View Full Version : GT6 GT6 engine vs. TR250/6 engine?

12-04-2008, 08:47 AM
Question - besides the displacement, what is the difference between them? In pictures they look nearly identical.


Ian F

12-04-2008, 10:36 AM
Unless I am mistaken, cam bearings. The 250/6 engine runs the cam directly on block. One of the reliability upgrades is to bore the block and book the small motor bearing insert in it.

12-04-2008, 10:53 AM
The only key difference between the later 2000cc GT6 engine and the 2500 TR250/5/6 engines are the crank rods and heads. All you have to do to get 2500cc out of the 2000 engine is swap the 2500 crank rods into the GT6 engine.

The 2500 also has a duplex timing chain that can be installed on the 2000 engine.

That's about it. Some folks like the 2000 engine because it can rev higher and is smoother (lighter rotating mass).

12-04-2008, 11:09 AM
From what i have read, the 2000 engine does not have the major harmonic weaknesses of the 2.5. in racing circles, it is considered the only 6 capable and worthy of making into a race engine. The 2.5 is hard to keep together for a long time because of the harmonics. The rev potential is also a significant factor. Try pushing the 2.5 to 8500 for very long and you will find out. The other triumph engine considered realy worthy of a race engine is the 1300 spit engine.
Now we all know that all the others are raced, and raced successfully, but from a hp/displacement viewpoint they are very hard pressed. Years of development has made them work (due to people like Kas Kastner,and others).

12-04-2008, 11:19 AM
Usually only seen in the earlier GT6+ blocks, but worth checking any way if you are swapping blocks around, is that they do not have the cut outs at the bottom of the bores for the con rods of the 2.5 engine to pass.
These blocks can usually be identified by the lack of 'bumps' on the header side of the block, where the casting was also moved out slightly to give more clearance for the throw of the crank.

12-04-2008, 11:48 AM
Ah, found the pictures.

The first block is the early GT6+, without bulges in the side or the horizontal strengthening rib and as mention before, it does not have cut-outs in the bottom of the bores.

For reference, the block below that is a 1972 block, with all the features mention above present.


12-04-2008, 01:14 PM

Since the 6 in my Spit 6 is a Mk I engine, I can assume it's a 2.0-only engine...

All good info to know, nonetheless... I suppose it will depend on what the car finally ends up being. If more of a "GT cruiser" (which with a 3.27 rear, is what it is now), then I may look into a 2.5 if/when the existing engine needs a rebuild. Some more low-end torque would be nice (providing the diff lasts that long...).

Next question: so I'm assuming the heads (at least the Mk II and III engines) are the same? e.g. - the GT6 intake manifolds and exhaust headers will bolt on either?

12-04-2008, 03:55 PM
Hey Ian,... I think there's an oil pan issue with the 2.5 liter engine into a Spitfire. GT6 is too shallow, and TR6 is too deep.
Spitfire and GT6 Magazine web site has lot of links to folks that have done this swap. Keeping a rear suspension under it may be a larger problem.
I also believe you have to use the GT6 intake to clear your bonnet. Doesn't breathe as well, but fits...
This has always been on my "why don't you do this list"...
Take care Bob

12-09-2008, 04:03 PM
The main difference is in the stroke. The bore size is the same. The crank and pistons are different and maybe the rods. I have a GT6 block to 2500 spec engine in my TR6. The Serial number says GT6 so I though the natural and figured at sometime the engine was replaced with a GT6 engine. Kind of bummed me but the early GT6s and TR6s put out almost the same power and I was fairly happy with the way it ran inspite of a bad cylinder. When I pulled the piston out the ring grooves were blown out but the cylinder walls were still good. When I went to order a new piston I noticed that the part number on the piston was not for a GT but for a TR. I checked the stroke and found it was the same as a TR. It took me a while for the light to come on! I checked the head casting numbers and found it was a late series lower compression type GT. I had bought a rebuilt GT head and at the last moment I checked the casting numbers and found it was an early high compression GT head. With the extra displacement and the higher comprssion would have given me a compression ration of about ohhh 500:1. Anyway you can make a 2500 out of a 2000 but you have to be carefull with the heads. Of course if you can find a good crank and rods and pistons....

12-09-2008, 05:02 PM
Pistons are the same. Just the crank rods and heads are different.

At least Dave Vizard and I think so . . . :yesnod:

12-09-2008, 05:26 PM
My '67 GT6 was faster than my '76 TR6 even after modding the engine in the TR6. I believe the Spit-6 would make better use of the RPM range in the 2000 than it would with the extra power of the 2500. Just my opinion after having both.

12-10-2008, 08:26 AM
It's the weight: GT6 weighs about 1800-1900 or so lbs. Very, very light with essentially the same power as a TR-6 which weighs a bit more.

Really, entirely different cars IMHO.

A stock MKII GT6 (without all the smog equipment they added to the MKIIIs) will outrun a stock TR-6 (U.S. spec) in a straight line and will consistently outhandle a TR-6 in the twistys/autocross, etc. However, TR-6 top end is generally better, with or without OD.

Having said that, the front end of a stock GT6 tends to get twitchy at speeds above roughly 105+ MPH. Airflow is very smooth over the GT6 body, plus a fair amount of air gets under the car, creating lift. Not good at high speeds. You lose steering input significantly at higher speeds, unless fancy suspension work has been done to the car.

In fact, the GT6 has a very low drag coefficient, lower than many custom-bodied race cars. Just don't go too fast in one unless you like being airborne! :lol:

12-10-2008, 08:53 AM
I agree about the weight and drag coefficient.
But I did regularly drive my MKI over 100 mph and it handled great. I could top out at 7,000 rpm on a long downhill in the area and it handled as well at that speed as it did at 50 mph. Of course it was a new car then. And the engine sound was unbelievable at that speed!!

I gained the rapt attention of a somewhat confused State Trouper there one time too!! But that is another story.