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Discussion Starter · #1 ·
Hello all,

I've been looking into a hybrid setup for my rsx-s, using a k24a1 bottom end with my k20a2 head and parts.

The assembly and ECU part of things is pretty much covered. But what I don't understand that maybe someone can clarify, I've read from a few places that this combo will "only rev to 8000rpm"

Is this because of the lower compression? (9.7 vs 11.?, i forget)

Or do people just mean there's no point reving past 8000rpm? If so, what's the limiting factor there.

Thanks for any answers
 

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The longer stroke is the biggest limiting factor. Piston speeds are faster than they would be in a k20 at the same rpm. I’ve seen posts stating that there isn’t much power to be had past 8000 rpms anyways, especially on oem cams. Some have even said there isn’t any past 7500. I also wouldn’t want to spin that fast on a k24a1 stock bottom end, especially without a PRB oil pump.
 

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The longer stroke is the biggest limiting factor. Piston speeds are faster than they would be in a k20 at the same rpm. I’ve seen posts stating that there isn’t much power to be had past 8000 rpms anyways, especially on oem cams. Some have even said there isn’t any past 7500. I also wouldn’t want to spin that fast on a k24a1 stock bottom end, especially without a PRB oil pump.
I was just speaking with the engineer of the “k48” build and I basically stated the same thing you just did when he claimed 10k rpm’s and 750 hp NA. Granted that engine won’t be stock whatsoever, i just find 10k rpm on an under square engine to be unrealistic.


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Discussion Starter · #4 ·
The longer stroke is the biggest limiting factor. Piston speeds are faster than they would be in a k20 at the same rpm. I’ve seen posts stating that there isn’t much power to be had past 8000 rpms anyways, especially on oem cams. Some have even said there isn’t any past 7500. I also wouldn’t want to spin that fast on a k24a1 stock bottom end, especially without a PRB oil pump.
Thanks for the input, that makes sense. The oil pump I'd be using is from the k20a2, which is apparently good to 8600rpm, same for water pump and housing.

What I don't understand is how piston speed limits the power band. The pistons are going faster that I get, but if the head was making power past 8000rpm on the k20a2 bottom end, how does piston speed change this? (Ignoring the added stress on the k24 bottom end, unless that's directly related)

Just trying to figure out what my best option is in building this. I've also considered using the k20a2 bottom end internals, and high-comp k24 pistons, but not sure how that would work out. Or boring out the k20 but I'm guessing that would be the most expensive, I could be wrong. A k24a1 would cost 600$, and maybe the pistons would be another $600.
 

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Discussion Starter · #5 ·
I was just speaking with the engineer of the “k48” build and I basically stated the same thing you just did when he claimed 10k rpm’s and 750 hp NA. Granted that engine won’t be stock whatsoever, i just find 10k rpm on an under square engine to be unrealistic.


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The k48 haha! Is it a real thing? I glossed over the articles but it would be an epic engine, I'd pay for a sound clip
 

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The k48 haha! Is it a real thing? I glossed over the articles but it would be an epic engine, I'd pay for a sound clip
It’s not a Honda produced engine but it is a thing…apparently. Idk i am very skeptical. The engineer is claiming 750-800hp with a 10k rpm redline and compares it to the 458 and the new z06…i don’t buy it. As an engineer, compromise had to be made somewhere on this design. I have a feeling it’s on longevity.


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Discussion Starter · #7 ·
It’s not a Honda produced engine but it is a thing…apparently. Idk i am very skeptical. The engineer is claiming 750-800hp with a 10k rpm redline and compares it to the 458 and the new z06…i don’t buy it. As an engineer, compromise had to be made somewhere on this design. I have a feeling it’s on longevity.


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I read up a little bit on it. He's designing the block from scratch, and my understanding is that it's a racing built v8 block that you can bolt k24 heads onto. It's still cool, but it's a far cry from any production k24, and I'm sure the heads are getting a full crazy build as well.

