To me a good car is a safe car. Predictable at the limit, enough brake and tire to match the power level, etc. There are plenty of cars that can be driven fast but will jump up and bite you if you get distracted. To me those aren't good track cars, at least for recreational use.
Build a good car.
When I was younger I was contemplating getting a motorcycle. I had never had one. A coworker said he would sell me his, ( a vmax or something I can't really remember but it was considered a very fast machine). I said "I think that will be too much motorcycle for me." He said "That's why they make throttles variable. It will go as slow as you want."
There is something to be said for building a car that needs to match how you intend to drive it. I've been having the same debate with myself of building up a new downhill mountain bike. Current DH bikes are basically race bikes that need to be ridden a certain way in order to work properly. If the rider doesn't have the skill to ride one that fast, it can go wrong quickly and painfully.
I've dropped any dreams of being a great driver. I'm a decent driver, but far from great. Even my basically stock R53 MINI is a far better car than I will ever be as a driver. While I've done a few autocross events with it, I'm still fearful of taking it to a track. Maybe in 2021...
So for the 917 build, I would lean towards the 500HP option in lieu of the 1000HP option. It just would seem to make sense that if you can push the car and have fun with the lower HP engine, then you may not feel the need to double it. But going back may be harder. Maybe.
If you enjoy the build process then by all means go ahead with the build. If the build process is really a way to get a car that you can spend track time with then you may want to rethink the plan.
High HP/wight cars can be easy to drive if they don't have non-linear power delivery, aero that doesn't start working until after mechanical grip has given up or just plain messed up suspension. If you deviate from the original 917k enough to avoid those pitfalls then you should end up with something that's enjoyable to drive even if you don't push it to its limits.
All of the various replies is what makes this forum great; so much really good advice.
So far our collective answer is "you like building cars, go for it, but you don't want to kill yourself, so build something that is a little less hairy and enjoy it".
Tom1200 said:All of the various replies is what makes this forum great; so much really good advice.
So far our collective answer is "you like building cars, go for it, but you don't want to kill yourself, so build something that is a little less hairy and enjoy it".
Well that is a succinct way of what I was going to say.
I am going to go back to the drawing board on the planning of this car in terms of power output. I think building something like this where I can feasible in the right conditions wring its neck out will be more of what I am looking for. That may mean a 300-400hp motor that spins to 9K over say a turbo motor that is just brute power. I have some very basic down force numbers as well now. Basically the car does not produce any until you are at ludicrous speeds IE 180 plus and even then just at 500lb's max.
It could also mean finding a old formula car and using that as a starting point for something in teh same vein but with more modern suspension. Or branching out on my skill set to build something in the vein of what Chris Rünge does.
FYI the motorcycle analogy is a very good one for me. The red mist descends very quickly and I want to see exactly what it will do. Which is why I don't own one. The ACR is very tiring to own and drive as I cannot exploit even 70% of what it is capable of
In reply to Tom1200 :
Plus, some things are just recalcitrant and un-fun until you get up past a certain level of commitment.
Friendliness at 5/10ths as well as 10/10ths is the quality I'd really like to define, refine, bottle up, and apply later.
I wanted to post this separate for my post above as it's going to be long winded..... I promise there's a point to it
Over the years I've spent an inordinate amount of time thinking about what I would buy with a 100K budget. There are some really fast, single seat cars and GT cars in that budget range. I say inordinate because I don't expect to ever have that kind of budget for a track toy.
From the driver standpoint; when I had the D-Sports Racer, the lap record I set was within a second of the Formula Mazda and Trans-Am lap records. I've also driven a 750hp race Viper and was comfortable with it. So having demonstrated my brain is capable of processing a Trans-Am or Formula Mazda level of lap time, the next driver logic step would be a step above those cars, actual experience tells me otherwise.
Someone once tested an Indy Lights car and went 13 seconds a lap faster on the same 1.5 mile course then I did with the aforementioned DSR. Again from a personal standpoint I'm OK with going 2-3 seconds a lap slower than the cars ultimate pace but at 10 seconds a lap slower I would not be happy with myself.
