Written by Carl Heideman
From the Jan. 2007 issue
Posted in Drivetrain
The TR6 can climb from zero to 60 mph in 10 seconds. At least, that’s what Triumph claimed early in the car’s production history.
These numbers have always seemed pretty ambitious to us, as 12 to 14 seconds seems normal for TR6s in the real world. However, even if we give Triumph the benefit of the doubt, those acceleration numbers are pretty lackluster in these days of 10-second minivans. While the TR6 is a beautiful car, it can’t get by on looks and iffy claims alone. To return some street cred to the TR6, we set out to make some tried-and-true performance improvements on the car.
Our subject car is a very nice 1975 model that came to us with bone-stock running gear, including the engine, carbs and ignition. Before starting, we baselined its acceleration and horsepower figures. The car covered the zero-to-60 sprint in about 13.5 seconds—disappointing, but what we expected—while its engine produced a maximum of 74 horsepower at the rear wheels. That horsepower number was also right where we expected it to be.
Our goal was simple: We wanted to improve performance without sacrificing drivability. Additionally, we didn’t want to rebuild the engine or make overly expensive changes. To that end, we decided on some mild changes that would give our TR6 the desired performance boost while maintaining its docile street manners.
The meat of our recipe would be a ported and shaved cylinder head. To help get more air and fuel through that head, we’d also be adding a new camshaft, a pair of side-draft carburetors and a performance exhaust. Finally, we’d give the engine a good tune-up.
Out With the Old, in With the New
While people are often quick to swap carbs and cams, the cylinder head is usually the key to any performance modifications. Putting big carbs on a poorly flowing cylinder head is like increasing only the big end of a funnel; if the small end is left alone, the funnel isn’t going to flow any better. Likewise, a high-lift, long-duration camshaft won’t do much if the head can’t breathe well.
To get our project going, we pulled the cylinder head and sent it to CD’s Engine Service for some work. For about $800, they performed a three-angle valve job, opened the ports and shaved the face by 0.150 inch—that last move would raise the stock 7.5:1 compression ratio to 9.2:1.
This order sounds simple, but it’s not. CD’s has an experienced staff that has ported a lot of cylinder heads through the years, so we knew we were in good hands. We’ve seen a lot of “ported and polished” heads that were done by less experienced people; as a result, we found they didn’t offer what they claimed. Since this is the most important part of a performance upgrade, we highly recommend seeking out a good shop for the work.
Our test car’s camshaft was showing its age, so we decided to replace it. We turned to Good Parts for their $299 GP2 cam and $75 lifter set. While many people have found great increases with the Good Parts triple-carb setup, we stuck with just the cam and lifters.
After installing the cam and head, we moved to the ignition system and carbs. University Motors rebuilt the distributor and made sure its advance curve was suited for performance over emissions. (For more on ignition systems, see Issue 116 of Classic Motorsports.)
While the University Motors crew was working on the distributor, they upgraded it from points to electronic switching via a PerTronix Ignitor sourced from Moss Motors. The rebuild and electronic conversion setup set us back about $250.
The Zenith-Stromberg CD-175 carbs that came on the car were in decent shape, but had enough wear that they were ready for a rebuild. Our baseline dyno testing showed that they were running quite rich, which is usually a function of worn-out needles (easy to replace) and tired jets (more difficult to replace).
To see how our improvements were helping so far, we put the old Zenith-Stromberg carbs back on the engine and recorded some zero-to-60 runs. Our times had dropped from 13.5 seconds to 10 seconds flat. Our Innovate Motorsports LM-1 wide-band air/fuel meter confirmed that we were still running rich, but we were obviously making more power. It was time to finally address the worn-out carbs.
While Zentih-Strombergs can be rebuilt and tuned to work and perform quite well, we chose to switch to new Weber carbs. In fact, we installed the first 45MCHH kit made for the TR6. The 45MCHH is a new carburetor that is basically half of the tried-and-true DCOE. However, where the DCOE is a twin-choke carb, the 45MCHH features a single-choke setup. The DCOE and 45MCHH share many internal parts, meaning the setup and tuning procedures are nearly identical.
Why did we only go with two carbs instead of three? Our math showed that our two carbs would provide enough flow for our engine’s 2498cc displacement and 6000 rpm redline.
The 45MCHH kit is a slick, easy-to-install setup. A set of adapters join the Weber carbs to the stock TR6 intake manifold. A supplied linkage and choke cable mate the carbs to the stock throttle linkage. Two chrome-topped, foam-core air filters keep things clean—and looking good, too. Installation of the carbs was quick, taking only about an hour and a half. The price for the kit is about $749.95 from Interco Products or one of its dealers.
