gears and rears

[diegokid]

The rear gears I had orginally wanted for the TKO600 aren't avaible. I'm having a FE 406 built and my question is will the TKO600 with .64OD be too high for a 3.42 rear or will about 1800 to 2000 rpm be too slow for this motor run well.

[RodKnock]

I believe the prevailing opinion is that the 0.82 5th gear ratio is the best for 3.42 rear gear. Stroked FE's with their torque might be able to tolerate low RPM cruising, but I would not recommend it for a SBF.

[Excaliber]

The cruise rpm range where your motor is 'happy' is strictly a function of the camshaft profile. Rear gear ratio, trans final drive ratio and cam profile should all be considered as a package deal.

[SSSammy]

I have the 3.42 with the .82 TKO 600. Verrry nice I have a fairly aggressive cam, so it does NOT like to run below 2000 rpm. I cruise at 65 mph at about 2200 in 5th, I think. The only reason to go with the .64 would be if you have a really mild cam, or if you intend to cruise mostly at 70 plus in 5th. I just wouldn't do it. There is really no upside unless you are determined to pinch your gas pennies.

Now you will hear from someone with a .64, but like Ernie says, it's all about the cam profile.

Sam

[xlr8or]

I have the .64 in mine with a 3.31 rear and it's fine. My engine doesn't mind light cruise at 2000-2200. Luckily my normal cruise on the freeway is 75-80 when using 5th so it sits about 2100 RPM. Doing it over I would go with the .82

[Mister]

I also have the .64 with a 3.42 rear. If I had it to do again, I would go with the .82. Too much cam, too low speed limits, too many radar traps.

Where, at what rpm, will your engine be happy? And what speed does that work out to be with a .64? with a .82?

Mike

[Excaliber]

The probem is GETTING a .8 to begin with, there hard to come by and often much more expensive than a standard issue .6 ratio. Not all trans will even have a .8 as an option, forcing you into a more expensive model. And well worth the price in my opinion. There is NOTHING more frustrating than trying to cruise and be happy and at the same time watching the tach like a hawk, thinking about down shifting, worrying about speeding to keep the rpm within range. I tell you, it will wear you out and you will never be happy if you don't get it right, right now when you can.

[diegokid]

I will try to get the cam profile from SA today and post it later, Its the standard 406 FE. The wife and I plan on driving this car quite a bit so we thought the power and torque would be fine. I think a .75 or so would be the best but Tremec hasn't seen the light.

[Cobrabill]

Originally Posted by xlr8or

I have the .64 in mine with a 3.31 rear and it's fine. My engine doesn't mind light cruise at 2000-2200. Luckily my normal cruise on the freeway is 75-80 when using 5th so it sits about 2100 RPM. Doing it over I would go with the .82

Your engine DOES mind it-you just don't know it.At 2100 rpm your advance isn't all the way in.Pay attention to what Ernie said.

[terry251]

My 418 has a semi-mild cam, and a 3.73 rear ratio. I went with the .64 thinking that it would be much more comfortable on the highway. It is great at 80 mph and up, but that turns out to be a very small percent of my driving. I essentially lost one gear from the 5 speed box in my normal driving. After getting used to the car, I find it much more enjoyable on secondary highways than on the interstate. One other observation - any acceleration below about 2200 rpm brings on scary levels of detonation; another reason I'm switching over to the .82.

Terry

[Excaliber]

I had not considered the timing advance issue with the low engine rpm that will occur with a .6 ratio, and that brings up an interesting point. The primary reason for the ultra low rpm was originally to increase mpg and factory motors were designed to work well at that low rpm. My jeep, automatic trans, lockup torque converter, cruises along nicely at 1800-2000 rpm, heck it loves ultra low rpm. But it's fuel injected, computer controlled and has a low rpm cam profile. You can also bet the timing advance curve has been optomized for this ultra low cruise rpm.

Consider the older 302 powered Mustang GT's. The stock motors (about 225 horse) were low rpm engines! They would hardly make power above 5000 rpm (if that high). They typically came with a .6 overdrive 5 speed as well.

Timing advance curve plays a HUGE roll in mpg.

