Tuning the GM Little Block | PART 2

993 PROJECT – DYNAMOMETER DEBRIEF

Whenever I have the opportunity to carry out seemingly random testing, I do so to fill the information bank…because as we move along in doing this work, all info fills in the puzzle just a teeny bit. So… when we fired up the 993 it made sense to fit the carb and inlet manifold (36DCD &tunnel Ram) straight off the 110Bhp 1273, knowing that the package would at least on part throttle have reasonable fuel mixtures. The added advantage was that if the setup needed some adjustment, this Weber is the easiest carb in the world to reset particularly for light throttle work.. That was needed in order to get the run-in done quickly.
Whilst the objective on the project was to run the 57 year old Opel Carburettor, the mods done and complexity of adjustment on the unit were likely to hold up proceedings. As it turned out I was able to reset the DCD quickly (primary too lean), do the run-in, and then fit the Opel carb…..basic tuning  took two days to get the driveability right. I also managed to get the carb swop with manifold and aircleaner and accel linkage switch down to just 20 mins to practise for Dyno Day …and then refitted the DCD to do the first dyno run.
For the run-in I did the trip to my workplace in Springs for two days (142Km round trip). It was then that I was stunned to establish that cruising at 100Km/h on the backroads through the cornfields I averaged 5.5 L/100Km for the period  and totalled around 350Km before giving the machine a good ‘whack’…. Two surprizes.

Firstly the engine buzzed to 7000rpm without fuss and amazingly could, for a 1000cc ‘Mini’ Little Block, handle the massive 27X24mm choke tube setup used on the 1273 brilliantly. Normally a little engine with big choke tubes would suffer all sorts of driveability hassles at WOT…especially at low rpm. This was the first sign of something special to come.

I explained in Part One the drama we had on ‘Dynoday’ …so the first run and setting up the Weber with smaller choke tubes had to be done quickly. This we did and ended with 77Bhp at 6500Rpm before time pressure forced us to switch to the Opel carb and get that done before the end of the day. At that point  unlike part throttle, the full throttle mixtures were still too rich, and the profile of the enrichment curve showed that we would need time with mains/airs and perhaps emulsion tubes to get things spot on….Not enough time…. so 78 Bhp would be the reasonable estimation for max power if optimised.

On to the Opel Carb….

THE ORIGINAL OPEL CARB AND MANIFOLD  –  CHOKE TUBE SIZE INCREASED FROM 21 TO 28mm… AN AREA  INCREASE OF 78%.       A 1000 ‘S’ INLET MANIFOLD PORTED AND MATCHED TO 32mm HEAD INLET PORTS.

For any of this to have a level of credibility, this ‘blind’ project was undertaken to be done in real time…and accept whatever the result. The 993 Little Block was put together employing the basic tuning methods covered in both the magic 1088 and Blydenstein Project posts and with one unknown….the camshaft a spec not used before….and that we will get to when we get to the camshaft section in later posts. However, I was fairly confident that the overall result would follow the previous work done and anyway, what is the good of not introducing some stuff from which you can learn…and that camshaft choice was a gamble …but it worked….just not exactly like I expected though..

The important message is it cements the philosophy of sticking to the basics. We achieved 70 Bhp from this little masterpiece… better than the target set…BUT…was I satisfied with the result?…overall, yes… but a few aspects we’ll touch on that need improvement.

Here is my Summary :

Top end power: Excellent…& to be frank …surprising…so much so that we could if we really wanted to, trade 10 Nm Torque at 5000-7000Rpm for 10Nm at 1500-to 4000 Rpm

Midrange 3000 – 5000Rpm:  On the money… This is a class act at these engine speeds…high speed cruising is outstanding . The stock 993 ‘S’ max speed of 130km/h is now an attainable cruising speed…take a look at the power difference at 5000Rpm in the comparative chart below. Top gear hillclimbing ability and fuel consumption extraordinary for a 58 year old 1000cc machine

Low speed 1000 – 2500Rpm: If you need more…. This engine, running a 10.7:1 static Compression ratio runs beautifully on 93 Octane RON fuel  (1600m) with absolutely no sign of knock. Introducing some over-advanced timing also shows good knock margins. So…having learnt as a young engineer never to be afraid of numbers, we probably need to venture into territory not seen before with a mild cam duration spec on a small bore OHV would jack up both sides of the curve by increasing cylinder pressures throughout the rpm range The alternative would be to adjust valve timing as noted below and that would trade top end for low rpm response