The carbs come with a baseline jetting that is pretty close, so we started there. Our air/fuel meter proved invaluable in quickly helping us get the carbs close to an ideal 12.5:1 air/fuel ratio when under acceleration. After some minor tuning, zero-to-60 times were down to about 9 seconds. The car’s transformation was well under way.
One Final Tune
To really dial in everything, we headed back to the dyno one more time. A few pulls revealed that we were running a bit lean under load—more like 13:1 than our desired 12.5:1. Power was looking good—we were up to 94 peak horsepower at the wheels—but we knew we could do better.
Before correcting our air/fuel mixture, we wanted to make sure our ignition timing was as good as it could get. We always work this way: optimize ignition first, then work with mixture.
Our initial timing setting was 30 degrees BTDC at 4000 rpm. (This was with the vacuum retard disconnected.) Since the compression ratio was still on the low side, we figured the engine would take more advance. We were right.
Before each pull, we inched up the timing one degree. For a little while, each degree added about one more horsepower to the rear wheels, with engine output peaking at 38 degrees. Since we were making the same power from 35 to 38 degrees, we backed the timing down to 37 degrees.
Now that the timing was set, we finalized our jetting choices for the carburetors. Richer main jets and air correctors brought our peak power to a very respectable 100 horsepower at the wheels.
Back at our test site, we found that our zero-to-60 times had dropped quite nicely. We were now down to 8.4 seconds, only a fraction of a second behind the latest Mazda MX-5 Miata. The TR6 was performing very well and drivability was great, proving that well-planned performance modifications and a good tune can really feed a choked classic.
Devil Is in the Details
People often ask us how one new engine part will improve a car’s performance. That’s usually a hard question to answer, since the engine is a system and not a set of independent parts.
We got our power increases by using the right combination of head work (porting and compression ratio increase), camshaft (one matched to compression ratio and desired power band), ignition work (timing and advance curve) and carbs (size and jetting). We knew from experience that head modifications made the biggest difference—about 15 horsepower at the wheels—but we were able to unlock even more power from the head through the other improvements we made.
Throwing parts at a car is fun. Tuning one takes a little more work, but the results are often quite impressive.
|Stock||Modified||Modified and Tuned|
|Horsepower at 3000 rpm||54||62||65|
|Torque at 3000 rpm||95 lb.-ft.||109 lb.-ft.||114 lb.-ft.|
|Peak torque||95 lb.-ft.||109 lb.-ft.||115 lb.-ft.|
|0-60 time||13.5 sec.||9.1 sec.||8.4 sec.|
The Basics of Porting and Polishing
Bench racers love to brag about radical cams, big carbs, and trick port and polish jobs. The poseurs will discuss the quality of the polishing job. Those in the know will talk about the port work, where serious performance starts.
The bottom line is that production cylinder heads are designed for a compromise between good performance and easy manufacturing. As a result, they almost always leave room for improvement on the performance side—it’s very hard to cast the perfect port in a high-volume production environment.
Enter the port and polish job. The key word here is port, not polish. Porting a cylinder head is a seemingly simple process of making air flow through it more easily. The trouble is, it’s not so simple. Some people will “port” a cylinder head by grinding away material to make the ports as big as possible. This will often hurt performance more than it helps; when it comes to head porting, flow path is more important than size.
Here’s a fact to remember when discussing cylinder heads: Air is lazy. It doesn’t like going around corners, squeezing into tight spaces, or expanding to fill larger spaces. Therefore, the best port jobs straighten out the bends as much as possible and keep the port size as consistent as possible throughout the airflow path. A good valve job also helps the air get into the combustion chamber.
Experienced head porters will know how to work the head to maximize flow, usually using a flow bench to measure the results along the way. They will also know how far to go without hitting water jackets and other areas that will ruin the head. The guy who has done hundreds of heads is going to do a better job than the one learning the ropes. It’s worth spending a few more bucks to have the job done right.
What about polishing? There are a few areas where polishing makes sense, mainly when it takes away unnecessarily rough spots or casting flash. But a little roughness around the edges of a port helps keep the fuel in suspension. For the most part, polishing is just bling that keeps the poseurs bragging.
We’ll be looking at cylinder heads and porting in much more detail in an upcoming story in Classic Motorsports—keep your subscriptions current.