Problem with high-RPM performance on my '94. Help?
 

I've only put about 200 miles on it myself but it has 84k total. Here are the mods done by the pervious owner: underdrive pulley set, Apex'i cat-back, GReddy blow-off valve, GReddy turbo timer, Apex'i fully adjustable suspension, R1 strut tower brace, and a GReddy boost gauge. My problem now is that after 4750 RPM the engine goes bonkers. In fifth it takes about a quarter of a second to hit redline, which means I top out at 100 mph. The power hit is MASSIVE. Any ideas?

Oh, and while I have your attention, I have a few more questions. What do you think should be my next round of mods? I'm thinking braided steel brake lines, heat wrap for the exhausts and turbos and whatnot, Pulstar Pulse plugs and wires because they're just so trick, and a voltage stabilizer / conditioner / grounding solution to complement my expensive sparkers. And maybe a new high-flow rad. That'll probably have to wait for the next round of purchases (bushings, mounts, fasteners, hoses, and maybe an engine torque damper). Your opinion? I'm learning more toward doing the rad / heat wraps and brake lines first and waiting until I NEED new plugs. Oh, and do any of you know a good shop for fabricating a eight-point SCCA and NASA certified cage and doing a full interior strip?

Thanks in advance...

Engine goes bonkers?? How can you expect anyone to help you with that stupid description.

Well, I said that in fifth it takes about a quarter of a second to hit redline from 4750 and that I'm topping out at 100.

Wow, your all over the place with this. Clutch is slipping it sounds like from the description of the 5th gear. as far as your mods goes I highly suggest you stop posting and start reading...for the next few months...Start with the FAQ, there is a ton of good info in there. Dont start modding the car until you understand a good plan of attack. What are you going to be going with the car?

Those spark plugs are not worth it IMO, and I for sure wouldnt be purchasing them before any of the reliability mods if your car does not have them yet. Still got the stock AST? Stock down pipe? The radiator would be a good thing to pick up, check out the classified section of the forum for a used koyo or fluidyne or something similar. WTF is a voltage stabilizer / conditioner / grounding solution? What ever it is you dont need it. Especially with the limited mods you have. Only need to do the brake lines if you think the pedal is a little soft. The stainless lines will firm up the pedal some though.

LOL. I concur with Cgotto6, read, read, read.

Spend money on supporting reliability mods first. Radiator. DP, heat wrap all good. If your clutch is slipping that would be a priority since nothing else matters if your clutch goes.

Those plugs aren't necessary, you can make a nice ground wires for chassis and engine yourself, torque dampner not a huge deal either.

Yea, forgot to add that pretty much everybody just runs stock plugs with no problems, or if your above 300-325whp people move to running 9 heat range (think its the stock trailing) in all four plugs. definitly no need for an engine torque brace unless you are making crazy huge power (above 400whp or so). Usally the better option over the torque brace is just a set of quality motor mounts.

Sounds like your clutch is slipping but since you said you feel loads of power, maybe you have a 4.7 crown and pinion. Make a video?

thewird

The clutch slipped on my old MR2 and it felt different. This just feels like like it runs through the whole rev range in the blink of an eye.

The engine could not do that if it's coupled to a load. You have a clutch slipping.

Make a video.

thewird

I agree it sounds like clutch slipping. But you say there is a big increase in power? "power hit" could go either way so please clarify. And don't let the increase in RPMs fool you into thinking there is more power there. Does the car actually accelerate faster or just increase in RPMS? 5th gear is a dog on these cars. Can you slowly increase past 100 mph without giving it as much throttle? If it had different gearing to lower your top end and increase acceleration the car would do it in all gears and at all rpms not just 4000 +.

Could be when the secondary turbo hits and with some really short gears. I'm not sure how much of an effect 4.7 rear end and JDM 5th gear would have on it. Might also not be OEM gearing.

thewird

Think about the math for a second - those gears would have to be insanely (impossibly) short for 100mpg in 5th. Like 9:1 short, and they don't make them in that ratio. Also remember that the speedo is on the tranny side so if it's reasonably accurate (how did the pster know he's going 100mph?) then it's not the rear end.

