I want even more power!

OK, so careful flowing, port-matching, better cams on the standard setup wasn't enough - what's the next step?

Bigger / More radical Cams

Here we're looking at hydraulic cams with up to 0.48" of lift. 

All of these camshafts require some degree of headwork to accommodate the increased lift.  

All require 0.1" removed from the top of the valve guide and either double springs or double springs with damper coils to avoid coil bind, bounce and resonance.  Valve followers are available with shortened tops and tapered ends for better flow and are not much more pricey than the standard item.  All will also require shim kits, adjustable pushrods or adjustable rockers.  Forged pistons and accurate balancing are a must.  Additionally, when building the engine, piston to valve clearance must be checked.  Test springs are available to make the process of rotating the engine against the valve spring loads much easier.  These are springs just strong enough to hold the valve assembly together and close the valve so that the entire rotating assembly can be rotated by hand.  The simple way is to place all the pistons at the top of the bores and have a strip of plasticine across the likely contact area on the crown of the piston.  The heads and gaskets should be fitted to the engine and lightly bolted down and the valve train built up.  The camshaft is then rotated by hand feeling for interference.  When the heads are removed, the pistons may have dropped down the bores slightly and an impression will be found in the plasticine.  A clearance of 0.10" is desirable although some people use less at their own risk.  Note that shortened bulleted guides will increase oil consumption slightly.  

Lifter considerations

Additionally,some of these cams may need hi-rev hydraulic or solid lifters.  There is an intermediate offering called the Rhoads lifter which bleeds down below 3000rpm thus reducing overlap and making an otherwise radical cam very soft and thereby improving tractability.  My experience of these is that they are embarrassingly noisy at low revs, sufficiently enough to make people ask whether there's something wrong with your engine. If considering these cams, you also need to bear in mind that these are semi-race cams and that as they have fast ramps with smaller base circles, they will wear themselves and the rocker gear at a higher rate than the softer camshafts.  You also run a greater risk of breaking something at high revs due to the increased load.  Attention to detail is critical.  Also since these cams are designed to raise power and torque significantly, unless the heads are properly prepared you are wasting your time.  Good gas-flowing makes a big difference here since we are asking for greatly increased flow.  Modified plenum and trumpets are needed to flow sufficient air to make the changes worthwhile.

Hydraulic cams

There are a number of offerings from various suppliers:

Most of these are available from Real Steel.  

In my view the best cams in this range are the milder ones, Piper 270, JE102, Typhoon, Stump Puller, 214.

V8 Developments supply the MC1/218 variant.  224 and 234 camshafts are really rather wild and best suited to the race track.  The others are quite usable on road or rally/hill climb and similar applications. Kent Cams and Piper can also supply all of them although some may be reserved to the suppliers above.  Personally I’d limit selection to these unless the engine will hold together past 6000rpm.

Solid cams

Beyond this we are looking at serious competition engines and solid camshafts.  Nothing very bizarre here if you are familiar with normal pushrod and tappet engines, same arrangement as hydraulic but using solid or roller tappets with adjustable pushrods or adjustable roller rockers.  None have a power band below 2000rpm but all produce extremely good power in their range and this can be extended downwards somewhat with judicious design of exhaust headers.  The mildest one, the 404, has a range of 2000-6000rpm with reasonable part throttle response down to around 1000rpm. This camshaft will make a 4.6L V8 produce 300bhp at 5500rpm.  Fitting the camshaft and rocker gear is not the hard bit, keeping the bottom end in one piece is and great attention needs to be made to building the toughest crank assembly possible. That said, the Stump Puller and 218 hydraulic camshafts aren’t far behind at around 270-290bhp.  The next up, the 348 has a range of 3000 to 6500 or so and from there on the camshafts increase the power band increments of about 500 rpm all the way to 8500rpm.

High RPM

Once you start exceeding 7000rpm you need to start looking at steel cranks rather than the standard cast iron along with forged rods to go with the forged pistons.  Ultimate headwork is required to get the best from these cams and the exhausts should be balanced open types with minimal back pressure.  Inlets need to be very open and multi throttle plenums with shortened flared trumpets and opened bores are required to get the best from an engine like this.  We’re approaching the ultimate limits of the block and cylinder head castings as well.  It is possible to get outputs well in excess of 400bhp with considerable effort and substantial strengthening, dry sumps, etc, but the risk of catastrophic failure of the engine is rather high and there is no chance of the engine being usable for anything except out and out competition.  I do know of a few brave souls who have reached over 550bhp with over 700ftlb of torque using a combination of turbocharging and nitrous oxide, you wouldn’t catch me standing anywhere near it on full song though. Realistically, for a fraction of the cost you could find an alloy block and heads for a large US V8 and get at least as much power without really trying.  These engines will stretch to 1200bhp or more.  The principles are exactly the same as the humble Rover V8; they just get rather bigger.  8.2 Litre big block Chevrolet, anyone? Eliot: Yes please, that's why i binned the Rover and dropped a chevy in - it costs thousands to get over 300BHP from a rover - bog std 350 chev gets you that for a fraction of the cost.


