The Blydenstein Project | Part 2

“26.15”

Careful attention to detail in building the ultimate OHV Little Block has resulted in the sort of performance we could only have dreamed of back in the day. Surprisingly though, everything that has been done is 60’s relevant. In Part 3 at the end of his piece we will cover the most important tuning advice for the OHV whether it be a 993, 1078 or 1196 and larger.

In Part 2  we look at optimising the 1273 for a quick road car by discussing dynamic compression ratio and seeing how we arrived at an optimum static compression ratio of 11.6:1 on a street engine, running on pump fuel, without any self destruct tendencies.  To get to that CR, the required combustion chamber volume was going to be… 26.15cc….  Read On….                                         

GM’s Little Block

In this series and as touched on last week, we are really combining two things in one… the tuning of the OHV Opel engine, combined with some general tuning factoids that will likely work on most if not all classic engine builds.

Being located in South Africa there are a few issues that are specific to our environment. Two of these, namely our significant altitude variations and local fuel octane values make for interesting tuning requirements for classic car engines …that is if you want best performance…so… to start with, we touch on the fuel issue first in order to position this work correctly.

FUELS

I am no fuel expert but have had sufficient exposure to understand the basics and given that we need to tune engines to the locally available elixir it is worth covering a couple of ‘rules of thumb’. It is also worth mentioning that if we are talking stock Hydrocarbon Based Fuels (no alcohol mixes or ‘octane boosters’) crude-oil refining processes worldwide are pretty similar and fuel characteristics very much the same. Here we need to remember that hydrocarbon fuel composition and function is heavily biased towards historical refining techniques. We have this rather interesting question of whether available fuels have directed engine design, or the other way round. The answer to that is straightforward… fuels are the constant, and engines have had to adapt ….and that is exactly what we are doing in tuning the ‘oldies’… we need to get our old crankers to love the available fuel.

The main difference internationally is just how the octane value of the fuel is numbered…ie is a 95 octane in SA the same as 95 octane in the USA? The answer to that is a simple… No It Is Not.

The reason for that, as already noted has little to do with differences in fuel characteristics but rather the fact that in the USA and European countries fuel octane is really an average of the two measurement criteria, RON and MON and in SA due to historical happenings we use the RON value as the communication tool.

What’s the difference between RON and MON then?

Firstly both figures are obtained by testing on a variable compression rig that measures the fuel anti knock characteristic (Octane) at two different and very specific air inlet temperature and engine speeds. This is an internationally standardised rig and test method. A typical local fuel (95 Octane RON) would have a difference in the two values of around 7 numbers…ie RON 95 and 88 MON, this difference is referred to as the octane sensitivity.

Our local fuels using the USA standard would then be in the 91-92 Octane range and truth be known the American method is probably more representative for the purposes of optimisation than our numerically inflated RON biased numbering. Either way, if we understand that, it solves the problem of differing numbers.

From this we also need to understand that the fuels we buy at our commercial pumps do not have a linear anti knock performance either. If we assume that we could have consistent cylinder pressure throughout the rpm range, knock margins will differ between low/medium vs high engine speeds. Lets keep it understandable again…as a rule of thumb the RON number tends to effect low to medium engine speed knock and the MON number is more likely to determine high speed knock at max torque and above. That statement is not directly lifted from the test method, but from practical experience.

The reason I highlight this is to identify that in tuning classic engines we have many variables wondering around aimlessly. Cylinder pressure fluctuations, octane performance and the characteristic knock resistance of the engine itself. When we add to that the altitude component and expect that by coming up with random camshaft profiles, unknown squish values and engine design variables… we then superimpose the fuel… and hope that all is going to be optimised?…unlikely.

If we do not apply some basics, it will only be ‘OK’ through two routes…the first is pure luck… or the other is through the school of hard knocks…pun intended.

SETTING UP THE 1273

In Part 1 we explained the background to dynamic compression ratio and also touched on the issue of having some knowledge of the engine being worked on. In the old days we would kind of accept the result at a best shot in building a “hot” engine and not having the best grunt at low engine speeds…but there are a few things we can do to optimise what we did back in the day. The first is to get much of that low speed response back. To do that we use the Calculation for DCR to help us. In the case of the 1273, the work done on the 1088 is the basis for setting up the bigger engine. In the 1088 application I used the opportunity to find the limits of compression ratio that the engine could handle on our 95 RON octane fuel, at both altitude, sea level and low, medium and high engine speeds.

