Vauxhall 8 Valve Twin Cam slant four (Part two)
Well I was surprised at the amount of traffic part one of this article created, and the feedback leads me to believe you’re hungry for parts two and beyond. As I said in the first part of this article I shall eventually show you the engine, and give specific history to the engine but for now, I shall continue the story where I left off in July 1970. This article deals wit the reports from Blydenstein dated October 1970.
A quick recap, is that the engine have been given to Bill Blydenstein for assessment by Vauxhall Motors, its been stripped inspected and placed into a frame, where it can be run and tested prior to fitting into a Viva GT for road testing.
So here we go Part two of four…..
Second Technical Report
This report deals with the initial road and bench test running of the engine as forecast in the preliminary report dated l6th July 1970. To date we have not received the outstanding spares requested of which the most important are a strengthened engine block, the second cylinder head, connecting rods and several camshafts etc.
The results of this initial test program are given below.
Completion of Test Car
The left hand drive Viva G.T. purchased from Vauxhall Motors has been converted to right hand drive and the Twin Cam engine installed. In order to get a reasonable clearance for the exhaust manifold we had to offset the engine towards the right of the car by 1/2″. This was done by cutting, modifying and re-welding the engine mountings and caused no problems. The exhaust system used was adapted from a 1968 Viva G.T., the near side system only being used; a 1970 type gearbox and linkages were installed.
Road Running and Testing
The engine was initially tested on 42 DCOE Webers. The GT1 camshaft profile (see graphs enclosed with this report) and the short induction lengths of the carburetors and manifolds in combination with the rather inadequate exhaust manifolds and system made it very difficult to find good compromise carburetor settings. As time was short we went over to 45 DCOE Webers as we believe these carburetors to be the best all round instruments.
After some trial and error it was found that the following settings proved adequate for road running: –
Main Jet 160
Air Corrector Jet 200
Emulsion Tube F9
Pump Jet 50
Slow running Jet 50 F8
Due to the excessive overlap of the GT1 profile the engine did not take full throttle below 3000 R.P.M, but road performance above this speed was beginning to show expected results.
A compression test gave 150 – 155, which we consider extremely low. The combination, of this low compression ratio and a rather stiff engine gave a very poor torque at the lower R.P.M, ranges. Running in was obviously required and it was decided to drive the car to Belgium where we had made arrangements to have the engine bench tested by Transeurop Engineering of Lummen. We choose this tent bed as they have full petrol injection facilities and our own test bed is not yet operational. Also, this firm specialises on the preparation and tuning of Opel and Chevrolet engines and can be trusted with confidential work.
As the engine loosened and the pistons and combustion chambers gradually coked up the performance improved.
When we arrived at Lumen the engine was running very well after about 350 road miles. As we still had very few spares the engine was cruised at only 5000 H.P.H on the Autoroutes and maximum revs kept below 6000 RPM. Bearing in mind the above self-imposed limits, it was nevertheless encouraging to find the engine running extremely smoothly and, apart from the carburetor roar, reasonably quietly. No oil leaks occurred from the camboxes, which proved that our drainage holes on exhaust lids were doing their job (see preliminary report). Oil consumption was, so far negligible and the oil was perfectly clean.
Although running pleasantly enough on Webers it was obvious that proper tubular exhaust manifolds would be required to give acceptable low speed performance for road use. Also, it would be necessary to effect a subtle change of camshaft profile either to the V.M. standard 2 O.H.C. profile (not yet received) or to our GT2 profile to get the best results even with modified manifolding. It was decided therefore to explore the potential of the engine using T.J. petrol injection, as it would also allow a short test of our GT4 camshafts, which we had brought with us.
The engine was fitted to the test bed after the necessary brackets had been made. Ignition settings Used was 9′ B.T.D.C. static. It was decided to run without the fan and the resulting readings are therefore not true net figures. After a short period of running to check all the services, the engine was tested from 3000 to 6600 RPM with the following results:
RPM Corrected BHP
RPM Corrected BHP
At each reading the maximum power was obtained by varying the petrol supply with a micrometer adjustment. Although we did not dare hold the engine at higher RPM for any length of time for fear of bearing or connecting rod failure the above figures are nevertheless time steady power readings.
