if you were going to build a FC with NA 250 HP
can you use a NA car
or should you start with a turbo II car ??
I cant find a cheep turbo II clean car

 

I think you would be fine to start with a clean n/a car. From what I've read you'd probabply want to ditch the 6-port engine in it in favor of a 4-port. If your pockets are deep enough you could go with a 13B-RE like this guy. Of course you're probably going to need deep pockets to ever hit 250whp n/a anyway.

https://www.rx7club.com/naturally-as...-gtus-1032737/

He has dyno'd at 230hp to the wheels with his setup. You'd have to read the whole thread but I think it is streetported with a Haltec ecu, Custom exhaust. His header primaries lengths I'd say are tuned for that engine as well.

Anyway you have to do so much to get there it won't make that big a difference which car you start with though there are some advantages to starting with a TII. 4-port motor from the start and LSD. Not all n/a's have LSD so do your homework and make sure you get one that has one. Preferablly the clutch type from the 86-88 cars, though it may need rebuilt by this point.

 

thank you,
but will the NA trans and diff hold up with 250HP
will the turbo II be a little better or a lot better?

 

Someone else will have to answer that as I am not sure. Probably would depend on how you treat it.

 

The turbo transmission is a lot better,+ more aftermarket and used clutch/flywheels for it. And the turbo engine is a better base to use in an engine build, then get a set of s5 Na rotors, good headers, itb intake and get a good bridgeport. If you use good quality parts and get everything balanced good, you shoul be able to get 250 hp at the flywheel fairly easy but NOT cheap.

 

I would definitely upgrade the Trans to a TII.It is cheap and easy to get.
The Plus side is if you ever go to an actual TII that it is just plug and play.No worries about breaking anything.

 

the NA diff should be fine @250hp unless you're going drag racing, then you want the T2.

the NA trans will work for a while behind 250hp, but its not really meant to handle that kind of power.

the T2 trans is stronger, but the gearing is terrible, it takes like 80hp to spin, and if you're road racing, its just as fragile as the NA trans.

putting the miata gear set into the FC case is a good option, its slightly stronger, but the gearing is better.

the Rx8 trans is an option as well, it needs the driver to be gentle, but the gearing is the same as the competition trans (with a granny 1st), and it can handle the power

 

The main issue I have with transmissions at 230hp is the synchronizers. The synchros hate life when shifting at high RPM and this fills the oil with junk and this eats the bearings and gears.

Turbo transmissions have more mass and are harder to shift at high RPM than non-turbo.

If you want to make 250 nonturbo horsepower, you're going to need RPM.

 

exactly. my buddy has a T2 trans behind a P port, and the engine shreds those transmissions like things that shred stuff.

it spits out a trans per event..

 

^yep

the 2 things I really every see transmissions rated for is torque capacity or RPM... horse power is just a side effect. Ive come across many "500hp" tranny's that destroy themselves at 9k RPM regardless of power levels

 

Hmm I bought a used T2 trans years ago for another Na build. Based on this info, it doesn't look like it's gonna work for me. Would you know the rpm shift threshold or the limit for safe trouble free hi rpm shifting without causing damage?

 

it depends on how old /tired it is to start with. it also depends on the driver technique, i'm sure a careful driver would have no trouble up to like 9k, but add some ham fisted driver, and i'm sure they can break it @the stock redline

 

Yeah it's not really a threshold thing, there are a lot of other factors, mainly driver technique and clutch disk weight.

I wouldn't want to run a 240mm disk. 225 better. 215 better yet. Best is to suck it up and get a 5.5 or 7.5 multiplate clutch.

I did a good one at the last event. Folded the 1-2 shift collar hard. What happened was I went for 3rd gear and got 1st instead, and immediately shoved it back into 2nd while the input shaft was spinning at probably 11-12k RPM. That hurt things. Then the next run, I was accelerating out of a corner and on the 1-2 shift, the synchro failed completely and I ground the gear HARD. Folded the splines in the collar over. No more 2nd gear and because the folded splines won't allow the collar to move back, no more 1st either. Finished the run by clutch-kicking it in 3rd. Then had an interesting drive home.

