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Tired Of Having Iron Atop Your Clevo?

There's never been a shortage of performance components for Chevs but if you're into Fords - Clevelands in particular - choices have been pretty slim. There are aftermarket race-style heads for the Clevo, but they generally don't bridge the gap between street and track all that well.

John, from Cylinder Head Innovations (CHI), could see the gap in the market and after the success of their Hemi heads (Head Games, SM June 2000); they created a new lead-free-friendly alloy head to fill the void. Target performance was around 500hp on Optimax. Street Machine gave the new offerings the once over to find out what they're capable of.

"I pity the poor person trying this for the first time!" laughs Con.

With these big heads being destined for the street, the final port shapes and sizes are a bit restrictive for an all-out race engine, but are well suited for hot street work straight out of the box. John says that "to get both ports and the chamber right, we probably pulled the head on and off of the bench about 50 times".

There are many dead areas through the inlet ports of the standard iron heads - which took lots of fiddling to find. One of these is in the bowl area, which now differs considerably from standard. On the cylinder-wall side of the port, a section had to be closed up then opened out on the side that's closer to the exhaust port. Many other adjustments were made and a more gradual curve on the short turn radius also proved beneficial.

Closing up and re-shaping the combustion chambers was also necessary. While this raises compression, it wasn't the sole purpose. It was a matter of achieving better airflow. Adding the peak to the chamber improves cylinder filling by directing the incoming air charge down into the cylinder. It also helps prevent the charge from moving directly across the chamber and out through the exhaust port during the long overlaps common in high-performance cams. The in-fills also create larger squish areas on either side of the chamber, which increases turbulence and improves combustion.

Good port design in modern engines promotes good swirl characteristics, which is essential for efficient combustion in current engines. Con explains, "swirl is why low compression (8.5:1) engines, like the Gen III, still make good power on unleaded fuels. Straight out of the box the Gen III's heads are better than any ported Fuellie". Naturally these desirable features are incorporated into this new Cleveland design, combining the effects of good squish and swirl simultaneously.

John and Con gave the new cylinder heads a more central spark plug location, which offers a number of advantages. Besides handling a little bit more compression and timing before detonation, it gives better flame travel with lumpy- topped pistons, due to the air/fuel mixture burning down either side of the dome more effectively. Con and John also managed to alter the angle slightly and get the plugs pointing in a more favorable direction.

Poor flow characteristics through the exhaust ports is a famous problem with standard Cleveland heads. The floor drops away too severely after the short term and the port opens up to a huge volume that severely slows down gas flow.

One drastic solution is high-porting the heads. This is where the last inch of the exhaust port is milled off completely and a piece of aluminium flat bar is added into which the new ports are carved (these are known as port plates). The resulting ports invariably have higher roofs, higher floors and generally smaller and more even runners all the way out to the exhaust flange. This type of drastic modification is no longer necessary with these new heads, as they employ a high-port configuration.

Maximum flow while maintaining the smallest volume possible is the aim of CHI's new port design. This keeps airspeed high, which is what you want. Even though the shape and angle of the exhaust ports in the new heads are considerably different from standard units, compatibility with standard headers has been retained to keep costs down and make sure everything will fit into your engine bay.

The deck on these heads are extra thick so that they can be shaved to attain any desired compression, along with allowing enough meat to accept things like O-rings if you are considering a nitrous or supercharged (or both) application. Strength has also been added in various places, such as under the rockers. The new units feature a raised strengthening "bridge" between the rocker pedestals and considerably more material has been added under the spring seats.

A good coolant jacket design is another aspect of head design that is vital for long-term reliability. While many believe that aluminium heads don't need as much coolant in them, John and Con think differently. They feel that a generous and free coolant flow is the key to reliable operation. Getting good flow through the coolant jacket took quite a bit of work and made it challenging for the caster, however, its importance cannot be overstated. On the matter of consistent mould making and setting, it's also essential for each chamber roof to maintain an even thickness across its span. If this isn't achieved, hot spots can develop and create a myriad of problems.

When searching for performance, you need a baseline for making comparisons. Here, Con and John used a set of heavily worked factory iron heads. The inlets had been filled (using bolt-in aluminium tongues) and the exhaust had also been fitted with exhaust-port plates. The aim in this test was to have similar flow figures in both types of heads. The iron units measured out at 235cc inlet port volume, while the aluminium versions showed comparable flow figures with only 215cc of port volume.

