Leafy wrote: Top end of a medium turbo, bottom end of an N/A car, linear blend of the two in the middle, in other words, terrible.
Not terrible. Certainly not as efficient or powerful as a turbo and less low-end grunt than a roots/positive-displacement blower. But that "bottom-end of an N/A car" is nice for around-town driving.
Centrifugal blowers are very popular with the Muscle-car crowd, since those engines usually have lots of low-end grunt to start with.
Sky_Render wrote:Leafy wrote: Top end of a medium turbo, bottom end of an N/A car, linear blend of the two in the middle, in other words, terrible.Not terrible. Certainly not as efficient or powerful as a turbo and less low-end grunt than a roots/positive-displacement blower. But that "bottom-end of an N/A car" is nice for around-town driving. Centrifugal blowers are very popular with the Muscle-car crowd, since those engines usually have lots of low-end grunt to start with.
Centrifugal blowers were historically popular with muscle cars because a lot of those big, cheap American V8s had lousy heads that couldn't breathe at higher RPM. The boost curve of the centrifugal is sort of a band-aid fix for that. They're physically smaller than positive displacement designs (and thus easier to package), play better with intercoolers or carburetors, and are cheaper and have a lot less plumbing than the pair of turbos you'd need for a V8.
If the budget supports it, I'd go with a turbo over a centrifugal every time. The boost curve probably isn't a big deal for a road race car where you'll see very little time below 60% of redline, but it seems like a poor choice for autox or a street car on a modern, small-displacement, high-revving engine.
One telling point about centrifugals -- you can find tons of factory turbo cars, quite a few with roots-type blowers, and a few with twin-screws, but I've never seen a car that came from the factory with a centrifugal blower.
I think TVR was playing with them. Of course, that might tell you all you need to know...
As if they needed help making their engines go bang.
Although they probably tried it on the Rover V8.
Digging this back up, talking about a different car this time.
We have a miata with a nice little built motor at the moment. It's currently wearing a Begi 2554 kit, which i'm not 100% happy with for a couple reasons.
Anyways, i'm considering selling the Begi kit and picking up the Project G Rotrex kit, sans fueling and electronics, upgrade to C30-94. (Rated to 425hp)
On paper, it looks like an easy and fairly cost effective way in my particular situation to get in the 300-350whp range while retaining a softer power delivery that should keep the 5spd trans in the Miata happy-ish.
I'm struggling with whether or not i'd be happy with it. It LOOKS like a riot from the videos i've seen, but it's definitely going to be the exact opposite of the current setup. Current setup makes HUGE low end power that comes on really hard, which is both fun and aggravating.
I just wish i knew someone locally that had a rotrex on a miata, dammit.
Swank Force One wrote: Digging this back up, talking about a different car this time. We have a miata with a nice little built motor at the moment. It's currently wearing a Begi 2554 kit, which i'm not 100% happy with for a couple reasons. Anyways, i'm considering selling the Begi kit and picking up the Project G Rotrex kit, sans fueling and electronics, upgrade to C30-94. (Rated to 425hp) On paper, it looks like an easy and fairly cost effective way in my particular situation to get in the 300-350whp range while retaining a softer power delivery that should keep the 5spd trans in the Miata happy-ish. I'm struggling with whether or not i'd be happy with it. It LOOKS like a riot from the videos i've seen, but it's definitely going to be the exact opposite of the current setup. Current setup makes HUGE low end power that comes on really hard, which is both fun and aggravating. I just wish i knew someone locally that had a rotrex on a miata, dammit.
Bro, you know its about the torque. You should be able to make 350hp (dynojet) on the 5 speed with stock valve springs, but you're going to need a turbo with an electronic boost controller to do it and its going to need to be tuned on the dyno. Hit 250hp asap, then futz with the boost pressure to keep it there all the way to 7500 rpm. You just cant cut it that close with the blower easily unless you want to keep dicking around with a bunch of different sized restrictors while on the dyno, which sounds really expensive.
What are you talking about, fool?
A rotrex doing 350hp should never have the torque hit to kill the transmission. In theory. I'm talking quite literally just throw a C30-94 on it running to compressor redline and see what happens. Motor will be fine. Trans... i have one spare.
The "logical" answer is EFR, but that's going to cost an astronomical amount more. A 2560 is also logical, but not much of an upgrade.
It will do that if you run it with a restrictor and pullied to actually do something useful down low. IE pulley it to run to redline, but add a restrictor to limit it to 350hp and see if that results in too much torque. You wont break 4th gear on the dyno.
I saw my CRX posted earlier in the thread.
