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Old 09-01-2015, 12:32 AM   #1
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Default Rotrex C30-94 or C38-61

I'm looking for high 200s whp on a 99 with stock top end and forged internals (9.5:1 CR). I'm considering the C30-94. While I'm not after a huger amount of HP, I sure would enjoy more midrange torque. Any drawbacks to the C38-61 outside of the need for custom mounting hardware?
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Old 09-01-2015, 01:55 AM   #2
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Originally Posted by BustedKnuckles View Post
I'm looking for high 200s whp on a 99 with stock top end and forged internals (9.5:1 CR). I'm considering the C30-94. While I'm not after a huger amount of HP, I sure would enjoy more midrange torque. Any drawbacks to the C38-61 outside of the need for custom mounting hardware?
umm,
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This unit is capable of over 500hp and works best on 2.5-4.oL engines.
you want to use a 500hp unit for 200hp? You'll need to bleed off so much air it will sound like you're trying to inflate the earth.......if it even works
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Old 09-04-2015, 12:14 PM   #3
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I know the Rotrex C30-74 is good for over 200 whp with a standalone ECU. These units are not known for big torque like a turbo, but the horsepower is there.
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Old 09-04-2015, 02:39 PM   #4
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I know the Rotrex C30-74 is good for over 200 whp with a standalone ECU. These units are not known for big torque like a turbo, but the horsepower is there.
Run E85 and a 75mm pulley on a C30-74 and you'll have plenty of power. No need for the -94 unless you hate transmissions.
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Old 09-04-2015, 02:46 PM   #5
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<p>You know, people keep saying that. But Mobius barely got 230 whp out of his with a 949 CNC head. Had the bigger pulley too IIRC. No E85 though.</p><p>I haven't looked into it much deeper but I don't understand the huge difference between your statement above and Mobius' experience. Does E85 make THAT much of a difference?</p>
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Old 09-04-2015, 02:48 PM   #6
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<p>You know, people keep saying that. But Mobius barely got 230 whp out of his with a 949 CNC head. Had the bigger pulley too IIRC. No E85 though.</p><p>I haven't looked into it much deeper but I don't understand the huge difference between your statement above and Mobius' experience. Does E85 make THAT much of a difference?</p>
I'm putting E85 in my car today along with 1000cc injectors, dyno next week before MRLS so I'll have a data point with the 85mm pulley. All the dyno runs of Mobius car that I've seen also show substantially less boost than I've recorded on my data logs.

EDIT:

I did some searching, based on Mobius dyno graphs he was seeing 7psi peak with a 75mm pulley...he should have seen over 12 psi peak. With my current 85mm pulley, I see 9.7psi at 7100RPM. E85 should make a substantial difference based on my experience...I see no reason why a properly running C30-74 with E85 shouldn't make a chunk over 200WHP.

Last edited by Lincoln Logs; 09-10-2015 at 05:17 PM.
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Old 09-10-2015, 12:13 PM   #7
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Run E85 and a 75mm pulley on a C30-74 and you'll have plenty of power. No need for the -94 unless you hate transmissions.
The 6 speed transmission from the MSM should be able to handle the torque of the C30-94.
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Old 09-10-2015, 12:14 PM   #8
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The 6 speed transmission from the MSM should be able to handle the lack of torque of the C30-94.
<br />FTFY
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Old 09-10-2015, 03:37 PM   #9
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Quote:
Originally Posted by my97miata View Post
I know the Rotrex C30-74 is good for over 200 whp with a standalone ECU. These units are not known for big torque like a turbo, but the horsepower is there.


Barely. With a lot of supporting mods that would probably make 160whp naturally aspirated.


I'd go C30-94 if I had the choice.
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Old 09-11-2015, 01:22 AM   #10
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With C30-74 I got 300whp on E85 with ATI pulley on well built 99 motor on E85.

Mid 200's should be easy on stock block with rods.
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Old 09-11-2015, 10:29 AM   #11
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Originally Posted by k24madness View Post
With C30-74 I got 300whp on E85 with ATI pulley on well built 99 motor on E85.

Mid 200's should be easy on stock block with rods.

Depends if OP is willing to run E85... Mid 200's is not gonna happen on pump gas with OP's stock head.
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Old 09-11-2015, 05:31 PM   #12
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Originally Posted by k24madness View Post
With C30-74 I got 300whp on E85 with ATI pulley on well built 99 motor on E85.

