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Learning how to build Lotus Seven replicas...together!
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 Post subject: Power to Weight
PostPosted: October 2, 2018, 5:05 pm 
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Joined: June 24, 2011, 4:42 pm
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Location: Boise, ID
I'm not sure which sub-forum this would be better in, so it's in here.

I couldn't find very much, if any, information about what power to weight people end up with. I was thinking a database could be made giving some information and specs on the builds. Something like engine, frame type, power (dyno verified would be great, either engine or wheels), and total wet weight.

I've got way more ideas than I could feasibly do, and I think a thread with some info like this would help people out when deciding what they want to do.

Thoughts?


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 Post subject: Re: Power to Weight
PostPosted: October 2, 2018, 5:45 pm 
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Joined: April 23, 2006, 8:26 pm
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Don't fixate too much on this ratio. It's good for what it is, but has its limits with "our" cars (Lotus Seven-type cars) due to aerodynamics. You can have a great power to weight ratio and still get beat on a high speed course by a more streamlined OEM car with less power.

For your question, you'll also get optimistic numbers (such as using dry weight instead of "as driven"). Anyway, for our cars it's typically somewhere between 4-10 lbs/hp.

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 Post subject: Re: Power to Weight
PostPosted: October 2, 2018, 6:02 pm 
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Joined: June 24, 2011, 4:42 pm
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Location: Boise, ID
Which is a massive difference. I'm not necessarily looking to compare to other mass-produced cars, more of an "even" comparison in a 7 body.
A Miata based car, compared to a v8 car.
BEC to Miata Turbo.
Like I said I have several different things I want to do, but in the back of my mind I probably don't "need" a 400whp v8 locost, if a well sorted 250whp Miata locost will be enough because of lightness.
With a good set of data, people would compare power to weight ratios and get a decent idea of what they'd be comfortable with, and throw in other variables to. If, for example again, a simple book chassis with a Miata Turbo base would be close enough to a more custom frame to fit a v8, then maybe the power gains outweigh the frame weight and complexity of the build.
If this really is a very hard idea to follow through with, that's fine. I'm just brainstorming things for when I get started, and if my ideas can help others, even better.


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 Post subject: Re: Power to Weight
PostPosted: October 2, 2018, 7:13 pm 
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Joined: August 27, 2005, 1:04 am
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Location: Kamloops, BC, Canada
I think the power delivery is as important as the peak number when you're talking about a car like a Locost. For instance, my car is around 200hp and 1300lbs, but with a torquey V6 and it will break the rear end loose in the first 3 gears, even with sticky tires (Toyo R888's). The sudden power burst of a turbo car could be even worse. A high winding bike engine with very little torque but tons of HP at 14000 rpm would probably feel a lot quicker than a more laid back car engine with less peak power, but might not accelerate any quicker in a drag race. I'm torn between building another one with either a small block Ford or a Duratec 2.3, not quite polar opposites, but close.
Kristian

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 Post subject: Re: Power to Weight
PostPosted: October 2, 2018, 7:21 pm 
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Power to weight is just a number that gets you an approximate comparison of 0-60mph times. What you don't get from that number is "handling, which can be an immeasurable parameter for most of us to quantify. The Seven, in its many incarnations, has always been about handling. If you want 0-60 then the biggest, baddest engine, turbo(or supercharger) and tires will get you there. HP/torque goes up faster than the weight necessary to attain that power. As for handling, the general rule is: the heavier you get, the worse it handles (all other things equal. So pick your target and work towards it.

One thing you can do is to attend the Midwest Gathering next year and get rides in various different builds to find out which one checks the box that YOU want and mimic that build. This is a much better way to get you started down a path that will satisfy your wants than some number on paper. I can't remember any BEC cars attending though. I will add that if you can get mid 100-200 HP in a 1300 lb Seven, it will be tough to wipe the smile off your face. Even better if you can get it down to 11-1200 lbs. My 160HP rotary powered, 1400 lb Locost was a blast and really came alive above 5000 RPM. BEC's and rotary powered Locosts are low torque and need to rev high to get big HP. If you are used to driving torquey engines, then you probably wouldn't like either driving experience, even if the HP/weight is in the same range as a prepared Miata or NA V6.

