Where to start?…Traditionally in our early days, we thought selecting the hang-on bits to be a good place to start… ie what carburettor/inlet and exhaust manifolds to use, then perhaps moving on to cylinder head mods and sometimes lastly the camshaft. Compression ratio, despite the fact that we thought we knew quite a bit, was generally a thumb-suck based on some level of historical ‘mumbo jumbo’.
Here are a few Tuning chestnuts we believed in:
- Long duration cams result in poor bottom end response
- Big ports result in poor low speed torque
- Large exhaust manifold pipe diameters kill engine response
- Compression ratios higher than 10:1 will likely result in detonation
- Short stroke engines suffer poor low rpm power and need to ‘buzzed’.
Whilst these could be regarded as accepted norms from the old days, this 1273 shoots down every single one of those handsomely.
First… a little background.
Very often have we seen engine mods done on classic cars where the result is not only below par, but in some cases combine all the nasties noted. These vary from less than best power, lack of mid-range, meagre response, early detonation (knock) and many others…. one of the worst being poor sensitivity to tuning on the Dyno. Don’t worry…I’ve been there and it is part of the never ending learning curve.
As I see it there are two basic avenues to this learning, the first is getting to know a particular engine type, the characteristic do’s and don’ts as it were…that comes with time. The second is about getting to know those very basic issues that tend to work on any engine…we will cover both in this piece on the OHV 1273.
A note. I am constantly surprised at the number of contacts requesting more tech stuff on old engine work. This comes from a wide spectrum of readers and particular interest is shown by young folk dead keen to know more about old school tuning. So as part of this piece on the 1273 I go into some detail on general tuning issues which I consider to be important. This is not meant to teach experienced engine tuners to suck eggs, far from it.. but rather to put out there what I find works well and the hope that it resonates.
CHOOSING THE RIGHT COMBINATION OF PARTS FOR A CLASSIC ENGINE JOB
Never underestimate this part of the work because the benefits here provide the basis for a bad, good or unbelievable engine and I am going to start by combining two issues many folk have little idea are in fact directly linked. Compression ratio and Camshaft Choice. We tend to think of an engine mainly in its mechanical sense… and that’s ok…but…an IC engine is, first and foremost, just a pump… No different in basic function to a bicycle pump in the way it that it ‘sucks’ in air (oxygen) and blows it out again. Yes, the IC Engine has the extra bit of kit that adds fuel and a spark to the process….but we need to focus on the first bit…FILLING THE CYLINDER. That is where the magic is.
Us oldies suffered from a syndrome back in the day where we accepted (and maybe even enthused over) the fact that a ‘hot’ cam would knock the bottom end out of engine response and come on like gangbusters when ‘on the cam’…never thinking of course that it is possible to have good response throughout the rpm range. Filling the cylinder at low and medium engine speeds is to a large extent about maintaining inlet charge velocity and that gets killed when we do not think hard about the correct matching of the bits we chuck into the mix. The current growth in the business of turbocharging everything in sight these days has also tended to mask the art of Normally Aspirated tuning on old engines.
For me, the learning was about connecting the dots on as many seemingly unrelated issues as possible so as to better understand engine characteristics…. as an example, I am going to repeat a story on the Firenza Can Am to make the point.
FIRENZA CAN AM… LEARNING ABOUT CYLINDER PRESSURES
Let’s start with that evergreen …compression ratio…. and very briefly shoot down most of what we have learnt as the do’s and don’ts.
Having started my career in the Service division at GMSA in the late 60’s when we had small block Chevy V8 engines in most of our bigger cars, I soon learnt that on 93 Octane fuel (highest octane available) some of the Chevy small-blocks were sensitive to knock. On engines used in the performance cars like the Monaro and Chevy SS (Holdens) the 327 and 350 versions ran 9:1 compression ratios and, even where timing was accurately set, would often be close to knock.
So…a few years later, having graduated to the Experimental Engineering division and the midst of the Can Am project… the specially built Z28 based 302 DZ V8’s were on the way from the Tonawanda engine plant … this Chevy small block runs an 11:1 compression ratio.