MP62 Coldside vs. Rotrex.

[Rdrcr]

I thought this might be worth a discussion.

A Miata equipped with the larger Rotrex C30-74 making an unrestricted 12 psi on a 150mm crank pulley, TDR A/A FMIC, with 350cc injectors, header, FM exhaust, and tuned using the Powercard/Timing cards made 231whp and 178wtq.

I don’t know. They are good numbers and it is nice to see that someone used a ‘piggy-back’ on their Rotrex install but, from all the hype, I would have expected better results with the larger C30-74 supercharger (The standard size Rotrex which is sold in the Kraftwerks kit is the C15-60).

Most of the impressive dyno’s I’ve personally seen from Miata’s equipped with the Rotrex superchargers have been on race cars with some engine work, limited power-robbing accessories, and free-flowing exhausts. I always wondered what kind of numbers they would produce on a street car without a fully programmable ECU. Now I have.

What do you guys think about these results…good, bad, or indifferent when compared to the MP62?

Mike

[FormerDatsun510Man]

Actually, what you want is the SMALLEST supercharger that will efficiently make the max CFM (hp). This will ensure the best torque. I would think this would be especially crucial on the Rotrex.

It is about what I would expect. An air/air IC'd setup on a stock engine, whether it be turbo, MP62, Rotrex, etc.... as long as properly sized should make 230-240rwhp pretty easily on a stock 1.8L. However, there is a price to be paid both monetarily and with overall impact on cooling. The Rotrex alleviates the driveability problems of an intercooled Hotside MP62, but it still has a big FMIC blocking the radiator. That is not too much of a problem on a track car, but on the street using your A/C in the hottest months... I found it is an issue. I suppose there is a workaround, but that is more cutting, fabrication and more $$$. With intercooler that 230rwhp Rotrex is well over $5K, just in the kit. With better engine management it will be over $6K minimum. That is not counting the required radiator upgrade (due to radiator block by FMIC).

The torque curve is great on the upper half, but below 4000rpm, it makes less than even the Lowcost FFS kit. Not that lowend matters to everyone, but heck, I do spend time driving my Miata on the street in the 3-4K rpm range and it is nice to have some punch to pass. 115ft-lb at 3000rpm is NOT stellar torque.

That said the Rotrex does have an efficiency advantage once in the powerband and should make more power per psi than the MP62. However, this dyno seems to contradict that. Perhaps the tune is less than optimal. Hard to know without seeing the A/F trace. Also we don't know how much timing (if any) is pulled. As it stands, it is making 10rwhp per psi just like I had seen with many FMIC'd Hotside MP62.

[Tom @ Fast Forward]

20 RWHP/PSI. Correct?

I'm with Bill. I love the low end torque of the MP62. What they have is great for a track car where you seldom drop below 5000. If what I think is true, we should be able to get a stock 99/00 engine to 240 with 190+ tq and still maintain the flat torque. Also think it has possibilities of making it on 93/94 octane?? We are just out a couple weeks from testing. If it works, it would be upgradable to the older kits.

I would trade some high end HP for low end torque any day.

As it stands, it is making 10rwhp per psi just like I had seen with many FMIC'd Hotside MP62.

[FormerDatsun510Man]

No, I mean 10 ADDED rwhp per psi. Dividing total rwhp by boost doesn't make sense.

[Tom @ Fast Forward]

DOH! My bad.

[Rdrcr]

That said the Rotrex does have an efficiency advantage once in the powerband and should make more power per psi than the MP62. However, this dyno seems to contradict that. Perhaps the tune is less than optimal. Hard to know without seeing the A/F trace. Also we don't know how much timing (if any) is pulled. As it stands, it is making 10rwhp per psi just like I had seen with many FMIC'd Hotside MP62.

Hopefully we'll get more information regarding the tune..or see how much whp it makes with a Megasquirt.

...you've seen FMIC'd MP62's making the same power as the example here, interesting. I'm trying to find the benefit of the said efficiency of the Rotrex.

