We are going to be going into detail on our Chevy 6.5L Turbo Diesel Bonneville Race Truck!
From the outset, we were determined to see how fast we could run with a full-sized, mostly stock Chevy Turbo Diesel pickup. Powered by a stock long block 6.5 diesel using factory issue, standard output model 5521 injection pump. The truck would remain street legal, retain its factory interior, tilt/power steering, cruise control, radio, power windows, and so forth.
We planned that after we had massaged this combo to its maximum speed, we would eventually move on with a few, select power upgrades, and go faster.
As with any engine build, the selection of the block is the first step. As concerns our needs for the Chevrolet land speed race truck and because we are GM 6.5 Turbo Diesel folks, it was a foregone conclusion we use the 6.2-6.5 block for our build.
For our Chevy 6.5L Turbo Diesel Bonneville Race Truck build, we chose the 1991- 1993 issue 6.2, with 599 casting number. This one has the one-piece rear main seal and the smaller 6.2 bore size. The same block was used in earlier 6.5’s by boring to the larger bore. Even with the 6.5 cylinder- bore, the walls are still quite thick.
A primary reason for doing the 6.2 is that the 6.2’s piston is a more robust design as compared to later 6.5 turbo diesel pistons. The 6.2 piston features a ring package that is positioned further down and away from the top of the piston than the latter, 6.5 engine. In a high output diesel, it is essential to keep the ring package as far away from the combustion heat as possible. The 6.5 ring pack was moved up closer to the top of the piston in the interest of improved exhaust emissions when the GM power-train took over the engine
Another equally important reason to use the 6.2 pistons in higher output builds is that the 6.2 piston has a much sturdier design wrist pin boss. These beefier pin bosses offer better support for the wrist pin, resulting in less pin flex. In this piston, the heavier pin bosses also contribute better support of the piston crown. While these factors are not critical in more typical applications, we feel they are essential when the engine is going to be making more than 500 hp.
In any build, the block must be thoroughly examined for any cracking in the main bearing webs. Cracks can occur alongside the main bolt holes. Cracking alongside the main cap bolts in the webs is the result of GM’s use of bolts rather than studs to fasten the caps to the block. When the main cap bolts are torqued down, they impart a friction-induced twisting load into the main web. It is this twisting stress which can lead to cracking alongside the bolt holes.
Left unattended these cracks will usually climb upward into the cylinder walls. The factory’s use of main cap bolts and the problems they cause led us to develop our main stud kit for the 1982—1997 6.2 and 6.5 turbo diesel engines. We offer another main cap stud kit for the latter, 506 block as well. This is a very important upgrade for these engines regardless of output level. We have found through many years of using this main stud kit that the cracking of main webs is solved. During the development of our main stud kit in the early ’80s, we experimented until we determined a torque sequence and step value that permitted securing of the main caps without introducing main web twisting stress. This procedure also prevents distortion of the lower portion of cylinder bores number 3,4,5&6 as had been noted when the factory main cap bolts are used.
We check the main bearing line bore and housing bore for proper dimension.
The like the mainline housing bores to be on the small side of the factory spec range for good bearing ‘crush’.
We machine the deck surfaces parallel and square to the crankshaft mainline. This can often require a 0.006 – 0.016” cut which leaves the pistons protruding up out of the cylinders a bit more than stock. Most factory deck heights come in at from -0.002” to + 0.006” or so.
A typical deck cut depending on the amount removed will result in a deck height that ranges +0.007” to something like +0.011”. Our land speed block came clean and square
We like to run the piston as close as we can to the cylinder head to achieve the best possible ‘squish’ action, highest possible pre-chamber filling velocity/tumble, and highest developed compression heat. So, after we determine the piston deck height, we juggle cylinder head gasket thickness to adjust for deck height. We will sometimes resort to using the 0.010” thicker head gasket to maintain a piston-to-cylinder head clearance of around 0.032—0.038”.
