Miata tubular subframe question

[Grant]

I've got a F20C (2.0L S2000 motor) powered NA (1990) Miata I'd like to run in TTB (weight is ~2340, power is ~210whp). Unfortunately due to the motor swap the steering geometry on the car has some issues. To fix it I'm having to get an aftermarket tubular subframe, raise the motor (and steering rack) up and install bump-steer adjustable tie-rods.

 

Unfortunately for the car's TT class, all the tubular subframes available also relocate the control arm mounting points, supposedly to NB ('99 to '05) specs. I'm told both the upper and lower arms are moved up the same amount, about 4cm or so.

 

Looking at the TT rules,

   Relocation of front suspension mounting points +6
...
13) Replaced or modified K-members that change the location of the lower control arms +8
14) Tubular K(cross)-members that do not change the location of the lower control arms +2
15) Bump steer kits or shimming of the steering rack +2

I'm not sure how many points this would cost me? #8 and #13 seem to overlap, but #13 doesn't cover moving the upper mounts (but then who would want to move the lower mounts and not the uppers?). Right now I'm guessing this modification would cost me 10 points (#13 + #15), which is a steep price to pay for something that might net me a few tenths if I'm lucky, but it'll probably be necissary for me to run the car at all.

 

Just wanted to confirm before I move ahead with the project. It might be possible to modify the OE subframe if I have to.

 

Thanks for any help.

[Meat]

Why not just have it reclassified by hp/weight ratio and not worry about taking any points for the subframe?

[Grant]

I guess I'm not clear on exactly how the power/weight classification works. I've read how it it applies to TTS/U/R, but there isn't much on how it works with the other classes. I need to have my car classed by power/weight (I haven't been able to scale or dyno it unfortunately, but I know it'll fall into TTB's weight/power range) because it is an engine swap. However, I thought that just determined its base class, so any additional mods would use the points system, possibly pushing it into different classes?

[Joshua]

Yes, a motor swap requires you to be re-classed by Greg, unless already mentioned in the rules.

 

My car is now classed by Weight:Power, and it means that you do not have to take points for anything related to engine performance (e.g. intake, exhaust, ECU, etc.), but you do have to take points for other items like suspension, steering, tires, etc. I would think that changes to steering geometry would still cost points.

 

A friend of mine did an F20C install in his Miata a few years ago, and he finally gave up on it mainly due to problems associated with steering rack flexibility and geometry issues. These concerns were related to front sub-frame design limitations. The front sump on the Honda motor really gets in the way of good rack placement.

[kbrew8991]

unfortunatley the motorswap relcass only takes care of power & weight modifications, not suspension/chassis/etc stuff - still need to take points there

 

I wish I knew for sure which one of those two combinations of points fits your situation, but when in doubt take more - I find it easier to add modifications later rather than try and pick something to take off the car. YMMV

[Grant]

A friend of mine did an F20C install in his Miata a few years ago, and he finally gave up on it mainly due to problems associated with steering rack flexibility and geometry issues. These concerns were related to front sub-frame design limitations. The front sump on the Honda motor really gets in the way of good rack placement.

Fyrestrike? I've been chatting with him on S2KI. The rack definitely needs to be moved forward, which of course affects Ackermann geometry. I think you can get all the bump-steer out if you move the rack up as high as possible and use adjusters on the outer tie rods. The Ackermann will still be off, but I don't think that is all that important. The car is still easy to push around in the pits at least.

 

Ken, I'm relatively certain you'd be much better off points-wise to modify the stock subframe in order to keep the OE attachment points and 8 TT points (assuming my assessment of the points cost is accurate?). To be honest the penalty for a subframe with relocated points seems very steep to me, but then I'm not familiar with how much that sort of thing benefits other cars.

 

The rules in general don't seem to favor cheaper cars with engine swaps, because we get classed by weight/power like everyone else, but then have to pay for things other cars may have as OE (like LSDs, proper steering geometry, etc).

[JamesL]

The rules in general don't seem to favor cheaper cars with engine swaps, because we get classed by weight/power like everyone else, but then have to pay for things other cars may have as OE (like LSDs, proper steering geometry, etc).

 

I agree.

 

I have an 02 Civic with an engine swap from an Acura RSX. The 02-05 Civics share basically everything(except engine and drive train) with the RSX. They have identical suspension geomety/layout.. ie: all the off the shelf suspension components from the RSX(coilovers, strut bars, swaybars) bolt up to 02+ Civics. The width and wheelbase are almost identical. The Civic does weigh ~100lbs less, though.

