Hartley H1 V8 for 300..500BHP
Ford Crossflow (a.k.a. Kent) and Ford Zetec engine
What's a Seven?
Zetec vs. X-Flow
Conversion to Zetec
|Popular engines for the Seven in
the US are the Ford Crossflow and
The Crossflow has a long and successful racing history. In the US it appeared modified (less efficient head design with recessed valves) in the mundane Ford Pinto. Although an OHV pushrod design, it's bowl-in-piston creates an inverted hemihead and makes tremendous power for a conventional engine, combined with intake and exhaust on either side of the head. This is implied for a DOHC, but at the time for an OHV engine a feature worth mentioning.
The preference for a tuned block is the later 711M version that has remained in production by Samcor (South Africa) until recently. This has stronger webbing, wider cam follower tappets, 6 vs 4 bolt flywheel and other detailed improvements over the older blocks.170-200BHP racing engines have been produced revving up to 9,000 and even 10,500. For the seven, more modest configurations are used and 135-150BHP at 6500-7000rpm is a reasonable option from 1700cc. It is not well known in the US, but did establish itself as the basis for Formula Ford, with 1600cc / 100..110BHP regulation limited versions at 5500-6000rpm.
Some Ford Fiesta's used this engine as well, in case you're hunting for one. The 711M is preferred over a Pinto derivative.
|Here some plots showing some
significant Crossflow engine power outputs
from Kent Cam's VTA configurations:
On the counter side: The Ford Zetec is the main stream engine of the '90's and is found in the Contour, (Mondeo in Eu), Escort and Focus. It comes in various displacements, but the 2 ltr is popular in the US. The modern 16Valve DOHC forms a good basis for tuning. Large production volumes ensure good availability and low parts cost. In the US a low-mileage long block can be obtained for $700-1000. It is very tuneable and has been pushed over 100BHP/ltr with natural aspiration. At 6500rpm, it's capable of 160-175BHP. An aluminum version is supposedly in production now, promising improved thermal behavior and lower weight.The first 4 Zetec pictures are courtesy of Steve K's engine collection for his Birkin - thanks Steve. Pictures of the Crossflow are from my car, while I converted it from the 1600 /110BHP to the 1700 /150BHP spec . This is the 1700cc version with various performance mods.
The earlier Zetec's have heavy cranks with modest balancing. The newer '02 ZX engines have slightly lighter cranks, but much better balancing and more important better guidance for the cam chain and the backwards running waterpump issue is also resolved in the newer engines. These can be recognized by the black ABS valve cover, vs the cast Alu one of the older ones. Note that few parts (e.g. heads) do not interchange (!) between the older and newer Zetec's - heads / cam drive / waterpump and various bolt holes are all different.
The bottom Zetec head-picture is from the ZX3 Focus version from my car. In May of 2002 I switched out the Crossflow for a ZX3 Focus-Zetec - see Crossflow to Zetec page
|Cross Flow (711M)||Zetec (Contour)|
Bore x Stroke (mm)
83.3 x 77.6
84.8 x 88
|Block crank side
cast iron for both
|Block head side
XF coolant flows around cylinders,
low inertia in the XF,
cast iron for XF / Alu for Zetec
Heron Xflow / Pent roof Zetec (ZX)
Lighter engine overall, lower intertial forces.
Lots of racing heritage (not the Pinto version...).
|Closely toleranced modern
engine. Very high durability, just now starting to build a racing
Focus ZX series Zetec is preferred over the Contour
Xflow Starter (Magneti Marelli) does fit on a Zetec, but only when used with a suitable flywheel (check Raceline)
Primary gearbox shaft splines and shaft.
|Weight - and CoG. The Crossflow is lighter
(approx 20lbs). The CoG is lower due to the lighter head and short
Height - The highest part on the X-flow is the oil filler neck/vent, which is lower than the Zetec cambelt shield of the valvecover. May need to trim this (and discard the belt cover) by 1/4" to get maximum sump clearance in a Caterham.
Some notions on combustion chambers:
The Xflow uses the "Heron" or "bowl-in-piston" chamber which allows for good manipulation of the chamber profile. In some versions the 1300cc pistons are used as they have a "pure" bowl, without valve cut-outs. For high lift cams, valve cutouts are needed to avoid valve/piston contact at high rpms.
A critical element of good Xflowing is the flow around the valves. The flat head helps, although the 90 degree entry and exit angles are a disadvantage compared to a pentroof design. Valve shrouding is less of an issue provided there is space between the wall. The Pinto Xflow head has a disadvantage as the valves are slightly recessed in the head, reducing peak flow.
With a DOHC design the valve timing profiles can be sharper (faster open / close times), so that the opening x time area is favorable compared to conventional pushrod implementations. Key is reduction of reciprocating valve train mass. Lighter valves can be accelerated faster and thus open and close faster. Reciprocating mass, the cam follower, pushrod, rocker, valve spring disc, top half of the springs, valve all contribute to the total mass, compared to tappet, top half of spring, valve in the DOHC. The XF Pushrod's other advantage is lower engine height - a major parameter for a fit in a Seven.
For high rpm tuning (beyond 6000rpm) the DOHC is the right choice. But, not without proper gas flow control. Managing flow inclusive from air filter upto the chamber and out of the exhaust manifold into the headers and muffler all determine how much gas can be flowed through the engine and determine VE or Volumetric efficiency or it's torque capacity.
Porting is important - the shape or flow of the intake tract is critical, with a minimal diameter variations other than a gradual tapering from carb venturi or throttle body to the intake valve. This will maximize the mixture velocity into the chamber. Avoiding turbulence in the intake tract, maintains maximum flow capacityl. Only rough polishing is recommended. A very smooth finish is not desirable. The rough surface supposedly helps to breakup smaller droplets.
Another consideration are the pistons themselves. A major drawback of the Heron chamber is that the top compression ring sits low and also, it makes for a heavy piston, limiting max rpm. The low sitting compression ring increases the crevice space between the piston crown and the walls above the first ring. It's a small space, but at max compression it contains considerable mixture, which will be late to combust if at all. For top tuning, the ring needs to move up. A good chamber however tapers at the edge allowing for squishing of the mixture, increasing it's speed during combustion. See here an 81mm bore piston from the 1600 XF , but the same principle applies in other Crossflows with larger bores.
A good indicator of a fast burning chamber configuration is the
amount of ignition
needed. Extended flame propagation time also means that some of
the mixture will burn while the piston is well down, meaning more
energy lost through the exhaust.