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Discussion Starter · #1 ·
Hi... Been lurking these forums quite a bit in the last year or so trying to educate myself as much as possible. I'm at a stage where I'm seriously considering a k24 swap into my Elise.

Long story, short...At this time last year I was debating k swap vs supercharger on my naturally aspirated Toyota 2zz motor. Many told me that the 2zz just won't hold up all that well to the boost; but I got a deal on a few parts towards that build and ended up going that route. A couple weekends ago I dropped a valve into the cylinder and trashed the motor. I believe it was my fault on a mis-shift over rev. And I also believe the valvetrain that was built for me wasn't built with the most ideal components. I was at 325whp on that build. I'd like to get back to a similar number with a k24 and I'm assuming that can be done with a more conservative level of boost and I won't be at the limits of the motor.

I track the car. I don't really care to compete in a certain class and don't care for any restrictions. I just do it for fun. The car still sees the streets once in awhile...I'd say about 10% street, 90% track.

Point of thread: I suppose I'm just looking for some guidance and feedback on a setup and parts. I'd love to re-use as much of what I put into the original build into this new build. I have a fuel surge tank for fuel starvation. I have a front mounted heat exchanger with a recirculating pump for a water to air intercooler setup. I have an AiM dash that's all wired and ready to go with sensors. I think I'll need to switch to a different AiM unit but that should be straight forward. Not sure what ECU to go with.

I have a ton of questions and thoughts but I don't wanna make the post too long for starters because everything is still developing. But looking to see if anyone's gone through something similar.

Thanks in advance.
 

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Get a K24A2, rotrex. Call it a day.
You may want to remove the balance shafts and run an RSX oil pump and pinned Cam gears, otherwise... leave it as is.

It should make 325whp without trying hard. It could do 400whp without much effort.

Or, do a NA build... K24A2 with K20A head, pistons, rods, valvetrain, big intake manifold, the works...and you should get close to 325whp NA, without the added complexity (and weight) of boost.

The biggest issue would be the gearbox. Once you're near that 400-450whp mark, you're on borrowed time with shearing gears.
 

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welcome to k20a.org and thank you for registering.

there are a few members here that have kswap Lotus's. I am sure they will chime in. Personally I have followed all the Lotus swaps done over the years here. i like the ones that have a smaller stroke crank due to the lightweight nature of the chassis. After owning a SCk24 and cammed K20 NA build I would have to say the K20 would be a better track engine. they are better at sprinting than the K24, K20 downshifts better, can stay in gear longer meaning less upshifts and better corner entry and corner set up with the K20. Honda made the 2.0L 86mm stroke their choice of road course engine for a good reasons. I believe a K20 will last longer on a road course than a K24.

these are all personal opinions based from actual experience on a road course with both engine set ups. In light weight chassis the K20 feels like a go-kart and does not need any extra TQ to move out quickly and go through the gears well. I am not anti boost nor anti k24, the K24 to me feels better in a heavier car.
 

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Discussion Starter · #4 ·
Get a K24A2, rotrex. Call it a day.
You may want to remove the balance shafts and run an RSX oil pump and pinned Cam gears, otherwise... leave it as is.

It should make 325whp without trying hard. It could do 400whp without much effort.

Or, do a NA build... K24A2 with K20A head, pistons, rods, valvetrain, big intake manifold, the works...and you should get close to 325whp NA, without the added complexity (and weight) of boost.

The biggest issue would be the gearbox. Once you're near that 400-450whp mark, you're on borrowed time with shearing gears.
So yes, exactly, here's where I'm at and where I'm leaning. Basically I'd like to keep the motor as stock as possible. A k24a2 would be cheap and easy to replace under the circumstances of another mis shift or whatever.

Then I've been looking at this kit:

The rotrex seems to run a lot cooler than the TVS and with the power band it seems to put a lot less strain on the motor/trans.

My next thing is to figure out the transmission. Likely a 6 speed out of I think the rsx? But I'd like to change the gearing to a more robust and close ratio set. Like a taller 1st gear that gets me to near 60mph. Then 2nd 3rd and 4th close to keep me in the power band.

