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For a bit less than an MD4-2D you can get many of the advantages of the MD4-2D for applications in the 200-350hp range.  The MD4E combination works good with turbo applications on motors of 2.5L or less displacement where the power comes on more gradual.  But if your motor is larger displacement, a highly exotic motor with over 325HP, or you plan to drive extra hard such as in race applications, you should opt for the MD4-2D that utilizes wider Weddle transmission gears.  

It's important to note that if your horsepower dramatically increases later you can always retrofit your existing MD4E with the larger Weddle gears, transforming your MD4E transaxle into the stronger MD4-2D.  Since both versions use the same case and differential carrier you can upgrade later without the added hassle and expense of changing chassis mounts.