Mike

[Rdrcr]

I'm with Bill. I love the low end torque of the MP62. What they have is great for a track car where you seldom drop below 5000. If what I think is true, we should be able to get a stock 99/00 engine to 240 with 190+ tq and still maintain the flat torque. Also think it has possibilities of making it on 93/94 octane?? We are just out a couple weeks from testing. If it works, it would be upgradable to the older kits.

I would trade some high end HP for low end torque any day.

I'm with you on the low end torque...I would not trade it for high rpm horsepower on a street driven machine.

I don't know what you have planned in regards to the testing that you are doing but, I am, as always, very interested in your results, especially if it is an upgrade to what I already have. I want more HP. :)

Mike

[FormerDatsun510Man]

Btw, how many years do you think they can have "CARB pending" until the customers wake up and smell the coffee?

http://forum.miata.net/vb/showthread.php?t=286952&page=35

[Tom @ Fast Forward]

The only bad part is that I still believe that the stock engine is a 225WHp engine. Taking a stock engine beyond that point is "Caveat Emptor". The built engine I will be testing is still a basic 1.8L 99 engine, just better rods (Carrillo?)/pistons (JE). Well, flowed head too. :)

[Tom @ Fast Forward]

"The first post in this thread, posted 16th June 2008, says "CARB pending", so.... any day now."

FM and Bell got away with it for years too.

Btw, how many years do you think they can have "CARB pending" until the customers wake up and smell the coffee?

http://forum.miata.net/vb/showthread.php?t=286952&page=35

[oldtimer02]

The low end torque is what makes autocross fun!
Also on the street, both passing ....and well, you know!

[Rdrcr]

I'm reading that the setup in question does not have enough injector to make the claimed HP, he's running 350cc injectors.

How much injector do we need to make power...or how much power can various injectors support?

Mike

[FormerDatsun510Man]

Here are some numbers... all rwhp are on a Dynojet with a correction factor very close to 1 (so it isn't like the case at Flyin' Miata where fuel requirements are WAY low compared to corrected rwhp):

180rwhp, 6-7psi MP62 Hotside (100mm pulley no IC): stock injectors (4x250cc) maxed out at 12:1 A/F ratio, '99 Miata: 1000cc at 62psi = 180rwhp (5.6cc per rwhp)
200rwhp, 9-10psi FFS Coldside, (105mm pulley), '00 Miata, 255 Walbro FP, stock injectors (4x250cc) at 85% duty + E-Cool (1060cc) at 35% duty, 11.5:1 AF ratio: 1221cc at 62psi = 200rwhp (6.1cc per rwhp)
210rwhp, 12-13psi FFS Coldside (115mm pulley), '00 Miata, 255 Walbro FP, stock injectors at 90% duty + E-Cool (1050cc) at 40% duty, 11.5:1 AF ratio: 1320cc at 62psi = 210rwhp (6.3cc per rwhp)
240rwhp, 13psi MP62 Hotside R4 (Air/Air IC), stock injectors + 4x270cc extra injectors, Walbro 255 FP, 12:1 AF ratio, Main Injectors at 80% and Extra Injectors at 60% duty: 1448cc at 62psi = 240rwhp (6.0cc per rwhp).

Thus it looks like 6cc per rwhp (Dynojet) for a '99-'05 is a pretty safe bet. Thus it comes out likes this:

200rwhp (injector size at 100% duty, recommended injector for 90% max duty): 300cc/min, 330cc/min
250rwhp : 370cc/min, 420cc/min
300rwhp: 500cc/min, 550cc/min

I rounded above to common injector sizes. Thus, for that guy with the Rotrex, with 350cc main injectors, they are likely static at 233rwhp if it is holding 11.5-12:1 at top end. So yes, his engine is right at the limit of the injectors. Is it holding the power back? Who knows.. I would need to see his AF trace. Likely it could go higher in power (slightly) if it leaned out to say 12.5:1... IFF there is no knock.

[Rdrcr]

^^^^
Awesome...that's the information I'm looking for so I can attempt to understand this stuff better.

