Yet another Triumph TR6
We seem to have had a glut of Triumph TR six cylinder engines lately. The latest one was a beautiful TR6 in stunning condition. Cars like this always make me nervous because I am afraid to go near them for fear of leaving some mark on the paintwork.
I know some people are dead against updating these classics and think they should be kept totally original, but I can’t agree with that myself. If we could only drive the cars in 1970 traffic conditions (and 1970s fuel prices would be nice) then there is an argument for keeping originality. But we don’t. We have to cope with current road conditions so more up-to-date tyres and brakes and engine improvement to help fuel economy are all perfectly acceptable in the world that I live in.
So here we have a TR6 with electronic injection and the original throttle bodies have been replaced with a single plenum type intake. This is a neat way of doing it and much cheaper than six after-market throttle bodies. The problem with trying to keep the standard bodies is that you can’t get a throttle pot on the spindle as it has no extension. The original linkage is a nightmare to balance and they tend to leak. Not surprising given their age.
By going to a single modern throttle body you can also add an idle air control valve. Now the bad news: If you fit any sort of performance camshaft to the engine you will get problems on light throttle and idle. Just as you crack the throttle, as in traffic driving conditions, you get a snatch or jerk from the transmission. This goes away as soon as you properly open the throttle but that’s not a normal driving style while creeping along in traffic at 1200/1500 rpm.
I have seen this problem on so many cars now, and I keep reporting it here in PPC, but nobody seems to believe me. You cannot fit a performance camshaft with a plenum intake and expect it to drive nicely on light throttle. That applies to any engine, not just six pot Triumphs. There is a trick you can do with the mapping to try to alleviate the problem, which involves retarding the ignition to force the throttle open but you are trying to work around the problem rather than solve it. Don’t say that I haven’t warned you!
When fitting performance cams I am often asked to “swing” the cam timing on the rolling road to optimise them. I don’t have a problem with this but it is always going to be at your own risk. When an engine goes together, the builder really should mark the safe limits of the cam timing so that I can move the cams to find the best position without anything coming together. If that hasn’t been done, then we are taking a big risk with clearance.
The Seven I had in recently had been here before when the Zetec engine had made over 190 bhp. Now it had been rebuilt with modified bits and new cams. The engine had been built professionally but the verniers were not marked and we had no instructions on how far we could safely swing the cams. After mapping the first power runs were showing less than 140 bhp!
A call to the engine builder confirmed that the cams had just been installed against the original pulleys, rather than timed with a degree wheel or with lift at TDC. Now we had a problem. Half of the day had passed and we had no power and the owner had come quite a way for the mapping session. We took the car off the rollers and pulled the cam cover off to check the timing with a dial gauge, measuring lift at TDC.
The exhaust valve had 50 thou lift at TDC which is what you would expect from a standard camshaft. The inlet had no lift at all. We had to remove the timing belt and shift the pulley two teeth to get 50 thou lift on the inlet. How could it be so far out? The answer is that the standard Zetec has variable cam timing on the inlet so if you use this to line up the new cam it is always going to be miles away from where the stock cam is with the engine running!
Once the timing was the same on both cams we had 210 bhp. There was probably more to come from more aggressive cam timing but without knowing how far we could go in terms of valve to piston clearance it had to be left at that. Disaster averted but no thanks to the engine builder who should have known better.
The amazing thing about the little A Series engine is not that it makes such good power; it’s that it makes any power at all! The shared intake port is a disaster for charge robbing where one cylinder sucks in all the mixture leaving the other one practically dry. With a carb the mixture simply follows the air so although the cylinder filling is uneven the mixture ratio stays about right for both shared cylinders.
Once you try to fit fuel injection all that goes out the window. The fuel is added in a very short time period and no longer follows the air flow. What you end up with is weak mixture on one pot and rich on the other. Rover had a way around this by keeping the injection pulses the same time scale apart; one had a variable start time while the other had a variable finish time. This works fine and Rover have a patent on it but there is still a problem in that you can only supply so much fuel before you hit a brick wall in terms of available injection time, which in turn limits the power you can have.
The answer, as I pointed out here some years ago, is to fit your throttle body in place of the carb and inject up-stream so that it works just like an electronic carb. The A Series turbo you can see here had a slightly different problem. After one false start the builder went away and fitted his throttle body up-steam, but on a fabricated plenum. When I tried to map it we had major drama with the mixture.
As you opened the throttle the mixture went weak. Add fuel and it suddenly went over-rich. If you just sat there doing nothing the mixture would eventually come back to where it should be. What I think was happening was that the fuel was building up in the plenum which had an off-set outlet to the engine. Eventually it would find its way into the inlet manifold but the system just didn’t work. The owner is now going to modify it with the throttle body directly to the manifold like a carb.