We typically build for decent horse power and that requires a cam profile that will work in the high 5000 or low 6000 rpm range. Compare that to my Jeep which runs out of power in the high 4000 rpm range!

Along with the advance curve question at low cruise rpm you also have to consider fuel/air ratio, I bet the engine is running way rich under these conditions and combustion is not going to be effecient for many of our motors. Perhaps a vacuum advance unit would be of help in this scenario (low rpm cruise). Perhaps a lower compression ratio and a leaner fuel curve would also help. Then get a really 'torque monster' ultra low rpm cam profile, like a truck, Jeep or motorhome would use. Something that makes max power at 4500 rpm.

Wait,,, are we building a truck or a Cobra? I lost track somewhere along the line.

And DON'T pay attention to Ernie, I tend to ramble and could be out to lunch on any given post.

[RodKnock]

Originally Posted by Excaliber

The probem is GETTING a .8 to begin with, there hard to come by and often much more expensive than a standard issue .6 ratio.

About 2-3 months ago, I called Jimi G at Standard Transmission and he had them (0.82) in stock at that time. I got mine delivered within a week or so.

[jwd]

Originally Posted by Excaliber

Perhaps a vacuum advance unit would be of help in this scenario (low rpm cruise).

A vacuum advance unit (connected to a CONSTANT vacuum supply) is mandatory for optimum street performance. It will not only cause your engine to run cooler, you'll get better throttle response and MPG. There are plenty of articles written on the subject. Those that disagree, don't understand basic engine tuning and vacuum advance operation.

Jim

[STEVE POTTS]

Hydraulic roller 486FE, makes close to 600 horsepower, 600 lb-ft of torque. The motor cruises very nicely on the interstate at 2000 rpm. Its perfect for my needs as I like to take 2 and 3 hour road trips w/ the cobra.

My rear ratio is 3.46 and the tires are 15". You can see my cam card/profile and dyno results in my profile

S

[Cobrabill]

Originally Posted by STEVE POTTS

Hydraulic roller 486FE, makes close to 600 horsepawer, 600 lb-ft of torque. The motor cruises very nicely on the interstate at 2000 rpm. Its perfect for my needs as I like to take 2 and 3 hour road trips w/ the cobra.

My rear ratio is 3.46 and the tires are 15". You can see my cam card/profile and dyno results in my profile

S

If you reach cruise RPM BEFORE your advance is all the way in-it is far from perfect.

[Excaliber]

Steve, thats not a bad example of a good low rpm torque motor. Looks like max torque is around 4000 rpm, the lowest rpm noted on the chart. Max horse power is under 6000 rpm. I built my motor for a lower rpm and better torque but have never dynoed it. It too pulls nicely from about 2000 rpm, but it likes 2500 better. My max rpm is now around 6200, down from 7000 +! I also run a vacuum advance now, by the way. I get 10 mpg or better when I cruise it at about 65 mph and 2800 rpm, thats UP from the 6 mpg I got with the old 'monster' motor. I suspect cruising at 2000 rpm with a .6 OD would not result in better mpg or a happy motor in my case, but I could be wrong.

The hyd roller is a great setup for a lower rpm engine, I'm running a solid flat tappet myself.

Top loader, 3:31 rear gear, dual fours.

[xracerbob]

You can always go with a G-Force t-5 and the .75 OD.

[xlr8or]

Originally Posted by Cobrabill

Your engine DOES mind it- you just don't know it. At 2100 rpm your advance isn't all the way in.Pay attention to what Ernie said.

Actually I do so don't assume.

Under light cruise the advance is not as muchof an issue. I have an initial advance of about 14 and by 2000 I'm up to about 30 with it all in by 2400.

[Cobrabill]

Originally Posted by xlr8or

Actually I do so don't assume.

Under light cruise the advance is not as muchof an issue. I have an initial advance of about 14 and by 2000 I'm up to about 30 with it all in by 2400.

I am not assuming anything.I KNOW.Because if you did know,you wouldn't have your cruise rpm below full advance.

And you are completely wrong with your"light cruise" comment also.You are either at cruise or you are not.

[xlr8or]

Sorry I forgot you were the all knowing authority.