What is the size of my current 5th injector?

Mike

[Tom @ Fast Forward]

The single extra injectors are 48 pounds/hour or roughly 500cc/min. The ratio is roughly 10:1. Typically it takes 0.5 pounds per hour per HP. That comes out to a little over 5cc/min. Bill is probably a little closer with 6cc as it needs to run a little richer. The dual injector setup is 315 and 750 rear and front respectively.

[FormerDatsun510Man]

Here is the long hand to determine BSFC in terms of crank hp:

1) 180rwhp + 26 dynojet to crank hp + 19 sc drive hp = 225 crank hp, 1000cc, 6psi boost, 62psi fuel pressure
2) 200rwhp + 26 dynojet to crank hp + 22 sc drive hp = 248 crank hp, 1221cc, 10psi boost, 62psi fuel pressure
3) 210rwhp + 26 dynojet to crank hp + 28 sc drive hp = 264 crank hp, 1320cc, 10psi boost, 62psi fuel pressure
4) 240rwhp + 26 dynojet to crank hp + 39 sc drive hp = 305 crank hp, 1448cc, 13psi boost, 62psi fuel pressure

1) BSFC = (((62-6)/43.5)^.5)*(1000/10.5)/225 = .48
2) BSFC = (((62-10)/43.5)^.5)*(1221/10.5)/248 = .51
3) BSFC = (((62-10)/43.5)^.5)*(1320/10.5)/264 = .52
4) BSFC = (((62-13)/43.5)^.5)*(1448/10.5)/305 = .48

As you can see, we are very close to a BSFC of .5 as Tom states.

[Tom @ Fast Forward]

Here's another number to remember. It is probably more important than the fuel number. HP is air, air is HP. Basically, ther is 1 HP in every 1.65 SCFM of air. The critical letter there is the" S". It means" Standard" which means one cubic foot of air at STP (Standard Temperature and Pressure). If you want to make 200 HP, for example, you must flow/burn ~330 SCFM through the engine. How do you get there? We know that stoic is 14.7:1. That means 14:7 pounds of air to every 1 pound of fuel per hour. That means 7.35 pounds of air to 0.5 pounds of air per hour. And that melts down to roughly 1.65 Standard Cubic Feet of air per Minute (SCFM). It's all about air.

[Rdrcr]

^^^^
How much air is the MP62 capable of delivering?

Here is the long hand to determine BSFC in terms of crank hp:

1) 180rwhp + 26 dynojet to crank hp + 19 sc drive hp = 225 crank hp, 1000cc, 6psi boost, 62psi fuel pressure
2) 200rwhp + 26 dynojet to crank hp + 22 sc drive hp = 248 crank hp, 1221cc, 10psi boost, 62psi fuel pressure
3) 210rwhp + 26 dynojet to crank hp + 28 sc drive hp = 264 crank hp, 1320cc, 10psi boost, 62psi fuel pressure
4) 240rwhp + 26 dynojet to crank hp + 39 sc drive hp = 305 crank hp, 1448cc, 13psi boost, 62psi fuel pressure

1) BSFC = (((62-6)/43.5)^.5)*(1000/10.5)/225 = .48
2) BSFC = (((62-10)/43.5)^.5)*(1221/10.5)/248 = .51
3) BSFC = (((62-10)/43.5)^.5)*(1320/10.5)/264 = .52
4) BSFC = (((62-13)/43.5)^.5)*(1448/10.5)/305 = .48

As you can see, we are very close to a BSFC of .5 as Tom states.

I have 280cc x4 and one 500cc injector for a total of 1620cc but, I can't run the fifth injector effectively to a 90% duty cycle. How much HP can my current setup support? I plan on adding the fifth/six combo and possibly going E85 but, I want to wait to see what Tom is cooking up.

Mike

[Tom @ Fast Forward]

The MP 62 moves 62 cubic inches of air per revolution. 1728 cubic inches in a cubic foot. Not a"Standard cubic inch though as the pressure and temperature are above STP standards depending on RPM and Boost. You have to factor in PV=nrT.