The test bed personnel suggested that lengthening the induction tract by 6” – 8″ would benefit torque as well as out and out power output to a considerable extent. As time was at a premium we decided, however, not to go any further with GT1 Camshafts as they would not give us the optimum configuration.
The GT4 camshafts were now fitted (see camshaft lift curves enclosed) As soon as the engine was started it was obvious that the exhaust manifolds were grossly inadequate for the .460″/320° lift/duration of this camshaft.
We did some brief runs above 5000 RPM but the engine laboured badly and the exhaust manifolds glowed white hot even at optimum fuel/air mixture settings.
A brief run up to 7000 HPM gave the following results.
RPM Corrected BHP
There was no point in going further as we were in danger of blowing up our only engine.
Our experience from the Tecalamit tests with the single can engine had shown that a well designed tubular exhaust manifold had improved power output by 25 BHP net, when similar camshaft profile was tested. Again further improvements are perfectly possible with a longer induction tract but time did not permit any more testing.
The engine was again fitted with Webers and installed in the car. Running the car back to England confirmed that the exhaust manifolds were hopeless for this camshaft. The 320° duration GT 4 profile runs quite adequately on the road in a single cam 2 litre engine provided the proper competition exhaust manifolds are used.
This exploratory test program had been extremely useful in that it has taught us a great deal about the engine.
1. No oiling problems have been encountered either on the road or on the test bed. The drainage holes described in our first report seemed to work perfectly.
2. It is fairly obvious that the exhaust manifolds are far from ideal when a .400″/290° lift/duration type camshaft is used.
3. Longer inlet tracts will give a noticeable improvement in torque and power.
4. A higher compression ratio, besides giving obvious power and torque gains, will also go some way to cancel out exhaust manifold defects at low RPM.
In view of the above it is now possible to map out the test program for the next couple of months. This will initially be mainly concerned with the road engine developments as our racing engine parts are still pending. In view of this situation we will outline our test program as follows.
Road Engine Development Using 45 DCOE weber Carburetors
The choice of the camshaft for a road engine is closely connected with exhaust manifold configuration. A tubular exhaust manifold will allow the use of .400”/290° lift/duration camshafts. Power output will be about 140BHP net, with useful torque curve peaking at about 135 lb. ft. It is fairly obvious however, that the tubular exhaust manifold poses great problems regarding:-
a) Difficulty in assembly and accessibility. As the manifolds must be fitted to the cylinder head before the head is assembled to the engine, the problems are obvious.
b) Use on left hand drive cars is virtually ruled out.
In view of the limited time available to us, we find it more practical and helpful to investigate a proper road type camshaft, which will work admirably with the existing cast iron exhaust manifolds. It is our intention therefore to fit camshafts profiled to the 2.3 litre exhaust and inlet profiles. These camshafts are on order. They will be fitted to our road engine and will be bench tested with both Weber carburetors and petrol injection before the end of the year. We would expect 130 BNP net. From this engine with splendid torque curve and a reasonable petrol consumption when the carburetors are properly synchronised.
Racing Engine Development
When testing a racing engine on the test bed one problem is for the test bed operator to “Catch” the engine as it “come son the cam” at around 5500RPM. The load must be instantly applied to stop the engine form over-revving. This means that an engine must preferably be safe to 1000 RPM over and above the maximum power RPM.
To reach the power we want form this engine i.e.. 200+ BHP at 7500 RPM, the engine ought to ideally to be safe to 8500 RPM, the absolute maximum being 8000 RPM.
In view of the weight of the stander pistons supplied, this is not possible unless a stronger engine block is used. Even then we may experience a blown engine when we start testing in the 7000 + RPM range.
We intend to build our racing engine as soon as the outstanding parts and assemblies have been received from Vauxhall Motors. We shall increase compression ratio to around 9.5:1 and machine larger valve clearance pockets and generally lighten pistons as much as possible.
Provided the outstanding parts are received in good time, power curves and a final report will be available in December.