Like resoling your favorite pair of shoes, we are going to refresh the engine of our project WRX. If you have been following our series on the remake of Ian’s 2002 WRX, you have seen us rebuild the suspension and the brakes already. So now we are going to turn our attention to the search for more power and are going to do some innovative stuff to the Subaru’s EJ205 with some help from Eagle Specialty Products.
Since we have a GD STI with a 2457cc EJ257, we will have easy access to do A to B comparisons of Subaru’s big engine vs its smaller 1994cc EJ205. The EJ257 with its 99.5mm bore, 79mm stroke and larger displacement has more torque and spools its turbo fairly quickly. However, it seems a bit rough and not especially happy to rev. Conversely, the smaller EJ205 with its 92mm bore and shorter 75 mm stroke seems smoother and freer revving. It’s a very pleasant feeling engine even if it lacks the torque and a bit of turbo response. Both engines share a 130.5mm rod length. The EJ257 sports a stroke to rod length ratio of 1.65:1 while the EJ205’s ratio is an impressive 1.74:1.
So in the pretext of doing something different and unique, we decided to see what exactly we could do with the EJ205. We studied the EJ22G, the engine in the JDM 22B GC8 STI. This engine had a bore of 97mm and a stroke of 75mm for a displacement of 2.212 liters. This looks great on papers except the 97 mm bore which makes it not too practical to emulate as it would require the EJ205 block to be sleeved. We have reservations about sleeving engines due to the blocks structural integrity and issues that we have had in the past with sleeves staying put in race engines and even hot street engines.
To keep our block intact we would have to resort to stroking. Welcome Eagle Specialty Products to the project. They happened to have a slick EJ20 crank available with a very reasonable 83mm stroke. Usually, long strokes mean more piston speed and more stress on the engine’s internals. Although this is a whopping 8mm more stroke than stock, the stroke is still relatively short. Remember that an SR20 has an 86 mm stroke, so we will still have decently low piston speeds.
Eagle Specialty Products also just came out with a longer 132.9mm rod for the EJ20. We plan on having JE Pistons make us some 92.5mm oversize pistons with a 26.3mm pin height to accommodate the increase in stroke and the longer rod. This will give us a displacement of 2231cc, which is slightly more than the EJ22G. The stroke to rod length ratio falls to 1.60:1 which is still greater than an SR20’s 1.58:1. This will be a very reasonable engine, as far as internal stress goes, with decent displacement for the numbers. Our hope is to boost the EJ205’s torque and turbo spooling ability while not killing off its free-revving character. The end result being a very well rounded engine.
What is the significance of all of this concern about rod ratios? Read on and find out!
In the illustration above we have plotted piston position in the bore vs crank degrees. With a very short imaginary rod vs an imaginary long rod. This is to exaggerate things so you can see it on the graph easier. In the diagram, the piston with the longer connecting rod spends more time around top dead center (0-degree crank angle) due to a slower acceleration rate towards and away from top dead center (TDC).
A longer dwell time around TDC makes better use of the combustion pressure and turning the pressure into torque. a rule of thumb is that you want peak combustion pressure around 15-17 degrees after TDC. This helps mechanical as well as thermal efficiency. This is because the combustion pressures are higher while the crank angle is lower. The longer rod positions the piston to push down on the rod while it is straighter and the angle of the rod to the crank is more acute resulting in less side loading of the piston into the bore and offering more leverage of the rod to the crank for better energy transfer. Less side loading results in less friction between the piston and bore, freeing up power and reducing wear. The reduced piston acceleration also improves piston ring life, as the lower acceleration equals less force on the rings. Remember F=ma?
More critically, the slower acceleration around TDC on the exhaust and intake strokes improves volumetric efficiency at higher rpm. This is because you have more time to fill and empty the cylinders on the intake and exhaust strokes. Other things we have noticed with longer rod engines is that the torque and power peaks become closer in rpm. The engines are less sensitive to timing, so you can often get away with a few degrees less advance with no loss of power. The engines also rev more freely, seemingly with less effort and vibration. We have found in experimenting with Nissan, Honda and Chevy engines that even small changes in stroke to rod length ratios can make very feel-able differences in how the engines react and how long they last under racing conditions.
A rod to stroke ratio of 1.7:1 or higher is a good general starting point for a performance engine. Although many respectable production engines don’t meet this. For example, the Honda B18C is 1.58:1 and the K20A2 is 1.62:1. The Chevy LS7 is 1.53:1 and the BMW S54 is 1.53:1. High revving 1000cc sportbike engines tend to be around 1.9:1 to 2.0:1 and typically have a redline around 13k rpm. 600cc sport bikes use a ratio around 2.1:1 to 2.2:1 and redline around 15k-16k rpm. Bespoke racing engines can have even higher rod ratios. For instance, the 2009 Toyota F1 engine had a ratio of 2.72:1! The F1 engines of that period spun to 18,000 rpm. With those extremely high performance naturally aspirated engines the longer rod to stroke ratio was very important to making power, because the piston spends so little time around TDC due to the extremely high revs. Anything that can be done to improve dwell time near TDC, improve mechanical energy transfer, and reduce friction will greatly benefit performance.
Not everything with long rod engines is all advantages. We have found a slight trend in a long rod engines having a slightly greater propensity to detonate. Especially when trying to get an engine to run well with California’s awful 91 octane fuel. This is largely mitigated by a longer rod engine’s less need of ignition advance to make the best power. Another thing is that when the torque peak and power peak become closer together, peak torque is made at a higher rpm. This can make the engines bottom end response soggy. We think that because the piston moves away from TDC slower, the intake and exhaust port velocity is reduced at lower rpm actually hurting VE at lower engines speeds. This is partly mitigated because the engine wants to rev freely. Lastly, because exhaust port velocity is reduced at high RPM there is a slight tendency to spool the turbo slower.