Talking to Mazcare in Atlanta today on a supercharged convertible I am planning to build as my 2nd rx-7 daily driver.
Already have a 92/91 TII.

These guys build rx-7 race cars so there pretty knowlegable.

One of the guys had done this before, but he didn't recommend it because:
- Intake temp is increased
- Intercooled supercharger requires insane pully gear ratios
- To get the "instant" boost & intercooled air, your running the supercharger out of spec (overspin) so it won't last long
- He's tried 2 different aftermarket superchargers that worked well, but didn't give you the "BIG @$$ NA ENGINE" feel I'm looking for.

I'm looking for MAX streetable feel, 200HP, and near OEM TII gas mileage. Note sure if a ported engine would help/hurt low end torque WITH boost.

I was thinking swapping a TII drivetrain/engine into a convertible, but wanted a supercharger for more low end torque instead of the 3500 rpm rush.

Any NA supercharged owners want to contribute to this discussion?

Keep in mind 260-270 FWHP (220RWHP) is very possible with a supercharger, but I'm looking for LARGER NA / streetable feel as the primary goal without killing fuel economy or reliability.

 

Okay, you pretty much know where I stand on the whole supercharging thing. Assuming that you can get the Paxton setup, you can talk to Craig Conley at Paradise Wheels 760-740-0954, he's the new owner of the SN series blowers originally made by Paxton. Paxton sold everything to him, parts, service, etc. I spoke at length with Craig, he's a real nice guy. He told me that he can upgrade the blower to 17psi for $100 extra during a rebuild ($700) or $250 on its own. So that would give you plenty of boost.

Now, the other thing I've been thinking about is an air/liquid intercooler right where the elbow between the sc and tb horn is. (See the pic below.) Only problem is its a tight fit, but if you moved the ac line that's running right behind it, and used the tb horn as the outlet of the IC, I think it would fit. Food for thought.

 

SuperchargedRex,

Actually I don't - please share your thoughts on the supercharger, throttle responce, adv. over a turbo setup, repsonces to the presumed disadvantages above, etc...

Thought I'd be different and swap TII tranny & drivetrain into a convertible, but go with a supercharger instead.

A few more pics:

 

In general, I like the supercharger, and the fact that everything pretty much bolts on is a big plus. I could readily transfer it to another NA motor if I wanted to. I hope that others that go down this road heed the advice and lessons learned by myself and the others (Phil Garrott, Travis Shrey, Infini) so they know what they're getting into. I had the luxury of going last of the four of us, and I listened to what the others had to say. I also understood that this is not inexpensive; even if you could get a used sc, and fabricate the custom pieces inexpensively, you're still easily into the $2500-3000 range. Even if you get into the $3500-4000 range with all new parts like I did, I still think it might be similar in cost to doing the turbo swap when factor in all the pieces.

First off, the Paxton is a centrifugal supercharger, so it is essentially a turbo where you have a belt spinning the compressor instead of exhaust gases driving it. Unlike a turbo where you can get all of your boost in any rpm range (once it spools up), the sc provides boost in a linear fashion relative to rpm; you don't get full boost till close to redline. Therefore, a similarly sized turbo should provide a flatter torque curve over a wide rpm band. Now with the sc, you don't have the lag, but you don't get all of the boost either; its a trade-off.

If you're looking to drag race a lot, get a turbo, but if you want something that can be docile on the street, give you near-stock gas mileage, but still give you great top end power when you want it then the supercharger might be a better option. Not to say that it doesn't give you low end, because there is a remarkable increase over stock. I guess it just depends on how you drive and what you use the car for.

I find the throttle response to be very crisp, especially in the mid range rpms (2500-5000). The car pulls very smoothly all the way to redline, although there's a bit of a surge when the 6 ports open up. Personally, I like the way it accellerates linearly, like a stock motor. From what I understand, some of the guys who autocross say they don't like having a turbo because if you downshift into a turn you spool up the turbo giving you more power than you wanted/expected. I don't know how true this is, but it makes sense to me.

And if you wanted even more power, then the upgraded sc would make sense, but you have to remember that you'll need more fuel. Even if you swapped secondary and primary fuel injectors for 550cc/min, I doubt if it would be enough fuel for that much boost, which means you're looking at the cost of a fuel computer and possibly even bigger injectors.

Bottom line is, speed costs, how fast can you afford/want to go? If you have any other questions, feel free to drop me a line. You can find links to the other superchargers on my website too.

 

SuperchargedRex,

Any good sites that explain the different types of superchargers (centrifugal is one type?) and what to look for (such as how boost levels are specified)?

I'm looking at a similar path - supercharged convertible as a daily (TII takes care of weekend dragging ). I'm craving that docile / larger NA / max streetability feel your describing.

A prerequisite is the TII drivetrain & engine I know. Just swapping in the supercharger in place of the turbo may work if the boost levels are the same (6.5 - 8PSI is all the OEM TII fuel system can take). Trick is keeping < 3500 rpm boost levels rich I guess.