I'm also now looking towards ecus and considering flex fuel. I know e85 isn't absolutely necessary at my power levels. But I think it'll just help things run cooler
 

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Discussion Starter · #5 · (Edited)
welcome to k20a.org and thank you for registering.

there are a few members here that have kswap Lotus's. I am sure they will chime in. Personally I have followed all the Lotus swaps done over the years here. i like the ones that have a smaller stroke crank due to the lightweight nature of the chassis. After owning a SCk24 and cammed K20 NA build I would have to say the K20 would be a better track engine. they are better at sprinting than the K24, K20 downshifts better, can stay in gear longer meaning less upshifts and better corner entry and corner set up with the K20. Honda made the 2.0L 86mm stroke their choice of road course engine for a good reasons. I believe a K20 will last longer on a road course than a K24.

these are all personal opinions based from actual experience on a road course with both engine set ups. In light weight chassis the K20 feels like a go-kart and does not need any extra TQ to move out quickly and go through the gears well. I am not anti boost nor anti k24, the K24 to me feels better in a heavier car.
Thank you. That's interesting and yeah I've read varying opinions. My thought is to start with a base k24 because I think that's my preferred engine of choice in a completely na application. From (semi) local guys who have swapped and stayed na, the lower end torque of the k24 does help a lot. Of course it does depend on what type of track etc...
 

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Welcome here to K20a.org @Gstomp06.

I suppose I'm just looking for some guidance and feedback on a setup and parts.
What kind of track is it? Has it long or tight and short turns? Will centripetal forces lead to oil starvation (> 1.0 g)?

The 99 mm stroke engine is a good torque booster approach from both, the lower R-S-Ratio (giving the air a bigger impulse) and the bigger displacement. This was the basis for Honda to introduce this for the heavier chassis Honda had that time. The alternation of load path was designed for low engine speed torque and made it as performance engine equally to the K20A2 level, because of the engine speed restrictions.

The K-series community open up this concept by displacing the RBB/PPA/RAA head by an performance type: RSP, RBC, PRB, which is a door opener for higher torque at higher engine speed and, depending on setup, down low too. But this engine has still that 99 mm stroke, creating bigger not balanced forces at same engine speed compared to a 86 mm stroke engine. As well as the higher side load force due to the higher R-S-ratio. On top of that, the tensional forces due to the oscillating inertia. Alone the latter needs a -800 rpm compensation. So while a stock 86x86 engine can race all day long at 8000 rpm, a 87x99 shouldn't exceed the 7100 rpm.

In terms of specific torque/liter, this means for the same power, the K24 needs -5 Nm/liter less, but because they use the same head ports even this will get a challenge. So finally power-wise at same maintenance intervals, same bolts on, a K24 powered car will likely fall back at very long straights compared to a K20 powered one. On the other hand the power down low is everywhere bigger compared to the K20. The more curves, the more advantage for the K24.

My personal choice will always be a high revving engine, less shift events, less misshift issues, less maintenance, better sound and much more. My own K20 does over 200 ftlb's at flywheel in NA mode in a 86x86 crank assembly. None of the stock K24 will reach that level, most of stock near franken K24 with bolts on also not. The Lotus Elise concept matches best with a hot high engine speeder (NA or FI). The K24 appeals with a lot torque on the base line, but comes also with a lot of non-tasty restrictions which don't match the light-weight character of the Lotus Elise. For me it is an decision by concept. 95 % here will recommend an K24, but only 5 % of them drove a Elise on track. This is my more emotional advice.

Markus
 

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Discussion Starter · #7 ·
Welcome here to K20a.org @Gstomp06.

What kind of track is it? Has it long or tight and short turns? Will centripetal forces lead to oil starvation (> 1.0 g)?

The 99 mm stroke engine is a good torque booster approach from both, the lower R-S-Ratio (giving the air a bigger impulse) and the bigger displacement. This was the basis for Honda to introduce this for the heavier chassis Honda had that time. The alternation of load path was designed for low engine speed torque and made it as performance engine equally to the K20A2 level, because of the engine speed restrictions.

The K-series community open up this concept by displacing the RBB/PPA/RAA head by an performance type: RSP, RBC, PRB, which is a door opener for higher torque at higher engine speed and, depending on setup, down low too. But this engine has still that 99 mm stroke, creating bigger not balanced forces at same engine speed compared to a 86 mm stroke engine. As well as the higher side load force due to the higher R-S-ratio. On top of that, the tensional forces due to the oscillating inertia. Alone the latter needs a -800 rpm compensation. So while a stock 86x86 engine can race all day long at 8000 rpm, a 87x99 shouldn't exceed the 7100 rpm.

