MikeC

Hey? They put out the most power at their power peak, not when you load them up. Drag cars are a different story and irrelevant to street/circuit cars in this case. They have less gears because they have so much power and compensate by slipping the clutch. An f1 car is probably a better comparison and they have 7 gears.

SPOautos

I dont under stand why someone would pay that kind of money for these gear ratios:
2.823
1.941
1.513
.958
.760

When they have this for stock:
3.48
2.015
1.39
1.00
.719

There are only 2 gears in there that would help acceleration over stock, 3rd and 5th. HOWEVER, the 5th gear in the stock trans can be lowered to be even lower than the "custom" set up for a 1/10 the custom price. At that point you'd have MORE acceleration on the hwy with the stocker. To me this leaves 3rd gear as the only benifit of customs and its not much benifit at that. Our 3rd gear is already very good....as is the rest of our gear ratios.

Where is the $5K-$7K worth of benifit??? MAYBE in reliablilty for a hard core or pro drag racer that doesnt care about how thier car drives in the street. But if your wanting to hwy race you can run prob 600rwhp thru the stocker with no problems.

STEPHEN

Kento

Today's F1 V-10s are normally-aspirated. Why do you think they need 7 gears? Because their torque peak is so comparatively narrow and high, due to the fact that the engine needs to spin upwards of 19,000 rpm in order to make its 800 or so horsepower.

Drag cars aren't irrelevant, because he said that he wanted his car to be "good in the quarter-mile", and he feels that having one extra gear will give him better acceleration with a forced-induction engine.

Think about your statement-- it's a bit contradictory. If drag cars have only one "gear" due to their large amounts of horsepower, then why don't F1 cars cut down on their number of gears and use some form of slipper clutch? They're not exactly hurting for more power, and shifting costs time on a road course, too. Or switch roles: if F1 cars (which would be considered by some to be the ultimate technology) use 7 gears to get maximum acceleration, then why don't the blown drag cars at least try some multiple ratios? But wait: ever hear of Pro Stock drag cars? Yes, they use a lot of gears to get down the strip in the least amount of time. What kind of motors do they use?

I didn't say that forced-induction motors don't have a power peak; I said they become more efficient with more load up to a certain point. I don't want to get into an overly long technical discussion of forced-induction motor characteristics, but you might try asking any of the top fuel or funny car engine builders about this.

In closing, this isn't a black & white deal; there are instances where having additional gear ratios closer together will help a forced-induction motor in some form of racing (think World Rally). But back to the original post concerning using a six-speed vs. the stock five-speed on an FD to get better acceleration in the quarter-mile: I seriously doubt that the $7K or more spent on having six speeds will pay the same dividends in acceleration that the same amount spent on other performance components would provide.

ForceFed

Kento, you bring up some good points. I'd like to clarify that im not looking to gain as much in the quartermile as i am on the highway. I remember awhile back when Steven Kan said: " If you have to go into 5th gear to beat a Supra on the highway, you're screwed" meaning that the fall in RPMs when doing 4-->5th gear is so dramatic the Supra will literally leave you standing still.

I'm hoping that with the 6 speed gearset i can help alleviate that with the closer spaced gears. I also believe the 6 speed box is fully capable of 200mph.

jimlab

True.

He won't be using more than the first 3 or 4 gears in the quarter mile, so a 6-speed would be of no benefit whatsoever. He'd be better off calculating the differential ratio needed to end up at or around redline in whatever gear he crosses the finish line in with the transmission and tire combination he already has. That's standard procedure for the strip.

You answered your own question above. They are extremely peaky engines, as all small displacement/high rpm naturally aspirated engines are, and even with a curb weight of 1,300 lbs. (or whatever), they don't have the low end torque to get the car rolling with only one gear.

With the exception of Top Fuel dragsters and funny cars, drag cars do have multiple ratios. Whether manually (or air-driven actuator) shifted or automatic, they have several forward gears. Some as few as 2 (Powerglide automatic), some as many as 4 or 5 (Lenco, Jerico, etc.).