Your clutch is slipping.

Even with a 4.7 and the jdm 5th there is no way he wouldnt be over 100 at that point. Its def a slipping clutch. Video would be nice though.

I should make a correction, 3.9 or 3.7 rear end. 4.7 is for higher speeds lol.

thewird

4.7 would have a lower top end then 3.9. It would accelerate at a faster rate through each gear. Rpms would be higher for a given speed.

Now I'm really confused. I thought 4.7 would give a higher top speed?

thewird

No sir. Higher number is shorter gearing (gear is physically shorter)= Lower top speed gets there quicker
Lower number is taller gearing = higher top speed but gets there slower

Shift points also have to be considered too, sometimes people run slower times with shorter gearing because it's not optimizing the power band, and/or an extra shift is required.

Try this calculator out. You can enter different gearing and tire combination to see the speed at each RPM point. Stock tire size is 25.5 4th gear is 1:1, I don't know what 5th is but I know it can be found easily. Stock rear end is 4.1 for manual, 3.9 for auto and 4.3 for some jap versions, the 4.7 is the shortest rear end gearing I've heard of for the FD.

http://www.wallaceracing.com/gear-speed.php

Found this.

Gear Ratios:
...............Manual.........Automatic
1st gear....3.483..............3.027
2nd gear....2.015.............1.619
3rd gear.....1.391.............1.000
4th gear.....1.000.............0.694
5th gear.....0.719 ----
Rear end.....4.100.............3.909

It doesn't list the JDM 5th gear ratio which is shorter (as an option I think), or the 4.3 rear end that was optional on some JDM models.

Wow, I had it reversed all this time... Thank you.

So back to the original posters response. Maybe he has 4.7 rear end with jdm 5th gear

thewird

nah it would still go faster then 100mph, and if really was geared super short, all the other gears would zip by as well not just 5th. It would zip through the whole rpm band too. It all points to a slipping clutch. 5th gear loads it the most so it slips in 5th and no others.

Well, the OP didn't specifically say it only happens in 5th. Whats throwing me off is the fact that he says his power goes bonkers.

If you let off the throttle after you hit redline, does your RPM drop?

thewird

I think the op is confused by the rpms sky rocketing, leading him to believe he has a sudden rush of power. Even with a 4.7 rear end and a jdm 5th which is .806 his top speed at 8 k would be 160 mph. He would need a rear end ratio of 7.5 to top out at 100 mph with jdm 5th. Or be sitting on tires that are 9 1/2 inches shorter then stock with jdm 5th and 4.7 rear.

Ha, gotta love the sled fd look on 13 in wires.

Seriously though, it has to be a slipping clutch, and why the hell has the op not said anything about this? He asked for helped then ran.

Fix your clutch.

It happens in all gears, not just fifth. It's definitely a slipping clutch, I just needed to drive it late at night again so I could run it through the gears without undue police attention. Sorry it took so long for me to reply. The reason I couldn't diagnose this myself is because my old car was a '87 MR2 with a worse-than-stock engine, so when the clutch slipped it almost completely halted forward momentum. I felt like the engine was still hooked-up in the RX7 because I was feeling the kick from the tubos spooling up right before the slip. Not used to this much power, really. Anyway, whenever something breaks I figure I'll upgrade it instead of going OEM replacements. Can anyone recommend an up-rated clutch?

Also, I took your advice re: first mods. I decided I'm going to do steel lines, wraps, hoses, a short-shifter kit (since I decided the throw is a bit long for my tastes), and then a rad and some rad diversion plates. In that order. I'd do the rad first but it's the most expensive.

I seem to notice vast price disparities among the short shifter kits out there. Why is this, and how much should I spend? Sorry for the bombardment of questions in one topic, but I figure I should try to contain the noob-storm to just one topic...