Friday, April 30, 2010

4AGE 20V to 7AGE 20V conversion

With the introduction of the 7A-FE, the 4A-GE tuners gained an easy option to use the longer stroke 7A-FE block to gain higher stroke figures previously reserved for the daring and the well financed. The 7A-FE made its debut on the more mundane Corolla sedans in 1992. The block design of the 4A was kept thus came the possibility to use the 4A-GE head. The birth of the 7A-GE hybrid.

The characteristic of the 7A-GE is generally more subdued than in the peaky tuned 4A-GE, but for all of the maximum revs lost to the 4A, the 7A makes up in torque. With longer crank stroke, the 7A-GE’s cannot be revved as far due to known failures of connecting rods and bearings. The maximum power band usually falls at as little as 6800rpm, a sign of internal stress, meaning it’s way-past the physical redline. Most tuners recommend its use below 7500rpm. Despite its conversion to the true-twin cam design and free-flow head, the modification still produces all of the 117ft/lbs of torque of the original 7A-FE configuration, and the power gains from the 7A-GE varies but is in the general area of 10% over a similarly equipped 4A-GE.

Note however, that in the 7A-FE, the peak power comes in earlier at around 5700rpm compared to the 6600rpm of the 4A-GE. Because this is a custom application, the figures may vary by individual project. Like the 5A-GE and more so, the 7A-GE is a very universal in drivability and is highly recommended for street machines looking for that little extra push, who see occasional autocross or ET traps. Those looking into more elaborate systems like turbo or high-rpm usage should, however stay with the 4A-GE design for it’s intended sporting nature and previous data and upgrade parts availability. Also, the flywheel attachment of the crankshaft is inherently weak on a 7A-FE so making it rev high or boosting too much power on it is a little bit risky. Having said these, let’s dive into the world of 7A-GE.

Conversion to the 7A-GE hybrid need not be as complicated as the 5A-GE. This is because the block internals need not be torn apart and modified. Starting with a cleaned, adjusted and polished 4A-GE head, it can bolt right onto the 7A-FE block. This process is fairly elementary in nature… There are few differences in peripheral components depending on the engine orientation and the chassis it’s installed in, but all of the components should be readily available from one of the 2 engines you should have on hand the 7A-FE and the 4A-GE.

The problem most people run into in this configuration is the timing belt. Because of the taller block of the 7A, the 4A-GE timing belt proved to be too short for the increased deck height. Why not use a 7A-FE belt? Well, the “FE” design head is a twin cam but is driven by the crank only on one cam. The other cam is driven off the powered cam by gears. A “slave-cam” twin. So what do we do? Traditionally, we had to go hunting in the bin at a parts store or junkyards. But, thanks to the few who visit Club4AG, a Porsche 924/944 has been rumored to fit. For this application, the crank cam-pulley from a 4A-GE has to be used… Obviously, the 7A-FE pulley will not drive the GE cams at the correct speed anyway…

All else being done, the engine should have 1800cc and compression ratio of between 9.8 and 11 depending on which piston and 4A-GE head you used. Most applications use similar ECU and peripherals from the 4A-GE and seem to work quite well despite it’s increase in displacement. 7A-FE ECU is avoided because of it’s low rev-limiter and complexities in placing it in a car originally equipped with a 4A-GE. Perhaps an aftermarket ECU like the “Freedom” is potentially helpful in making it perform at it’s peak output with a particular grade of gasoline.