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PostPosted: January 31, 2018, 7:05 pm 
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Joined: January 26, 2018, 9:43 am
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Long time lurker and now, participant. A huge word of thanks to everyone on this site: I've learned so much from various interesting builds and even though I'm not building a Locost roadster (I'm deep into the planning phase for an unusual build... a tube framed, mid-engined 1979 Mini) it's really applicable to what I'm looking to do.

I'm hoping that I could get some advice regarding the trade-off of engine weight versus performance (and to a lesser extent, cost). Specifically, I've narrowed down my powertrain considerations to either a Honda K20 4cyl or a Honda J series V6. Between all of my reading which includes the 2 books by Kurt Billinski, I understand the benefits of keeping the car's weight under control but what I've not been able to find is any sort of rule of thumb regarding additional weight (in this case the engine and tranny) vs power gains.

The Honda K series with a manual transmission supposedly weighs in at 405 lbs which includes the intake and exhaust manifolds, axles, and major accessories. With the right unmodified version you can expect 200 hp and about 140 lb-ft of torque.

The Honda J series weighs approximately 550 lbs with the same component considerations. A box stock J35 from the modest (read "affordable from a junkyard") Odyssey mini-van will yield about 255 hp and 250 lb-ft torque.

TLDR question: all other things being equal, is the additional 145 lbs in drivetrain weight (still in the same mid-engine location) worth the 50 + hp and the additional 110 lb-ft or torque?

I understand that the performance parts selection is a bit limited for the J series but as more guys start swapping them into their Civics, the aftermarket will no doubt respond. The K series aftermarket is considerably larger but the popularity of the swap also makes the cost of a good engine/trans MUCH higher than the J series. It's not my primary consideration but it does factor in to some degree especially if one is looking to make a K series motor put out J series like numbers.

Truckloads of thanks to any thoughts or experience somebody has to share on the topic.


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PostPosted: February 1, 2018, 8:32 am 
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A simple comparison 200/405 = .493 HP/lb. Vs 255/550 = .436 HP/lb. The V6 will also have additional wt such as cooling and exhaust systems.
BUTT :P what is the main use? drag racing or driving down back country roads.
The real question is Torque, what is the best advantage for your selected application, acceleration or handling.
If you are racing, torque typically wins.
Another BUTT, is packaging and wt distribution.
I would start the decision process by making a list of needs, wants and goals.
Than look at the technical aspect and est cost for each item.
A little up front planning in any project goes a long way.
Dave W


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PostPosted: February 1, 2018, 9:45 am 
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I had the same choice with my Miata powered Locost. I wanted to keep the Miata trans but get more power. There are conversion kits to put both the K series and the J series in a Miata. After A lot of research and head scratching the K series was my answer. Very happy with the choice, I picked up a K24A2 for about $700 after I sold the unnecessary parts. The parts availability is amazing and it should not be under estimated. From sumps to custom ECU's to dozens on inlet systems, the choice is amazing. And the icing on the cake was 218 rwbhp with what I now know was a inefficient inlet setup and a restrictive exhaust. I have been told I should be over 235. The headroom on the motor is also huge with 350 NA possible and turbo and supercharger kits 400 is easily obtainable. I did not regret my K series setup.

Graham


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PostPosted: February 1, 2018, 10:11 am 
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Dave - You're right, I should have mentioned my intentions. Storming through twisty mountain roads (a bunch of them here in western North Carolina) and the occasional auto-x is what I have in mind. Course cones are one thing but a track day and the possibility of trading paint with another novice racer like myself is somewhere south of appealing. Also, drag racing is not a consideration...the occasional stop light to stop light grand prix is fine but if I really wanted to go drag racing, I'd probably choose a different platform than the Mini.

I have been working on a blueprint of a front engine RWD configuration featuring a Honda F20 (gotta love those high revs) but needless to say there are numerous obstacles to wrestle with,
not the least of which is space. The necessary engine setback really cramps up the pedal box area and requires a substantial rearward placement of the driver's seat (nothing quite like having the B pillar in the middle of your side vision). There's also the concern of having so much weight above the front axle centerline (seems like a recipe for understeer). Then you get to the back end: a live axle rear adds unsprung weight and designing such a narrow IRS seems quite complicated. I haven't ruled out the possibility but KB's mid-engine Kimini build is seriously capturing my attention.