As I mentioned above, given the gear ratios in the transmission used, the differential gear is selected which puts the car at or near redline as it passes through the traps in the highest gear. It is far easier and less expensive to change the differential ratio than it is to play with transmission ratios.

500 cubic inch naturally aspirated big blocks making about 1,300-1,350 horsepower, on average. They use 3 or 4-gear planetary gear transmissions (Lenco, Jerico, Liberty, etc.) and are manually shifted without releasing the clutch. Each gear has its own shift handle, and they're pushed forward or pulled back sequentially.

Which would be pointless. Turbocharged engines may rely on load to make more power, which is why turbo cars can have difficulty making full boost (the same boost they make on the street or track) on a dyno and make more boost in higher gears, but a crank-driven supercharger (like NHRA Top Fuel engines use) doesn't rely on load for power production, and neither does a naturally aspirated engine.

At least we agree on something...

David Beale

Woops, the MPV 5th gear replacement to lower ratio was not on Steve C's site, it was a message on the big list:
"Howard Coleman wrote this long ago:
As previously posted, one of the best mods for our cars is a shorter 5th gear. it adds 15.5% more revs and negates the HUGE drop with the stock 5th gear. the gears come out of the MPV and 929 5 speed. part number R504-17-308 and R504-17-610A."

Rhode_Dog

Jason from www.jt-imports.com ?

Chronos

If the stock gearbox can hold out to 600rwhp in road racing conditions, I don't think it's worth it for me to upgrade. So I suppose the only practibility of a 6speed would be for the freeway racing to hit higher top end speeds and have less of a power dip going into 5th....however, if you really are planning to go that fast, you better invest just as much into aero to make sure you don't kill yourself!

Chronos

Anyone know what the top speed of 5th becomes with this?

SPOautos

If you want to go 200mph you need to do a WHOLE lot more than change that trans. Personallly I think just lowering the 5th gear in the stocker is the way to go. I think it can still hit right around 175ish which should be WAY fast enough for hwy racing.

Cam at Pettit racing can tell you, I installs them.

STEPHEN

David Beale

Even with the extraordinary price and all the above, the 6 speed sounds enticing. I have searched and searched and have found very little info. on the kit that is installed in the FD box. Anyone have a URL? I was kinda hoping for better sycros. Maybe I should just put in a motorcycle xmission ;-).

ForceFed

I agree. I've been looking at the RE Amemiya underbody trays. Hopefully these will do their job and direct air under the car so that the car does not get any "lift" at high speeds. We all know what happens when air speed under something is greater than air speed OVER something. (RE 20B FD@ Salt flats anyone?)

I like the idea of the underbody trays as i would like to keep the exterior of my CYM stock appearing.

ForceFed

Stephen you are probably right in that i would never do 200mph, but it's nice to know you CAN do it should you want to. 175ish is ok, but i'm swinging for the fences here.
If you dont get it, just say to yourself " My car has the potential to go 200mph" and wait for the grin on your face LOL

In all seriousness though, i was originally thinking GT35/40 but now i think i want big dick-swinging power. A T88 or T51 KAI or SPL may be the way to go, or a pricey ball bearing unit from Garrett or some other place with comparable specs to the T88 or T51. Throw in water injection and i think you could squeeze 18-20psi and 500RWHP out of the 13B. Funnel that through a 6 speed tranny and you've got a recipe for fun and a lengthy prison sentence

Ah yes, just the rush i'm looking for...

Darril

jimlab

OK, let me try. "My car will have the potential to go 200 mph".



Car Test 2000 says 231 mph, but I'll never find out. I've pegged the old 180 mph speedometer once and I don't need to repeat the stunt. I've seen what happened to an FD that left the road at 150+ mph... even a cage, harnesses, and helmets wouldn't have saved them.

ForceFed

Perfect
Ouch. When you go fast, you have to understand that these things can happen and accept it. You have to be willing to take responsibility for your actions...

