I've been looking at the differences in port timing and intake runner lengths between the 4 port and the 6 port Renesis alot lately. The 4 port engine has longer intake runners which tune lower in the rpm range. As a result the 4 port engine makes more torque than the 6 port engine up to about 7000 rpm or so. The auto transmission just sucks up so much of it that it isn't noticable. Since the port timings between each engine are relatively close up until the auxillary ports open on the 6 port engine, it would seem logical that we could get alot of this power difference back. It is the midrange that gives us daily drivability and gas mileage anyways. Remember that the faster the engine gets past this range, the faster it gets to where it really makes power.

Both engines utilize VDI (Variable Dynamic Intake) which changes the effective length of the intake manifold to tune to a different rpm. This should mean that we could lengthen the ultimate manifold length for better low end as long as we didn't change the VDI system length. This would be very easy to do. The 4 port engine averages about 9" more intake runner length than the 6 port engine. Lets just get that back.

The answer is of course with a new upper intake manifold. Simple really. The easiest way to do it would be to actually use a plenum behind the throttlebody. We would then have 3 seperate runners feeding it. One from the auxillary ports and the other 2 from their respective rotors. The tuning effect is based on ultimate length that the returning wave from one rotor has to travel to react with the other rotor. This would add about 8 or 9 inches back to the length. this should only alter the powerband in the sub 7250 rpm range which is right where we need it the most. Above this point we should still have the same power levels and ultimate top end. My only concern is how a plenum will affect the powerband. I honestly don't think it will hurt it as long as it is large enough. As with any mod, the best results will be with proper tuning.

This is just another in a long list of things I am going to try when I get my RX-8 later this year. I can't figure out why so many companies are only concerned with ultimate horsepower when there is so much that can be improved upon in the rpm range that we stay in most of the time. Anyone else have some thoughts on this?

 

the article in road and track about the mazdaspeed rx-8 mentions that it just came out of the shop from getting power tweaks. they mention an T increase to about 165 i think. they said it was mainly from tweaking the intake manifold and a slightly different exhaust.

 

Dude, you know alot about rotarys. I'd have to agree with what you're saying.

 

In many ways this much like the old days with American muscle cars. Performance intake manifolds (Edelbrock to name one) effectively "moved" the peak horsepower within the RPM range of that particular engine. Depending on what effect you were looking for, you would get the appropriate manifold. Some where designed for low end torque and some worked better for high RPMs(not compared to the rotary of course). Same thing with aftermarket cam shafts. You cant take physics of the equation. When you gain something on one end of the spectrum....you often times lose it on the other. A perfect example it final drive ratio gear changes.

So...the ultimate goal would be to "put" the peak HP and torque where it can be utilized the most. Thats rarely 8500 RPM.

 

Rotarygod,

You are completely wrong, shut up.

I'm just kidding, I think you are onto something again. Hurry up and get your 8 so you can try out all your very good ideas! Or you can just build your own renesis from scratch the way YOU would have done it. We all know you can do it.

 

I had actually suggested something similar in a past posting. I had used the example of the intake runner length on the 302 V8 Mustangs... the GT40 upper and lower intake manifold, Holley, Edelbrock and others Vs stock. The basics are the same... the power and torque band was changed/moved based upon the length of the runners... equal length, unequal length, long runners, short runner, etc. And so I thought that it may be possible to tune some more power out of the 8 by making changes to the diameter and runner length of the intake manifold...

 

Gyro: I'm fully aware that you can't gain without a loss elsewhere. Luckily we effectively have 2 manifolds in one. One for low rpm's and the other for high rpm's. This is what the VDI system does. I only want to alter the lower end for more power through the lower range. The VDI crossover point may have to change slightly for a smooth transition though but I'm willing to bet that it won't be much if any. Proper tuning will smoothen out any rough spots as well.

More low-midrange power will give us some much needed gas mileage as well. I'm trying to figure out how to get the best of both worlds; good top end and better low end. I'm really limited by physics but I'd like to get as close to that limit as possible and Mazda did do a pretty good job on this manifold already. It doesn't mean it shouldn't be tried though!

 

My comments support your point and are aimed at the large group of enthusiasts who believe peak HP is the ultimate goal.

 

Now I guess the question is why can't manufacturers think this way... unconventionally and bring out more innovative products... in this case giving us more power where we need it.

 

Just throwing out a blue sky idea here.

Why not use a piezoelectric transducer to generate pulses in the intake. It would take a bit of research on the front end to determine the correct phase for each RPM. You might need to generate 2 pulses, 1 to cancel the existing pressure pulse generated and a 2nd to induce the one you want. If this worked you would effectively have an ifinitely variable intake tract length in a fairly small space. Timing and aplitude of the pulses could be varied to determine the maximum benefit. Once you had it, you would just program it in an ECU in a look up table.

 

Just a question on the 4 port VDI. When the VDI valve opens and when it closes? Always concerning rpms

 

Wow old thread! It's interesting to see how my views have evolved over the years.

I'm not sure what the question is. Are you asking if it's always based solely on rpms as opposed to rpm and load?

 

^ Well I know that the VDI valve in 4port opens at 7250rpm is this true?

If this is true the VDI stops exactly when the rev limiter comes at 7800rpm (4 port).

So my question is the VDI comes alive for only 550 rpm? From 7250rpm till 7800rpm ???

 

Threadsurrection!
The 4-port is rev limited to 7800?

So when are you getting that '8, Fred?

 

I'm not. I'm content with a 7.

 

Can't have both?

 

Owning one rotary is expensive enough. I've had more than one at a time. The biggest problem is me. I can't leave them alone. Why have 1 non running money pit when you can have two? I always take them apart and play with them and am then slow to put them back together again. I've got other things that money is getting saved up for right now anyways.

 

Does one of those things you're saving for happen to be shiny and circular?

 

RG,

Put that big brain of yours to work coming up with more power for us 4-ports! The 6-ports get enough attention already!

 

Install the GReddy turbo. It fits and it works. Problem solved.

 

Ok. How bout some new non-FI options? Realize it is pretty much a pipe-dream, but hey - if I can get you big brains thinking about it, maybe something will come up...

 

A chrome turbo?

You're a smart guy Travis! I don't care what anyone says about you.

 

Send me some 4 port housings and I'll look into it.

 

True, yet some things still remain the same

 

Only problem is I need those housings for my car to run