Must the cylinder head be modified to attain good horsepower?
The answer is YES….and again YES…The single most important issue in tuning the engine…..critical on 1966 and later B body cars. Without this work there is little point in doing the rest ….we cover the basics in this series of posts. Get the head professionally done if you are unable to yourself. This is the one area where spending a few $ is worth it.
Are big valves essential to getting good horsepower?
The answer is no. Keep the initial valve sizes whatever the spec of your engine unless you are looking to extract the max. Big valves help but only become effective and to be considered once cylinder head work is done properly and in conjunction with camshaft and carburettor upgrades. My real-time tune of the 993cc package we are busy with right now uses stock (tiny) 31X27 valves….and from the initial road work done this week, produces surprising engine response..
What compression ratio is best for a good road spec engine?
The answer is that the static compression ratio depends on what camshaft has been chosen to do the job. Broadly speaking with the use of the high-end factory cams (SR for Opel and “90” Series Vauxhall) C/ratio would be in the order of 10:1. C/ratios of up to 11.8:1 are required when using camshafts with around 275 – 280° of duration in a road car application. Please remember that when these engines were first designed available fuel octanes were as low as 87/93. The availability of 98 octane changes the game completely.
I recommend you read the Blydenstein Project posts to get a handle on Dynamic Compression Ratio because correct DCR largely determines low and medium engine speed response.
How much material can be removed from the head face to increase compression ratio? Here is a rule of thumb:
Firstly up to 3mm can be removed from the heads.
On Opel the reduction in Comb Volume is around 3cc per mm…. also for 1057 Vauxhall
On VM 1159 and 1256 this moves to around 3.8cc per mm.
So typically a 2mm cut will increase compression ratio by about 1.5: 1 … ie move an 8.5:1 compression ratio to 10:1
However… increasing compression alone is not the answer….read on.
What carburettors are best suited to tuning the little block.?
The Answer is any carb that flows more air. In fact the inlet manifold design has a greater effect on power output than for example whether one uses a Weber sidedraught carb or a set of high-end ‘bike carbs. The downdraught DCD range if Webers is excellent both for power/response and good fuel economy but again heavily dependent on manifold design and inlet port configuration.
What power output improvements can be attained?
A good road spec tuning job will net a 50% improvement in power with the basics in place… However attention to detail will net as much as 70%. My current 1273cc Opel is producing a solid 110 Bhp and fitted to the A body Kadett gives mind-boggling engine response. Check out the video in this post.
Are ‘wild’ 280/300°+ camshafts necessary to get good horsepower.
The answer is no. What you want is a flat torque curve with good bottom end response…these engines provide high rpm power with practically any of the longer duration cams anyway. For road work I recommend around 260 – 270° duration with inlet valve closing points between 62 to 68° ABDC. That type of spec will give huge mid range power and pull strongly to 7500 Rpm.
What are the mechanical limitations?
For road spec applications not much. The Opel is inherently stronger due to the use of an all-steel bottom end and excellent individual component quality. However both engines are good-to-go with standard major components (Pistons, rods and crank) in 1300 cc form able to handle up to 90 Bhp and 7000 Rpm without any difficulty. This is where the Opel gets to have the advantage and that figure moves to over 100Bhp and 8000Rpm using basically stock components..