My son and I are having a generational discussion about the stock twin turbo set up of the 3d Gen RX7 versus the single turbo conversion we are seeing on many after market modifications offered in used cars. I prefer the factory, original twin turbo setup. My son said that there is no reason to do a single turbo conversion, as the factory twin is road tested, factory engineered and gives the most consistent and reliable engine performance over the single turbo. What do you think? I would pay more for a factory stock car over a modified single turbo conversion car, regardless of the quoted horsepower increase. What do you think? Thank you. Respectfully, John J. Garvey

 

Modern singles will easily outpace the OEM system in response and output (simultaneously!) & be more reliable from a lack of vacuum lines standpoint.

That being said, well maintained stock examples will always be worth more.

 

Most people go single turbo on the FD because they can't figure out how to keep the sequential twins working (it is complex) or they want more than the 350-400rwhp the stock twins can provide.

Not many single turbos can match the stock sequential response.
Most quality ball bearing single turbos (GT3576R, Apex RX6, etc) of the same 350-400rwhp range as stock twins will be as responsive above 3,000rpm.
A very few (I have experience with EFR 7670) can be as responsive as the stock twins with bolt ons from 2,500rpm on.

For the Gunsai Touge competition in Hot Version videos RE Amemiya has stuck with stock twins for response while Feed (both have been "Touge Monsters"/winners) uses small singles (T04S or the new Garret GTW they put on).

 

I think the 7670 is slightly oversized if 400whp is the ticket. a 6758 is good for 450whp and would likely provide the response the OP is looking for. You certainly have a lot more headroom with the 7670 though! If you were just looking to match the stock twin performance, I'd even consider a 6258 for ~325whp.

On the other hand.. if you're booking it, you'd never be at 2500-3000 rpm anyway

edit: disregard this! I am in piston engine land when it comes to turbo sizing..

 

I may be biased as I just made the switch, but I would vote single turbo all day long. But your end goal will greatly influence which turbo to run. I had sequential twins with bolt-ons up until I chewed up an apex seal. During the rebuild decided to go with the Turblown 7670 kit. Still breaking the set-up in so i can't give a true apples to apples comparison yet but given the choice I would pick the single turbo.

The sequential twins are very complicated as everyone has said. They are a massive hunk of castiron that pukes out heat. Beyond the headaches of the miles of vacuum lines, multiple solenoids that can fail, countless gaskets and potential leak points, they run much hotter than the newer single turbos. I made no changes to my oil cooler, radiator, intercooler set-up when going single, but I did switch to an electric waterpump. With the twins, I regularly sat at 190-200F idling and it wasn't uncommon to see 210+ driving around. With the single set-up I have yet to go over my set temperature of 185. And after driving around for an hour I can open the hood and grab the hot side piping of the intercooler. I could barely touch the cold side before.

The only upsides I really see sticking with the OEM set-up is if you live in a place that is very strict about modifying a street car, you're a collector and want a piece of late 90's engineering, or you want to keep the car as quiet as possible. The 7670 is a very quiet single turbo but it make more noise (Turbo whistle, BOV, etc...) all of which enhance the experience IMO, but aren't for everyone.

 

 

Thanks for the corrections! I am thinking of piston engines when I think about turbo sizing. Didn't realize the VE differed so much on a rotary.