NyX_Nick

Hi All,

I'm still pretty new to Rotaries, I don't really have any experience working on/building them all though I do fundamentally understand how they work and have seen videos of them going together but, what exactly is the main cost difference in building something like a 13B-REW to safely handle 400whp vs like, an inline 4?

When I was originally going to do a built engine in my MR2, I remember pricing out cams, springs, valves, headwork, block work, figuring out what rods/pistons/etc would work best. Stroker Vs. Oversquare, etc. And then of course other flow mods like what turbo, manifolds, fuel etc. All the standard stuff I guess that would be required to get a once 200hp motor, safely and "reliably" up to the 400whp mark (on pump)

But from what I've seen, Rotaries are mechanically a lot simpler. There's no headwork that needs to be done, but porting instead. The stock irons/rotors and eccentric shaft seem to be able to handle a lot of power, although I have seen some videos where the Rotors have material removed and are balanced, but it doesn't seem like that's always necessary.

In my eyes, it seems like for the most part, stock Mazda internals, with some port work and good apex seals makes for a strong block, and the stock intake manifold seems to flow well enough for those numbers, although I'm not sure about exhaust. It seems to me that most of the cost comes the turbo setup, making sure you have sufficient fuel for your needs and cooling (Water, air and oil) I've seen a lot of builds that have either twin pump setups, or one really powerful pump like a Walbro 450. Big Koyo rads and v-mount setups to keep temps down, and oil coolers for oil temps, and water/meth kits seem to be used fairly often on these builds as well.

That brings me to ignition stuff, is it normal to need an upgraded coil for 400whp, and is it standard to gap plugs, or step down in the heat ranges vs. what's recommended for stock, as power increases?

I understand the drivetrain needs to be upgraded (mainly clutch, fork and diff brace) from what I've seen, but the transmission itself seems pretty stout.

Is it safe to say that most of the cost of building an engine like this would come from the mods I'd need to install to keep it running reliably, vs building up the block/internals?

I've liked the idea of a high HP Rotary since I was a kid, and I'd love to learn my way around these engines. From the little bit of research I have done, parts wise it looks like it would be a fair bit cheaper to build one of these vs. an i4, or i6 or something. But I know that a build is more than just the sum of it's parts. So does the extra cost come from all the "While I'm in there's" that I'll run in to along the way, or paying for a shop to assemble the engine or build/tune it if I were to decide to go that route?

I know this probably get's asked a lot and I have tried googling but I come across a lot of old threads vs. new ones so I figured I'd ask in the new member section

Sgtblue

iTrader: (16)

Full disclosure...I'm not a builder, just some experience with rebuilds and maintaining my FD...

Yes, IMO it would be easier and less expensive to 'build' a rotary to your hp level over a small displacement piston engine. As you mentioned, the BIG deal is cooling (radiator, bigger IC, AI) and fuel (pump, lines, injectors, management). The rotary doesn't give second chances for detonation. Exhaust flow mods are fairly easy and a good street porting is all you should need. I think radical porting and lightened rotors and balancing is (or should be) reserved for dedicated very high rpm track cars. And though you can get there with the stock twins, if you have the budget and really want to make things simpler a well-chosen modern single turbo is probably the way to go. Drivetrain will need some reinforcement...engine mounts, differential bushings and probably some aftermarket transmission bracing. And an upgraded clutch from OEM...but not necessarily puck-style.

One thing you didn't mention is what will be your use of the car? Drag strip? Fun street car? Road track? Remember the FD was a sportscar, with a fairly sophisticated chassis and suspension for it's time and price segment. The rotary engine's characteristics fit in with that well. Plus, 400 whp is, IMO, getting close to all that's useable on the street or a tight road course. I guess my point is I always hear guys stating hp goals instead of performance goals. And those are two different things...IMO.

If you haven't seen this thread yet it might be of interest to you. It gives you a good idea of how fast the 13bREW responds to a few staged bolt-ons and gets very close to your power goal----> https://www.rx7club.com/3rd-generati...added-1104322/

NyX_Nick

Thanks! I'll read through that when I get off work. My intended goal for the car is a fun street car, and some mild track use. I set that goal cause I feel like it's a pretty good spot where I can set the car up so that it still has good road manners and the turbo doesn't need to be too big, so it should still spool fairly quickly. Most of the tracks around me have a lot of long straights and not too much in the way of tight and technical, a lot high 2nd or mid 3rd gear corners. Likewise, the backroads here while there are some twisty portions, they're not particularly tight either, more long straights, and sweepers. So I'd be trying to build the car to work well in that sort of environment. 90% of the time on tracks and backroad blasts will be spent above 4-5k, and while I appreciate what can be accomplished on the stock twins, or even the BNR upgraded twins, I like the idea of a single mainly because it adds some simplicity and cleans up the bay.

Suspension wise while I probably won't go as crazy as my MR2 and use heim joints for everything, I'll definitely look in to poly bushings all around, as well as trans/diff bracing and I actually haven't checked if shifter bushings are a thing on this car, but so then probably those too.

Before any of this I'd like to start with a base that is fairly clean. I'm not against high miles since everything would be essentially getting rebuilt anyways, but as long as there's very few gremlins I'd have to chase down before getting in to the meat of the build. I'd like to keep the "While I'm in there's" down to a minimum, but as with every build, I understand that they'll show up

Sgtblue

iTrader: (16)

If you want a better understanding of the rotary and how to stage modifications, I'd recommend this book. After 17 years of owning my FD I still pull it out occasionally for reference. --->

NyX_Nick

Just ordered that, thank you