I have a 87 na 13b with a 91 engine and I was wondering what the difference in the two intakes were and if you could use the 87 intake with the 81 engine thanks in advance

 

*91 engine

 

The S5 intake breathes a little better, and uses VDI to optimize the intake geometry/resonance for better high-rpm breathing, as well as using the air pump rather than exhaust to actuate the 5-6 aux ports.

If you are using an S5 in your S4, did the engine/ECU harness all get switched over, and an S5 ECU and and MAF? Usually these need to be kept series-consistent for everything to work correctly. Presuming that to be the case, it won't be completely plug-and-play to go back to an S4 intake with otherwise S5 hardware and electronics. And it would be a bit of a downgrade in power, at least at the top end.

 

I'm pretty sure it was all switched over but I'm not sure I didn't do the swap but I will look into that do I need an adapter plate at all if I were to go that way or will it all bolt up? And thanks for the help!

 

I'm confused about why you'd even consider doing this? The S5 intake is well understood to breath significantly better than the S4. Stock, the S5 makes 14 more hp, with a broader torque band, but the suggestion is made the gains can be substantially more, because the S5 runs pig rich above 6000 and can make substantial added gains with tuning. Swapping to an S4 will effectively drop your rev limit to 7000 (due to not breathing well past 6500) and cost 14 or more HP. https://www.rx7club.com/2nd-generati...ake-s4-813390/
All this is discussion of why the S5 intake is better and how much (and a lot of uninformed flaming; I'd concentrate on the posts from Rotary God and Banzai Racing). No one ever goes the other way, S5 to S4. If you do, you also would need the S4 lower, because otherwise there are drilling and grinding issues to match up the studs and have clearance to tighten one nut. With the complete S4 upper and lower, you will also have to figure out how to make its exhaust actuated aux ports work (not super hard, but another nuisance job... on the way to making less power).

 

Having done the S5 upper with S4 lower swap, the S4 manifold is punchier down low and I prefer it for a city driving. The S5 makes way more power above that and has all the benefits mentioned above. The basic principle is that the S5 manifold can change its effective runner length via the VDI system. This allows it to flow better over a broader rpm range. The S4 is optimized toward the middle rpm range and really chokes out up high.
The stud mismatch between series is an issue, but I ended up keeping the S4 lower for aux port simplicity. I just used 4/5 of the studs and a little extra rtv. Everything, (including the smog test) says it worked just fine.

 

This isn't entirely true. I've seen all kinds of combos and for some applications the S4 intake is preferred to the S5.

Just to clarify, a few points:
Yes, S5 engines make 15 more HP than the S4 engines. However, the S5 engines also have higher compression and lighter mass. A LOT of the S5 power comes from improved rotors and improved CPU.
The S4 intake has strait runners that are vastly superior to the S5 in terms of FLOW VELOCITY. This is why the S4 engines always have more punch off the line.
The S5 intake actually creates positive pressure waves in the intake. If you hook a vacuum gauge to the lower intake, you will see +1-2 PSI from the VDI effect above 5000 rpm.
So in summary, the S4 intake doesn't actually choke the engine. It actually breathes better than the S5. The S5 intake cheats above 5000 rpm by actually working like a very very very small turbo.

There is a video on YouTube of a tuner running a streetport NA with an S4 intake. They hit 210RWHP and 160 torque. The S5 intake will never make more than about 190-195RWHP due to the crazy twisty design. For my autocross car, I've actually considered swapping back to the S4 intake because you need that low end punch to pick you out of the corners.

 

Found the video... it was on facebook. https://www.facebook.com/forcefedperformance/videos/1230380690312572/
http://www.facebook.com/forcefedperf...30380690312572

 

^ This

The advantages and disadvantages depend on the application. IMO the conversion is a complete waste of time unless you are broke and that is all you could scrounge from the junk yard. If you want a screaming NA engine, install a race intake manifold, ITBs, performance exhaust system, and a professionally-tuned standalone EMS.
http://www.borlainduction.com/media/carb_vs_fi.pdf