Cosworth XD  Rebuild    Email Questions or Comments  Updated 2/8/09

I'm a bit of a nerd and as such, I've always strived to keep all the engine work in house.  I worked with the same engine builder for twenty years on my motorcycle stuff and convinced him to join me to run the cars.  He did all the YB engines for the Chevron, the DF and small block Chevy engines for the two Spice and the XB and XD Indy and ChampCar motors.  There were no mistakes for over twenty years then, within a three month period, he failed to check valve clearance on one SB Chevy Spice engine and one Cosworth XD.  Both had new pistons and both failed due to valve to piston collision.  The reason for the poor build quality is still a mystery and I may never know exactly why it happened but the net result was that I now had the opportunity to build the engines myself as opposed to offering technical assistance and just watching the process.  Luckily, my builder had done a very good job of teaching me the fundamentals of quality engine refreshing especially in critical areas like prepping and assembling cylinder heads and, yes, even checking valve drop to confirm proper piston to valve clearance.

Over the next few weeks I will attempt to document the XD build process.  I will try to include detailed pictures to augment the Cosworth XD build manual and to visually explain the more complicated elements of the build process.  Before I get started with the build it is helpful to understand the XD development progress through the 1999 specification which is the only specification that Cosworth in Torrance ships. (Ed Pink Engines just purchased the XD program from Torrance and it is unclear just yet as to how the engines will be offered).  Knowing the engine's weaknesses helps you to focus your attention during disassembly inspection which is a critical part of the refresh process.

A good bit of progress has been made on the issues facing today's users of the 1999 specification XD.  The first is ring seal.  If you take a look at the 99 specification road course XD piston below, you will notice that Cosworth did a tremendous job of reducing friction resulting in increased horsepower.  One of the ways they did this was to use one 0.026" compression ring!  This approach is great for competition but not so good for us vintage guys looking for more than three or four hours of ring seal.  The solution is to use the 500 mile specification piston which uses a more standard twin 0.039" compression ring setup.  This approach has provided in excess of forty hours of ring seal in the XBs.  I have always run my Cosworth engines 6% rich and have never had a problem with cylinders going out of round or rings loosing seal.

The second task was to address the relatively short valve spring and cam drive life.  Broken springs and failed cam drive gears/chains were a primary source of failure in competition and remain the major weak point of the 1999 specification XD.  The primary elements of long spring life are the forces that the spring must generate to control the follower as it traverses the cam lobe, the range of movement of the spring and the quality of the spring (especially the steel).  My first thought was to take duration out of the cams as I was no longer running the 14,500 to 14,700 RPM limits used in competition.  A quick conference call with the leading spring designer at PSI and one of the premier specialty cam grinders in the US quickly fixed that notion.  As a result, I packed the intake and exhaust cams to send to the cam grinder to be profiled and the valve train components up and sent them off to PSI for a kinematics analysis.  As suspected, PSI used the cam profile data from the grinder and came to the determined that the existing spring package was of high quality but was very highly stressed.  The solution to the problem turned out to be rather simple and a plan was put in motion.   

PSI attacked the quality by special processing and design followed by manufacturing using only the best wire from Kobe Steel in Japan.  The result was a very high quality spring but that did not address the fundamental problem that the package itself was a highly stressed sprint design.  Another quick conference call with the grinder and the spring designers resulted in a regrind of the intake cams (the exhaust were already fine).  The over the nose accelerations and valve spring travel issues were addressed by reducing intake cam lift by 10% and smoothing the nose to reduce maximum deceleration over the nose by 13%.  This had two direct positive benefits.  The first was the reduced valve deceleration over the nose of the intake cam combined with the reduced peak rpm (from 14,700 to 13,200 with over rev margin) resulted in less full load spring tension being required.  The second benefit came from reducing the cycle range of the spring as a result of less lift.  The guys at PSI explained this by comparing a valve spring to a coat hanger.  If you bend the coat hanger back and forth over a 45 degree angle it will take a certain amount of time to break.  Start again and use a 60 degree angle and the wire will break much sooner.  The same applies to valve springs so reducing the net travel range of the spring dramatically increases spring life.  The result is a spring design that goes from a "sprint" design to an "endurance" design and which lasts five times longer.  The couple of percent drop in peak horsepower is easily compensated for by increasing boost pressures 1/2".  Cosworth's top notch boost control manages any reductions in torque.  More to the point, I've noticed no decrease in terminal speed in the cars using the new package.  I've also had no valve spring failures and cam drive gear life has gone up significantly.

One very big benefit to the above approach was a massive reduction in refresh costs.  I replace springs at every refresh and valves every other.  Cosworth sells the XD spring set for $125 per valve or $3936 per engine set.  I am currently at 12 hours between refresh and climbing on the XDs (going for the 18 I was getting on the XBs) so the spring costs alone are $328 per hour!  I had to do twelve engine sets of springs with PSI but my per engine costs were cut by 75%!  Combine that with buying valves directly from Xceldyne and the goal of reaching the $500 per hour mark achieved by the XB seems more likely.

The remainder of this page is devoted to the XD refresh process.  One quick note: It is interesting to look at the way Cosworth has generated room for valves in a relatively small displacement motor.  Bore is 92mm (same as the XB) making the stroke 50mm to yield about 2.65 liters of displacement.  This massively over square configuration allows for a large amount of valve area as shown in the pictures below.  I have been told that the Honda and Toyota motors are even more over square.