In terms of specific torque/liter, this means for the same power, the K24 needs -5 Nm/liter less, but because they use the same head ports even this will get a challenge. So finally power-wise at same maintenance intervals, same bolts on, a K24 powered car will likely fall back at very long straights compared to a K20 powered one. On the other hand the power down low is everywhere bigger compared to the K20. The more curves, the more advantage for the K24.

My personal choice will always be a high revving engine, less shift events, less misshift issues, less maintenance, better sound and much more. My own K20 does over 200 ftlb's at flywheel in NA mode in a 86x86 crank assembly. None of the stock K24 will reach that level, most of stock near franken K24 with bolts on also not. The Lotus Elise concept matches best with a hot high engine speeder (NA or FI). The K24 appeals with a lot torque on the base line, but comes also with a lot of non-tasty restrictions which don't match the light-weight character of the Lotus Elise. For me it is an decision by concept. 95 % here will recommend an K24, but only 5 % of them drove a Elise on track. This is my more emotional advice.

Markus
Thank you.

I'll need a bit of time to process what you just wrote on a more technical level. I'll research some more tonight.

I will also post up some of the tracks I frequent nearby. A lot of them are smaller and more technical tracks. My driving style also favors that a bit...also partly because I back out of high speed corners/sweepers unless I'm feeling like everything is clicking. I'm also not caring to compete. I just like to challenge myself and improve incrementally.

I understand your message about the Elise and the high revving lightweight character of the car. I did track the na 2zz for several years prior. I did find myself shifting more frequently just to stay in the high rpm range (lift). A bunch of the guys here in the states that have gone k-swap on their Elise/exige.... almost all have gone k24. And they couldn't speak anymore highly of the switch over.
 

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LotusElise just explained what I said in so many words easier to follow. The K20 is a racing engine platform that is happy performing and revving engine. It takes the abuse better than the K24 in my experience. After road coursing both, and even having more TQ with the FI K24, I still prefer the camm'd K20 in a lightweight chassis. I feel like I am driving (racing) something that can do this all day long happily. I could hear the engine speeds in the K24 rev matching crying during downshifting in competition situations.
 

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I understand your message about the Elise and the high revving lightweight character of the car. I did track the na 2zz for several years prior. I did find myself shifting more frequently just to stay in the high rpm range (lift). A bunch of the guys here in the states that have gone k-swap on their Elise/exige.... almost all have gone k24. And they couldn't speak anymore highly of the switch over.
If you were gonna stick to NA and want rev-friendly... K20.
Want boost and ability to go angrily into the universe... K24.

I track my boosted-K24 in, essentially, a big brother of the Eliges. Rev it to 8500rpms, no problem. I short shift though, because I have a ton of power (610whp), almost too much for a 2350lb car.
I also use a quaife sequential, so my gearing is pretty long... 1st goes to 62mph.
 

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If you were gonna stick to NA and want rev-friendly... K20.
Want boost and ability to go angrily into the universe... K24.

I track my boosted-K24 in, essentially, a big brother of the Eliges. Rev it to 8500rpms, no problem. I short shift though, because I have a ton of power (610whp), almost too much for a 2350lb car.
I also use a quaife sequential, so my gearing is pretty long... 1st goes to 62mph.
Want boost and ability to go angrily into the universe... K24.
🤣
this is true. 99mm stroke a great deal of pop. more than Z06's, 68 GTO's or any V8, V10 for that matter. .except the one viper that has a stroker making it greater than 99mm.
 

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If you were gonna stick to NA and want rev-friendly... K20.
Want boost and ability to go angrily into the universe... K24.

I track my boosted-K24 in, essentially, a big brother of the Eliges. Rev it to 8500rpms, no problem. I short shift though, because I have a ton of power (610whp), almost too much for a 2350lb car.
I also use a quaife sequential, so my gearing is pretty long... 1st goes to 62mph.
Whoa that must be a wild ride, haha.

I was "only" at 325whp, and I had a long 1st, up to about 59mph, 2nd was 82or so, 3rd 104ish, 4th like 125....it worked out fairly well for most of these road courses near me.