Here's the dyno graph for the motor so you have an idea on what power delivery is like around 350hp> Stock K20a + c30-94
The way the power builds is controllable and addicting :)
Leafy wrote: It will do that if you run it with a restrictor and pullied to actually do something useful down low. IE pulley it to run to redline, but add a restrictor to limit it to 350hp and see if that results in too much torque. You wont break 4th gear on the dyno.
You're not getting it. The C30-94 probably isn't going to make more than 350whp under any scenario at peak on a BP.
Unrestricted.
I'm really not concerned with the torque down low after a fashion. Not an autox car. 
Or at least, i think i'm not. This is why i need to drive one. The 2554 is annoying. I don't need or want 5psi by 1400rpms.
In reply to wrongwheeldrive:
That looks berkeleying delicious!
Swank Force One wrote: In reply to wrongwheeldrive: That looks berkeleying delicious!
Its also a k series, so imagine that, but E36 M3tier down low. the c30-90 should be able to hit well over 350hp if you actually pulley to redline at the miata redline and have enough knock resistance to run any decent amount of spark advance. Based on the compressor map you'll hit about 18psi at redline and be flowing enough air to hit 400hp if you pulley it to hit 100krpm at redline.
Swank Force One wrote: On paper, it looks like an easy and fairly cost effective way in my particular situation to get in the 300-350whp range while retaining a softer power delivery that should keep the 5spd trans in the Miata happy-ish.
350 hp at 7200 RPM is 255 lb-ft. That's enough to shatter a 5-speed.
codrus wrote:Swank Force One wrote: On paper, it looks like an easy and fairly cost effective way in my particular situation to get in the 300-350whp range while retaining a softer power delivery that should keep the 5spd trans in the Miata happy-ish.350 hp at 7200 RPM is 255 lb-ft. That's enough to shatter a 5-speed.
It'll take that for a little while. I was assuming that he was spinning to 7500 like most tuned miatas. 7200 is a weird number to spin to, its above the factory redline but lower than you can safely go.
If 255ftlbs is going to trash a 5spd, then i don't know how long it'll last either way, for that matter.
Leafy, are you talking whp or crank hp numbers? The blower is rated for 425hp. Not whp, just hp.
Anyways, thanks for reminding me that i need to work on my tune. Currently only set to 7200rpms. I'm sure that extra 300rpms will be winsauce on my enormous turbo.
I roughly plotted where I think the miata would end up at stock redline at 100krpm on the blower and it came out to be flowing ~47lb/min. That should be enough for north of 350whp as long as the IATs arent silly even with the stock intake manifold. Remember soviet on the EFR6758 was doing 400 on 93 flowing about the same.
Interesting.
I suppose the next thing to figure out is how much i can sell the Begi kit stuff for.
Leafy wrote: It'll take that for a little while. I was assuming that he was spinning to 7500 like most tuned miatas. 7200 is a weird number to spin to, its above the factory redline but lower than you can safely go.
7200 is the factory rev limiter on an NB. I don't bother to rev mine above that because there's little point, the torque falls off significantly (cams?).
250 dynojet wheel ft-lbs is definitely into a significantly shortened life for a 5-speed. Mine stripped 3rd gear at 85K miles (50K of them boosted) around that torque level. Mine's a turbo, though, so it makes peak torque significantly lower than a centrifugal blower would. A rotrex would probably not see that torque level as often (at least, not in street driving). Some cars have killed transmissions with less torque than that, some have survived more, it's variable. One theory I've read suggests it has to do with manufacturing tolerances for the axial tooth engagement and deformation of the case allowing the gears to separate thus reducing the radial engagement. The 6-speed is generally agreed to be stronger, and has a visibly reinforced case compared to the 5-speed.
codrus wrote:Leafy wrote: It'll take that for a little while. I was assuming that he was spinning to 7500 like most tuned miatas. 7200 is a weird number to spin to, its above the factory redline but lower than you can safely go.7200 is the factory rev limiter on an NB. I don't bother to rev mine above that because there's little point, the torque falls off significantly (cams?). 250 dynojet wheel ft-lbs is definitely into a significantly shortened life for a 5-speed. Mine stripped 3rd gear at 85K miles (50K of them boosted) around that torque level. Mine's a turbo, though, so it makes peak torque significantly lower than a centrifugal blower would. A rotrex would probably not see that torque level as often (at least, not in street driving). Some cars have killed transmissions with less torque than that, some have survived more, it's variable. One theory I've read suggests it has to do with manufacturing tolerances for the axial tooth engagement and deformation of the case allowing the gears to separate thus reducing the radial engagement. The 6-speed is generally agreed to be stronger, and has a visibly reinforced case compared to the 5-speed.
The stock redline on the NB is 7000 rpms. The tach reads 7200 because its wildly inaccurate at high rpms.
IMO, if you're going to run a supercharger, it should be a twin-screw supercharger.
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