I'll continue to use an Indy Lite car as my example; Besides being slow. running it on my own would be a challenge and having to have a crew is one extra logistical thing to worry about, I take solace in being self reliant. I also don't want to have a car that needs anything more than refueling and tire pressure checks track side.
Additionally I would start to skip certain tracks as given the cars performance some tracks that are now perfectly safe would become less so given the higher velocity and the potential for actually hitting things that were formerly well of the track. The car maybe built as safe as it can be but there are certain amounts of physics that can't be overcome.
So my long winded point is why build something so far outside your comfort zone and driving ability that you are already questioning how safe it / you may be?
I'd build a 400-450hp 917 (originals 525hp range? ) which would be plenty fast enough to be fun but not insanely beyond your ability to drive it. It would be reliable and super user friendly.
EDIT: it appears wearymicrobe has already come to this conclusion..........LOL
The other thing that crossed my mind-
If the car is for fun and not competition, then reliability is a major factor. Fun is turning trouble-free laps. Un-fun is trying to fix things while laying on the dirty paddock asphalt in the 100 degree heat. There are lots of ways to get a reliable car in the 300-400hp range. There are fewer ways to get one in the 1000hp range.
ShinnyGroove (Forum Supporter) said:The other thing that crossed my mind-
If the car is for fun and not competition, then reliability is a major factor. Fun is turning trouble-free laps. Un-fun is trying to fix things while laying on the dirty paddock asphalt in the 100 degree heat. There are lots of ways to get a reliable car in the 300-400hp range. There are fewer ways to get one in the 1000hp range.
I have done the whole 800-1000 reliable horsepower. Its not hard to do you just throw money at the problem. Lots and lots of money.
I will report back once I get my head on a little straighter.
I've owned several cars that were beyond my driving ability. You don't have to drive it to the limit.
I had a nicely-prepped E30 that was way beyond my ability at the time, but it was certainly fun driving it at 100% of my capacity (which was about 80% of its capacity)
The only caveat is that some things that make a car capable on a track can make it less safe on the street for some drivers. My E30 for instance had a little more camber than was safe with H-rated rubber. The difference between grip and "oh god I'm facing the wrong direction" was not an enjoyable part of driving it. Stickier rubber and pulling 1 degree of camber out of the rear was a much better solution. It may have given up a few tenths on the track where race rubber and the extra camber would have been better suited, but it made it safer for ME to drive on the street.
Otherwise, go for it. I'm a big fan of having more car because it pushes me to learn better driving. I would straight up drive a NASCAR on the street if I could, but I would set it up for street; less camber because I won't be driving 180 mph and getting downforce and I would have street rubber, but nothing wrong with having too much car... as long as you don't drive it above your skill level.
In reply to Curtis73 (Forum Supporter) :
Not to belabor the point BUT......
The level of car wearymicrobe was originally talking about would be easily 5 seconds a lap faster than a NASCAR cup car. Just as an example our DSR had minimal to moderate down force yet pulled 3Gs on the brakes and over 2Gs in a corner. Couple this with the ability to out accelerate a standard Viper and things can go off the rails very quickly. The mistakes also compound themselves very quickly.
A friend had an off in the DSR, he was driving the car about 75-80% of what I did. He got ever so slightly off line (maybe a foot) going into a 45 right 90 left section and by the time he realized the mistake he was traveling way to fast to keep it on the road. Someone asked him where he went off and he started to point to the exit of the 90 right and I interrupted and said "back there" pointing to the 45 left that's 100ft before the right hander. Basically his timing was off by a split second and he missed the first apex by a foot, which then caused him to miss the next apex by about three feet, which then caused him to run about six feet wide on the exit. He then tried to save it, which caused him to shoot across the track and do some 60mph offroading in a car with 1" of ground clearance.....past that point he was a passenger.
This is the dilemma with these types of cars, if you motor around at about 50-60% (nothing wrong with that) pretty much any competent person can drive them but once you start getting into the 75-90% zone they are just as unforgiving as they are at 100%. It's not that the balance of the car is tricky it's just purely a matter of arriving at the scene so quickly that your brain and skill level doesn't keep up.