Our Chevy 6.5L Diesel Turbo Diesel Bonneville Race Truck Build was assembled using standard thickness Fel-Pro gaskets which, in combination with the piston deck height we had, provided a nice tight 0.032” piston to head surface clearance.
It is interesting to note that Detroit Diesel originally engineered the 6.2 marine version of the engine to use studs to fasten the main caps in place. While it is unfortunate, the change from studs to bolts was done as a last-minute, cost-cutting measure. This move away from studs proved a fatal decision, as many engine failures happened as a result. This engine was fitted with our main stud kit and cylinder head stud kit.
You always want to check your connection rods for cracks and flaws before replacing the bolts and resizing the big ends. We purposely size them to the small side of the factory spec range for best bearing ‘crush’. Then We install our special wrist pin bushings in the small end before sizing them to provide a 0.0004” (yes, 4/10,000”) clearance to the wrist pin. This is a critically important dimension that must be adhered to.
Mahle pistons feature wrist pin bores that are perfectly sized to the wrist pin right out of the box.
It is a big mistake to ‘hone’ the pin bores of these pistons, as this will destroy the factory ‘wave’ finish Mahle intended. The Mahle bores are sized to provide 0.0004” clearance to the pin. Honing these bores will result in premature failure as the scratches caused by the pin hone stones will result in point contact welding and lubrication breakdown.
Our cylinder bores were treated to a very smooth, plateau type finish before we thoroughly scrub them with hot, soapy water. We use Dawn dishwashing soap for this. After the block is thoroughly washed, we use lacquer thinner to do a final cleaning of the cylinder bores. This is a very important step that must be done to assure removal of any honing grit. Next, we massaged the cylinder walls with PowerKote, a special anti-friction powder. We use a patch of microfiber cloth to work this stuff into the fine scratches created by the honing process.
This friction-reducing coat is effective for about 7,000 miles of running. During that time, there is a significantly reduced twisting force being imparted into the piston rings as they slide up and down the cylinder walls. This allows the ring-face to run more square to the walls to allow them to mate with the cylinder wall for an excellent sealing of the rings to the wall.
The rotating assembly was balanced to perfection and fitted to the block. It is interesting to measure the torque required to spin the rotating assembly (minus cam).
This combination will usually rotate with only 14-18 lb/ft torque. Unless the cylinder wall is prepared as described, the rings will be influenced by friction against the cylinder wall and suffer a torsional twist. This results in a rounded face and reduced quality of seal with the wall. We also massage PowerKote into the rod, main and cam bearing surfaces while we are at it.
For our Chevy 6.5L Diesel Bonneville Build we fitted our block with the high-quality Mahle pistons. Mahle creates these pistons using special alloys, alloys that provide a vastly superior expansion control rates as compared to others. This allows a close fit to the cylinder wall. A closer fit provides superior transfer of heat from the piston to the cylinder wall, a more stable piston and best possible control of the ring pack as they contact the cylinder wall. The particular Mahle pistons for our 6.2 were fitted at 0.0027”, which may, to some, be way too snug. In our application, they did an excellent job at that clearance. After we completed our first season with these particular pistons, they were returned to Mahle engineering in Olive Branch, Mississippi for a thorough examination.
The folks there were impressed with the thrust-side pattern on the skirts and the fact that there was no degradation in the structure. They noted with interest that the piston crowns had sodium embedded in them. This must have been salt dust that had been pulled through the air filters.
All 8 of these land speed survivors ended up on desktops there at Mahle Engineering where they retired as pencil holders.
We opted for one of Scat’s 9000 series cast steel crankshafts for our land speed racer. We micro-polished the bearing journals and fitted the crank with Clevite bearings set at 0.0025” clearance on both rods and mains.
Except for the use of different piston-to-wall clearance as dictated by the particular piston being used. We do all our engines exactly as we did the land speed racer 6.2.
Our plan from the outset was to see how fast we could run with a stock long block and standard output, factory issue fuel injection pump.