Overall comment: Low speed engine power does not match the stock 993 S, however, the engine feels good at these rpms on part throttle. What is stunning is the completely flat torque curve from 3500Rpm to 6500Rpm … which keeps a strong feel at modern highway speeds…most unusual and gives exceptional overtaking capability , especially at medium speeds when cogging down a gear to third. This is one of those conundrums in car usage. For about town and low speed work I would advance the cam timing about 3 degrees to bump low speed response at the loss of some top end. For my driving style however, it is perfect just as it is. Perhaps with raised static C/ratio. Here is the point…there are so many folk out there running their stock 993, 1078 and 1196 classics…. and missing the opportunity of having this kind of power and fuel economy under the hood with an engine that remains absolutely stock from the outside….err well the dual exhaust is slightly different on the ‘A’ but that would only knock off about 5 bhp anyway. The way that power curve just continues to grow after the stock 993 goes to sleep, is what makes this machine magnificent to drive as a cruiser. Third gear is also special…with standard gearing, 80mph is a breeze at 7000Rpm and on those country backroads would frighten many 1000cc more heavily modified Minis.

Somebody please tell Richard Hammond to give Oliver a birthday present….

Here is the Power Curve compared to a stock 993 S spec engine and note all figures are corrected to sea level which is the industry standard for quoting engine performance. Figures rounded up or down to whole numbers for clarity….The graphs tell the story…and this does not look like the same engine!!

So, to most of you these may just be numbers …and many experienced engine folk out there will say that that level of improvement, particularly using mainly original components, is just not possible…but…here’s the proof of the pudding…and this video shot is at 1600m above sea level….

Apologies for the focus on the camera at the start of the video but its getting on a bit and has a mind of its own !

I remind you, every single component on this engine is Opel, not one aftermarket part used at all. MY message to you is very simple. Doing the internals of these wonderful pieces of engineering is where the magic lies….This performance is impressive enough…you can add 16% more thump to this at the coast

SO HOW DID WE DO THIS?

 The Cylinder head – The key to real performance. To unlock the potential, we go back to that FIRST RULE OF TUNING AN OHV…. DO THE INTERNALS… and …. MODIFY THE CYLINDER HEAD.  Here are the steps to a good basic head job

As noted in Part One, modifying the Cylinder head is critical to good power and engine response but within that there are a few key issues. We will cover these as follows.

1. General layout – Do things fit together properly in the first place?
2. Piston Valve pockets
3. Combustion chambers, valve sizes and volumes.
4. Inlet ports – The Good the Bad and the Ugly
5. Valve throats
6. Exhaust ports.

First Issue…available fuel octane… and here we are going to target using  98 RON octane fuel (equivalent to 95 RON at 15-1700M) as the basis for the work. Please appreciate in all things good there are some compromises to be had and the OHV likes good compression to have fantastic engine response. Believe me it is worth the extra few $ per tank of fuel.

 So….again….what have we learnt?

For those that do have experience in this work you may appreciate that all we have done is applied the basics…something said time after time in most spheres of activity, however having done so we highlight not only the very impressive result but expose the elephant in the room as to why these little engines were not treated with the respect they deserved so many years ago. The Little Blocks will match and most probably in many cases beat any of the popular engines of the era at their own game. So lets take a peek into how we can spread the info.

We are talking the best route to a very quick road car engine here and as already mentioned the first temptation is to bolt on bigger carbs or that set of tubular pipes ….that is not the way to go. Think of it this way. The engine is simply a pump…air in…air out…and the piece in the middle controlling this air flow is the cylinder head. By adding the carbs, wilder cam and exhaust yes there will be an improvement…but relative to what is possible, this will be very limited because the head is actually the major ‘choke’ in the process. A further problem is that on B bodied cars, starting from 1966 onwards, the cylinder head resembles more of stranglehold on proceedings than a simple choke… SO…here we have spent the major effort on the cylinder head and kept as many original parts as possible in the build…I think the result proves the point very well.

