Honda / Acura K20a K24a Engine Forum banner
41 - 60 of 70 Posts

·
Registered
Joined
·
24 Posts
Hows the progress going?
I am curious as well. I've been getting slow played by Innovative for much of my stuff. If that is where you ordered from I am guessing you are probably having the same luck and hence no update of the thread. I ordered over 6 mos ago and still waiting for parts to show. Tim at innovative has been helpful, just really slow in producing any parts.
I'll be working on wiring this weekend, since that finally showed up, no fun.
 

·
IMAKEBOOSTATIDLE
Joined
·
6,913 Posts
Rotrex this bish!!! Excellent build subscribed
 

·
Registered
Joined
·
533 Posts
Chris,
From my experience with forced induction and the K-series trans I'd advise you to use a supercharger rather than a turbocharger in your Exige. Couple of reasons: Its well known that the gears Honda builds aren't that strong (especially 4th in the K-trans), and while cryo-treating can extend their lifespan the bottom line is you really need to look at the kind of torque curve different methods of FI produce. Worst case scenario for drivibility and time to distance is a centrifugal compressor, because boost and therefore torque production are directly linked to engine RPM. Conversely this rather weenie torque curve, where the torque peak is generally right at the redline, is good for trans longevity since drive-train parts only feel torque and not HP. Torque production in a turbocharged engine is weak at low RPM simply because turbine housing sizing is always a compromize, and to get great low-end torque the rising backpressure kills the top end. Using a bigger turbine housing is great for top end power but makes the engine lazy until spool-up and then watch out because the torque curve is going to climb like a Saturn rocket. Most all of this is negated with a VT (variable turbine) turbocharger, but the turbo manufacturers have been thrashing on making this setup work reliably for about 35 years without a lot of success. Its the spool-up part of the torque curve that's troublesome for the trans because that torque spike is always somewhere in the mid-range, and then the torque falls off as engine RPM increases. (This can be negated to a degree by using very agressive boost control, but its expensive and take a lot of dyno-time to get it right.)So in most cases your peak HP production happens at an RPM where the torque is far lower than at its peak, so the trans is taking a lot more abuse than it needs to for the HP generated. Best case scenario from a drivibility standpoint is a positive displacement supercharger. Bottom end and midrange torque production is higher than with a turbo without the spike, and since it keeps making more boost as the RPM climbs the torque curve stays relatively flat all the way to the redline. Secondly, its a whole lot easier to drive a supercharged car fast because there are never any surprising spikes in the torque curve so the HP production is very linear relative to RPM. An example you'll be able to relate to: The 2,308cc "K23A" (M90 supercharged) engine I have in my Elise makes 454 [email protected],000 RPM with 12 psi on Arco 91, and is only producing a maximum of 288 lb/ft of torque. If that engine were turbocharged and making the same HP the torque peak would be more like 340 lb/ft and the trans would last nowhere near as long. I started turbocharging cars in 1975, and I LOVE Turbochargers, but I've never even considered using one in an Elise or Exige and I've been converting them to Honda Power since 1989.
Torque curve manipulation is really what engine building is all about, and if its done properly you always get the most HP from the least Torque.
Take care and have fun,
Joe
 