As far as K20 vs J series packaging is concerned - it's more or less a wash. The engine placement and weight distribution should be fairly comparable.

So I guess I come back to the comparison. As you've noted, the HP/lb comparison favors the K20 but the torque disparity is pretty large. My intended usage would seem to welcome the additional torque however considering the goal weight (under 2000 lbs with 1/2 tank of fuel and me in the driver's seat), at what point is the additional torque just a tire smoking device?

davew wrote:
A simple comparison 200/405 = .493 HP/lb. Vs 255/550 = .436 HP/lb. The V6 will also have additional wt such as cooling and exhaust systems.
BUTT :P what is the main use? drag racing or driving down back country roads.
The real question is Torque, what is the best advantage for your selected application, acceleration or handling.
If you are racing, torque typically wins.
Another BUTT, is packaging and wt distribution.
I would start the decision process by making a list of needs, wants and goals.
Than look at the technical aspect and est cost for each item.
A little up front planning in any project goes a long way.
Dave W


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PostPosted: February 1, 2018, 10:48 am 
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There's no black-and-white answer because much of the decision is subjective:

1. What fraction of the overall car weight is the engine?
2. What will the car be used for?
3. A heavier engine increases weight and moves the CG, but how much do you care? Subjective
4. What engines are available in your area?
5. Is it worth getting a more desirable engine shipped to you even if cheaper alternatives are available locally? Subjective
6. How important is a source of aftermarket parts? Subjective
7. Can you afford a "better" but more expensive engine? Subjective
8. How important is engine reliability and can you get replacement parts locally. Subjective

And on and on.

BTW, thanks for buying my books!

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PostPosted: February 1, 2018, 11:46 am 
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What is the wheelbase you expect to have?

I put 125 HP in my 1978 Ford Fiesta and it would easily go fast enough to suck the top of the door frames out several inches. I could put my whole arm out thru the crack.

If you want to know how fast you can go and still smoke the tires, you compare the weight on the tire times it's coefficient of friction to the torque of the engine multiplied by the transmission gear ratio and then the final drive gear ratio. The coefficient of friction for a rubber tire might be somewhere between 1.2 and 2.0 for a drag racing slick. I don't remember the number well, but you can search for it.

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PostPosted: February 1, 2018, 12:36 pm 
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KB - Thanks for the input.

Quick calculations assuming the 2000 lb finished weight works out to the K series engine/trans/accessories being 20.25% of the car's total weight. The J series option works out to 27.5%
of total weight.

I'm not sure how I could go about calculating the CG short of having both drivetrains and mocking them up. I'm thinking that the transmissions' placements would be very similar although the weight may be different (need to research that) however the K 4cyl vs the V6 weight placement is a much tougher guess.

Access to engines is a wash as I can get either from a variety of sources.
I've been seeing K20 engine/transmissions for about $4400. K24 is considerably cheaper.
A J35 comes in at around $700 for engine only. There are plenty of resources and they are not used as swap material as much however the challenge is finding a manual trans as only 5% of the J series motors/vehicles came equipped with one.

Replacement engine parts for either should be reasonably easy to source.

Your open questions included the idea of a "better" but more expensive engine. I haven't looked into many options beyond the Hondas mentioned, the Ford Duratec and GM's Ecotec. The Honda's have better aftermarket support. Curious what you have in mind as a "better" option?

KB58 wrote:
There's no black-and-white answer because much of the decision is subjective:

1. What fraction of the overall car weight is the engine?
2. What will the car be used for?
3. A heavier engine increases weight and moves the CG, but how much do you care? Subjective
4. What engines are available in your area?
5. Is it worth getting a more desirable engine shipped to you even if cheaper alternatives are available locally? Subjective
6. How important is a source of aftermarket parts? Subjective
7. Can you afford a "better" but more expensive engine? Subjective
8. How important is engine reliability and can you get replacement parts locally. Subjective

And on and on.

BTW, thanks for buying my books!


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PostPosted: February 1, 2018, 2:52 pm 
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Joined: April 23, 2006, 8:26 pm
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I can't recommend my "best" for you - that's a very personal decision. The only thing I'll add is that a turbo K24 results in about 63% rear weight bias; On-track that's about the limit. Point being, the V6 will make that worse from a handling perspective, but what defines "handling feel" is subjective as well.