Is there any concern about the rotrex and the broken shafts that I kept reading about? Is that an older design issue that was eventually resolved? A TVS option from mercracing is pretty robust and won't likely run into any issues like that...but I do get concerned about the heat. Although at my power goals, I'm assuming relatively low boost and with an air to water intercooler inserted into the manifold, it should run fairly cool.
 

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Sounds great. I am looking forward to see them.

For V8 guys this is still a half engine ;).
Palmer Motorsports: fun track...some good high speed sweepers...only issue is that its built into a mountain and an off can literally mean you're into some boulders.
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Thompson Speedway: Pretty short and technical. Really only one higher speed corner as you enter onto the main straight.

Ecoregion Map Organism Land lot Urban design


Club Motorsports: Really fun track with elevation changes. Has a little bit of everything. Safer than Palmer up top.
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Canaan Motor Club: Small technical track. Some challenging decreasing radius and off camber corners. One high speed banked corner
Plant Land lot Urban design Grass Residential area


NHMS: Can be run in a chicane setup or with the big nascar oval setup. Rough track with curbs and pavement changes. Relatively short

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Lime Rock: Fairly well known track. Relatively fast paced and horsepower wins here.

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These are all pretty close to me. There's a few others that are a bit of a drive away, that I will be going to as well. Watkins Glen, New York Safety Track, and NJMP.
 

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Gear-x make a brilliant close ratio gear set for the 6-speed that can take 600Nm. More than you want in an Elise.

Watch Marc Julians videos from canada. 395 WHp k20a2 turbo with that gear-x kit. Love it.
$2.5k though. Long 1st and then close ratio to 6th. Sounds like a liter bike motorcycle tranny.
His power was originally a bit higher, but he had a hard time maintaining charge temperatures.
He then lowered boost from some 14 to some 12 PSI and got much more consistent power.


if you overdo it powerwise, handling becomes challenging. K24 Rotrex C38. You need to know what you are doing on track as it get seriously fast. :cool:




Some prefer the torquey nature of the k24 while others like to rev their build k20 to 9k.
The k20 revs scream race car while the big lower reving k24 sounds like a meaty big inline 4.

For a track car, I’d go k20. if k24 and stock, don’t rev them higher than 7200 or 7300. Road users rev them higher, but on track, you’ll spends miles on end reving higher than 7k.

I have a currently SC build K20 in my Elise, but also helped converting a 2004 Toyota Exige to a JRSC k20.
Much better drive train than the SC 2ZZ.

The Rotrex kits will make a lot of power and provide for a thrilling experience.
if you like to add low down torque, go for the charge cooled big TVS kits (TVS 1320, 1900 and 2300 available) from Mercracing. This covers power from 350 wHp to well above 500.

Supercharging either a k20a2 or a k24a2 in complete stock form expect of a decent baffled sump or eventually removal of the balance shafts provide a path to well above 400 HP.
Just to rev them too high as sometimes suggested or done by the my stock k24 revs to 8500 crew (only on the highway for 10 seconds). The cast pistons will eventually give in.

if you build them with rods and pistons, ported oil pumps, you open the path to a durable 8k rev limit for the k24 or like 9k for the k20.
 

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Palmer Motorsports: fun track...some good high speed sweepers...only issue is that its built into a mountain and an off can literally mean you're into some boulders.
Canaan Motor Club: Small technical track. Some challenging decreasing radius and off camber corners. One high speed banked corner
Wow, there are some nice sections, which need attention on the pedals. I like the Palmer and Canaan from see just the straight turn mixture and shape. Looks like short gearing or a bunch of torque makes it easier to hold pace high.

For a track car, I’d go k20. if k24 and stock, don’t rev them higher than 7200 or 7300. Road users rev them higher, but on track, you’ll spends miles on end reving higher than 7k.
Yuup, that's the weak link of the low R-S-ratio and 99 mm stroke and should be included into project plans.

Thanks for the video link of Mark Julian's FI'd K20, that is what it is, driving a K20. It's like racing, not just driving around a course.
 

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Discussion Starter · #19 ·
Gear-x make a brilliant close ratio gear set for the 6-speed that can take 600Nm. More than you want in an Elise.

