Hello,
Has anyone installed the T700R4 transmission in their HE, and if so what is your fuel mileage. Especially cruising 70 ish with the cruise control if yours works.

Thanks, Chris

 

I guess my basic question is are the V12s just gas hogs that no matter what you do burn twice as much gas as other engines of the same HP?

I looked at the epa ratings for the 6.0 with the 4sp auto and they are rated at 15 mpg hwy.

 

well since you asked, an overdrive gear will do nothing for your city economy. someone said something about the 700 having a better 1st gear ratio but pics or it didnt happen. also the TV cables are hard to tune properly and cause 3rd gear failures on a regular basis.

400's are bulletproof so thats what im sticking with. plus i dont mind the sound of a V12 with glasspacks at 3000rpm cruising down the highway.



Gas hog? ive had SBC's that got way worse mileage with WAY less power and less displacement.

how much are you gonna spend on a 4spd and install?

how much more are you gonna spend on gas for a 3spd for the lifetime of the vehicle?

 

As said.

I am down here, and the specs are very different to the "Stateside cars".

Last 13000km trip in the XJ-S we returned 11.2L/100, or for the non-google people out there, 21mpg (USA), 25mpg (Brit/Old Aust). That was a myriad of roads/conditions/speeds (cruise is boring down here), and the car was FULLY loaded with our "sruff", as we were away for 6 months.

Around the "burbs" 15L/100 IF I am lucky. That is 15mpg USA, and 19mpg Brit.

The car is 5.3l, with TH400, cold air intakes, NO balance pipe, larger throttle discs, 16CU ECU, NO AAV, openish exhaust, balance engine (2000), twin thermo fans, Sanden a/c comp, etc.

I looked at the TH700 swap when the engine was out, but the $$ involved (as M90 said) and the estimated return due to fuel savings alone would have me DEAD and buried waaaaaay before it got in the black, and that just made NO sense, even to me.

A couple here have done the conversion, and they also swapped the rear end ratio to 3.54 to reduce the clutch pack burning (also mentioned). Also understand we have 110KPH blanket speed limits here, and 50KPH in the burbs, so the 4 speed is struggling at 2.88 in my opinion.

 

I have a 700R4 in my car. I do like it. The fuel economy nets me about 20 MPG US on a high trip. I cruise at 75-80MPH. I have not really tracked my mileage except on one trip just to see.

The 700R4 is a good transmission if you get the correct unit. Just like everything GM ever built the early ones are prone to failure and the later ones are better. You can buy parts that are much stronger than stock if you are concerned.

The TH400 is a good strong transmission. The gear ratios are less than ideal but certainly it is drivable. The unit is very heavy and as such it takes power just to turn the mass. The lighter the rotating mass the better. It is a reason aluminum driveshafts are popular.

Adjusting the TV (throttle pressure) cable is not a problem at all. However if you try to do seat of the pants you WILL burn up your unit. You set the TV cable using transmission line pressure. You need a gauge to do it.

 

good point, i should have mentioned that. i forget what year exactly, but i remember reading specifically that after a certain year, the reliability really improved.

ive heard, but not researched, that there is virtually no aftermarket for these transmission. are you talking hardened parts, shafts and upgraded clutch packs?

i believe that most people who do something like this and a job like this themselves would be inclined to do "seat of the pants" tuning.
as you said, tuning it properly really is a requirement as theres not much room for error.
this brings up another question that i feel should be asked. does the Johns 4spd kit come with a throttle linkage for the TV cable?

I have a built BOP TH400 that i LOVE that i used in a couple of previous builds. firm shift kit, etc. and cost me next to nothing to build. i prefer it because ive got one that i like. its heavy, yes, but ive had heavier.

 

Grant, please explain more about the balance pipe removal. Also do you just keep it going on the throttle on cold mornings with no AAV, and where do you take off the ECU vacuum feed with no BP?

Completely agree with you and M90 about the GM400.

Greg

 

Bugga, now the brain has to kick in, this may be fun.

Basically I got sick of the AAV issues, even with rebuilt ones, they worked, but I always had that gut feeling, has the bugga closed, or is something else causing idle issues, and decided there must be a better way.

The cold start injectors had been removed and plates covered the holes, sooooo, simply tapping those holes to take a neat spigot and 2 equal length hoses to a "T" at the rear of the fuel rail and connecting to the ECU vac hose.