Mid 200's should be easy on stock block with rods.
The appointment is set, I'll be at the dyno with my car tomorrow. E85 feeding a stock 99 block, BP5A cam, C30-74 with an 85mm pulley (9.7psi peak). Once I'm done I'll report here with the numbers.
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Old 09-11-2015, 09:25 PM   #13
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So with the C30-94 what's the draw back to this over the C30-74?
I know with a bigger turbo there is the issue of more lag.
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Old 09-12-2015, 04:40 PM   #14
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Originally Posted by my97miata View Post
So with the C30-94 what's the draw back to this over the C30-74?
I know with a bigger turbo there is the issue of more lag.
Generally speaking, a bigger Rotrex just means more airflow, more power and more torque at any given RPM. It'll just feel like an even bigger motor compared to the same set up with a smaller blower. TrackDayHookey has a huge thread on Miata.net with dyno plots showing what you can do with bigger Rotrex units on BP motors.

Time for some results on the C30-74. This morning I had an appointment at the dyno facility Brian Goodwin has been using for many years. It's a privately owned Dyno Dynamics tucked away in San Diego 65ft above sea level. With good weather, it doesn't get any better for making power. Conditions this morning were not the best with air temps at 80* and ~65% humidity.

Because this isn't a DynoJet (what I normally tune on and for comparisons sake) I spoke to the dyno operator at length about how this compares to a DynoJet. Thankfully he reported they have run back to back with the same conditions on local dynojets, so he has a firm grasp on the correction factor compared to other shops in town. First run on the Dyno Dynamics reported 220WHP at 9PSI, not bad!



After getting the fuel curve dialed in and I started to play with timing...the results were impressive to say the least. Last run of the day put down 235WHP and 190WTQ at 9psi peak. The car would have made more power, but given it's age (3 seasons on a junkyard block) and the fact it is completely stock I stopped here. Based on the correction factor the shop gave me this should be an easy 240-250 on a DynoJet given the same conditions. I should also note, I am running a Racing Beat header, Racing Beat high flow cat and a Racing Beat exhaust...little more power may be had by deleteing the cat.

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Rotrex C30-94 or C38-61-21175672649_595355ca44_o.jpg   Rotrex C30-94 or C38-61-21351800652_61ba4e3e1b_h.jpg  

Last edited by Lincoln Logs; 09-12-2015 at 04:51 PM.
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Old 09-13-2015, 06:46 PM   #15
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Nice results, I'm sold on the C30-74. How much extra is the C30-94 anyway? Just curious.
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Old 09-13-2015, 07:32 PM   #16
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Originally Posted by my97miata View Post
Nice results, I'm sold on the C30-74. How much extra is the C30-94 anyway? Just curious.
Thanks! No cost upgrade, it just comes down to your power goals.
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Old 09-16-2015, 09:58 PM   #17
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IIRC TrackdayHookey ultimately installed a C38-71 after the C30-94. That's how I got the idea of the C38. There's another guy who installed a C38-61, but not a lot on his results. Seems to me the reason more people aren't using the larger units is due need to build a custom mount. No kit for the C38.
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Old 10-03-2015, 09:27 PM   #18
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A C38 would be total overkill on the 1.8, like filling your tea kettle by holding it under Niagara Falls. A few years ago, a guy on the other Miata forum (aaronbc) was getting 300+ whp on a built 1.6 using a C30-94. I watched a couple of his hillclimb videos, the car was a rocket.

I want to replace my C30-74 with a -94 and restrict it to limit max boost to about 10 psi. My goal is to fatten the midrange a bit but keeping max whp around 215-220. I recently changed the 90mm pulley on my setup (made a shade over 200/176 @ 9 psi) with a 75mm. The change in usable power was substantial. By "usable", I mean the powerband started 1000 rpm lower with the 75mm than it did with the 90mm, making 3 psi by 3000 rpm and rapidly going past 12 psi by 6000. I don't know what the numbers were, didn't dyno it, but it felt like I had an extra cylinder.

Last edited by Burnout; 10-03-2015 at 09:37 PM.
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Old 11-03-2015, 07:24 PM   #19
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Default C38-61 it is then...

Thanks to a former NB owner running a C38-61, I'm now in possession of his old setup. Will report back in 2 years when up and running.
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Old 11-05-2015, 11:41 AM   #20
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^^^ We are counting the days.
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