 

The RSX is baseclassed in TTD with the 6 speed tranny stock. I am baseclassed TTD*(due to the weight difference, which I see as completely fair), but have to take 3 points for upgraded # of fwd gears.

 

It seems no consideration was given to other differences between the chassis... such as the Civics miniscule front brakes and rear drums, while the RSX has ~11.8" front/10.3" rear discs.

 

Regardless, I like my car and still plan on competing with it... but there are "little things" like this that put engine swapped cars at a disadvantage. I guess the easiest solution is sticking with a common car that can be competitive out of the box. It is very difficult to make rules that are equally fair for every chassis and type of car.

[kbrew8991]

and here I was thinking some motorswap/motorswap rule cars had a better advantage than us "normal" cars - often the reclasses seem to come out ahead points-wise vs if you modified the "normal" car to achieve the same power & weight levels to me.

 

maybe if each thinks the other side has the advantage its probably balanced??

[Grant]

I didn't think engine swap cars had to take points for engine/driveline stuff? So the forward gears thing would be free? Greg hasn't gotten back to me on my classing yet though, so I'm not entirely sure how this process works.

 

It might make sense to give some classes 'free' mods? So if you increase your car's power and go from TTD to TTB you get a free LSD, since all the stock cars in TTB come with LSDs and have the same power/weight ratio as you.

 

It seems no consideration was given to other differences between the chassis... such as the Civics miniscule front brakes and rear drums, while the RSX has ~11.8" front/10.3" rear discs.

I'm not sure why upgraded brake calipers cost points anyways? I can't see brakes improving fastest lap times on any modern car? Its just nice for them to work well for more than a few laps at a time, not crack rotors after just a few sessions, etc.

[JamesL]

and here I was thinking some motorswap/motorswap rule cars had a better advantage than us "normal" cars - often the reclasses seem to come out ahead points-wise vs if you modified the "normal" car to achieve the same power & weight levels to me.

 

maybe if each thinks the other side has the advantage its probably balanced??

 

Perhaps. And Im sure there are some cars which have an advantage when they do an engine swap. But there clearly are some that do not.

 

Like I said, Im sure it is very difficult to make the rules equally fair across the board. I think NASA does a pretty damn good job as it is.

[JamesL]

I didn't think engine swap cars had to take points for engine/driveline stuff? So the forward gears thing would be free? Greg hasn't gotten back to me on my classing yet though, so I'm not entirely sure how this process works.

 

It might make sense to give some classes 'free' mods? So if you increase your car's power and go from TTD to TTB you get a free LSD, since all the stock cars in TTB come with LSDs and have the same power/weight ratio as you.

 

Not according to Greg. Any changes vs the stock configuration of the drivetrain must be assessed points... meaning tranny differences.

 

Im guessing this means you will have to take an additional 3 pts for the S2K's 6 speed?

[Grant]

Im guessing this means you will have to take an additional 3 pts for the S2K's 6 speed?

Yup. The car won't be competitive anyways (up against C5 Z06s on 295 A6s ), so I suppose it doesn't really matter. The F20C is also hit because of its peaky power band (since weight/power is based off of peak horsepower).

 

I'm not really sure where an engine swapped car would have an advantage? In any case it makes sense for the rules to err on the side of penalizing cars with a lot of work done to them, because the series will get more participation if it encourages vehicles that don't need a ton of work to be competitive.

[kbrew8991]

(broken record) often motorswap cars come out ahead power-wise vs what it would take points-wise to modify the original engine to hit that same power level - don't forget about the "give-back" you get as your base class goes up from the "base" tire width changing (/broken record)

[Shawn M.]

(broken record) often motorswap cars come out ahead power-wise vs what it would take points-wise to modify the original engine to hit that same power level - don't forget about the "give-back" you get as your base class goes up from the "base" tire width changing (/broken record)

 

 

Ken, you sound like a broken recor.. a broken recor.. a broken recor.. a broken recor..

[Grant]

(broken record) often motorswap cars come out ahead power-wise vs what it would take points-wise to modify the original engine to hit that same power level - don't forget about the "give-back" you get as your base class goes up from the "base" tire width changing (/broken record)

Well thats the case for all cars classified by power/weight, isn't it? Which I thought everyone was moving towards anyways?

[JamesL]

(broken record) often motorswap cars come out ahead power-wise vs what it would take points-wise to modify the original engine to hit that same power level - don't forget about the "give-back" you get as your base class goes up from the "base" tire width changing (/broken record)

Well thats the case for all cars classified by power/weight, isn't it? Which I thought everyone was moving towards anyways?

 

Exactly. Those same non-engine swap cars could come out evenly power-wise if they decided to be reclassed based on dyno readings.