Kento

OK, well then you're not going to have a car that's "streetable". Yes, everyone has their differing opinions on what kind of car behavior can be labeled "streetable". But IMO, the big lag that will come with slapping on such a big turbo, the vicious hit when it comes on boost if you're not careful, plus the cantankerous manners the car will have at lower revs (and that's after spending days getting the fuel/ignition curves dialed in) puts it out of the streetable category. But you have the prerogative to do what you want.

Hey Jim, I was just referring to Darril's original premise, not the "standard procedure for the strip." But your statement about needing to "end up at or around redline in whatever gear he crosses the finish line..." is what I was getting at with my statements about forced-induction motors being "different animals" than normally-aspirated. Maybe at the lower boost levels a stock/near-stock FD runs at, running to redline is probably best. But at higher boost levels, not all forced-induction (especially turbo) motors react the same; many like it better (and get quicker ETs with more MPH) if you ride the torque curve just past its maximum, not wringing the motor out to redline in every gear, like you would with a built-to-the-nines normally-aspirated motor. I have seen it happen (and experienced it myself) with turbo dragbikes.
Oh yes, note that when I refer to "redline", I'm talking about the engine manufacturer's set engine rpm limit. I'm sure that when many forced-induction racers have a set "redline" on their car/bike, it's the power peak, not the engine's rpm limit.

I wanted MikeC to figure that out. I know that I had already answered the question....

With the current fuel and metallurgy technology available, normally-aspirated motors need revs to get the most power out of a given displacement; forced-induction engines just need more boost, which even with a turbo, doesn't necessarily require more rpm. This means the naturally-aspirated motors need more gears to keep the engine on the boil as rpms rise and the powerband narrows. But forced-induction motors don't need more gears as power rises, because they have the accompanying torque to get away with it.
This is what I meant when I asked the question "what about Pro Stock drag cars?" Although not needing a whole handful of gears, they do need comparatively more to get down the strip quickly (although a big displacement V-8's excellent torque negates a lot of this concept).

Here I may have been mistaken in believing a gas-supercharged motor may benefit from increased load; I could be wrong here. However, blown-nitro motors are definitely "load-sensitive"; refering to setting the car's progressive clutch engagement, the blown-nitro motor makes better power the more load from clutch/traction is applied. Please, ask any of the top fuel engine builders/car/bike pilots about this. I'm not saying that the supercharged motor relies on load for power production; I'm saying that load assists in making power. Oh, and I never said that naturally-aspirated engines need load to produce power.

But turbocharged engines definitely benefit from load to produce more power, which is the point I was trying to get across with the comparison between naturally-aspirated and forced-induction motors, and trying to "ease" the load by shortening the gearing through more gears.

JIm, I've got a lot respect for your knowledge, and I don't want to get into a tit-for-tat exchange. I just think there were some misunderstandings about what I was trying to get across, so I tried to clarify them here. OK, I've used enough bandwidth here.
I am now officially out of this thread. (good riddance, eh?)

jimlab

Fair enough... one correction, though...

I can't speak to bikes, but for cars, maximum acceleration is achieved by shifting at the point at which torque at the axles in the current gear falls below the maximum torque (torque peak) at the axles in the next gear higher. If that point doesn't occur until after redline, then you should hold the gear to redline. Hence my statement "at or near redline".

A stock FD peaks at about 235 lb-ft. of torque at 5,000 rpm. In first gear, maximum torque at the axles is ~2,850 lb-ft. (235 x 3.483 x 4.1 x 0.85). The engine is down to 137 lb-ft. of torque at 8,000 rpm, but torque at the axles is still ~1,660 lb-ft. (137 x 3.483 x 4.1 x 0.85), higher than it would be at maximum in second gear (235 x 2.015 x 4.1 x 0.85 = 1,650). Not by much, but you're still faster holding 1st gear to redline, assuming traction, than you are shifting. Every time you shift, you lose time. Each higher gear accelerates more slowly than the gear before.