I had a PreHE inlet end plate for the LH manifold rear, which sorted that. A 1" core plug blanked off the RH end plate, where the balance pipe attached, and JB Welded just because. This now gave me 2 hoses, siamesed for the brake booster, one from each inlet manifold.

The throttle bodies were lathed to accept the 2 XJ40 3.6ltr discs I stole, which basically opened them up from 63mm to 71mm.

I found a fuel pump blanking plate lying around that fitted the AAV opening perfectly.

I had long ago removed the solenoid valve from the backing of the RH air filter, and found a second one in the boxes, and tucked them up under the inlet manifolds, one each side obviously, basically attaching them to the steel coolant transfer pipe. Inlet air comes via the 2 backing tubes that once fed the over-run valves (also removed), and the vac hoses are via 2 of the "hollow" studs that were used for vac to the fuel pressure regulators (also not used anymore). these "hollow studs" are at the rear of the inlet manifolds. Electrickery is via an ign powered relay with a switch under the dash panel switching the earth terminal of that relay, simple.

Idle speed, well, basically guess work. I set them at 0.002" and then wound the "set screw" out 2 flats each, and that is where I started, and basically is where it is now, pretty cool. The throttle rods were obviously reset once all settled down.

That is the basics of it, and I bet I have forgotten something, so be it, ask away, I will recall it eventually, or go and look at the beast. There was a LOT of outside of square thinking, and a little trial and error, but I basically copied the carby V12 set up, and if it did not work, PUT IT BACK.

Air filters were home made with foam and chicken wire, using the Jag filter "tin plate", which then cleared the larger discs as apposed to the tin plate cover shield that the standard filters have, which will JAM the disc at full throttle, FUN, you bet.

The over run valves were removed loooooooong ago, just plain dumb in my opinion. Maybe OK in the Northern hemisphere where stuff rotates differently to downunder.

 

Tons of aftermarket support for the 700R4....which is good because in standard form one won't last very long if you ever plan on taking your engine to the 6500+ rpm it is capable of. You'll need some advice on how to beef it up.

Once that is done it would be a great trans behind the V12. The 400 (and 2.88 diff) is the biggest hinderance to the car's performance.

Anyhow.....

My XJS V12 would give 20-22 mpg highway with the 400 (gentle driving) so I'd expect at least a 10-15% improvement over that.

I beleive the "good" 700s are about 1990 and later. Funny thing but I can't think of *any* GM trans that worked well right outta the gate. Seems like all them were trouble for the first couple years and then GM straightened them out.

Cheers
DD

 

I'm hearing two fuel mileages for the HE with 400 transmisions. Cruising 70 on the highway are they getting 17mpg or the 20-22?

I expect to wind up in Maine, where they only depreciate cars to 6 years for excise tax puposes. They base value on original msrp. I would keep a V12 around for weekend blasts, but the excise tax makes that prohibitive. (excise tax on an old XKE is cheap because their new price was $5+k, but of course I can't afford to purchase one) So what ever car I have has to also be able to make multi-state highway trips. (And with expansion of the money supply I expect the price of gas to inflate big-time, but am skeptical my income as a wooden boat carpenter will also) I also have an outback as a winter and abuse sufferer, and will also have a truck.

PS. I have mechanical/welding/metal fabrication skills and equipment, so I can upgrade issues that don't require expensive kits.

 

Doug,

Seems GM has that problem. They make a piece of junk at first and then it turns out to be a terrific piece. Even cars. Remember the first Fieros? The later ones were actually pretty good. The TH350 and TH400 certainly had their share of issues at first.

There is tons of aftermarket support for the 700R4. You can buy upgraded and different gear ratio planetaries, billet drums, better sprags, etc. I have a friend who builds 200R4 transmissions with trans brakes for drag racing. He runs an 8 second Grand National with a 200R4. The key is the upgraded high quality aftermarket parts.

 

Never understood why mpg would be a primary concern in a car of this nature. Would you seriously do a transmission swap for better fuel economy? Greg Edge, what about the performance benefits?

0-60
5-60 rollling start
40-60
60-100

 

I for one wouldn't go thru all the expense only to increase the fuel economy. I also doubt that I'd do it until the 400 trans dies. But if the there's no shame I considering fuel economy when deciding on upgrades or conversions.

As for acceleration I'd expect the 700-equipped car to be quite a bit faster on the 0-40 mph sprint, possibly so much faster that even 0-60 would be faster. Problem is, the 1-2 shift is like falling off a cliff in terms of gear ratio. Once in 2nd-3rd-4th gear I wouldn't expect much, if any, improvement in acceleration.

