So why is rod ratio even important? Well, it has to do with how the rod moves as the crankshaft rotates. A longer rod will have a smaller angle if deflection than a shorter rod at the same stroke. This is most apparent when the crankshaft is half way between TDC and BDC. The smaller the angle, the less the rod has to rotate on the wristpin axis, and the more force from the piston is applied downward on the crankshaft. See the illustrations below:
Here you can clearly see that the long rod engine on the right has more force from the combustion directed through the rod to the crankshaft and less thrust force pushing the piston against the side of the cylinder. By reducing this piston trust force, you are also reducing the amount of friction between the piston and the cylinder. The maximum angle of deflection shown is for the standard 2.2L rod ratio of 1.64 (left) and the 1.80 rod ratio of our tall block 2.2L engine (right). While this difference is not that significant for many performance applications, it is significant for an all out drag racing performance engine. It is of great significance when compared to the short rod 2.5L engine with a rod ratio of 1.45. Here, the maximum angle of deflection is 20.12 degrees. Consider this when decided between a 2.2L and 2.5L engine! The 2.5L crankshaft that is normally put into the tall block has a rod ratio of 1.51, which is slightly better than in the short block, but still worse than 2.2L short block.
The reduced angle that the rod has to travel reduces the rotating intertia of the crankshaft and rod assembly. This reduces the parasitic drag of the rods and sends more power to the output of the engine.
Another advantage of the increased rod ratio is the amount of dwell
time of the piston at TDC. While technically, the pistons are at
TDC and BDC for an infinately small amount of time on both engines, the
effective amount of time they stay at TDC is increased on a long rod engine.
For example, if you consider 5 degrees before and after the actual TDC
on the crankshaft to be the effective TDC, then in a long rod engine the
piston will move less during this 10 degree sweep than in a short rod engine.
This is again because the long rods have a lower change in angle for any
given change in angle of the crankshaft. At BDC however, the dwell
time is actually decreased with longer rods, though this is not as big
of a concern on forced-induction engines. So, there is a "happy medium"
for the rod ratio here, which seems to be about 1.80. Increasing
dwell time increases the amount of time that the valves can stay open,
which increases the volumetric efficiency of the engine (the effectiveness
of the engine to move air in and out of the cylinder).
The 2.5L tall block is actually a stronger block than the 2.2L Turbo I block, even though it is a non-turbo block. Because of the increased torque output of the 2.5L engine, the tall block was given stronger main bearing supports and slightly thicker cylinder walls. The 2.5L tall block should be less prone to the block torquing of the early 2.2L Turbo I block. The trick is to find an early 2.5L N/A engine, which are a dying breed. It's best to take a trip to the local salvage yards and keep an eye out. If you get the block from a salvage car, be sure you get the oil pan, front bearing support, dip stick and tube, and all timing belt sprockets (they use the round-tooth belts). These parts are specific to this engine, so be sure to get them all.
Since the block is a non-turbo block, some modifications have to be made to it to accomodate the turbocharger. First, a 23/32" hole has to be drilled in the back of the block for the oil drainback from the turbo. According to Garry McKissick Jr., the tall block has the boss needed in the back of the block for the oil drainback tube, but it is not at the correct angle. So, the hole has to be drilled perpendicular to the boss provided, but some modification of the tube is necessary to get it to mate properly to the turbo. Also, the hole for the coolant supply line has already been drilled and tapped. All that is required is the pipe-to-flange adapter for the turbo coolant supply line.
It is important to have the block bored and honed with a torque plate
to remove any disformity in the bores. The torque plate allows the
bores to be honed round while torque is applied to the head bolt holes,
since this torquing actually disformes the shape of the bore. It's
always a good idea to give the machine shop the pistons that you are going
to use so that they can hone the block as close to the piston-to-wall spec
as possible. Another good thing to do is to cross-drill the block
between the bores, which allows coolant to flow between the bores and keep
them from disforming from the heat. O-ringing the block also helps
maintain a positive seal between the headgasket and the block as the block
expands from the heat.
The diameter of the pistons depends on how large you are going to have the cylinders bored out. Stock bore size (A size) is 3.440 inches. Typicially, a high-mileage block needs to be bored 0.020" or 0.030" oversized (most go with the 0.030", just to be safe). So, a 0.030" oversized bore requires a piston diameter of 3.470". Be sure to give the pistons to the machine shop that is machining your block. This way, the correct piston-to-cylinder-wall gap can be achieved (these specs are supplied by Venolia).
The compression height of the piston determines where the pin bore is placed. This is calculated based on the stroke of the engine, the length of the rod, and the deck height of the block. For rod ratio of 1.8, a rod length of 6.520" is required. With the 9.83" deck height of the block and the 3.622" stroke of the crankshaft, a compression height of 1.499" is required. This is slightly less than the stock 2.2L Turbo piston (1.604"), but more than the later stock 2.5L Turbo piston (1.370").
A stock 1986 and later Turbo piston has a dish volume of 14cc. When overboring a cylinder, the piston dish volume has to be increased to maintain the proper compression ratio. You can give Venolia the dish volume, and they can determine the best way to implement it. Otherwise, you can calculate the diameter and depth of the dish you desire.
As stated in the above sections, two pin diameters are popular: the 0.936" Chevy pin or the 0.912" Ford pin. While the Chevy pin is stronger, the Ford pin is lighter. The stock Turbo II pin is 0.900" in diameter, but is known to flex in high output applications. Also, Venolia forged pistons use spiral locks to retain the pin instead of the stock cir-clips. The spiral locks sit in a square-shaped groove and they can't be pounded out by the wrist pin like the cir-clips can. A pin length of 2.500" is required.
The stock ring groove dimensions are 1.5mm x 1.5mm x 4mm. Just tell the piston manufacturer that you want to use the stock ring dimensions. This is by far the easiest route because they have all of this information on file.
If you would rather go with a stock piston, the you should go with a Mahle cast piston. They are the best cast pistons that you can get for your engine. Two types are available. The stock Turbo II piston has a compression height of 1.604", which limits your rod length to 6.415" and your rod ratio then becomes 1.77. Better than stock, but not quite 1.80. If you go with 2.5L Mahle cast pistons, they have a compression height of 1.370", which allows a rod length of 6.649" and your rod ratio then becomes 1.84! Better than the desired rod ratio, but the 2.5L cast pistons have shorter skirts and higher rings. The shorter skirts provide less support for the piston to guide it across the bore and the higher rings encounter more exposure to the high temperatures inside the combustion chamber. If you are going to be running this engine hard and hot, go with forged pistons. Exposing the ring to very high temps can blow the moly coating off of the ring. Also, Mahle's floating pin retainer clips are prone to failure. They use the cir-clip design, which can be pounded out by the pin. Venolia forged pistons use spiral locks, which can't be pounded out.
There are several types of rings available. Companies like Sealed
Power, Total Seal, and Hastings
sell gapless ring sets that have a double ring for the lower compression
ring to greatly reduce blowby, which is important in any performance engine.
They also have high performance rings for the upper compression ring that
can take high temperatures and even detonation.
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This page is maintained by Russell W. Knize and was last updated 05/17/99. Comments? Questions? Email minimopar@myrealbox.com.
Copyright © 1996-2003 Russ W. Knize