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IPM 04-09-2007 08:46 AM

Turbo 400 tranny rebuild/upgrade option
Maybe when it comes time to rebuild my turbo 400 tranny I'll spend an extra few $$ and add the switch pitch feature. Could use like a gear splitter.

If I later swapped the rear differential ratio from the current 3.75:1 to 3:08:1 I could have lower engine speeds at cruise and use the switch pitch to launch my rig from stop.

Switch pitch on, 2600 stall, 2600/1800 x 3:08 = 4.45 so plenty of mechanical leverage to start.

Switch pitch off to cruise with slower engine speeds.

Rig remains basically OEM with no modified driveshaft, overdrive unit etc.

Just a thought should I ever need a Turbo 400 rebuild even if I never swapped the rear ratio. It would be nice to have more "ratios" to split when chugging up a steep mountain. Even 3.75 don't pull the tallest mountains so well, wind up in 1st just to keep the engine revs high enough.

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scottanlily 04-09-2007 09:58 AM

With the heavy weight of the motor home ,you don,t want to have a stall
speed of anything near what your talking about. High stall speeds get the engine RPM up before the mechanical engagement occurrs completely, but
with the heavy load it will not aggressively take off ,it will have a high rev slipping sensation happening and extreme heat will build up in the trans from the converter higher stall .In the original application the switch pitch worked pretty good.A heavy truck chassis needs a very low stall speed such as the
Allison converter commonly installed in motorhomes and heavy trucks. It has
heavy torrington bearings inside and furnaced brazed stator fins and so forth
also has 6 bolt flexplate mounting as opposed to only 3 with a standard converter. The low slippage of the allison or RV type converter builds up far less heat than a stall converter does ,and the heat is whats the destructive factor in any automatic transmission. You need low RPM lockup to get all your 454 torque moving the mass of weight and get the rig up to speed ,thats the
thing needed with heavy trucks .Stall converters or a higher stall speed works
great in a camaro or mustang or high performance street vehical thats fairly light weight not in any heavy vehical .Remember that any type of converter
slippage creates heat and poorer mileage . Thats specifically why the lock up
converter is a great idea and all manfactureres have used it for many years,
it goes into full lock up (no slip) in overdrive 4th or third in the older 3 speed units ,but locked up gives 100% engagement and better fuel economy.
If you want a great converter ,get the Allison heavyduty unit .

Scott of scottanlily

RUSSELL 04-09-2007 09:59 AM

1. Forget changing your rear end.
2 When rebuilding your trany tell them you want a tow and go shift kit in it.This will firm up the shifts and stop a lot of slipping and sliding in the trany.
3. Put in a slpliter with low,direct and overdrive. This will give you a lot more gears to play with.
4. Make sure who ever rebuilds your trany has your torgue converter rebuild.
Do not let them put in a regular torgue converter. Yours is heavy duty. If you know a rebuilder of torgue converters take it there.Demand that yours be rebuild. You need a low stall T.C. This will give you a better millage becouse of the low slip.
I hope this will help you.
Regards from Russell in sunny Tucson Az.:brows:

scottanlily 04-09-2007 10:06 AM

One other thing I wanted to mention is ,that alot of these transmissions
in these motorhomes may actually be turbo 475 s ,not 400s .They apear identical ,yet the 475 has straight cut heavier duty planetary gearsets for
superior strength ,and may have the allison in them already ,but that is not necessarilly the rule .you never would want the 3:08 gear ,better to have a
gear vendors installed and be done with it ,you will be happy with that .


IPM 04-09-2007 06:49 PM

sounds good thanx

nickglase 04-10-2007 09:35 PM

any of you guys know what rear differential we have on the p30 chassis? i'm about to replace the fluid and repair a leaky rear gasket on my 88 345. the current gasket appears to be the goo type. am wondering if they make a 'to-fit' gasket. i also have an issue with lack of engine breaking in second --i think it could be the front servo. with luck i'll fix it, otherwise it will take $$.



IPM 04-13-2007 09:44 AM

Hey, the classic GMC motorhome owners have worked out a way to use switch pitch. Interesting. Heat build up is still an issue to switch out but they have a solution.


IPM 04-13-2007 10:06 AM

More GMC comments on switch pitch. Heat has to be managed but it's working for them I guess.

Quote from GMCnet: GMCnet => Brake pedal fade

" I've had mine in now for about 3 years. I got it from Manny Trovao.
I've probably got over 30,000 miles on it now and its is performing
extremely well. I also have a 3.55:1 final drive from Jim Kanomata.
I was pulling a 3800 pound Jeep Grand Cherokee in the mountains and
it got a lot easier when I got the switch pitch installed and working.

Right now we pull a 4 door Geo Tracker and hardly know its behind.
I have the Bruce Roe switch pitch control box and usually leave it in
the automatic position. One thing that the controller does is to cut
the switch pitch off after about 20 seconds so as to prevent
overheating the transmission. But, with my external transmission
cooler, and my dash transmission temperature gauge I find that on a
long grade I can switch it to manual on and continue up the grade
without overheating.

The switch pitch is also great when you are in city traffic. It
helps get you up to speed, especially when towning.

It sure is nice when you are on a long mountain grade and get behind
a tractor and trailer. The switch pitch is just like taking an auto
out of overdrive. It allows the engine to rev up about another 300
or 400 rpm and keeping it closer to the peak torque level."

IPM 04-13-2007 10:11 AM

from another gmc motorhome site:

Manny Trovao - Transmissions (including Switch Pitch) built specifically for our motorhomes. 3.50:1 Power Drive chain and sprockets.
San Jose, CA 408-937-1583

tsunami 04-13-2007 10:42 AM

Instead of re-working the 400, look for a 200R4 and have it re-worked. That way you can have overdrive and the 400 and 200 are identical in size. Might have to have the drive shaft shortened, but you'll get better fuel economy.