·
Registered
Joined
·
153 Posts
Chris,
From my experience with forced induction and the K-series trans I'd advise you to use a supercharger rather than a turbocharger in your Exige. Couple of reasons: Its well known that the gears Honda builds aren't that strong (especially 4th in the K-trans), and while cryo-treating can extend their lifespan the bottom line is you really need to look at the kind of torque curve different methods of FI produce. Worst case scenario for drivibility and time to distance is a centrifugal compressor, because boost and therefore torque production are directly linked to engine RPM. Conversely this rather weenie torque curve, where the torque peak is generally right at the redline, is good for trans longevity since drive-train parts only feel torque and not HP. Torque production in a turbocharged engine is weak at low RPM simply because turbine housing sizing is always a compromize, and to get great low-end torque the rising backpressure kills the top end. Using a bigger turbine housing is great for top end power but makes the engine lazy until spool-up and then watch out because the torque curve is going to climb like a Saturn rocket. Most all of this is negated with a VT (variable turbine) turbocharger, but the turbo manufacturers have been thrashing on making this setup work reliably for about 35 years without a lot of success. Its the spool-up part of the torque curve that's troublesome for the trans because that torque spike is always somewhere in the mid-range, and then the torque falls off as engine RPM increases. (This can be negated to a degree by using very agressive boost control, but its expensive and take a lot of dyno-time to get it right.)So in most cases your peak HP production happens at an RPM where the torque is far lower than at its peak, so the trans is taking a lot more abuse than it needs to for the HP generated. Best case scenario from a drivibility standpoint is a positive displacement supercharger. Bottom end and midrange torque production is higher than with a turbo without the spike, and since it keeps making more boost as the RPM climbs the torque curve stays relatively flat all the way to the redline. Secondly, its a whole lot easier to drive a supercharged car fast because there are never any surprising spikes in the torque curve so the HP production is very linear relative to RPM. An example you'll be able to relate to: The 2,308cc "K23A" (M90 supercharged) engine I have in my Elise makes 454 [email protected],000 RPM with 12 psi on Arco 91, and is only producing a maximum of 288 lb/ft of torque. If that engine were turbocharged and making the same HP the torque peak would be more like 340 lb/ft and the trans would last nowhere near as long. I started turbocharging cars in 1975, and I LOVE Turbochargers, but I've never even considered using one in an Elise or Exige and I've been converting them to Honda Power since 1989.
Torque curve manipulation is really what engine building is all about, and if its done properly you always get the most HP from the least Torque.
Take care and have fun,
Joe
I agree totally that turbochargers are fantastic and most all cars I have built have been turbo however its just not a good form of power for a mid-engined well balanced car designed for turning.

Do you by chance have a dyno of your k23a setup? I have quoted your build once or twice but never actually had a dyno to show.

As for a centrifugal blower argument... I don't totally agree. An interesting setup to note that is quite popular is taking a large centri blower spinning it to its max (at revlimit) and then regulating the pressure via a wastegate on the charge piping. This ends up delivering a centri curve in the low end however somewhere in thr mid range, where the boost reaches set wastegate pressure the torque curve flattens and it acts basically like a positive displacement blower. The upside to this is having a nice predictable flat torque curve in the mid to upper end, while still maintaining the much greater efficiency of a centri blower. As you know (and this is coming from a fellow elise k powered m90 guy) a roots blower leaves something to be desired in terms of both compressor efficiency as well as parasitic loss to some degree. Also of note, packaging a rotrex many times is easier than a large PD blower.
 

·
Registered
Joined
·
49 Posts
I guess i should start posting up my K20A/C38 rotrex Exige S2 progress pretty soon :) I might have to push her to limit before limiting it with wastegate control just to see what it puts down!

oh BTW

whatsADSM is the MAN!!!:mj:
 

·
Registered
Joined
·
533 Posts
WhatsADSM,
Here's a dyno chart from back in July '09 when I finally got around to putting my "K23" on the dyno. I machined 4 differently sized 8-rib pulleys, but only 3 of them are on this chart. 12 psi came from running the M90 at 58% overdrive, 14 psi at 69% overdrive, and 16.5 psi at 81% overdrive. This was done more for personal data gathering than anything else, so we didn't even rev the engine to its redline which is 9,200. Basically I wanted to see what shape the torque curve would be with the little IPS supercharger cams since they're small enough to give really sweet mileage at a cruise. Its my street car so that's important, and with the gearing I'm using I'm hoping to see somewhere close to 40 MPG at an 80 mph cruise. From other S/C dyno testing I've done bigger cams would flatten the torque curve out above 6,500 and make substantially more top-end HP, but considering the car only weighs 1,820 lbs I think this setup will be OK as a starting point. The 12 psi run was done using Arco 91, so that's the street setup. The other runs were done using MS-109 since I had some and that's our standard dyno fuel for N/A and low boost testing. The highest post intercooler temp in the manifold was 138 F. but since this was done on the dyno the airflow through the heat exchanger was piss poor, so it'll be substantially lower with 80 mph airflow.
On the subject of centrifugal compressors, you'd need a drive system that would be variable relative to crank rpm to achieve what you were talking about, and as far as I know nothing like that exists. I saw engineering drawings of one that used a planetary drive to get the wheel up to a usable speed quicker, but there was no consideration given to maintaining that speed as the engine rpm increased. Generally with a crank driven centrifugal compressor the torque and HP peaks are at the same rpm, right at the redline, so the torque curve is often curved in the wrong direction, concave rather than seriously convex like with a positive displacement blower. Centrifugal compressors need to be driven by something other than the crank to make a decent torque curve. Something like really hot exhaust gas.
Take care and have fun,
Joe
 