Toward that end, our long block was treated only to careful assembly with no internal performance modifications. And, this includes the use of a factory stock camshaft, timing chain and sprockets, pushrods, and rocker arms.
For our Chevy 6.5L Turbo Diesel Bonneville Build, we deviated from this pattern a wee bit with the cylinder heads. We used standard 1995 6.5 castings which we modified by installing intake and exhaust valves that were one-year-only 6.2 one-ton pieces. These are larger than 6.5 turbo valves at 1.935” and 1.63”, intake and exhaust.
We machined the heads to accept these larger valves by exactly duplicating the factory 6.2 geometry that surrounded these larger valves in those first 6.2’s—-exact in every detail.
We also re-profiled the backside of both intake and exhaust valves to assist flow. The valves were all set to a negative protrusion of -0.040”, pretty much in the middle of the spec range.
The valve guide clearance was set at the small end of the factory spec range. We like them snug for best control of the valve as it interacts with its respective seat.
We used a set of Comp Cams ‘beehive’ springs and retainers in lieu of the more conventional factory type spring/retainer set up. The downside is that the exhaust valves are not being rotated which is very important in a conventional application. In this application, the rotation of the exhaust valves was not necessary.
We applied a ceramic coating to the exhaust port runners as well as the combustion chamber surfaces. Then we used the standard ‘diamond’ pre-chamber inserts as were used in the F series engines.
We used a production, standard-volume oil pump. It was a well-used (tested), original mid 90’s 6.5 pumps that we inspected and adjusted clearance on the gears. The oil pickup and screen were also used factory issue piece.
While the injection pump was a standard output 5521, we did run a set of our HO fuel injectors.
The intake manifold was a standard, F series model with zero modifications.
We built a set of tube type headers, one for each bank of the engine. These were fabricated of a heavy wall (0.125”), seamless, DOM (drawn over mandrel) tubing that measured 1.625” inside diameter. The tubes were blended together just before being joined to the turbocharger inlets (2 inlet-ports per turbo). These pairs were arranged to separate their pulses by another cylinder in the firing order. This separation of the exhaust blow-down pulses made it possible to have nice even flow into each of the turbochargers.
The turbos were free-spinning (non-waste gated) Garret S200 models that were outfitted with unique, billet compressor wheels.
The turbocharger compressor output was plumbed through individual aluminum tubes to the top of the factory intake manifold. They dumped into that factory manifold through a simple plate that bolted on.
We were short on time the first time out, so we used simple cone type filters, one for each turbo. It would have been better to take advantage of a ram-air set up if time had allowed.
While we were feeding the engine heated, under-hood air, we were not concerned about that heat because we were using water injection to lower intake air temperature.
The exhaust was unique in that we ran both turbos exhausts through 3” mandrel-bent piping and blended these into a single 4” intermediate pipe which fed one of our Super Quiet mufflers and regular over axle tailpipe. It sounded great delivering an exhaust note that speaks with authority and yet is refined in note.
In terms of engine output, the computer program we did for the racer was not different from a performance tune we would do for our customers. Where we did deviate from a typical performance tune was with the engine rpm limit and transmission control features.
In sticking with the aforementioned ‘keep it stock’ theme, we also imposed a 3950-rpm limit in engine speed.
We set the transmission parameters to provide torque-reduced, automatic upshifts at 3800 rpm, eventually achieving an exit speed of 163 mph.
This engine outfitted exactly as it was run on the salt flats produced 532 hp at 3950 rpm with 778 lb/ft torque at 2700 rpm corrected to standard conditions.
Interestingly, during a post-season dyno session, we decided to see what would happen if we simply raised the maximum rpm to 4800. We were quite surprised at the change in power output. At 4800 rpm it produced 643 hp! If we had run that tune on the salt things would have been a bit better. Live and learn!
Here are some more pictures on our Chevy 6.5L Turbo Diesel Diesel Bonneville Build
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