The 4-banger turbo 718s haven’t gotten much love from the aftermarket because they are just the lowly 4-cylinder engines instead of the all-mighty 6-cylinders in the Porsche lineup. Those of us who grew up tuning turbo-4s from the 1990s and 2000s know how much potential they have. Enter RaggDoll Motorsport.
RaggDoll has a shop 718 GTS with the 2.5L engine. As no one else was really making high quality performance turbocharger upgrades for the 4-cylinder family of engines, RaggDoll decided to go at it themselves. I have typically shied away from hybrid turbos as many companies just do them wrong, but I’m comfortable with what RaggDoll has done in creating their RENNEN turbocharger upgrade for the 718 Cayman 2.0L base engine. For one, they rebuild with new OEM Borg Warner bearings. They also don’t hog out the turbine housing to stuff in a too-big diameter turbine wheel which can cause numerous issues.
From my estimations, the stock 2.0L engine turbocharger is good for about 39 lbs/min. The RENNEN gets a compressor wheel upgrade capable of about 53 lbs/min which is basically what the 2.5L GTS turbocharger compressor flows. So, the factory compressor housing is already well-sized to the new bigger compressor wheel of the RENNEN. To go with the higher flow compressor is a higher-flow design of turbine wheel. The RENNEN is sized to be a modest upgrade in order to retain good response characteristics. The goal with the turbo upgrade is to be able to make good power while only using 91 octane fuel. The Race Gas Concentrate is great stuff, but I don’t want to have to depend on it.
I’m just going to highlight some of the tips and tricks of the install because there are a bazillion steps. To swap out the turbo on the 718, you need access from both above and below. Removing the bulkhead panel behind the seats gets you access to the compressor outlet tube.
With the compressor outlet tube removed, you need a long and straight T30 driver to loosen the screw holding the oil feed line in place. Later, I found out that it’s easier to pry the oil feed line off the turbo after the turbo has been unfastened from the header giving a bit of free play; I wasted about an hour trying to pry out the oil feed line before I figured out the trick to loosen the turbo from the header. You will need a very short and very thin pry tool; I used feeler gauges to start. After I took this picture, I did stuff rags in the two open pipes. It would be very bad to drop a fastener into the turbo.
To keep from dropping the oil feed line screw multiple times and having to crawl under the car multiple times when reinstalling, I had the bright idea of using a little bearing grease to hold the screw to the torx driver. This was after crawling under the car multiple times due to dropping the screw. The tub of motorcycle bearing grease is the same one I’ve had for almost twenty years. Considering I don’t even have a motorcycle anymore, the tub of lube will probably outlive me. Hey, that’s a nice grade 8.8 stainless steel screw.
I found an 8mm ratcheting close-ended wrench came in handy in a number of tight spots. These two oil lines need to be removed from the turbo oil drain reservoir.
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I personally think you can actually hear those minute surface flow improvements in the exhaust note. At this level, you’re not chasing performance gains, but rather aural pleasure, instead.
Way back in the day, I did gas stand testing to quantify the turbine efficiency improvement due to extrude honing a turbine housing. The gain was measurable and worthwhile for those seeking maximum turbine efficiency. The two major areas where surface smoothness mattered were the turbine inlet section and at the narrow throat section where the flow exists the housing volute and enters the turbine wheel. Basically, wherever flow velocity was highest. I couldn’t smooth out the throat section on this turbine housing, but I got the entrance section done and I bet the difference is measurable on the gas stand. The compressor side, not so much. I also like to call it arts and crafts time and it’s relaxing. Sure, the hand smoothing of the turbine housing is a small incremental improvement, but so is a drop-in air filter, thermal barrier coating, compressor inlet, or my x-pipe exhaust tip. Keep on adding them all up though and it comes meaningful 🙂
Awesome and thorough write-up! After a bunch of 4 cylinder miatas, I don’t think I could upgrade to a Porsche with a 4 cylinder though lol
I had glanced at the GT4, but it was essentially a $50k premium taking into account the ADM dealers were asking. The 4.0l GTS was roughly a $25k premium over my T. But in the end, I still really like turbo-4s.
Initial engineering meeting for the 718 6 cylinder : “What would be the best engine layout and displacement to give customer’s high levels of power with a linear powerband?”
Initial engineering meeting for the 718 4 cylinder: “Who has a copy of China’s engine displacement tax regulations?”
For the higher trim GTS, I don’t disagree. For the base trim, the 2.0l turbo stomps all over the 981 base 6-cylinder in torque, power, and fuel efficiency.
That said, the 2.0l with this turbo upgrade, good fuel, and some exhaust mods is 350wtq and 400whp capable which puts it in the range of a bolt-on and tune 4.0l 6-cylinder. I’m straddled with Cali 91 octane and keeping the stock cat. The 2.5l engine with larger upgraded VGT turbo is 500-550whp+ capable, over 400wtq, and still good response to boot because of the VGT. No NA modded 4.0L can touch that.
Not that everyone cares about fuel economy, but I can get 32-34mpg highway no problem. The 4.0l can’t touch that.
“The 2.5l engine with larger upgraded VGT turbo is 500-550whp+ capable, over 400wtq, and still good response to boot because of the VGT. No NA modded 4.0L can touch that.”
565>500-550 https://demanmotorsport.com/4-5l-cayman/
“I’m straddled with Cali 91 octane and keeping the stock cat.” –> We forge the chains we wear in life.
Mmm… A bit extreme as that’s well past bolt-ons and a $50k+ engine package. But it’s possible. RaggDoll is also working on a non-VGT big turbo package that will be good for 600-650whp on the 2.5l. A built bottom-end is recommended at that point. Of course, it’ll have some lag that the NA engine won’t.
Did you cerakote the compressor housing? Also, since the boost was holding with higher flowing wheels, did you notice a mass flow change in the data logs?