An important point to start with, unusually for an old 60’s design and very different to the ‘A’ series BMC, the little block cylinder head castings can be modified to produce almost any power output within the realm of classic tuning.. Most manufacturers from the era would do the extra yards by producing specialist castings to enable greater power output…the ‘S’ range of Minis a perfect example. What I am trying to point out is that a stock 998 Mini cylinder head casting will never do the job in air flow on a modified 1293 Cooper S….(yes & I understand the bore spacing differences on these engines) Yet here we have available VM and Opel castings able to do the business.

Here is an example of a stock Opel 993 casting optimised and shown at the start of this piece to indicate the  potential of what it is that can be achieved with this old piece of iron.

This Cylinder head makes maximum use of the available casting. Potentially a 140 Bhp package

….but….there are two issues that need to be cleared from the get-go….firstly the need to ‘pocket’ the pistons for valve clearance and secondly how to correct casting/machining offsets which affect combustion chamber alignment and valve positioning to the bores.

The first issue on the table and one that many ohv tuners tend to avoid is the issue of valve to piston clearance. For those who know the ‘A’ series Minis and Ford Kents, this is generally not an issue… but the way I do the Little Block, understand that cutting valve pockets into the pistons is mandatory in the majority of all properly executed mods. Firstly you will note that we remove a substantial amount of material from the cylinder head, we then add increased valve lift but more importantly longer duration camshafts and eventually bigger valves, so things get very tight.   Get your minds around that issue first  and you are on course for a well set-up engine.

There are other ways to solve the problem by using specialist aftermarket “pop up” piston similar to those designs used on older small block Chevys and which provide for adequate clearance…..I steer well clear of those because in my opinion ‘pop ups’ do not work as well as a compact chamber using flat tops. Besides they add unnecessary weight.

I will illustrate an easier way to get valve pockets cut when we get to camshafts but first the basics in modifying the heads.

MODIFYING THE CYLINDER HEAD

I am assuming at this point that your engine is disassembled and we can start from first principles, because this is where tuning the OHV engine provides the basis for a spectacular result.

A trial assembly of an engine no matter how experienced you are is an absolute must…because we need to know the devil in the detail. Today’s modern engines owe as much to modern tech in multivalves and electronics as they do in the beautiful casting consistency, machining and tolerancing. The last three we can get right on the oldies, so a number of things need to be done to upgrade the old OHV monsters to match the modern stuff in those three aspects.

Firstly just bolt the cylinder head to a bare block and take a peek at the combustion chambers from the crank case. For the purpose of illustrating this point in real time, I today took a stock 1196 cylinder head and bolted it to a 993cc block. What is important here is that the engine blocks are consistent and bolt location to bore centres are well controlled on all Opel/Vauxhall blocks. By fitting the 1196 cylinder head to the small-bore block, it illustrates the  chamber mismatch more clearly….However… we have those two mismatches, Chamber to Bore….and….Valve centres to chamber…They do not have the same reason for offset, so which one do you choose to provide true centre?

 Typical Bore/chamber offset. This shown on an Opel Block/Head trial assembly.

 Typical Bore/chamber offset. This shown on an Opel Block/Head trial assembly.

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).

Measure the chamber to valve outer diameter gap. From the two pics you will see that these two random cylinder heads (from units in my garage)…both have offsets but to different sides. It is therefore important to do this check before starting work on any cylinder head. I have found some that are spot-on ….but very few.

I will show the Head work first and the following pics demonstrate the valve offset in the first steps to centralising the chambers. Please note that I am working on the head ‘as found’ without any clean up procedures before starting work…this gives a clearer idea of how the work progresses.

To correct this, simply position the head gasket (used in final assembly) centrally to the valve positions and not to the chamber machined positions, mark the head as in the pic and ensure that when combustion chambers are opened, the chambers are centralised to the valve positions ie valve head to walls the same on all cylinders.

This is an area where much of my work is different to mainstream tuners. I have always removed 2mm or more from the cylinder head face to start with and opened the chambers to the required volume. This ‘drops’ the floor of the chamber closer to the piston and immediately helps in unshrouding the valves.

