide which GT6 motor you have? If it's an original 1972, it's very likely much lower CR than 9.0:1...more like 7.75:1 as I recall.

Alan_Myers
06-19-2006, 04:12 PM
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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tomshobby
06-19-2006, 04:31 PM
[ QUOTE ]
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.

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.


[/ QUOTE ]
My experience with many early GT6 miles verifies the premium fuel recomendation. I had a problem with burned valves after a limited time with regular fuel.

And the gearbox, differential weakpoints were proven out in my car.

If you are going to increase power to any considerable degree I would find ways to beef up the drive line too.

All that said. The GT6 is a very fun car and mine handled beautifully at high speeds.

Flinkly
06-19-2006, 07:23 PM
ok, now that i have the manual and parts book in hand, i'd like to try to restate my question.

i'm not trying to increase my compression, although the domed pistons would regardless of what my ratio is now. i was going off the catalog page for that info. my question was about what pistons should i run in my gt6. everything i have read so far states either nothing about it or that i should have domed pistons, instead of the flat ones in my engine now. it does sound like flat is somewhat better, so you did answer my question, but i wasn't looking to up my compression ratio to 9.5 from whatever i have now (supposively 8.5 or 7.5 with domed pistons). i would rather have adequate power and good gas mileage than overstress my engine and have a rocket. thanks for the info though, it looks like i'll stick with flats and dave that 400 dollars.

Eleven
06-19-2006, 08:08 PM
I have a Rimmer Bros late model engine with domed pistons. I could not see any head changes that were necessary. Would not go the expense of changing to them, overall, you are missing a little but not much

Alan_Myers
06-19-2006, 09:32 PM
Hi again,

You're right! And it appears I have to correct a lot of what I wrote previously!

Looking at the Vicky Brit catalog again closely, I now see that they appear to show higher compression and domed pistons on the later car, not the other way around.

Looking also at their more detailed info on valves, etc., which is on page 59 of that same catalog, it appears the later style head (larger valves) was used with GT6+ (or Mk II, if you wish) and flat pistons *and* that head was used in GT6 Mk III, but now with domed pistons. According to VB catalog, the early/narrow head (smaller valves) was only used on GT6 Mk I, with flat pistons. This jives with a notation in the Sevice Manual at the end of the Engine section, which shows and describes the head changed with the second model.

I suspect some of VB's info though, because we know the compression was decreased from model to model, it didn't increase as they show it! This obvious error in the vendor catalog is what threw me off when I read it earlier.

I've got the Bentley GT6 manual that combines factory service manuals and handbooks for all three models. In General Specifications area it shows GT6 Mk 1 compression ratio as 9.5:1 and all subsequent cars as 9.25:1. Yet, just a few pages earlier in the general specifications for the GT6 Mk III, it lists that car's as 8.0:1! So, I think there were at least 3 different compression ratios and I tend to trust the info in the Service Manual, over the VB catalog.

Too bad the Service Manual doesn't say anything about the piston design, to really clarify that point. Here, I suspect VB is correct, simply because they would get a lot of customer complaints and returns if the items they are selling were so obviously different from what's in the car. They would correct that sort of error quickly, to save themselves grief, while a mistake about the compression ratio data that's largely academic might go unnoticed and uncorrected for years and years.

In spite of the confusion about CR in the Service Manual, I tend to generally take its information much more seriously than that in the vendor's catalogs. So, while I suspect that domed pistons probably would be *correct* in your car, but that they were part of lowering the compression to around 8.0:1, not increasing it to 9.5:1 as VB seems to show.

However this also completely confirms that either head *can* be used with either piston, contrary to what I wrote earlier. Mk I with flat pistons had the early head. The same pistons were used on GT6+/Mk II, but now with the improved head. And the domed pistons were used with the Mk III and the improved head.

What we can guess from the flat top pistons in your car is that the engine has either been swapped with an earlier one or it was rebuilt and the person who did the rebuild either used the wrong design of pistons in error, or deliberately used the flat tops for some reason, perhaps hoping to get the higher CR of the earlier cars.

Andy asked a good question. Have you confirmed if the engine is original to the car? You might be able to tell if the engine has been swapped by its serial number. A Mark I engine will have a "KC" prefix. GT6+/Mk II will be "KD" and a Mark III will be "KF".

The engine serial number might give another clue (or serve to confuse us some more). Engine numbers are suffixed with "E". Is there a letter just before it? On at least Mk I and GT6+/Mk II engines, the sevice manual indicates there was another letter preceding the "E". This would be either "H" or "L", indicating high or low compression engine in each of these models! (In prior years, I know Triumph built lowered compression engines, or offered them as options, destined for sale in countries that had poor quality gasoline. Perhaps that was true of the GT6 models, too.)

If "correctness" or originality is your concern, I wouldn't worry about internal parts that can't be seen, so long as the car runs well or, better yet, is improved by them. Even externally visible but very subtle modifications such as a milled cylinder head don't typically devalue a car (i.e., no one would ever know unless they measure it or run a compression check on the engine).

On the other hand, if the car is a 1972 and it has an earlier model's engine with an "incorrect" serial number prefix, that might effect the value of a car a bit in the eyes of a very particular "concours" buyer. However, I suspect the vast majority of people wouldn't be concerned and many wouldn't even notice.

So, you seem to have a choice whether to replace or keep the pistons. If it were me, I'd try to find out more about the car's performance with them installed, before spending the money for another set. If they are equal to or better than the dome top pistons, in terms of performance, I'd just keep em and enjoy the car.

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Flinkly
06-19-2006, 10:24 PM
ok, after looking at my numbers list, my engine number is KF10255UE. KF denoting MK3 and american market. UE denotes an emissions controlled engine, if you go off of the info from spitfore & gt6 magazine:

https://www.triumphspitfire.com/enginenumbers.html

so my engine seems to be correct. but like my english tranny, i'll just have to wait till my heritage certificate gets here to really know what is right and wrong.

billspit
06-20-2006, 07:51 AM
Call the Roadster Factory tech line if you have any more questions. I don't have the number handy and it's not toll free.

IIRC Triumph changed the pistons mainly to commonize the head or something with the TR6 to save $$$. I have heard the domed pistons cause problems. I would stay with the flat tops and save the cash. If you rebuild later, then see if there are high compression flat top pistons available.