Attachments

·
Registered
Joined
·
153 Posts
WhatsADSM,
Here's a dyno chart from back in July '09 when I finally got around to putting my "K23" on the dyno. I machined 4 differently sized 8-rib pulleys, but only 3 of them are on this chart. 12 psi came from running the M90 at 58% overdrive, 14 psi at 69% overdrive, and 16.5 psi at 81% overdrive. This was done more for personal data gathering than anything else, so we didn't even rev the engine to its redline which is 9,200. Basically I wanted to see what shape the torque curve would be with the little IPS supercharger cams since they're small enough to give really sweet mileage at a cruise. Its my street car so that's important, and with the gearing I'm using I'm hoping to see somewhere close to 40 MPG at an 80 mph cruise. From other S/C dyno testing I've done bigger cams would flatten the torque curve out above 6,500 and make substantially more top-end HP, but considering the car only weighs 1,820 lbs I think this setup will be OK as a starting point. The 12 psi run was done using Arco 91, so that's the street setup. The other runs were done using MS-109 since I had some and that's our standard dyno fuel for N/A and low boost testing. The highest post intercooler temp in the manifold was 138 F. but since this was done on the dyno the airflow through the heat exchanger was piss poor, so it'll be substantially lower with 80 mph airflow.
On the subject of centrifugal compressors, you'd need a drive system that would be variable relative to crank rpm to achieve what you were talking about, and as far as I know nothing like that exists. I saw engineering drawings of one that used a planetary drive to get the wheel up to a usable speed quicker, but there was no consideration given to maintaining that speed as the engine rpm increased. Generally with a crank driven centrifugal compressor the torque and HP peaks are at the same rpm, right at the redline, so the torque curve is often curved in the wrong direction, concave rather than seriously convex like with a positive displacement blower. Centrifugal compressors need to be driven by something other than the crank to make a decent torque curve. Something like really hot exhaust gas.
Take care and have fun,
Joe
Joe,

Thanks for the update. I appreciate the dyno plots! You must have quite the cooler on there to keep the temps at 138, that is actually quite reasonable for 14psi out of a positive displacement blower.

As for the centrifugal blower thing I wqs talking about. It's actually very real, done frequently, and very straight forward to achieve. No variable transmission units or anything of the sort.

Start by buying a larger blower than you woukd expect like a c38-91 rotrex unit. Setup it up to mechanically overspin a few percent at redline. Then put a wastegate on the charge piping. Have it open at let's say 13 PSI. What will happen?

The blower is large and pulleyd to spin so it will hit 13psi somewhere in the midrange. Normally it woukd continue to create boost and torque would continue upwards as you pointed out. However in this case the wastegate will open and regulate the boost charge to 13psi. You end up with a torque curve that goes up until the midrange and then goes roughly flat (actually follows the VE of the engine, which in the case of a k series means nearly flat).
 

·
Registered
Joined
·
533 Posts
WhatsADSM,
It was 138 degrees at 16.5 psi.
Now I see where you're going with the Rotrex thing. I'm basing what I've said on the experience we had with Hondata's Bonneville RSX. Oscar supplied the biggest thing Rotrex made and it simply wouldn't flow enough air to get the job done, so in the end they ended up using 2 of them. Even with 2 Rotrex's the torque curve was concave! Max torque was something like 390 lb.ft. at 9400 rpm.
Now if you were only interested in an extremely mildly boosted engine I'm sure what you talked about would work OK, but you'd still get a sweeter torque curve with a positive displacement blower.
I've taken broken Rotrex's apart and its obvious what their problem is, they're appealing to people with limited available space so their compressor housings are small in diameter. Good marketing but poor engineering. The compressor wheels they're buying are designed to be efficient in MUCH larger housings, so overall its a very inefficient package.
Quite a few of them went into self-destruct mode at as little as 5% over what they say is the safe RPM. The shaft snaps off where the diameter is reduced to accept the compressor wheel, and the dummies in England who make them say its just fine to produce that part with a zero root radius even though there's plenty of room for a very generous root radius. Not an impressively designed or built unit in my opinion.
Take care and have fun,
Joe
 