To do that, the 1088 was set up to promote detonation using a Dynamic CR higher than one would sensibly do at sea level… and got some surprising results… showing that we were in fact somewhat conservative on valve timing.
This is a screen shot of the Wallace racing DCR calculator that I find the most informative and consistent. In this pic it shows the result of the 1088 in its final set up. Please remember that the top of the mind Static CR for this type of engine in this application would normally be around the 10:1 figure.

A few things…note that the full stroke of 2.401” (61mm) is in fact 1.88” (47.8mm) at the point of IV Closing and that the Dynamic compression ratio difference between sea level and 5500ft altitude is 8.75 vs 7.65 :1 The engine would have to be run at both altitudes on 95 RON.

OK, now we have a dynamic compression numbers for sea level and altitude….what do we do with them? We talked about compression ratio giveaway and so what we are going to do is minimise that giveaway at both low and high engine speed… we do not have the capability to have a variable compression ratio engine but we can use the theory to optimise what we can to ‘adjust’ within the envelope.

To start with, I used the US rule of thumb where their 91 Octane fuel (equivalent to our 95 RON) would be acceptable on an Iron Head Small Block Chevy at an 8:1 DCR (At sea level) The thinking here was to start at around 7.7:1 for the 1088  at altitude. …equal to 8.75:1 at Sea level. By testing both on the 95 RON we would get a very good idea of overall knock sensitivity. The calculator told us to set the static CR to 10.9:1 to achieve this.

I need to clarify that the use of the Dynamic compression ratio calculation is mainly to establish whether the engine has enough functioning compression to respond well at low and mid-range rpm…essentially at engine speeds prior to maximum torque. If the value here is below 7 ie in the 6.5 area (at either altitude or sea level respectively) you will most likely have soggy low speed engine response.

Before we get to an overall solution… we need to understand the problem:   Firstly, long duration cams cause low engine speed cylinder pressures to drop due to that inlet charge already in the cylinder and where volume is being reduced because of upward piston movement…having enough time to bleed back through the open inlet valve and into the inlet tract… that happens before that inlet valve shuts out flow… and before proper compression begins. At higher rpm there is less time for the reversion to happen and we end up with a full slug of fuel/air ready to be fired.  That answers the question of how the engine recovers to give good power at high rpm. Given the relatively high RON value, the Compression ratio giveaway at low engine speeds is  therefore massive and to put it bluntly, well-built road engines running long duration cams scream out for more compression in that 1000 to 3000Rmin range…and by increasing the static CR one can recover a chunk of that ….However…we need to take heed of the fact that when approaching max torque and beyond, engine knock response is now no longer in RON sensitive territory.
MON values are now more in control and if not matched properly can run into high speed knock…a lot more destructive than the ‘pinking’ we are likely to encounter if we are borderline at low engine speeds.

The first eye opener on the 1088 was the fact that the engine wanted large timing at low rpm. Normally on a high compression engine, static and low speed timing needs to be a bit conservative…not on this animal…static timing ended up at 18°BTDC with full advance of 30° all in at 3500rpm with absolutely no sign of knock anywhere. That was at altitude. So then a run down to the coast…still no knock with the only noticeable difference apart from the massive change in engine response being a mild tendency to run-on with the engine hot at shut down. I could live with that and an idle solenoid would solve it anyway.  There was much more we learned though, and I cover that in the 1273.

This approach to tuning an Oldie seems to be bearing fruit….

Back to the 1273.

Now I am well aware of the fact that we have not spent too much time on the performance tuning aspects of the engine…we will get there in Part 3 next week but issues around combustion and knock sensitivity provide the basis for the power work…without this attention, power does not arrive reliably on a NA classic.

I used the same DCR calculator for the 1273 but this time the values were different. Work on the 1088 told us that we needed more cam duration because the balance between high and low speed knock margins were in favour of increasing high engine speed cylinder pressures and keeping the DCR the same. (Camshaft duration was up to 278 and Inlet closing point to 70 deg ABDC. ) The longer Connecting rods also changed the dynamics, so the DCR was kept similar to the 1088 purely to keep at least one point of direct comparison. The DCR calculator told us that with these new values we would require a static compression ratio of 11.6:1 The calculation is one thing, but to get a 60 year old design to stretch its legs properly using a compression ratio that high, one has to take care of all factors affecting engine function.

This is the calculation for the 1273

Again to make this understandable, at this point we had, theoretically anyway, pretty much secured low speed (1500 -3500 rpm) engine function to being similar to the 1088…and, assuming that changes to combustion chamber shape and squish area were not going to make too much difference, all we needed to do now was make sure that the engine could run at WOT (Wide Open Throttle) at high rpm to make full use of the increased static CR and longer cam duration.