M Hohnstein 04-13-2007 11:00 AM

V/P experience
I had V/P turbos in my 79 Chevy crewcab and my 84 SOB 27' class A. They work pretty good as long as you remember to turn off the high stall on the road. Build up the heat real good if you don't.
Consider this experience. I had the SOB in podunk Idaho once up on time, the gravel back roads are real steep in some places. I was on one that kept getting steeper until I figured I better turn around. Had a small heavily loaded utility trailer on the back, didn't quite make the cut into the side drive, hill was so steep kept moving back with all the wheels LOCKED, skidding in the gravel. Pretty exciting but it did stop, hit the high stall and the rig would not move. HUH!! Flipped the switch to low stall, floored the throttle and it started to move up the grade very slowly, made it to the top and got out of there.
When I got back to the shop some time later I took out some V/P parts and started pondering. The V/P stator or switch pitch stator if you prefer, greatly reduces the flow through the converter to effect the higher stall. When low stall is called on the control piston fills and the stator blades open to allow flow through which results in less reduction or "torque multiplication".
The reality of the switch pitch is these parts are getting pretty rare and frankly now that I have tried a few of them over the years, I would go with the Allison type 500 series converter, forget the straight cut planetaries, not stronger in my experience and get a GearVendors.
I'm installing a 496 stroker kit in my 454, using a 700R with GearVendors in the 34.5, used this drive train before, works stellar and you get a lock up converter. Now before some 700 haters get their shorts in a bunch, double OD is not the idea, that does NOT work. The ability to select 22% or 30% OD offers great flexibility and kicks up the intermediate range which is great on some grades. I used this drive train in a class C pulling a 26' tag trailer grossing 24k #s, climbing some pretty good eastern 6% grades and powered by a 520 4.5" stroke BBC mind you. Now I was not trying to be the first to mountain top, but I kept up with the semis and I never used an auxillary cooler, with the lock up converter the oil never got over 200 degrees. It works for me and IMO it's a good bet in a Classic due to the light weight and air flow efficiency.

M Hohnstein 04-13-2007 11:07 AM


Originally Posted by tsunami
Instead of re-working the 400, look for a 200R4 and have it re-worked. That way you can have overdrive and the 400 and 200 are identical in size. Might have to have the drive shaft shortened, but you'll get better fuel economy.

You might want to inspect the hardware in a 200 4R before engaging in a project using one. You may want to focus on the forward drum input shaft and the mass, or lack of same in the input roller clutch, not to mention the spindely 28 spline turbine shaft. Brrrrrrrr. Might not be the best idea.

scottanlily 04-14-2007 09:54 PM

Im seeing that the switch pitch idea has caught on ,so Ive learned somthing
new here on that ,but its being utilized differntly than would be in a car application .The launch you desire from stalling the converter is to get the RPMs up to where you are making optimum horsepower .That is the foremost
reason behind a stall converter.In the 70s we took the 10"vega converter
remember the chevy vega? well, that 10" converter rebuilt with the furnace brazed fins and heavy duty stator assembly allowed 3500 RPM stall speed.
Your camaro would launch off the line .or my 64 327 chevy nova 2 door
(no post)and a powerglide trans ,FAST was the word for it ! Anyway ,it
sounds as if these guys have been able to use it in a different manner
as reported here .I do not think going to a taller gear is the answer though
in any case .You guys also need to look out for thrust bearing wear do to
increased converter pressures (not line pressure) that excessively loads the crankshaft thrust bearing in the engine ,common with turbo 400 transmissions
the cause usually being a restricted cooler (not plugged up necessarrily) but a restricted cooler causes converter pressures to be excessive and forces the converter forward againts the engine destroying the thrust of the crankshaft .So a
word of caution when making these modifications ,and use the BEST cooler
and the biggest one they make .True Cool is one brand that offers big sizes
of trans coolers .As for the 200R4 ,forget that idea ,its not at all heavy enough for truck use ,and was marginal for cars .I know that some guys are building parts to strengthen them to some degree ,but they are NOT anything
remotely close to a turbo 400 .The 400s or 475 units are extremely heavy
duty ,can really take it .If you guys are interested in these modifications ,you
should go look at some wrecking yards at cores and trans shops as well to
get informative info on these transmissions ,this way you can see the size and strength differences in these GM trannies .The gear vendors still is the best way to get the gearing and power needed without stressing the trans or
converter to the extreme .A plate and fin style cooler is the best ,not the cheaper types of coolers such as the traditional Hayden coolers ,although ANY cooler is better than just the radiator cooler by itself.

Scott of scottanlily

scottanlily 04-14-2007 10:31 PM

Hello M hohnstein, the turbo 475 is the heaviest duty 400 style trans ,
the planetery gears sets are stronger by design ,thats the point ,Im certain I wouldn't say not to use that transmission .How or in what manner have you found they are not worthy to use ? If you mean that extreme high loads or abuse have broken them ,anything can be broken if its exceeding its strength capacity .Your
weights you are pulling certainly will break alot of transmissions .If you want
to really have the didge setup ,forget the 700R and figure a way to get an
allison 5 speed or 6 speed from behind a 6.6 duramax such as a 5500 GM
truck with a custom PCM to run it ,they are electronic so they need a computer to operate it or some version therof to get it to work .The 4L80E
is a turbo 400 overdrive ,so GM built it already .It needs a PCM or
power train control module as its called to run it also .The best choice above
all else is the Allison transmission if you really want a serious transmission.
I know you got the 700R to work ,but it was never designed for any type of loads your pulling ,it will break .

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