Bigger Capacity Rover V8's

There are four standard offerings, 3.5, 3.9/4.0, 4.2, 4.6 litres. There are currently four well established big bore/big stroke arrangements around, 4.3, 4.5, 4.8, 5.0 and 5.2 although various other sizes exist between 3.0 and 5.5 litres. The standard 3.5 uses a bore size of 89mm and the 3.9, 4.0, 4.2 and 4.6 use 94mm.  With the exception of the 5.2 which uses 96mm, the other conversions use one or other of these bores with various stroke lengths.

The 4.3 conversion uses 89mm with custom crank and custom 89mm pistons and one of the 4.8 conversions uses this crank with larger custom 94mm pistons.  These conversions utilize the 3.5, 3.9 and 4.2 blocks.  Note that 3.5 blocks can be machined to take the larger 94mm or 96mm liners.

The 4.5, 4.8, 5.0 and 5.2 conversions

use 94 and 96mm bores with a reground 4.6 crank or a specially made crank, which usually have larger big end and main journals.  One way to get the 4.8 is to start with a 4.6 crank.  The big ends are machined to the "old" 3.5/3.9 size with a greater offset to increase the stroke.  They are normally built on a 4.6 block since the other blocks are not really strong enough.  The 4.8 uses custom pistons and 3.5/3.9 rods and the 5.2 uses special rods.  Some 4.8’s use a custom crank similar to the 5.0 and 5.2. Personally I would  be extremely wary of using a 5.2 for competition since a bore of 96mm implies removing nearly all the material in the block which ties the top and bottom of the bores together to fit the liner in place, thus compromising strength. In addition, the gudgeon pin sits very high in the piston and the rod angles are quite extreme due to the long stroke, thus generating high piston side forces and odd secondary forces in the rotating assembly.  Further, because the heads just won't flow much more moving to 5.2, the 4.8 and 5.0 can be teased into giving almost as much power with just a bit less torque.  In summary I think the 5.2 engines are a bit flaky.  The 5.0 seems to be quite sound and is used by TVR without issues. Using the previous tuning techniques with additional headwork for the increased flow required will allow reliable engines to be built which can produce well in excess of 300BHP without major departures from standard components.  Standard pistons and rods are not safe above 6000rpm or 280BHP on a 4.6 Litre engine, but engines using mild cams running to 5500rpm  will get away with it.

Power expectations

Here a table showing four stages of tuning and various sized engines.






















As you can see, it's not extremely difficult to get high output from these engines.  However, getting them smooth, reliable, tractable and reasonably economical is the hard bit and this is where attention to detail is important since all of the production engines are likely to come apart in various ways above 6000rpm.  Favourites are piston crowns separating from skirts above the gudgeon pin boss, big end bolts shearing and causing the connecting rod to be pushed through the side of the engine block, main bearing caps working loose and damaging or cracking the main bearing supports in the block, rocker toes shearing off and distributor drive gears wearing at extreme rates.  All of these require suitable alternative parts or remedial techniques to avoid possible destruction of the engine. Attention to exhaust dimensions and particularly intakes is very important and gas-flowing is the key to getting the best from it all.   Note how the increase in capacity tends to soften the camshaft characteristics.  Consequently, the 4.6 and bigger engines actually need a fairly radical camshaft to work well.  The RC87, MC1(218), 404 and 348 camshafts work very well on these engines and coupled with modified plenum, free-flowing exhaust and comprehensive porting, some can achieve in excess of 300BHP on a 4.6 while remaining smooth and tractable with a good idle and low end torque.

