gus5891

ok i looked in the faq..it showed the 86-88 rx7 n/a with 146hp...an the 89-91 with 160hp...so my question is will i end up with the 160hp if i put the internals from an 89 engine in my 87?or will it vary an

NZConvertible

It's not the internals that made the extra power, but the externals. Specifically the intake manifold, which incorporated the VDI (Variable Dynamic effect Intake) system. This is basically a switchable dual length manifold.

RETed

9.4 versus 9.7 compression and 7kRPM versus 8kRPM redline also helped...


-Ted

gus5891

well i already have the89 intake on my car...but i'm abbout to take the motor apart anyway an i have an 89 engine sitting in a shed seems like it would be to easy an almost stupid if i didnt do it

NZConvertible

The compression increase would've done almost nothing for peak power, 1hp tops. A the S5 made peak power at 7000rpm, so you should get pretty close to S5 power at the S4's redline. But revving to 8krpm would probably make you faster, since average power between shifts would be higher.

I don't really get why the redline went up though. Redline has nothing to do with an engine's ability to breathe, only the mechanical limitations of the moving parts. The only significant internal change was the slightly lighter rotors, but that doesn't seem enough...

Soma

Obviously you're aware you can run the na s5 rotors in the s4 rotor housings as long as you use the s5 front and rear counterweights.

I imagine the decrease in rotational weight from lightening the rotors added to the increase in the redline. How they determined a full 1k increase is beyond me but I bet someone did the math for it.

Sideways7

iTrader: (7)

I have a hypothesis, and it goes like this:
Generally the redline is the max the engine can rev safely (generally speaking). In this case the rev limmiter is usually close to the redline. For instance my wifes 6 has the rev limmiter only 200 past the redline. On my RX-7 the rev limmiter is way past the redline, something like 1000+ past (I can max out the tach at 8k and it still revs a bit). Why? Because the engine can run that high safely. The reason the redline is at 7k is because power drops off around 7k due to the way the intake manifold is designed. They could have gotten more power at high RPMs but it would hurt lown end power which they didn't want to do. When they created the VDI manifold it allowed them to use the runners tuned for low rpm but still have the shorter ones for high rpms. Thus it could make power higher and justify the higher redline.

That came out like a conspiracy theory, didn't it? Oh well...

NZConvertible

They're less than 5% lighter, which doesn't seem likw it would make that much difference to me.

A little. But your idea isn't a a silly one. It does seem odd that the rev limit is so much higher than redline.

gus5891

yeah i know i can put the internals of an s5 into an s4 with the counter weights...an i know that the higher redline was not only based on lighter rotors but also on better balanced ones...but as i said with an s5 engine just sitting here an i'm about to strip mine down it would be stupid not to go for it...an little more power...a little more performance...an a little more hell on the honda civic's lol

NZConvertible

The S5 rotors are not "better balanced" than the S4's. If Mazda did one thing well it was balancing rotating assemblies. Or dis you mean the rotor recess volumes were better matched to each other?

If you have the S5 motor there you might as well use it. If you want to have a functional OMP you'll need to swap the S4 front cover and TB onto it. Otherwise you'll have to premix as the S4 ECU can't run the S5 electronic OMP. You'll also need an rpm switch, solenoid valve and air pump to operate the VDI system.