S13 vs. S14 chassis; Weight vs. stiffness

[djsilver]

This questions is SCCA autocross related.

Weight vs. stiffness is the only meaningfull difference between the two that I know of, related to this subject. If there are others, feel free to discuss.

Neither car is going to be competitive with the 6-cylinder BMW's or the turbo-charged XR4Ti (boost is now unrestricted) in DSP.

My S13 coupe has the complete 5-lug S14 suspension (including rear subframe) under it but I'm severly limited in methods of stiffening the chassis. I previously welded in some fender braces from a convertible using the "update/backdate" rules but the rules have now changed and the section that stated "engines and transmissions must only be swapped as a unit" now also includes "unibodies", so even those little braces are now illegal. A cage would stiffen either chassis nicely but the additional weight is a killer. I will run a rollbar to make it legal for time trials. I've done most of what I can under the rules for the motor and measured 168rwhp and similar torque at the last dyno trip.

The next obvious class up is Street Modified. I can make up the horsepower difference there and there's a lot more freedom in suspension setup but chassis bracing is still quite restricted. Most all the parts I have laying around will fit either chassis so that's not a deal breaker.

The only disadvantage I've heard discussed on the S14 chassis is that it weighs more. The only advantage that I've heard discussed is that the chassis is significantly stiffer in stock form. I know Logr and Jim Bennett are running the S13 and Ace-in-Hole has done well with the S14 and there's a few more that have played in the class, although I don't know how many have done it seriously.

The cost of picking up an S14 chassis can be manageable if being a running car isn't a requirement. The cost of changing classes is going to be about the same on both.

Opinions?

[Equinox]

What exactly makes the S14 chassis that much stiffer? I've noticed small things like extra bracing under the fenders and the like.

Has anyone done any measurements to work out how much stiffer it is? I have no idea how this would be done but I guess putting weight on one point and measuring deflection along an axis would kinda work.

[logr]

168whp sounds pretty good for dsp. I haven't looked at all the mods you could do but really light rotating weight would sure help. Light wheels and brakes would go a long ways. What about the 275 15's on 13 lb wheels. I would think you could get to 2400ish and do pretty well. Rev to 7500 and you have 66mph in second. Fixing my front roll center and using an s14 subframe with no give has made a world of difference in the handling of my car. A good friend drove it at a T&T this weekend and said he had more fun in it than his zo6.

Not sure if that is what you are looking to hear. I believe handling and lightness is a big part of autox and I would think an s13 coupe could do well in dsp as well as sm and xp. I don't know about the wimpy unibody thing. My coupe picks up 2 wheels pretty easily from 1 jacking point.

[Black R]

I chose an s13 coupe with no power options specifically because the coupes are stiffer and s13's are lighter.

At the end of the day, weight is your biggest enemy IMO.

I say suck it up and cheat like all the bimmers do in sm.

seriously tho - didn't daddio win Sm this past season with an evo9 rs?

I'd be worried about light awd turbo cars more than the bimmers at this point.

[jmauld]

I think an S14 has the potential to do well in DSP.

[Robertftw]

"275 15's on 13 lb wheels" what wheels are these.

I'm looking to start autocrossing this year but the thought of SM is kinda scary for a first timer.

[Def]

At least in the Atlanta region n00bs were encouraged to either run in a special novice class or just do Time Only(TO) if you were just starting out and nervous or didn't know where your car fit. Just get out there and drive, not a big deal if you aren't competitive, no one really is their first time out.

[djsilver]

"Has anyone done any measurements to work out how much stiffer it is?"
I haven't seen any research results but I've seen it on the internet! I also think it was included in the sales literature when they updated the model.

"What about the 275 15's on 13 lb wheels?"
I have a 7200 rev limit now but power peaks at 5600 and steadily drops off after that. The 15's in DSP trim would have a realistic limit in the mid-hi 50's and require frequent shifts to 3rd, although that isn't a killer since most BMW's top out second in about the same place. The main reason I haven't tried them yet is that I can find 245-275/40/17's or 245-265/45/16's lightly used used at a good price from roadracers but the 275/35/15 is an autocross only size and us solo guys use them down to the cords so it would mean dropping a grand on a new set for an experiment.

"Not sure if that is what you are looking to hear"
It's exactly what I want to hear; opinions related to the question. I'm not asking folks to tell me what I want to hear.

"...,didn't daddio win Sm this past season with an evo9 rs?"
I think so a couple of years ago but last year Mike Simanyi won in a BMW E36 M3 with a non-turbo motor! David White and PJ (Ace-in-Hole) took 4th and 6th, respectively in PJ's S14 (way to go!) Randall Wilcox also pulled off 3rd in XP with Jim Bennetts S13.

"I'm looking to start autocrossing this year but the thought of SM is kinda scary for a first timer."
For a beginner just run whatever class your car fits in and have fun. The fastest way to learn is begging rides with the faster guys. Most regions allow riders and you can do it two ways; Within your run group just hop out of yours and catch a ride with someone already on grid. If you run with a group that has at least 3 run groups, you'll drive one and work one, but you'll have one free and you can go beg rides in grid!

Keep it coming guys!

[Solo_S14]

Man, your DSP sounds much more restrictive than our DSP. Given that we have a lot less classes, and DSP is the last class before DMOD (highest class).
We actually run PAX so that everyone in any class is competing against each other (PAX SUCKS).