The first thing to be determined was exactly what effect the reduced port volume had, and second, what the different chamber shapes meant to power readings. Both designed were mated to the same headers, intake manifold and modified 750 Holley. However, for the alloy units, timing was reduced by 3 degrees and jets two sizes smaller were used in the front and rear halves of the four-barrel (this seams to indicate that the alloy heads are more fuel efficient). With the view of eliminating variables, all the same valve gear was also retained and compression was kept constant.

Flow testing certainly gives an idea of what's going to happen but you're not completely sure until the operator pulls the string and the numbers pop up on the screen. Therefore, getting on the dyno with a new product is always the moment of truth. As can be clearly seen in the accompanying dyno graph, both the alloy heads and the heavily worked 4V iron units made a healthy 525hp. Better still, the alloy units achieved this output straight out of the box and at a very street able 6700rpm.

Torque output for the alloys was far from shabby too, with a relatively flat (which is perfect for street work) torque curve that peaked at a tyre-frying 450lb/ft. Drop this into a lightweight Falcon and you'd potentially be looking at a low to mid 11s - remember this is achievable with local petrol station convenience!

As yet, the guys don't know what the maximum power figures will be for the new heads, as there are many ways to build a good motor. One approach might be a small cam and a bigger intake port or perhaps a bigger cam and a smaller port, it all depends. In terms of a general recommendation for building an engine using these heads, a Crow Cam of 248 - 250 degrees duration on the inlet and 258 - 260 on the exhaust (at 0.05-inch lift) would be a good Cleveland street cam that would turn something like 450 - 500hp at 10.5:1 on Optimax with good torque and drivability.

At a lumpier flat-tappet cam, TFC single-plain manifold combined with a bit of mild port work and 11:1 and you should see 500 - 550 hp without much trouble using a good many factory parts. Going further, not a lot of porting, 12:1 compression and the right cam should get 600 - 650hp without too much trouble. A roller cam with internals capable of turning 8000rpm could see figures rise to 700hp pretty easily - the potential is there with the right work.

Keep an eye out for a look at a high-performance variation- which includes most of the alterations mentioned - in the near future. If you're interested, a bare set of heads will set you back $2350 (supplied with un-cut seats) with complete heads starting at $2850. You can reach John at CHI on 0419 245 265 and Con at PRE on (03) 9357 7668.

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01

As these heads were developed as a direct replacement, all the good bits from your factory iron 4Vs can be transferred straight onto CHI's new aluminium units.

02

New ports look like peashooters compared to the yawning 4V ports, yet manage to achieve similar flow capacities while maintaining high airspeed. While somewhat restrictive for an all-out race application, they're perfect for hot street duty. Besides, there's plenty of metal left so things can be opened out for maximum power.

03

Sure the chamber shape is radically different, but take a close look at the bowl area. Opening up the short turn radius and shifting the port walls have achieved dramatic improvements in port flow.

04

No need to add port-plates to your Clevo to achieve decent flow characteristics. New design incorporates a high port floor, yet utilises off-the-shelf extractors to ensure everything fits into your engine bay.

05

If a picture can tell a thousand words, here's two thousand. Clearly visible are the dramatic differences between the stock, closed-chamber, 4Vs and CHI's aluminium offering. Relocated spark plug, added peak, rounded chamber shape and better squish area came as a result of years of cylinder head experience and many hundreds of hours spent with a welder, die grinder and flow bench.

06

Fitting huge valves into Clevelands has never been a problem and this hasn't changed. These heads will accept up to a 2.190- inch intake valve and a 1.71-inch exhaust, however John believes they will work better with slightly smaller valves (as it enshrouds them). Here the valves are set at 600-thou of lived.

07

The beauty of ground-up redesign is that extra metal can be added to the faces so that the heads can be milled to create as much compression as needed and / or O-rings can be installed for sever-duty nitrous, supercharger or turbocharged applications.

08

The same bolt tensions as the iron heads are recommended. Redesigned intake gaskets are available from CHI to suit the revised ports, as well as the custom rocker covers.

09

In standard form, the factory iron 4V closed-chamber heads are good for around 380hp. These mule heads had been heavily worked and managed to achieve a peak of 525hp on PRE's engine dyno.

10

You'd be pretty chuffed too if you were about to drop a set of new heads onto your 351 Cleveland that made 525hp on Optimax straight out of the box!

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