My favorite place on my local track is a 30 degree kink over a crest taken at 82-83mph in my Datsun, it's 86 mph in my friend's Spec Miata, it's 93-95 mph in a 911 GT-3RS and it's a 100-105 mph in a medium down force sports racer. I've seen some onboards from higher down force cars and they are doing between 105-108 mph there.
Beauty is that modern technology can make things much more dial-a-evil
It's boosted; I have driven turbo cars that had a switch on the dash where one setting was low/no boost and you could flip the switch and unlock the ability to go to plaid. You can make it so you dial the power, trim it to your ability. There's also no shame in it. I had a friend who autocrossed a boosted FFR cobra who was in his prime. He shredded the blower belt and went faster without the blower. There is a point in application where more is less fun.
ABS: aftermarket systems are available now. Would take some evil out of a fast car.
Traction Control: available aftermarket now as well. More evil reduction. (Racetronics is one such system I think it can combine abs and TC)
Now, it might have just added to your budget, but you just probably made a car with a whole lot of ability to be evil (in all the good and bad ways) when you want and a Bob Costascat when you don't.
Wheelie control, abs and traction control have made modern superbikes much easier for average riders to cope with so obviously all of those are good things but here's what the electronics won't cope with:
So using all my info from my previous post about my favorite corner at my local track; if you're timing is off by say a tenth of a second, at 83mph you've gone 12 feet and at 105 you've gone 15 feet past the proper turn in point. It's a 150 feet from turn in to the apex and another 100ft to the exit.
At 83 mph it will take 1.23 second to reach the apex, minus the .25 average reaction time and you've got a full second to sort it out.
At 105 mph it will take 1 second to get to the apex, minus .25 leaves you 3/4s of a second to sort it out.
You make a correction and if you haven't done it correctly it will take another .25 before you realize it. At 83 mph you still have .75 left to save it and at 105 your down to .50 of a second.
Now throw in the fact that it's 100ft from the apex to the exit, you've likely missed the apex by 2-3 feet, subtracting the reaction time you've got 80 feet at 83 mph but only 65 feet at 105mph. This is what I mean by mistakes compounding themselves in faster cars.
While I'm a fan of the electronics no amount of electronics will fix things if you miss your mark.
In reply to Tom1200 :
That's why I was also talking about boost control.
A friend has a fast 70s aircooled 911 and also a late model GT3 Porsche. On backroads I'll take the aircooled 9 times out of 10. The current obsession with horsepower bores me. But what turns you on is what's important. We're just bystanders.
In reply to Apexcarver :
Understood on the boost control, I was speaking more to the higher corner speeds relative to production cars. Production cars in general have more suspension and tire wind up then purpose built race cars, be it a Formula 500 or 917 replica.
If you actually start and drive that car you will.be 1000% more involved with that car than pretty much every owner of actual 917s. Whatever limit you choose, you already won.
In reply to Tom1200 :
The vintage replica/mod aspect makes things interesting... It isn't a downforce car, well at least not one by modern standards. Aside from being lightweight, the charastics depend on wheel/tire selection. I'm not sure I follow what you might mean intrinsic to the car. I've driven a 1800lb miata that was telepathic in it's forgiveness and 3300lb ponycars that needed to be lead through a turn by the scruff of the neck or they would bite. With power a controlled variable (dial a evil) I'm not sure it would inherently be too bad, provided good geometry, dampning, and tire selection(perhaps trading peak grip for a forgiving breakaway behavior)..
The above isn't meant to sound argumentative, I'm curious what chassis aspect you are intoning with windup.
In reply to Apexcarver :
Not argumentative at all. In his original posts wearymicrobe mentioned using a formula car as a starting point and having mild amounts of downforce.I'm making the assumption that he's going to end up with a 1400-1600lb car with around 500-600lbs of downforce.
My comments are based driving a 1056lb car with around 500lbs of downforce, an older Formula Super Vee and a circa 2008 Stohr P2 car. These chassis have zero & tires don't take a set (wind up)n the way production cars do.
I've driven some heavily modified Miatas that exhibted zero roll and provided instantaneous feedback but they still take longer to set than the aforementioned single seat cars.
The best analogy I can give is some cars shout "don't do that" and others (single seat cars with down force) whisper "that's not s good idea".
Life is too short to drive boring cars. 
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