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 Post subject: Re: Power to Weight
PostPosted: October 2, 2018, 11:35 pm 
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https://www.reddit.com/r/cars/comments/ ... heet_of_a/

I wouldn't worry too much about it. Just decide what you want your car to do, pick your engine to suit and build the car as light as you can. One of my requirements was to be able to keep up with my friends in their big HP camaros. I opted for the high power/weight build partly because I knew to get the performance I wanted out of a BEC would require some skill/experience to keep the curb weight low enough. I also wanted to be able to drive the SNOT out of the car with some sort of reliability.

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 Post subject: Re: Power to Weight
PostPosted: October 5, 2018, 3:28 pm 
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Joined: July 17, 2006, 3:09 pm
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Location: Holly, MI
Your original post says this information will help people decide what to do. This is where I'm lost. For instance, my car weighs roughly 2100 pounds and has a dyno verified 500HP at the crank. It's a mid-engine car that looks nothing like a 7.

Now, how does that information help anyone decide what to do? When you say "what to do", do you mean what kind of car to build, or what type of driving to engage in?

Ken


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 Post subject: Re: Power to Weight
PostPosted: October 5, 2018, 8:26 pm 
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Joined: April 23, 2006, 8:26 pm
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Location: SoCal
Another thing not mentioned is the center of gravity (CG). A buddy built a 500 hp Locost and said that when he'd race his buddies, he had trouble putting the power down, and, once things got over 100 mph, aerodynamic drag killed him. With his front engine placement, having a ton of power makes for a great smoke machine with marginal controllability, but put that same engine behind the driver and more traction is to be had. The point of mentioning this is that both CG and drag make power to weight less meaningful.

I also agree that the original post seems to be an answer looking for a question; there is no black and white "do it this way."

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Last edited by KB58 on October 6, 2018, 2:19 pm, edited 1 time in total.

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 Post subject: Re: Power to Weight
PostPosted: October 5, 2018, 10:30 pm 
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Joined: June 5, 2016, 7:03 am
Posts: 215
Location: ontario
Jeimuzu wrote:
I'm not sure which sub-forum this would be better in, so it's in here.

I couldn't find very much, if any, information about what power to weight people end up with. I was thinking a database could be made giving some information and specs on the builds. Something like engine, frame type, power (dyno verified would be great, either engine or wheels), and total wet weight.

I've got way more ideas than I could feasibly do, and I think a thread with some info like this would help people out when deciding what they want to do.

Thoughts?

You seem to imply that the power to weight ratio is a deciding factor for seven builders. To a degree perhaps. But not as much as you may think. I have been lurking around this forum and others (on seven building) for ten years+ and built two sevens myself. My impression is that most folks want to retain the light weight character of this car but they also want it to be safe, street driveable and not break the bank.


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 Post subject: Re: Power to Weight
PostPosted: October 7, 2018, 8:52 am 
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Joined: December 7, 2012, 8:28 am
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Location: Sarasota
Personally I think power to weight rations mean nothing. Here are a few examples why using my car Scrap Metal.
Dry weight:=1380lbs
Race weight =1450lbs
Race weight with driver =1650lbs

Rated Power =244HP
Magic math Power = 312WHP (244rated + 10 for air filter, +15 for my exhaust, +15 lighter stuff +10% for tune)
Dyno (dynocom) = 249WHP
Dyno (Mustang) = 215 WHP

Power to weight is anything from 4.4lbs/hp to 7.7lbs/hp

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 Post subject: Re: Power to Weight
PostPosted: October 7, 2018, 9:50 am 
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Joined: April 23, 2006, 8:26 pm
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Another thing, everyone cheats. That is, nearly everyone leaves out driver weight (and sometimes fluids), which gives a more impressive number. In such a light car, driver weight can skew the result by 15%. The number is almost entirely for bragging rights.