So….we needed huge attention to issues possibly affecting knock outside of just compression…we compiled a list and actioned each one. Here is that List:

• Engine operating temp
• Inlet air temp
• Spark scatter – conventional distributor
• Combustion squish values
• Fuel temperature/circulation
• Localised block and cylinder head coolant boiling or boundary layer steaming.
• Fuel mixtures
• Spark plug heat range.
• Inlet manifold temperature
• Combustion chamber shape, volume, consistency.
• Piston crown shape.
• Radiator function.
• Water pump pressure/cavitation.

Engine Operating Temperature

 The first rule in tuning classic engines is simple…find that 76°C thermostat before laying a spanner on anything, hang it up in the garage/workshop where you can see it and let it be a constant reminder of what you need to do to keep things cool while you do the rest. We need a decent thermostat to get to operating temperature quickly… and after that…. The 2^nd rule….. make sure the cooling package has enough residual cooling capacity to keep the coolant temp below 80°C under all conditions. How serious am I?…..very, don’t go any further if you don’t do this first up.
Some of the biggest advances in the development of engines over the last 30 years have been in the area of core engine temperature control. Sadly this has probably been driven more by the needs on the emission front than just generating horsepower, but it teaches us that it is important and probably the most neglected part in tuning classics.

• We are working on old cast iron that undoubtedly has many years of scale, rust and general gunk in those coolant passages. Both cylinder block and ‘head internals were given an intense Hydrochloric acid wash, neutralised with a similar caustic soda wash thereafter. We are down to virgin cast iron here.

• Given that the average engine rpm used in tuned applications is at least double that of designed-in shopping trolley average, water pump rotational speed reduced by 30% to 40% (Depending on how often the tacho hits 8!) keep pump rpm in the best pressure window and prevent high speed cavitation. C/shaft pully dia reduced.
• Fitted a piggyback radiator, (Nissan Patrol Heater Core) plumbing water off the back of the highest temp area in the cylinder head and returning cooler water to the pump… intro to front of the cylinder head.

NISSAN PATROL HEATER CORE USED TO SUPPORT CYLINDER HEAD COOLING ALONG
WITH MOTOR CYCLE COOLING FAN.

• Coolant Tube (Flute), rolled sections soldered closed to prevent leakage and delivery hole areas opened by 20% to improve coolant delivery to valve seats and combustion chambers.

INCREASED COOLANT FLOW TO COMBUSTION CHAMBERS WALLS

• Total Cooling capacity increased (larger main radiator) to maintain coolant temp at or below 80C at full throttle.
• 1 bar radiator pressure cap fitted. This helps in reducing the possibility of localised boiling
• Water flow through the engine treated in exactly the same way one would porting a cylinder head, removed sharp edges and ensure that head gasket to block apertures matched neatly before assembly.

Why all this stuff?….because we want to minimise this….

This is your common or garden kettle at 60 – 80°C…with the element operating at around 250°C below that steel bottom….and yes there in no residual pressure and no water flow…but… in an engine running at full throttle …on the other side of that cast iron combustion chamber wall…the temperature is up to 2000°C. The transfer of heat through a chamber wall can be calculated and the whole process is a lot more complex than my simple diagram below…But Cooling systems need TLC and the first step is to maintain good coolant flow, run a high pressure cap and keep the coolant temperature below that 80°C That’s a rule of thumb only because in other parts of the engine temperatures are higher than that.

and if we do that well we can translate the gain into horsepower

All these actions collectively reduce the propensity for localised boiling or gas formation on the boundary layer where one requires solid coolant contact with the cast iron or aluminium. One must remember that where the above actions are not taken, heat transfer into the coolant will be reduced and although coolant temperatures would seem ‘normal’ on our dash readout, the running temperature of critical areas can be considerably higher than they need to be. This due to the fact that heat is not being transferred effectively.

This is very fundamental stuff and but remains a key starting point in the task of minimising the possibility of engine knock. Just remember the gauge on your dashboard is only there to make you feel good…it tells you Coolant Temp…not combustion chamber outer wall temp….that’s a gauge that should be up there right next to the big oil pressure and shift lights…

Combustion Issues.

Given that we are working with a 60 year old design here, there are options available to improve combustion consistency not available in original manufacture, the biggest of which is combustion squish and combustion chamber matching.

Let’s start with Squish.

Getting the squish right and ensuring that initial fuel burn takes place in the combustion chamber only and not across the whole piston face is another critical factor in minimising the propensity to knock.