Here's a quote: <In my engine, my pistons are kind of like drag pistons without vertical gas ports—only lateral ones—and are custom ones I designed,” Williams says. “They only weigh around 265 grams [each], with a 77-gram piston pin. My rods are titanium, and right now they are at 374 grams each>

I don't know how much the stock pistons weigh, but I'm guessing somewhere close to twice that lol $$$$$$
 

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I read up a little bit on it. He's designing the block from scratch, and my understanding is that it's a racing built v8 block that you can bolt k24 heads onto. It's still cool, but it's a far cry from any production k24, and I'm sure the heads are getting a full crazy build as well.

Here's a quote: B]piston pin. My rods are titanium, and right now they are at 374 grams each>

I don't know how much the stock pistons weigh, but I'm guessing somewhere close to twice that lol $$$$$$
Oh yea it’s all based on the k24 dimensions though. The recipe to high rpm has always been to have a shorter stroke relative to the bore. The comparison of an all out race engine that ignored the basic principles of an over square engine to that of any production car was kind of silly. I am really excited to see how it does, i just feel like 750-800 HP NA, 10k rpm and any sort of reliability is a tall order.


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I was just speaking with the engineer of the “k48” build and I basically stated the same thing you just did when he claimed 10k rpm’s and 750 hp NA. Granted that engine won’t be stock whatsoever, i just find 10k rpm on an under square engine to be unrealistic.


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Yeah that seems a bit excessive.
Thanks for the input, that makes sense. The oil pump I'd be using is from the k20a2, which is apparently good to 8600rpm, same for water pump and housing.

What I don't understand is how piston speed limits the power band. The pistons are going faster that I get, but if the head was making power past 8000rpm on the k20a2 bottom end, how does piston speed change this? (Ignoring the added stress on the k24 bottom end, unless that's directly related)

Just trying to figure out what my best option is in building this. I've also considered using the k20a2 bottom end internals, and high-comp k24 pistons, but not sure how that would work out. Or boring out the k20 but I'm guessing that would be the most expensive, I could be wrong. A k24a1 would cost 600$, and maybe the pistons would be another $600.
I believe it comes down to the cams. Found the below info on a post from k20a.org. See how the K24 piston speed is higher at redline than the K20? If you don't use a larger cam, you won't be moving any additional air through the head.

Engine Code: K20A2
Bore/Stroke: 3.38" X 3.38" (Edit: 85.851mm x 85.851mm)
Redline: 7900rpm
Piston Speed: 4450.33 Ft/min

Engine Code: K24A2
Bore/Stroke: 3.43" X 3.90" (Edit: 87.122mm x 99.059mm)
Redline: 7100rpm
Piston Speed: 4615 Ft/min
 

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Discussion Starter · #10 ·
Yeah that seems a bit excessive.

I believe it comes down to the cams. Found the below info on a post from k20a.org. See how the K24 piston speed is higher at redline than the K20? If you don't use a larger cam, you won't be moving any additional air through the head.

Engine Code: K20A2
Bore/Stroke: 3.38" X 3.38" (Edit: 85.851mm x 85.851mm)
Redline: 7900rpm
Piston Speed: 4450.33 Ft/min

Engine Code: K24A2
Bore/Stroke: 3.43" X 3.90" (Edit: 87.122mm x 99.059mm)
Redline: 7100rpm
Piston Speed: 4615 Ft/min
Oh yeah that seems obvious now that you say it lol, you have more displacement, so the air flow will max out at a lower rpm. So higher lift would be something to keep in mind as a future mod. Good to know, thanks!
 

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Oh yeah that seems obvious now that you say it lol, you have more displacement, so the air flow will max out at a lower rpm. So higher lift would be something to keep in mind as a future mod. Good to know, thanks!
It’s not necessarily more displacement. It’s limitations in the head flow and piston speed with the long stroke putting more stress on the crank and wrist pins. Obviously there’s a lot more to it than that, but if the bore and stroke were flipped around on the k24, you could much more easily rev it out.


