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Performance upgrade plans and an SU question.


Jedi Warrior
I've decided to do just a few small performance modifications to my car as opposed to going the whole other engine route. It was going to be waaaaay expensive, a little over 15K and I'm certain I can maintian the stock appearance by carefully making these minor modifications.

My plans are to install a new aluminum head that has been milled to 10.5:1 compression. It's been ported, the combustion chambers have been flowed and it's fitted with new larger inlet and exhaust valves, as well as new silicon bronze valves and titanium keepers. I'm also using a roller rocker assembly, phosphated and drilled lifters and chrome moly push rods.

To help the new head out, I'm installing a new 280 duration Newman camshaft with an adjustable gear and a set of fast road headers from Racestorations.

The last thing on my list is to replace the Z-Stromberg's with a pair of performance tuned SU's from APT (maybe). I say maybe because I've seen wild price swings on SU's, some new, some used, and a bunch of different model numbers. The one thing I know for sure is that my intake will accomodate the HS6 SU's with no mods, but what I'm not too sure about is exactly which model of HS6 I should be considering.

What I'm referring to is the "AUD" classification of the HS6.

Can someone here point me in the right direction?

BTW, I'm going to be selling the used short block and head I recently bought for the rebuild. The head is in decent shape but needs new exhaust seats, valves, guides, etc. I dissaembled the block and the head and had them looked at by a local racing prep machine shop. I didn't do any magnafluxing or anything, but the guy that runs the shop said the block and crank were in excellent condition. I just want to sell it for what I paid for it, so I'll post it in the classifieds here.




Darth Vader
Higher compression will require higher octane fuel. Is that available in in your area? If not, timing will have to be retarded to eliminate pinging. That in turn will eliminate the power gain from increased comp.

Just a thought, and I might be wrong. Others please tell me if I am.



Country flag
Hi Tab - Apt Fast as a pair of HS6 (rebuilt) for $550.

2XHS6-TR6 HS6 SU Pair Carburetors For TR6/TR250 w/ linkage PR $549.00 (https://www.aptfast.com/)

I don't think there were that many variations of the HS6. I think you'll find them pretty easy to tune.

Usable cores come up on eBay frequently if you want to rebuild them yourself.


Great Pumpkin - R.I.P
Tabcon said:
What I'm referring to is the "AUD" classification of the HS6.
My book shows the TR4A used AUD 209F and AUD 209R with TW needles and red springs.

But with a modified engine and modern fuel, you're probably going to want to retune them with different needles and possibly different springs as well.

I believe most HS6 can be made to fit, if you get the right float bowls and the little insert that sets the bowl angle relative to the carb body. But there are lots of other variations as well that may make some more suitable than others. For example, I'd stay away from anything that was originally fitted with a "waxstat" or an automatic choke.

Simon TR4a

Jedi Knight
Hi Tab, I think the inlet manifolds were the same casting whether the car was equipped with SU carbs or Strombergs (mine has SU, I think the H6.)

With regard to the other mods; I think the compression will be ok because of 2 factors, the heat dissipation properties of the alloy is better than that of iron heads, and because the 280 degree cam you are using will give quite a bit of overlap which lowers the effective compression ratio. I have an iron head, 290 degree cam, and static compression around 9.6, which seems fine on Sunoco 94.

On my car the porting was not done to my satisfaction, the guy concentrated his work on the areas that are easy to get to, hope yours is done better! I have the oversize valves, but the car dynoed just as much power with standard size Triumphtune valves with a 3 angle valve job in the previous head, which was more skillfully ported. Response was also better under 2,000rpm, so clearly there is a velocity/atomisation problem with the head as it is now.

I have tubular chrome moly pushrods, they are certainly strong and will not bend like a pole vaulters pole as the follower rides up the ramp, but they are heavy, more a reliability mod than performance. My pushrods were made 100 thou shorter to keep the geometry correct because the head is pretty thin, you may find the same-I don't like using spacers under the towers that support the rocker shaft.
I also have hardened light alloy retainers for the valve springs, these offset the weight of the pushrods to some extent.
i hope at least some of this is helpful.


Jedi Warrior
Exactly Simon. Great info.

The engine builder I initially brought the used cast iron head to, told me that if an alloy head is used you can easily bump the compression to 11:1 using premium pump gas. I will only be raising it to about 10-10.5:1.

I may retain the stock push rods I have since I don't really plan on spending any time at high speeds or unusually high rpm's. I'm just trying to achieve some low end power and sprint type short duration acceleration.

I'm fairly certain the combination of the Racestoration headers, Newman camshaft, new head and SU carb's will give me all the power I want in the TR. I'll also be making some subtle suspension and brake upgrades.

I really want to keep the car looking original, so all the mods will be sympathetically applied:smile:

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