As doggy as the 400/2.88 combo is, it does have the advantage of very long legs in 1st and 2nd gears.....1st being good for about 65mph and 2nd to 110 mph. So, on a 0-60 run, where the 700 has taken the dive of the cliff into second gear and the engine dropped out of its power band, the 400 is still in first gear

Countering that to at least some degree is the 700 being less parasitic than the 400....which is a notorious power hog. That extra power could be felt in any gear or at any road speed.

Cheers
DD

 

Remember it? I lived it :-). The 350, 400, 125, 200, 325, 700, and 440 transmissions. The early X-cars. I could go on and on. :-)

Too bad about the Fiero. By the time GM sorted them out nobody wanted one! Cool little cars the last 2-3 years of production.

Cheers
DD

 

"Your mileage may vary" :-)

To get 20+ mpg a V12 has to be running right. Many aren't, although their owners might not realize it.

Also, all three of my Jags seem to hit a wall at 70mph where the fuel economy drops off quite a bit. If driven at 80 mph my 20+ mpg dropped to about 17 mpg. OTOH, steady cruising on a long trip at 65-68 mph would yield good results. A boring drive, yes, but great fuel economy.

Cheers
DD

 

Doug is right most people do not have their car in a really good state of tune. Many try to short cut or second guess all the adjustments. Plus without an exhaust gas analyzer you never will know if it truly is running at the correct mixture.

I did not do the trans swap for better fuel economy. That is why I really have not tracked it more than one trip just to see. I swapped it mainly for the better gear ratios, overdrive and lock up torque converter. Eventually I want to change the rear gear. In the USA with our roads and speed limits cars are more fun that can accelerate quickly. Once you get to about 80MPH you are looking for a ticket.

My original transmission had a problem where it would puke the fluid out at random times just sitting. The first time I took it out I replaced the pump o-ring, torque converter seal and shift shaft seal. I had it out a second time thinking I may have nicked the pump o-ring. No such luck. I then decided I needed to tear it down and try to find the root cause. Since I had in the back of my mind to do the conversion I just bit the bullet.

@Flint,

It does accelerate better for sure. I still have the stock 2.88:1 rear gears. I am thinking something like 3.30:1 to 3.50:1 range will be good. I have not done much research on what rations are available. I do know replacing gears in the 2.88:1 is not feasible. I can't say I ever measured times for acceleration. I honestly do not care. I just want the car to feel faster than it did when I got her.

 

I had found a 4.0 but it turned out to have rust issues. Then I saw a pretty V12...

Lately I've been looking at the thousands my Subaru has been drinking and gas mileage does count. A car that burns twice the gas would wind up not earning its keep because it wouldn't get used, so that's why I wanted to know if it could be made resonable, 22 mpg is close enough to rationalize indulging.

A few years ago I did twenty odd coats of laquer on an 1800ES and afterwards at the time concluded for the useage gas mileage didn't matter (but the reliablity sure did, I could get in that car any time and drive anywhere with complete confidence)

I haven't bought yet, I'm still looking and will especially look down in Florida where my father is (I willing to mechanic but rust is a nightmare). Transiting RI to Maine I set the cruise. Running with the herd up 95 you're going about 70mph.

 

OK, better understood a tad.

On mine, somewhere at about the 140+KPH mark is where that micro switch on the throttle capstan is activated for fuel enrichment, and that is "about 12% enrichment", so I know if I run mine at that higher end of the scale the economy is waaaaaaay off, so to sit down and work out at what revs that switch is activated, and then what road speed that equals will tell anyone the point of economy, versus thirst.

If this is your first Jag, and you really want a V12, GOOD ON YOU, but the HUGE/MASSIVE amount of "catch up" maintenence is really going to teat your patience, TRUST ME.

You use the word "indulgence", good one, I would be using PASSION, and then some more PASSION, follow by Jack Daniels.

Once you get it RIGHT, as reliable as any car out there, but getting to that point can be tough, and the passion wears off, and the car suffers once more.

Enough ramblings from me, coffee is cold, see ya.

 

Thanks everyone for the education. I think I will look at 4.0's when visiting Dad in Florida as a practical car for when there's no salt on the roads, and maybe adopt a V12 as a pet some other time (unless one comes along that's fated...)

 

For a transmission swap I would look at a 200R4. Many muscle car enthusiasts prefer it over the 700R4 because it's a more straightforward swap for TH350/400s. It can handle quite a bit of torque and horsepower and doesn't have the TVS issue.