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PostPosted: February 1, 2018, 3:44 pm 
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If I may add, the engine and weight distribution are only two components of performance. Braking and your tires/springs/shocks combination are also big factors. In my planning stage (I humorously think of it as my Walter Mitty stage of the project) I set goals for all the factors I felt were critical.

I realized that I'm the engineer in fact, even if I'm not an engineer by profession, and don't have an engineering license from some sanctioning body or government. I decided that I didn't want to be an accidental engineer, designing something that had gobs of horsepower, but was unmanageable, or unsafe, on the road. You sound like you're an analytical type too.

I found it useful to construct a "Design and Engineering Specification" document where I took each subsystem and established goals for it's performance. I used a number of magazine road tests of high performance sports cars to set braking and acceleration goals, for example. Knowing I would have street tires that are affordable (no $600 per tire exotica) I found the coefficient of friction was going to be around 0.90G maximum, and likely a little less. Well, that's a big factor in designing your braking system. And, so it goes - thigh bone connected to the hip bone, etc., etc.

In my opinion, you have to have an overall plan if you really want to ensure you get a good performer when all is done. It's work, but it focuses your thinking, and the research helps you make good decisions and save money.

If you have any skill with a word processor, it's pretty easy to set up.

Cheers,
Attachment:
File comment: One page out of my design & specification document.
Locost Spec.jpg
Locost Spec.jpg [ 114.15 KiB | Viewed 697 times ]

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


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PostPosted: February 1, 2018, 4:49 pm 
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Quote:

I've been seeing K20 engine/transmissions for about $4400. K24 is considerably cheaper.
A J35 comes in at around $700 for engine only. There are plenty of resources and they are not used as swap material as much however the challenge is finding a manual trans as only 5% of the J series motors/vehicles came equipped with one.


$4400 for a K20 and transmission seems expensive to me. A friend of mine just sold a 2006 Civic SI with the K20Z3 and 90 thousand miles for $5000. The 2006 civics in Red had terrible paint and it was coming off his car in big sheets and nobody wanted the car because of it. If that is your budget you could probably find a complete running SI if you look.
He sold the Civic to get money to build his Midlana and he is running an Ecotec and F35 transmission. You can get good power out of them if you turbo or supper charge them. You can get standalone ECUs and wiring harnesses, and in these parts you can get Ecotec engines from the junk yard for 99 dollars and and the matching transmission for 79 dollars. The last time I was there you could taken your pick from close to 20 of them. One possible downside is a lack of factory limited slip in the 5 speed transmission.


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PostPosted: February 1, 2018, 4:51 pm 
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PostPosted: February 1, 2018, 5:24 pm 
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carguy123 wrote:
PM sent

Unless there's a "To", such a post is either ignored or makes everyone on the thread have to check their PM Inbox :?

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Last edited by KB58 on February 1, 2018, 5:28 pm, edited 1 time in total.

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PostPosted: February 1, 2018, 5:26 pm 
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WelderLee wrote:
Quote:

I've been seeing K20 engine/transmissions for about $4400. K24 is considerably cheaper.
A J35 comes in at around $700 for engine only. There are plenty of resources and they are not used as swap material as much however the challenge is finding a manual trans as only 5% of the J series motors/vehicles came equipped with one.


$4400 for a K20 and transmission seems expensive to me...

I agree. I bought my used K24 and transmission about six years ago and paid around $1500 for both. Even a used built engine for that price seems too high.

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Midlana book: Build this mid-engine Locost!, http://www.midlana.com/
Kimini book: Designing mid-engine cars using FWD drivetrains, http://www.kimini.com/book_info/


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PostPosted: February 1, 2018, 5:45 pm 
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FastG wrote:
218 rwbhp


Shouldn't it be either rear wheel horsepower or brake horsepower? I'm not sure that it can be both.

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PostPosted: February 1, 2018, 6:29 pm 
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KB58 wrote:
carguy123 wrote:
PM sent

Unless there's a "To", such a post is either ignored or makes everyone on the thread have to check their PM Inbox :?


It is generally accepted that it is to the OP unless otherwise noted with a "to"

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