QuestionQUESTION: I have a 72' GT6 and want to increase the HP/torque over stock. I also don't want to put a ton of money in the car as I am not sure I will keep it long term (already have a 66' TR4A for that). I was thinking of building up a low cost head and wanted your opinion. I have access to a 70' GT6+ head for cheap. I am pretty sure this will bolt right up and increase the CR from 8:1 to 9.25:1. I wanted to know if that is correct and if everything else will fit the same... intake & carbs, header, valve cover, rocker assemble, etc. Can you help? Thanks a bunch.
Craig
ANSWER: While I believe the 70 head will bolt right on, the issue becomes is it actually a 70 head, did it come of a 70 car, and did some PO change it for a 73 or TR6 head at some point before you?
Short of knowing what castings to look for, it may not be the CC volume you are looking for.
All the wide port TR6 GT6 heads are interchangeable, from what I understand. Early GT6 (narrow port) and early TR6 (different intake) may be the exceptions to that rule. I think a wide port head off a KD or KE engine is what you are looking for, with a suffix "H" for high compression.
The only way to know for sure is to measure the chamber volume on your current head, and then measure the chamber volume on the head you want to put on.
If all you want to do is raise the CR, then you can do the math yourself, as there are plenty of tools on line, and measure your current chamber, and then figure out how much to mill off your current head to bring up the CR to the desired value. That may be less $ and effort than rebuilding someone else's head.
You can look to www.triumphspitfire.com and look at the "interchangeabel & substitute parts" section to see what other owners have replaced...
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QUESTION: Thanks for the reply. I don't know if it has the H on the head or not. The numbers on the head are #312388 #218225 #V3267. If I do shave my existing head, don't I have to be careful how much I take off before I need to have the push rods milled? Or if you have any other low cost ideas on how to get the HP up I am open to anything. I thought about just changing the cam but not sure how much I would pick up off of that alone. Also, not sure if I can run some different carb needles in the ZS carbs I have. Any ideas. Thanks again.
AnswerYou'll probably have the same issue with the pushrods regardless of whether you mill or swap; as that is likely the only difference from the factory. Some shim up the rocker pedestals vs. go to tubular pushrods, which go for about $150 a set for custom lengths.
The "H" would have been part of the engine number on the block. I don't have a reference for the GT6 head #s, so I can't tell you whether that is actually a 70 head or not. Sorry.
Carb needles themselves aren't going to change much; You can go with a more "aggressive" cam, but now you are getting into a more involved upgrade.
For more 'oomph'
1) better intake & flow (larger carb or webers)
2) change to an exhaust header (better flow out)
3) increase compression ratio
4) change cam timing.
#2 is probably the cheapest, #1 the most, given the GT6 has a low hood profile and you need offset manifolds to go to different carbs.
But you are also getting into the range of an engine rebuild, as if the pistons & rings are "tired", you won't develop the compression you need to take advantage of the cam/intake/CR. Check your cylinder compression, and if you have a blowby gauge, check the blowby on the cylinders. If the compression is too low, or you have too much blow by, changing the head and/or cam will introduce new stresses that the engine won't be able to take advantage of...