Compression Ratio

Referring back to Boyles law for a moment, the main reason we can't go on raising the compression ratio as high as we like is because if you squeeze a volume of gas, it's temperature will rise by a proportionate amount.  If we raise the compression ratio too far we will reach a point where the air/fuel mixture self ignites before we want it to and at that point, due to the particular way the mixture burns in this circumstance, cylinder pressures and temperatures can reach destructive levels very quickly.  Anticipate the piston either to melt or break. Diesel engines are designed to operate in this region but the consequence is mechanical noise and particulate production.   We can dodge the issue to a certain extent by using fuels that have a higher flashpoint, by running the engine at a lower temperature and by cooling the intake charge.  All of these methods are used on competition engines but are tricky to engineer on a road vehicle unless you have access to a ready supply of Avgas, which is 105 octane petrol used on light aircraft.  Avgas will run with a compression ratio of 11:1 or more.  Even so, the pistons usually need to run with extra clearances to deal with expansion and some cooling of the underside of the piston using an oil jet may be necessary in extreme cases. It is possible to get around another 10BHP and 10lbft of torque by raising the CR; 10.3:1 is the sensible limit if using LRP, super unleaded or unleaded with an antiknock additive. 9.8:1 is the safe upper limit for unleaded.  If planning to use LPG mostly then you can happily run 10.5 as long as you accept that you may only be able to use LRP, Optimax, super unleaded, high octane race additives with unleaded petrol or avgas.  You will have to retard the ignition timing significantly and expect a noticeable power loss, or drive with great caution around the areas where pinking occurs otherwise.  I have an engine with LPG on it with a CR of 10.5:1 built for avgas and LPG and I use Optimax or LRP with 6-8 degrees less ignition advance quite happily, but there is a minor power loss.  Actually it’s about 40bhp down but when your engine makes 320bhp it’s not much of an inconvenience.  It can be more of a problem if you plan to tow a load and have consequently higher engine temperatures. The main point is that if you raise the compression too far, even if you retard the ignition timing to correct it, you will either have destructive knocking or the engine power will drop dramatically, more than if you had less compression and more aggressive ignition timing. See the ignition timing section later for an explanation of peak pressure in relation to ignition timing and it’s relationship to power production. There is also a discussion about raising LPG performance later on.

Ignition timing

Ignition timing also has a dramatic effect on output, getting this right is difficult and time consuming but worth it.  The standard distributor and its vacuum and advance curves are about right on production engines but are still only approximate.  Even a standard engine will benefit from detail here and a performance engine is wasted unless you take the time to match the ignition advance to the engine.

Why does it matter? When the fuel air mixture is ignited it takes a certain amount of time to ignite and for the flame to propagate across the entire mixture.  When this has happened the burning gases will be at maximum pressure and we want to make sure this coincides with the best position for the piston to apply the maximum force on the big end and hence generate the maximum torque.  This moment is normally at 15 degrees after top dead centre for that piston regardless of the load, speed or throttle opening.  The problem is that different fuels and grades burn at different speeds and the compression ratio, mixture ratio, temperature, density, load and rpm all cause the mixture to burn at different speeds.  Petrol and LPG both burn relatively slowly under normal conditions so the mixture is ignited well before 15 degrees ATDC to get best efficiency.  

Vacuum and Centrifugal Advance

The humble distributor has two mechanisms to cope with the varying conditions, the most important of which is the inertial advance or bob weights.  Because a given mixture will burn with a relatively constant speed, the mixture needs to be ignited sooner and sooner as the speed increases to make sure the peak pressure still occurs at 15 degrees ATDC.  So the initial advance is set at say 8 degrees BTDC and then the inertial advance provides the correction as the speed increases out to a maximum of 36 degrees usually.  The adjustment is performed with springs restraining the bob weights which alter the position of the trigger or points cam but these springs are only set for ordinary engines of ordinary size running under the design conditions and modifying an engine immediately voids those conditions. Standard springs normally permit a substantial amount of early advance up to around 2500rpm followed by a slower advance up to 3500rpm where they stop.  This is not ideal on higher performance engine where ignition advance needs to vary more smoothly and progressively out maybe 4500rpm or more.  Fortunately there are a variety of springs available for making adjustments, usually this means running with more initial advance and allowing advance to develop more slowly with less early advance, particularly on larger engines or when using LPG or forced induction.   

Adjustment

Getting the vacuum advance right is more difficult, the standard vacuum units have no scope for adjustment although after-market units are available with it.  The vacuum advance is intended to compensate for varying load and throttle openings which will affect the density of the mixture and does the job reasonably well, but cannot correct itself for camshafts which alter the vacuum at idle and low speed and small throttle openings.  It does nothing when the throttle is more than about a 3rd open on most engines and can apply adjustments of up to around 8 degrees usually.  Since most performance and large engines still end up spending most of their time at idle and very low rpm with small amounts of throttle, this is a sensitive area and can effect tractability and economy quite noticeably. It's impossible to get the ideal timing with the distributor even using a rolling road, but careful experimentation can achieve reasonable results.  Short of using a fully mapped electronic system like GEMS or MOTEC there isn't really much that can be done although there are programmable systems available from MSD and Mallory which can replace the inertial advance mechanism for racing use.  For LPG there are ignition amplifiers available from Iwema, RPI and others which can advance or retard the timing by a set amount to compensate for the slower flame propagation of LPG.  These can be particularly useful on high compression engines where LPG can run substantially more advance than petrol.