We must face the fact that many manufacturers never considered the issue of squish to be critically important in the classic engines back in the day. On the GM side we learned that the Small Block Chevy and all four and six cylinder derivatives had squish values in excess of the 1mm maximum regarded as the effective value. The situation has been further confused by the fact that in the fight for good emissions, very loose (open) squish values were found to help marginally, to the extent that  later Small Bocks were being produced with squish values of over 1.5mm. In Hot Rod/Tuning speak that might as well be a mile….The Opel Little Block is one of the exceptions from the era and in stock form runs a squish value less than 1mm…something that tuners back in the day promptly destroyed after going to the trouble of doing head mods.  Later factory supplied and aftermarket head gaskets some 0,6mm thicker than stock and this took the value out to over the magic 1mm limit.

SQUISH

So … what is ‘Good’ or ideal?

‘Good’ is the piston just missing the cylinder head at max rpm, forcing the fuel/air mixture into the more compact combustion chamber. That sounds glib and we do not really want that to get that close, so there are sensible values that get the job done. Doing this achieves two things, firstly forces the initial burn to take place in the confines of the chamber and as the pic shows, forces the air fuel mix into a tumbling motion in the process “mixing” the air/fuel more evenly and forcing this lot towards the spark plug. The “squished” area of the piston also runs cooler.  These features just make combustion more specific, reduce knock sensitivity, improve energy released and in addition, help to make the engine more responsive to timing adjustments during those Dyno runs. Doing all this just allows far higher compression pressures before random detonation sets in. Whilst all tuners will have target value of their own, I ventured into an area where we needed to know just how rigid the OHV engine assembly is… and how small we could make the minimum clearance safely. For this test I built a separate 1088 (75mm bore) using stock internals that included the original thinner web crankshaft, large bearing and piston clearances and squish set to 0.017”… that’s just 0.43mm between the cyl head and piston in a static mode …then went on to using a consistent max rpm of 7500…and occasionally 8000.  The attached pics tell the story….and remarkably for a three main bearing sub.. .no contact…this is one tough son of a gun.

From this, the Magic 1088 and the Magic 1273 were set at 0.7mm or 0.028”.

Combustion Chambers and “26.15”

I have noted the fact that ‘worked’ combustion chambers on the Little Block end up being smaller clones of the V8 big brother so …‘do not try to reinvent the wheel’ …has been my motto from day one. The one variation to the small block theme though, has been that I have never utilised pop-up pistons to achieve the required compression. The logic is simple, given the info in the previous paragraph I have never seen the value of putting a chunk of aluminium in the way of a very clean burn process. In the limited V8 work I have done, have preferred to mill both the block and cylinder heads and where some form of ‘lift’ is required on the piston…doing so by milling a chamber shaped, flat section onto the piston. I did this as far back as 40 years ago and have always wanted to build that DZ 302 with 6” rods, small chamber heads, flat tops…and keep the 30/30 cam with optimised DCR. I absolutely guarantee it will kill a stock DZ and contrary to popular belief with some mods to the inlet manifold, pull strongly at low and mid-range. (The 1273 has an identical Bore/Stroke ratio at 75%…so soft bottom end response is not just about that short stroke stuff and wild cams.)
Fortunately, In the case of the 1273 using the cylinder head from the 1088, combustion chamber volumes have to be increased from 23.5cc to 26.15cc to achieve the target 11.6:1 static CR…so no need for Pop ups at all. In order to attempt achieving similar Anti Knock performance, kept the Squish area % as close to the same as possible.

OPENING THE CHAMBERS IN STAGES MOVING BORE SIZE FROM 75.29 TO 81.5mm

SET THE VALVE SEATS FRACTIONALLY DEEPER WITH A WIDER 30° TOP CUT AND OPENED THROATS

ARRIVED AT 26.15cc EXACTLY FOR EVERY CHAMBER…hard work for the elderly

By ensuring that combustion chamber volumes were identical I only achieved what is standard practise on modern aluminium C&C milled chambers in mass production. It is a long standing belief of mine that classic engines would give vastly improved performance without major tuning if manufactured to modern standards of finish and consistency. Nevertheless, the chamber consistency is but one step in dodging the random detonation bullet.

TIMING

Yes, an electronic crank sensor to drive the ignition system and the ability to do miraculous things with spark consistency is the best modern solution…but…I bet we can get pretty close as far as spark scatter is concerned using the conventional distributor. Naturally we converted the stock Bosch distributor using available bits to an electronic trigger for the system. Again not necessary to head for a fancy product and simply used a VW polo module with a known compatible high capacity coil.

