THIS IS A TYPICAL BORE/CHAMBER MISMATCH ON AN OHV OPEL. HERE THE DIFFERENCES BETEEEN CYLS 2&3
On the Opel engine I can practically guarantee you two mismatches, firstly the combustion chambers not correctly positioned above each cylinder and secondly a machining offset in the head which misplaces the valve centres relative to the combustion chambers. On the VM I have seen both good and bad. Either way, we will go through a process to modify the combustion chambers making them identical dimensionally (chamber to chamber and valve positions to chamber) and then to locate the head in the right position on the block. I do this before starting work simply to get an idea of how good/bad the situation is. This is particularly relevant in the small-bore applications 993/1078/1057/1159 where the chamber is in fact shrouded by the bore. On the 1196 Opel or 1256 VM, valve shrouding as a consequence of bore interference is less of a concern because the bores are bigger than the chamber width …but… should be correctly positioned anyway so that chambers can be opened to the max to unshroud the valves.
There are two methods to sort this out. For road car applications I recommend using the existing valve positions as the datum and cylinder head modifications carried out on the combustion chambers to follow steps noted below under Centralising Valve Positions .
On a serious race engine application we centralise the casting by moving the valve guide positions to the centre of the casting before any work is done at all. I will cover how we do this in future posts on race engine preparation.
Again at this point there is an argument of…why bother ?….and will it make much difference? The answer to that is to look at all the other detail issues such as static compression ratio, dynamic compression ratio, squish, camshaft choice, cam end-float and timing chain set-up just to mention a few….and if no detailed work is done, the result will not be what you see in the videos.
CENTRALISING VALVE POSITIONS AND OPENING CHAMBERS..
Opening the combustion chambers is a critical step in the process to improve air flow and at the same time matching chamber shape and valve positions. There are a lot of comments regarding the use of bigger valves and the truth of the matter is that whether we are talking a 31X27 mm small valve application on a Opel 993 or the other extreme
being the 34.5X 30mm package on a VM 1159/1256, I would recommend cylinder head work first before considering larger valves for any application. The example we go through on the 993 project head by working on chambers and ports first in this piece adequately demonstrates what can be done using stock components. Put simply and using the two examples quoted, the 31x27mm valves in a 993 can give up to 80 Bhp and more… and the 34.5×30 valve sizes on a 1273/1289 version of either engine type will give up to 110 Bhp in a road application. …So…focus on head work first…. as a separate comment, the valve shape (profile) along with careful throat work will have much greater flow benefit than a diameter change with stock valve profile anyway.
The following pics illustrate two things. Firstly the valve to chamber offset which needs to be corrected and secondly using large valves to illustrate the offset also shows just how bad valve shrouding can become with a stock chamber and big valves.
Insert inlet or exhaust valves into chambers with adjacent valves …. ie cyls 1&2 for inlet or cyls 2&3 for exhaust. The pics below show inlet valves fitted to two different cylinder heads (large valves selected to illustrate the offsets more clearly).