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Discussion Starter · #12 ·
It’s not necessarily more displacement. It’s limitations in the head flow and piston speed with the long stroke putting more stress on the crank and wrist pins. Obviously there’s a lot more to it than that, but if the bore and stroke were flipped around on the k24, you could much more easily rev it out.


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Well after researching about it for a few days, I've decided against it. The cheaper k24s aren't made to rev at all, and the franken motors of the k-series run into the same exact problems as the LS-VTEC of old. Maybe a TSX motor could survive at 8000rpm, but def not a k24a1 crv motor. And changing out internals would require costly parts and re-balancing, which would make it more expensive than a TSX motor in the first place. And then if you're going to use a TSX motor, is there really big gains to be had using the k20 head? In the end you're basically just trading for the same power sooner in the rpm range, which could be achieved with a smaller (bigger?) final drive ratio. So if I spend money on any parts, I think it's wiser to upgrade the transmission. Mine's got some issues anyway.

Also just finished making the car lighter, holy crap what a difference. Didn't expect ~150lbs to change it that much.
 

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Well after researching about it for a few days, I've decided against it. The cheaper k24s aren't made to rev at all, and the franken motors of the k-series run into the same exact problems as the LS-VTEC of old. Maybe a TSX motor could survive at 8000rpm, but def not a k24a1 crv motor. And changing out internals would require costly parts and re-balancing, which would make it more expensive than a TSX motor in the first place. And then if you're going to use a TSX motor, is there really big gains to be had using the k20 head? In the end you're basically just trading for the same power sooner in the rpm range, which could be achieved with a smaller (bigger?) final drive ratio. So if I spend money on any parts, I think it's wiser to upgrade the transmission. Mine's got some issues anyway.

Also just finished making the car lighter, holy crap what a difference. Didn't expect ~150lbs to change it that much.
Pretty accurate assessment, However, the tsx engine does offer some gains NA. You have a massive boost in torque with a much flatter power band. It’s basically a poor mans stroker kit. The tsx head actually flows better than a k20 head if you port both as there is more room to port. Stock vs stock, the k20 head does flow marginally better. The biggest difference in head design is in the cam gear (VTC specifically) where you’re limited on the stock tsx cam gear vs a k20s vtc. I forget the exact values but you essentially can get more aggressive timing from the k20. You’ll also want to use a k20 oil pump if you go this route. You should see 235whp/190tq with bolt ons and the mentioned vtc. Whereas an equivalent 2.0 L will make 220 whp and 165 tq. Obviously results vary. The hp/L isn’t the same as a k20, but the power is much more useable with more potential on the table when you start talking about after market head, cams, VT, etc.

Otherwise, you are correct. The other k24s that aren’t 3 lobe vtec are economy engines.

If you go boost, the k20 is the better engine imho. Hopefully that clears things up a bit.


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Discussion Starter · #14 ·
Pretty accurate assessment, However, the tsx engine does offer some gains NA. You have a massive boost in torque with a much flatter power band. It’s basically a poor mans stroker kit. The tsx head actually flows better than a k20 head if you port both as there is more room to port. Stock vs stock, the k20 head does flow marginally better. The biggest difference in head design is in the cam gear (VTC specifically) where you’re limited on the stock tsx cam gear vs a k20s vtc. I forget the exact values but you essentially can get more aggressive timing from the k20. You’ll also want to use a k20 oil pump if you go this route. You should see 235whp/190tq with bolt ons and the mentioned vtc. Whereas an equivalent 2.0 L will make 220 whp and 165 tq. Obviously results vary. The hp/L isn’t the same as a k20, but the power is much more useable with more potential on the table when you start talking about after market head, cams, VT, etc.

Otherwise, you are correct. The other k24s that aren’t 3 lobe vtec are economy engines.

If you go boost, the k20 is the better engine imho. Hopefully that clears things up a bit.


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Cool, thanks for the info. Now I understand why people ask about swapping the variable cam gear parts. What about valve clearance using the k24a2/a3 and k20a2 head? Isn't that an issue? And are the valves same diameter?
 
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