In DSP, any conceivable chassis/body bracing/reinforcement is allowed.

Our suspension mods basically look like this:
Any mod/removal/substitution/addition sway/strut bars - 1 pt each
Any other suspension mods - 3 pts total

This basically allows you to do all the suspension mods you want (except strut/sway bars) as soon as you touch your springs (dampers are free - subject to adjustment limits). Having coilovers gave me the freedom to upgrade my entire suspension front & rear. Things like metallic bushings eliminate you from the lower SS classes, but an engine swap got me in DSP anyway, so I ran with it.

The thing that gets you is engine/boost mods, they bump you to MOD pretty quickly.

[stewiefied]

You sort of forgot Mr. Rhoades in there He (imo) proved that just about any car can be competitive with the right person choosing parts.
I don't like the part of autocross where everyone drives the car that is doing best. I think just about any car enthusiast has an opinion on vehicles and they should not ignore that.
I would say s13 coupe would be best in dsp or more restricted classes, though. They are the lightest of all s chassis and probably close to the s14 in stiffness. The only other negative is how much tire you can stuff in the wells, which becomes less of a damper in SM or XP.

[djsilver]

Man, your DSP sounds much more restrictive than our DSP. Given that we have a lot less classes, and DSP is the last class before DMOD (highest class).
We actually run PAX so that everyone in any class is competing against each other (PAX SUCKS).

In DSP, any conceivable chassis/body bracing/reinforcement is allowed.

Our suspension mods basically look like this:
Any mod/removal/substitution/addition sway/strut bars - 1 pt each
Any other suspension mods - 3 pts total

This basically allows you to do all the suspension mods you want (except strut/sway bars) as soon as you touch your springs (dampers are free - subject to adjustment limits). Having coilovers gave me the freedom to upgrade my entire suspension front & rear. Things like metallic bushings eliminate you from the lower SS classes, but an engine swap got me in DSP anyway, so I ran with it.

The thing that gets you is engine/boost mods, they bump you to MOD pretty quickly.

Yep, your DSP sounds more like our Street Modified class. I can run STB's but no triangulated bracing. I can swap parts between models/years of 240sx's as long as they can be bolted on without modification but I can't mix/match engine components and I can't modify anything inside the long block, including the cams. I can run coilovers and aftermarket suspension arms (upper or lower but not both) but I can't use spherical's and I can't correct roll-centers with extended ball-joints or modify the knuckles. I can flare/trim the fenders and run big wheels and tires and an LSD but I can't change the diff ratio so that limits the effective sizes I can run. I can swap seats but there's a minimum weight of 15lbs and I have to keep the full interior so there's not a lot can be done to lighten the car.

[hai1206vn]

If you run autocross, I believe chassis stiffness is a minor factor. The Civics in ST are wet noodles but lightness and suspension make them king. So for a 240, pick the s13 coupe, even the single cam if you want the lightest model.

From what Nissan published for the '97-98 s14, they claimed 100% increase in bending rigidity, 50% in torsional over the previous generation - this is likely comparing it to the hatchback. (http://www.nicoclub.com/tech/240sx/97-240sx-specs.pdf) Of course nobody knows any comparison to the s13 coupe.

One possible reason the s14 is stiffer is the increased use of ribbed steel panels, like on the strut towers and under the fenders. Toward the rear, where the s13 trunk meets the rear seats and between the shock towers is essentially flat, compared to the s14's funny gas tank shape connecting the shock towers, which probably helps against torsion a bit. The s14 subframe and its braces are also stronger. There's a little brace at the end of the driveshaft tunnel, basically tying the 2 rear seats. Also, I might be wrong, but the member under the rear seats connecting the 2 b-pillars looks bigger.

[djsilver]

You sort of forgot Mr. Rhoades in there He (imo) proved that just about any car can be competitive with the right person choosing parts.
I don't like the part of autocross where everyone drives the car that is doing best. I think just about any car enthusiast has an opinion on vehicles and they should not ignore that.
I would say s13 coupe would be best in dsp or more restricted classes, though. They are the lightest of all s chassis and probably close to the s14 in stiffness. The only other negative is how much tire you can stuff in the wells, which becomes less of a damper in SM or XP.

Jason is a killer driver and he built a killer car that won street touring a few years ago but the class is way past that car now. He converted it to street modified but never finished developing it when he changed jobs and sold it. I lusted after it when he had if for sale. The guy that bought it put a cage in so he could do Time Attack and then (I think) lost his job and had to sell it. I often wonder what happened to it....,

[jmauld]

I can swap seats but there's a minimum weight of 15lbs and I have to keep the full interior so there's not a lot can be done to lighten the car.

I have to look through my notes, but I'm fairly sure that I was able to "legally" knock 150lbs off of my car in DSP, and that wasn't difficult to do.

[SM240]

Really cause civics dominated since 2004 iirc in st(formerly sts). I dont think many other chassis have done that and that much has changed.Although all I look a is SM nowadays so I could be missing alot. I did really well in sts in 03-04. STX in 05 but a e30 M3 always edged me out(top end aroung 70). I did my best in sts with my car, went to SM in 06 and have always played cachup to my previous pax spot average.