[FormerDatsun510Man]

Mike,

The max duty I like to see on the main injectors is 90%. For E-Cool with a single injector, you are looking at approximately 40% max duty for 12-13psi boost. So you have 200cc worth of fuel from E-Cool with the single injector setup. Maybe you could go to 250cc, but that would be the absolute max. In comparison the dual injector setup runs the same duty where I see a similar EGT spread. That means twice the fuel. That is the big advantage with the dual injector setup.

Anyway, the most rwhp your current injector setup would support would be approximately 200rwhp:

280cc * 4 * 90% + 500cc * 40% = 1208cc
1208cc/6rwhp/cc = 201rwhp

Going with the absolute max, which is a little over what I consider "safe":

280cc * 4 * 95% + 500 * 50% = 1314cc
1314cc/6rwhp/cc = 219rwhp

I could do the longhand approach with a BSFC of .5 and calculating sc drive hp for the pulley used, drivetrain losses, etc. but as we have seen 6rwhp per cc (with 62psi fixed fuel rail pressure) comes out essentially the same.

Edit: On E85 with the max pulley size I recommend of 125mm, using the 420cc + the dual injector setup is required. I found it puts the dual injectors at 50% duty (the max I could safely use) and the main injectors at 90% duty (again the max I like to see and the perfect match). I suppose you can look at it this way, for E85 at 225rwhp you need 420cc mains and the dual extra injector setup.

[Rdrcr]

^^^^
As always, thanks again for the wealth of information. Does the WI, injecting only water, change anything with regards to fueling my setup.

I want to see what Tom is doing before I make my next decision on performance upgrades but, I'm already budgeting for the dual injector manifold and injectors either way.

Mike

[FormerDatsun510Man]

Typically on a WI setup the a/f ratio is about 5% leaner for the same CFM because the water is taking up that part of the liquid cooling function. Thus, one could run perhaps 5% higher (10rwhp) with the same fuel injector size. That would move the 200rwhp estimate to 210rwhp I would suppose.

[Rdrcr]

^^^^
Cool. I don't want to run out of injector when I'm in boost.

I have about 4 gallons of E85 in the car right now...I have not adjusted the cards...was that a bad idea at this point in time? It is a bit leaner in boost but, not much...11.8-12.4.

I was thinking about making a couple runs on a local SuperFlow Dyno to see where I'm at.

Mike

[Tom @ Fast Forward]

With the 90, I put in 3 gallons of E85 and filled the rest with 91. Thenupped the fuel pressure with the AFPR by 15 PSI. Worked great. Ran low on fuel and put in 3 gallons E85 and could only fit 5.3 gallons of 91. Ran lean as it wasn't getting enough fuel. Ran it down a bit and added 2 gallons of 91 to bring it back to close ti the same ratio and, voila, right back where it belonged. The volume thing is really important. As the mix was only 91, it did have a slight issue with knock because of lean even though the octane was probably higher as it had more E85 and less 91. When I added the 2 gallons of 91, that reduced the octane but increased the fuel and all was well with the world again. I guess I could have bumped the AFPR a bit but wanted to see if my % theory was correct. Evidently, it was.

[socal pat]

^^^^
Cool. I don't want to run out of injector when I'm in boost.

I have about 4 gallons of E85 in the car right now...I have not adjusted the cards...was that a bad idea at this point in time? It is a bit leaner in boost but, not much...11.8-12.4.

I was thinking about making a couple runs on a local SuperFlow Dyno to see where I'm at.

Mike

From my experience running 40% E85 the only thing "bad" about what you are doing is maybe missing out on some power. With the new found octane rating and probable lower intake and combustion temps I'd say you can safely play with AFR's a bit. Just go one light at a time on the mains and see what happens. You'll end up doing just that on the dyno anyway.

[tommyo]

Do the latest kits come with dual E-cool injectors?

[Tom @ Fast Forward]

All full ($3995) kits shipped for the last year or so have the dual injectors.