Ideally, the drop between gears would also result in the engine falling only as far as its torque peak. This would result in maximum acceleration through all gears.

Freaky Monkey007

Im sure i dont fully understand this.. but wouldnt any forced induction engine make more power with a higher load b/c that is how they make power? Or am i mistaking load for rpms??

On the 6th gear thing.. Its pretty apparent that money is not a real concern here. He wants a 6 speed he is gona get a 6 speed. I also dont see y you would spend 7g's on this b/c its just not worth it but that is just me..

On a side note, wut about a rx8 trans or a adaption plate for another trans? Just a susgestion, you could get a gm trans much cheaper but im not sure of everything involved in the swap so in the long run it could be more expensive.

MikeC

Drag cars go for around 5 seconds, after which the clutch is red hot. An f1 race is 2 hours (i think), it just wouldn't work. F1 cars need to conserve fuel, drag cars don't. F1 cars don't really have that much power compared to a drag car (800 hp compared to 5000). F1 is probably not 100% relevant but it's far more relevant than a top fueler.


What do you mean by "more efficient"? Do you mean it uses less fuel? Sure, but what relevance does that have. There is no disputing that a car will accelerate best when it's engine is kept near its peak power.

For a drag car I think the best way to choose first gear would be to determine the highest possible ratio at which the car can still break taction. Say that first gear ratio will get you to 80kph then you slide the clutch up to 80k's. What the top fuelers have done is gone "hey, that ratio will get us all the way to the end of the 400m so we only need one gear". Unless you can get 5000hp from your rx7 then you are going to need more than one gear.

Anyway, I do agree with you that a six speed box won't get you much or any gain over the 400 metres and I also agree that the money would be better spend elsewhere. I think a six speed box is more for circuit racing.

SpiritR

I like the idea of a 6-speed. I mean you can do a lot with a final gear change, but then you lose your top speed. This was with a 6-speed, you can do drag, track or just super high speed.

rx7tt95

Just from my personal experiences with a fairly big single turbo (modified T78 w/bigger compressor), I'm in need of another gear or rather different ratios. I often find the spacing of 2nd through 5th to be less than ideal with my current setup. A taller 6th would just be nicer from a cruising/hwy point of view and allow 5th to be more performance oriented.

ForceFed

And there you have it folks. Bottom line is, we can throw formulas, mathmatics and gear ratios around all we want, but what really counts IMO is genuine driving impressions and real world experience/test data. I truly believe that a T88 or T51 matched with the Trust gearset would be a good combination.

Darril

MikeC

That idea was tossed around in another thread. It seems that the gearbox will bolt up to the engine but the clutch is very different and the gearlever will come out in the wrong spot. The rx8 box is also meant to be weaker and 'only' do about 400hp.

If you use the standard 5 speed or a 6 speed gearset into the standard box then it would probably be the best way to go because it would require the least modifications to the cross member etc and all the standard parts would presumable still fit.

Chronos

I'm gonna have to agree with you there, the high speed capability and better cruising ability would definately be nice if nothing else. However, I do think a 6speed would aid you somewhat in road racing as you'd be more able to stay in a proper rpm around the track, instead of falling below 4500 with the 5 speed.

Actually, I plan to run 550rwhp with a GT35/40 on my pineapple ported engine using water injection and race gas at the track (road race) and about 450rwhp max on the street with pump gas. A T51 Kai or equivalent sized turbo would probably be too much to control on a road racing course with it's spool pattern and also be very aggrevating to drive on the street. I consider those turbos to be primarily for drag applications. Also, do you realize what the real advantage of water injection is?

Chronos

Jim, how does your program arrive at these figures? I'm sure you know that at high speeds aerodynamics are the cheif factor governing how fast a car will go. How can you rely on your program's results when it can't calculate these variables?

Why am I harassing your with this? There's only one way to find out how fast that monster of yours can go...don't wimp out on us!