Where I'm lost is how air intake temps help/hurt performance with a supercharger (mazcare's point against). Are there supercharged - intercooled options out there (intercooler increases responce lag) or is that not necessary?

I'm assuming though as with a turbo both the exhaust and intake gasses are co-accelerated creating higher air pressure levels within the engine.

In addition I'm assuming a streetport would NOT be good in a supercharged setup for the docile feel.

Thanks

-- vaughnc

 

Actually, there was an article in a car magazine a few months back. I haven't done my recycling yet so I'll try to find it for you. In the meantime, here's a pretty good link with a bunch of info: http://www.superchargersonline.com/techarticles.asp

The whole "hot air hurts performance" is actually a fairly simple physics concept. Cool air is more dense than warmer air, so there's more Oxygen to mix with the fuel and burn, creating relatively more power than warmer air. Everyone accepts the fact that a turbo runs very hot (and subsequently heats up the discharged air) because the exhaust gases are what's driving it, and many people believe that is the only reason why you need to cool the air coming out of the turbo, but their wrong. Its only one of the reasons. The second reason goes back to physics; when you compress the air molecules together, they give off energy in the form of heat. So the physical act of creating the boost pressure will increase the temperature as well. And this is why you *should* use an intercooler with a sc because you're still compressing the air and heating it up. Lastly, I can tell you from painful experience that even after a 30 minute drive, the housing on my sc gets hot enough to burn my hand.

There are some sc out there with built in intercoolers, but none designed for the RX-7. Like I said in my other post, I've been thinking about an air/liquid IC mounted right between the sc and the tb horn, this way there isn't any piping to create lag, but its an extremely tight fit. You could always run a front mount IC, and if you had the higher boost sc the lag might be tolerable.

As for how much boost the fuel system can take, there are a lot of opinions and not enough data for my liking. The NA motors come with 460cc/min primary and secondary injectors, and the TII come with 550cc/min primary and secondary injectors. I left all the stock NA computers alone, changed my secondary injectors to 550cc/min, upgraded my fuel pump to the Rotary Performance http://www.rx7.com unit, and I'm using the fuel pressure riser that came with the sc kit. I run rich and do not have any problems with detonation. Check out the fuel calculator on the RP site while you're their, it'll give you some insight into what kind of hp you can run with the factory injectors.

Just my opinion, but I think a TII motor wouldn't perform as well as my NA motor for two reasons. First, the TII motors have lower compression rotors, so you'll make less power. Second, I believe that the intake ports on the TII will flow more air than those on the NA (assuming the NA's 5th and 6th ports are closed at low rpms) and you might actually get bogged down off the line. Remember, the centrifugal sc doesn't give you all the boost at low rpms, so you've got these great big intake ports with not enough air coming in to keep up the scavenging effects in the combustion chambers. Now if you had a roots type blower, I think that would be okay because you'd get nearly full boost at low rpms.

Similarly, it is my opinion that even a mild streetport would sacrifice some low end torque relative to a stock motor, but since I opted not to port my motor when I got a rebuild, I have no data to support the statement. The other reason I didn't streetport is that I was concerned that I'd need more fuel upgrades, which would've necessitated even bigger injectors/fuel controller which means more $$ which I didn't really want to get into right now.

 

How in the world does the air charg going to the intake get hot?

I understand that a turbo would heat the air, but how would a supercharger heat the air?

Do they get that hot?

If so, wouldnt an oil cooler for the SC fix this?

I will start working on the variable rate pully if there is demand for it.

What say you SC fans?

 

I'll check out the site tonight for sure.

Note I was thinking the 89-92 years had higher compression rotors equal to the GTU, but I could be wrong.

Interesting in choosing the NA over the TII setup. I was thinking since the TII motor is tuned for boost that would be the ideal candidate. I see your point with low RPM performance but in truth "off the line" performance would be 3800 rpm takeoffs min. if you were dragging / racing. For lower RPM takeoffs I would think exhaust backpressure would be the key so the NA would have the advantage there.

As far as fuel delivery at the lower rpms, that would take some good tuning to get right. Obviously you would want enough fuel to take advantage of the boost at those speeds so electronically "wiring the ports open" would be one option.

Definitly need to do more R&D on the various supercharger types and get a wide range of opionions from many users.

Note Mazcare's originally was thinking a fast spool turbo setup would be bettter and get "near" the superchargers improved streetablility / docileness. Haven't rulled that out either.

 

The charge air is heated in two ways:
1. The act of compressing a gas heats it (basic physics)
2. The sc impeller spins at a very high speed, calculated as follows:

impeller speed = (crank pulley diameter/sc pulley diameter) x 4 x rpm

so at a modest 3,000rpm on the motor you're looking at ~20,000 for the impeller (5" and 3" pulleys), and that's gonna generate some heat. Yes, you can get an oil cooler for it, but it will still get hot during highway use.