The steps:

The preassembly of the final camshaft and distributor to be used in the build was installed in the block to do three clearance checks:
Installed the Cam and checked end-float. This had two issues that need to be fixed, firstly the location groove has, on some cams, been machined with a very mild lateral offset, this results in the cam moving in and out of the block to the tune of a few thousands of an inch as it rotates. If you have one of those cams ditch it before reprofiling or correct it by machining the location groove properly. The new 278° cam was good.

Next, limited end float on the camshaft to 0.004” or 0.1mm by milling two stock location plates to give the required clearance.

CAMSHAFT END FLOAT REDUCED TO 0.1mm

Next fitted the selected distributor without internals (installed shaft and gear only) without the stock rubber seal and any spacers between the block and Dist. Locked into place. Checked two movements… Gear clearance and the vertical shaft movement on the dist. Fixing this is something of juggling act because there will be zero lash to the gear and zero vertical shaft movement. The spaced distributor as noted in the pic below.

The next step in the timing loop is camshaft timing chain and for this some specialist work is also required. You will note that a duplex chain is not used…that is the situation for two reasons, first of which is as I have said repeatedly…this is not a Mini or a Ford Kent where specialist parts rain down from the sky…there are no readily available bits to do this… and that leads us to the second reason and that is that despite knocking the blazes out of these engines for decades…never had a failure…Whilst it would be sensible to this, I do the next best and tighten up the package to ensure that the camshaft does what the crank is commanding it to do. Naturally with camshaft locked in to doing what it is told, not only is valve timing taken care of but ignition control is also first class.

TIGHTENING UP ON THE TIMING CHAIN IS A MUST. TO DO THIS, THE FIRST STEP IS TO REVERT TO THE OLD STYLE HYDRAULIC AUTO ADJUST TENSIONER….WITH MODS.

ADDITIONAL CHAIN RUNNER SUPPORT PROVIDED ON THE TENSION SIDE.

The reprofiling of the tension side chain runner along with spacers to keep the chain in constant contact with the runner is the final control measure in tightening up both cam and ignition timing. The end result provides almost zero spark scatter … and with the distributor shaft driving the oil pump as well provides an additional direct damping effect on possible spark variation.

WELL WE ARE GOING TO LEAVE IT THERE FOR THIS WEEK ON THE 1273 AND CONTINUE WITH THE FINAL …PART 3 …IN WHICH WE WILL COVER THE REMAINING KNOCK CONTROL ISSUES AND THE ALL IMPORTANT MODS TO GET THAT AMAZING POWER CURVE.

MORE ON THE 1088…

I AM INCLUDING SOME STUFF WRITTEN ON THE 1088 BEFORE THIS …WORTH A READ IF YOU LIKE SOME BACKGROUND

…. All that DCR stuff was one thing…but what would the engine do at max torque and power…here I felt we could optimise the bottom end response within the knock curve, could the raised static CR survive the knock test at high rpm??….

. NOW WE GET TO THE CRITICAL BIT…. Nothing sorts things out like objective evaluation.

The Best was yet to be found…In a trip down to the coast described in the Magic 1088 story, I did my best to provoke the machine on the detonation front. It is a long way from Pretoria (5500 ft) to Port Elizabeth and on to George (both at sea level) A total of 1500Km one way, so a 3000Km round trip. As one can imagine there was ample opportunity to run absolutely flat out in top gear on those extended hills with Rpm in the projected danger zone around 5000Rpm…absolutely clean.*

At no time were the horses spared… the extended full throttle periods the hallmark of this run. If this machine was going to self destruct it would have happened somewhere on that magnificent piece of road along the garden route between PE and George.

In a nutshell we were spot-on at low speed dynamic CR but conservative at WOT top end, the engine could have delivered more. Ideally this engine needed a slightly longer duration camshaft with IVC at around 65 ABDC with compression upped to around 11.3:1.

WE will go on in this section to cover the detail of what additional work was done on the 1088 and what of critical importance did we learn. Firstly, power figures beat the objective of 70Bhp/Litre (achieved 77) but the most satisfying aspect was the bottom end torque and engine response. The application of this thinking had worked. Despite the 267° cam duration (way more than 220° stock), the 1088 had better torque in that important-for-response area, 1500 R/min through to 3000R/min, than a stock Opel 1078 cc SR twin carb engine. From that point onwards the 1088 said goodbye and at 6000R/min torque was up 40% and at 6500 84%.

The objective had been to at least maintain low speed response and pile on the power after that…Job done.