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 Post subject: Re: Power to Weight
PostPosted: October 7, 2018, 12:54 pm 
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Joined: June 5, 2016, 7:03 am
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Location: ontario
KB58 wrote:
Another thing, everyone cheats. That is, nearly everyone leaves out driver weight (and sometimes fluids), which gives a more impressive number. In such a light car, driver weight can skew the result by 15%. The number is almost entirely for bragging rights.



....as well as omitting the spare wheel systematicaly


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 Post subject: Re: Power to Weight
PostPosted: October 7, 2018, 12:59 pm 
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Joined: June 26, 2012, 7:12 pm
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KB58 wrote:
Another thing, everyone cheats. That is, nearly everyone leaves out driver weight (and sometimes fluids), which gives a more impressive number. In such a light car, driver weight can skew the result by 15%. The number is almost entirely for bragging rights.


I'm glad you bring that up.

Caterham in particular is known to be loosy goosy with their numbers to achieve the R400/R500/620R

1) They use a metric ton vs regular ton (2240 pounds)
2) I believe and I could be wrong that the weight is dry and typically without other options. (If you order your R400 and add heater option, obviously weight goes up, P/W goes down but it remains R"400")
3) The engine numbers are crank numbers, while most of the people who build and modify their cars use wheel HP numbers
4) Even if the numbers don't add up exactly, R400 sounds a lot better than R393...

I personally have a what was sold to me and believed by the previous owners to be an R500. Zetec HP with ITB's and other mods is estimated to be ~200, which would make it roughly an R400 technically.

At the end of the day do I really care which moniker the car should have? Not at all. It's a blast to drive and I almost never go WOT.

The much bigger issues with the car other than the P/W are the other elements that come with a car like this. Strapping yourself into the harness every time you go a block, looking out for rain, debating on putting top/doors on, looking out for pot holes, etc etc.


My advice? Pick front vs back, pick a motor, pick a reasonable HP that you can easily achieve with that motor and go at it.

Edit: For fun math

R420 = 420hp/ton
~210hp Duratec crank (likely optimistic to begin with)/1240 lbs = R378 (using crank #)

We area already at way lower number than the plaque and that is stock, using their own numbers provided on the website.

Start adding other common features that don't come with that 1240 lbs standard spec. + dry sump + LSD + weather equipment + heater + windshield washer + carpet + SAE Roll bar (thicker) etc etc.

Lets even say this will add 100 lbs, we are already an "R350".

Point is, my "superlight" (In quotes as I have carpet, wet sump, windshield washer, tools in the back, weather gear + gas/fluids) as it sits now could likely be the equivalent 300hp/ton or less an I won't care cause its still a blast and its still fast.

Edit X2: I dug deeper and this is what I found (see attachment)

They actually openly list that they are realistically 375ps per tonne for the R420. 620S is "Power-to-weight: 508bhp-per-tonne"

270 is one of the most "honest ones" Power-to-weight: 254 bhp-per-tonne

Source:
http://us.caterhamcars.com/cars/seven-270


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 Post subject: Re: Power to Weight
PostPosted: October 8, 2018, 1:08 pm 
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Joined: June 24, 2011, 4:42 pm
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Location: Boise, ID
I think it would be interesting and possibly helpful in some situations. If it's not feasible, or people don't want to do it, or it doesn't matter, no big deal. It was just an idea I had.


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 Post subject: Re: Power to Weight
PostPosted: October 9, 2018, 7:31 pm 
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Joined: January 31, 2012, 12:49 pm
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Location: Louisville KY
There are lies, damned lies and HP numbers.

Having said that... I wonder if the weight thing doesn't "matter more" than the HP thing, especially at different points in the curve? For example... adding 500 hp to a locost that already has 200 would produce a wow number of roughly 2 hp / pound. Not sure that the last 500 is at all usable tho, right? Burning off back tires while pushing thru hopeless Seven aero?

Now, if you take a 1400 pound 200 hp locost, cut 400 pounds off of the thing, you'd have 5 pounds per hp, but it would all be usable. So at that point in the graph, it would mean more than the prior example?

And as wiser voices have said, the driving differences would be vast.

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