·
Registered
Joined
·
153 Posts
WhatsADSM,
It was 138 degrees at 16.5 psi.
Now I see where you're going with the Rotrex thing. I'm basing what I've said on the experience we had with Hondata's Bonneville RSX. Oscar supplied the biggest thing Rotrex made and it simply wouldn't flow enough air to get the job done, so in the end they ended up using 2 of them. Even with 2 Rotrex's the torque curve was concave! Max torque was something like 390 lb.ft. at 9400 rpm.
Now if you were only interested in an extremely mildly boosted engine I'm sure what you talked about would work OK, but you'd still get a sweeter torque curve with a positive displacement blower.
I've taken broken Rotrex's apart and its obvious what their problem is, they're appealing to people with limited available space so their compressor housings are small in diameter. Good marketing but poor engineering. The compressor wheels they're buying are designed to be efficient in MUCH larger housings, so overall its a very inefficient package.
Quite a few of them went into self-destruct mode at as little as 5% over what they say is the safe RPM. The shaft snaps off where the diameter is reduced to accept the compressor wheel, and the dummies in England who make them say its just fine to produce that part with a zero root radius even though there's plenty of room for a very generous root radius. Not an impressively designed or built unit in my opinion.
Take care and have fun,
Joe
138 at 16.5# is even more impressive. I can only imagine what kind of core/heat exchanger that was. I'm surprised you were able to package the core in the S2 Elise. Were you running the heat exchanger up front?

As for the Rotrex thing, I hear what you are saying for sure, and yea my suggestion was for a more moderate boost/power level car (think 400 at the wheels or a little more). Interestingly I am helping someone with a S2 Exige (Federal) now with a swap and he is going to be running a larger Rotrex unit (c38-81 I think) on a mostly stock K20A2. It will be interesting to compare his torque/power curve and mine as well as some time to mph. I have a M90 (3.25" blower pulley stock 5.98" crank pulley) on a basically stock K24A2 with a little drop in IC core (not really big enough but at least it does something), and E85.
 

·
Registered
Joined
·
49 Posts
It's my exige whatsadsm is referencing and its a c38-71 I would have went with the -81 or even -91 but I got this baby for an amazing price. I'll post up my results once it's done.
 

·
Registered
Joined
·
153 Posts
It's my exige whatsadsm is referencing and its a c38-71 I would have went with the -81 or even -91 but I got this baby for an amazing price. I'll post up my results once it's done.
Ahh the man himself. :up:

It's going to be bad ass setup once complete! :worship:
 

·
Registered
Joined
·
533 Posts
Murray,
My water/air intercooler core is 7" long, 5.3" wide, and 2.25" thick. I started with a JR manifold, put it on an angle plate in my milling machine and cut the bottom half of it completely off, everything below the lower manifold to head bolt bosses. Then I built a new manifold using the M90 and that I/C core as guidelines. Space above the core, below the core, as well as smooth flow into the ports were also optimized, especially the #1 port which is absolutely horrible on both JR and Comptech manifolds. I machined all of these parts and then welded it all together, so the core itself has zero leakage. 100% of the discharge air HAS to flow through it. I machined the end tanks so the water flows through the I/C core twice, and sealed the bottom plate to water tubes using O-rings. All of this took about 60 hours. It fits with the stock firewall in both the S1 and S2 Elise/Exige, just barely but it fits.
In the Elise I use the stock up-front oil coolers as my heat exchangers. Derek followed my lead and did the same thing. Its not ideal because they're essentially cheap English oil coolers and rather small, but Lotus was having trouble cooling the 2ZZ motor with just water so they added the oil coolers to help with that problem rather than making a bigger radiator. At some point I'll have Ron Davis make a pair of bigger heat exchangers that are actually made for water rather than oil, but when I was using the M62 S/C K20A setup in the Elise they worked pretty well. Only 325 HP (stock motor with 9 psi), but the air into the engine never got over 106 F and that was on a LONG WFO run out in the desert at 90 F ambient.
With your 5.98/3.25 setup you're at 84% overdrive, so just a little higher than the 81% I ran to get 16.5 psi. The E-85 helps in your situation. You just don't have the rpm potential I have because of the 'tractor motor' 99mm stroke. That's one of the biggest reasons I built my "K23". Its 90mm x 90.7mm (2,308cc) and has a 1.75:1 rod/stroke ratio. No matter what it'll always make more power than a K24 even though its 46cc smaller. The only reason Honda used a 99mm stroke in the K24 was to get the displacment over 2,350cc so they could call it a 2.4 liter motor. Pure advertising, not engineering.
I'd imagine Derek has told you how much he dislikes the K24 in his Elise because it can't rev. Eventually he'll get around to building his 'real' motor, which will be 89 x 90.7 and R1900 supercharged. Then he'll be a really happy camper!
Take care and have fun,
Joe
 
41 - 60 of 70 Posts
Top