 

I have had the same NA S4 box in my car for about 4 years. What I "FINALLY" learned to do to make it live behind a 220 hp motor was all of the above. I treat the box like it was made of glass. I never drop the clutch, power shift, bang shift, whatever. As I am revving my car to 8500 rpm, I always slow the shifts down a hair to give the synchros a chance to catch up and I am gentle on the downshifts. I use the best quality gear lube that I can (Redline 75-90 NS) and change it often.

Even so, after about 10 minutes of hard loading I can smell the gearlube in the car, so this year it is a trans cooler, filter and pump setup to try to keep it a lot cooler. A buddy of mine spoke with Roger Mandeville and Roger told him that the greatest killer of gearboxes in an FC was heat and not from the loads. It was radiating heat from the exhaust and motor that heats up the boxes. He suggested heatshielding and tranny coolers on all cars used for more than 5 laps at a time. I believe that as I have seen my gearbox get up to well over 250 degrees after a 1 hour race without a cooler.

Regarding using a TII box on an NA car, I have to agree that the more robust gear setup is structureally better, but the weights work against you when it comes time to shift. No gearbox that uses synchros likes to shift at the 9,000 rpm level.

I also have a super lightweight flywheel, clutch and pressure plate setup on my car. All of these help to keep an NA box alive.

Eric

 

the racers around here like to teach shifting with 2 fingers, and it looks a bit odd when they show you, but it does seem to work.

 

LOL, thats exactly how I shift the car.

Eric

 

Mike, do they mutter, "by the grace of God, and these two fingers..." as the engine sings to the angels?

 

Info is starting to get really good.

 

You ever play with an Rx8 tranny? They are slightly stronger but not as strong as the turbo versions. The turbo II tranny I have may be perfectly fine because I engineer my set-ups for street level power and rpm. So 8,500 redline helps keeps cost down and at a manageable level.

 

What do you do to make it safe for the engine to rew to 8500rpm, just balancing?

 

lol, let me put it this way, if a girl demonstrated it, it could be construed as sexy. so its a little creepy when its a man

it does seem to work though.

 

Petersen, Factory 13b is balanced to around that level so additional balancing isn't needed for street and the occasional hi rpm shifts. Now I would consider balancing if your going to be doing sustained hi rpm runs. My car is a street car so my soft rev limit starts at 8k and limits at 8,500. Plus I'm nearing the oil flow limits of my factory oil pump. More power under the curve is more important to me than building some hi rpm monster that's virtually useless on the street.

 

and rotor clearancing.

 

8500 is within the realm of stock, especially with 2mm seals.

I feel no problem taking my unbalanced S4 rotating assembly (S3 non-race stat gears, too) to 9500-9800 *momentarily* on a regular basis. It's just clearanced and has more oil pressure.

Now. I'm not pulling huge RPM at the end of a long straight, just quick zips up to peak RPM and back. If I was doing that, I'd try to keep the thing under 9000 as an absolute max.

 

In mine, I am using a S5 rotating assembly in an S4 TII keg with 2mm seals. With my induction system, it will pull hard to well over 10,000 rpm, but I limit the rpms to 8500 rpm, but I do squirt it to 9,000 a few times a race to cover for the funky gear splits when I am chasing someone or defending from someone. The compression is still good after 5 years of use and it stills pulls like a mother.

One race a few years ago I checked the telltale after another driver ran the car and it read 10,200 rpm!!!! I was scared that it would grenade right away the next time I drove it, but it has gone for another 3 years, so I guess a burst to 10,000 rpm isn't bad once.

Regarding the RX8 box, it is very interesting, its just a balancing of the cost to do it versus the speed I would gain.

Eric