Watch Marc Julians videos from canada. 395 WHp k20a2 turbo with that gear-x kit. Love it.
$2.5k though. Long 1st and then close ratio to 6th. Sounds like a liter bike motorcycle tranny.
His power was originally a bit higher, but he had a hard time maintaining charge temperatures.
He then lowered boost from some 14 to some 12 PSI and got much more consistent power.


if you overdo it powerwise, handling becomes challenging. K24 Rotrex C38. You need to know what you are doing on track as it get seriously fast. :cool:




Some prefer the torquey nature of the k24 while others like to rev their build k20 to 9k.
The k20 revs scream race car while the big lower reving k24 sounds like a meaty big inline 4.

For a track car, I’d go k20. if k24 and stock, don’t rev them higher than 7200 or 7300. Road users rev them higher, but on track, you’ll spends miles on end reving higher than 7k.

I have a currently SC build K20 in my Elise, but also helped converting a 2004 Toyota Exige to a JRSC k20.
Much better drive train than the SC 2ZZ.

The Rotrex kits will make a lot of power and provide for a thrilling experience.
if you like to add low down torque, go for the charge cooled big TVS kits (TVS 1320, 1900 and 2300 available) from Mercracing. This covers power from 350 wHp to well above 500.

Supercharging either a k20a2 or a k24a2 in complete stock form expect of a decent baffled sump or eventually removal of the balance shafts provide a path to well above 400 HP.
Just to rev them too high as sometimes suggested or done by the my stock k24 revs to 8500 crew (only on the highway for 10 seconds). The cast pistons will eventually give in.

if you build them with rods and pistons, ported oil pumps, you open the path to a durable 8k rev limit for the k24 or like 9k for the k20.
Thank you. You guys are a wealth of information here. I'm typically on the lotus talk forums which are primarily US-based owners with the Toyota drivetrain. So it's been hard getting the right info.

My goals are to build a relatively modest and cost effective solution. I will be the first to admit that I'll need to improve my driving skills to be able to put down serious power in a small car like the Elise. I had gotten fairly quick with the na motor at 180whp, but was finding the 325whp to be a learning curve. I had to spend bigger money to build the 2zz bottom end, ported head, and cams to get it to 325. So a dropped valve into the cylinder did some expensive damage. My thought is that a stock k24/k20 should be able to sustain 325whp without as much strain. And the Honda motors are plentiful and cheap. I'm not looking to build the Honda motor for high revving and high strung applications.

Merc racing warns me of broken shafts on the rotrex units. I'm not sure if that was a design related thing that was later solved though.

So my thoughts (and I may be absolutely wrong here) is that a TVS blower was a good solution for a higher revving k20 as is gives it the torque down low. And that a rotrex was a good solution for the k24 because it helps it with the power up top. Now considering that I may only be revving a k24 up to say 7200rpm, I had no idea whether the c30 or c38 makes sense here, both are options straight from a TTS kit.
 

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the C38-series Rotrex units suffer from shaft brakages hitting the often hard rev limiters of Honda engines.
The newest C38R is also improved in this regard, but way to big for your power goal.
There are replacement shafts available with proper chamfers where the shaft diameter reduces towards the turbine wheel.
They put a step change of the shaft diameter in there to lead from the bigger center section of the shaft into the smaller threaded part for the wheel.
The torsional vibrations hitting a hard rev limiter shear them off. The bigger C38 turbines having a larger rotational intertia than the C30 wheels lead to fatique at this spot.

As of 2016 it seems Rotrex have addressed this themself:

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"A major design change was introduced in 2016 as a direct response to customer feedback. The high-speed spindle shafts in our C30 and C38 range superchargers were redesigned and significantly strengthened to avoid spindle snapping under certain conditions."

"Customer feedback" as in pissed customers thrashing $2k superchargers and within the warranty period dealers having to replace lot's of them.

I do trackdays with a chap that killed 2 C38 of a F20c engined Lotus Seven before the shop downgraded to a C30-94 that then lasted.

So be aware of cheap older kraftwerks kit with C38 SCs. At least map them to not hit the limiter that hard.

The C30 units do not suffer from this. For a K20 or K24 you would want at least the biggest C30-94 unit. This will get you to over 400 crank HP.

The roots or screw blowers are very reliable albeit with a flatter torque curve than a Rotrex. Both will give you as much top end power as you dare. Power essentially climbs continuously to wherever you set the limiter.

Stock 2ZZ tend to brake valves if you rev them higher than 8000 for longer than a few moments. It is very common. With a stock 2ZZ stick to 8k max.

K20a2 are getting harder to come by. K24a3 seem to be plentiful.
 
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