STOCK 993 COMBUSTION CHAMBER AND 31X27mm VALVES. This is what we started with

Bore size on 1015 engine is 72.73mm (.030”O/size) Stone Dia to suite bore

By Using the Stone cut method The Chambers are centralised to valve positions and radii blended to rest of chamber

This is the final Chamber…no rocket science…stock  31x 27mm valves (reprofiled) . Note the Open chamber. Open Radius to chamber walls and chamber kept “tight” to maintain max squish. Chamber volume 21.5cc. Valves are completely unshrouded giving excellent flow.  Target Static CRatio of 10.6 ended at 10.7:1
The point of this info is to demonstrate that going the route of big valves in a stock chamber will never work as well as the “worked” chamber/valve combination shown.

On the smaller bore 993/1078 Opel and 1057 VM engines the above chamber shape and size works brilliantly on a road spec engine with a 33x28mm valve combination.

Big Valves in a stock chamber…this is an extreme example….does not work.

I decided to use the original valves fitted to the chosen cylinder head and clean them up as best possible (original 1963 parts)…remember this exercise is also to show what can be done without shelling out big $.

But before we go there lets talk about required compression ratio and squish. In the prebuild of the sub, the most critical measurement is the clearance between the cylinder head and piston at top dead centre. On all OHV’s this measurement is to be 0.035”  (0.9mm) maximum. I run 0.030” Max on all my engines and go as low as 0.025”. One test engine was run at 0.018” just to see the result, amazingly with no adverse outcome except for some debris rash on the pistons…and that engine buzzed to 7500 relentlessly.

If the above is not done, you will not be able to maximise compression ratio or alternatively will be forced to run retarded ignition timing. So…measure the proposed cylinder head gasket for thickness (use a compressed gasket of the same make to get an accurate installed thickness) then measure piston height to block surface (@ TDC) and machine the block deck to arrive at an assembled clearance as noted. Remember that piston rock at TDC is significant if big piston clearances are used as this reduces clearance, so my recommendation is to use stock pistons (I use Mahle for both VM and OM in street car applications) and keep piston clearances within the top limit recommended which should be around 0.0015” or 0.038mm. (Mahle recommended min clearance is 0.02mm)

Here are some pics of combustion chambers …and you will note that the key is to keep the chambers as small as possible to maximise squish. The secret on the OHV is to have good wall radius from the valve seat outwards…and opening the shallow end of the chamber….massive reshaping of the chamber is, from experience, not critical to flow.
The 1273 finished chamber below is on the large side and a bit of a compromise. This done to get the head as used on the magic 1088 to the right chamber volume for the 1273. You will note that instead of opening the inlet side of the chamber as is common practise, I opened the exhaust side as an experiment. In the process the compromise affected knock margins slightly due to reduced squish area ( compared to the 1088)…but  in the [process along with the ‘headers’ improved cylinder pressures to the point that the 1273 almost made the very special figure of 100Nm/litre.

CYLINDER BORE RELIEF CUT

Going back to that first pic taken from the crank case to show the Combustion chamber offset, we see the shrouding of the valve by the cylinder bore. To fix that I have ground these Chamber reliefs into the top of the bores to improve flow. This rework has history in that my first 1000cc race engines back in the early 70’s had this cut, because it made sense to do it,but I never knew how deep one could go…until I read a write up on the Hillman Imp race engines which have a similar issue. There the best tuners machined a similar relief into the top of the block which went down to a depth of half the width of the top compression ring!… I duly incorporated that mod into my race engine along which with other developments eventually out-powered the 998 Minis and significantly, a locally modified 970S racer.

BORE RELIEF TO UNSHROUD  CHAMBER OVERLAP

Thats all for this week folks…Next week we take a look at ports, especially the Inlets. The Good the Bad and the Ugly…

Finally after many years of development and fun with this machine I have made the decision to put the car up for sale. The car will be sold with the fabulous 1000cc project engine. Mechanically sound with an excellent and unique powertrain This is a one time opportunity for anyone wanting an absolutely unique piece of motoring history.

The engine is the original built into the car in Port Elizabeth in 1963 so has original numbers.

For the rest it is a project car and an ideal opportunity to give the rest of the car the TLC it deserves.

  Price R 59 500. With that comes a spares package which can be negotiated but more importantly the ongoing technical advice from the seller.

Please complete the contact form below for enquiries or call me on +27 71 887 8207