Community Forums

Gary - K7GLD · 08-22-2009, 12:11 PM

As a new owner of a '88 27 foot Super Chief Winnebago class A, I'm looking for a few more and better details as to the mechanics of the drivetrain - is there a decent online source for internal details on the GM 454 as used in these MH's - stuff like 2-bolt or 4-bolt mains, and whether they had roller cams or not - what differences between the P-30/37 class 454's and those used in passenger cars and light trucks - if any?

Same with the transmission - some say it's a regular TH-400, others say it's an upgraded version of that transmission, with more clutches and a better torque converter - how can I find out which I have in my rig?

Any info from informed and knowledgeable members greatly appreciated.

JimElliott · 08-22-2009, 01:01 PM

Mortec Home Page: Chevy Engine Codes

Try the above for your engine...Look at the rear left hand side for your block number.

Jim

Gary - K7GLD · 08-23-2009, 07:10 PM

Quote:

Originally Posted by JimElliott

Mortec Home Page: Chevy Engine Codes

Try the above for your engine...Look at the rear left hand side for your block number.

Jim

Thanks for the pointer Jim - but what I'm after is more of a breakdown that describes or illustrates internal details - some heavy duty engine applications use 4-bolt mains, heavier duty valves and seats, roller cams, and such - not sure in the P-30 series GM chassis WHAT I have - same for the tranny - it looks to be a TH-400 - but some guys have said in this application, it could be a heavier duty version with more/better clutches, and a "non-ballooning" torque converter. Being a sort of curious type, I'd like to know a bit more detail for this rig - I suppose some of the Clymers and similar manuals would have the info, but hoping for an online source...

JimElliott · 08-23-2009, 08:43 PM

You will never know if it's a 2 or 4 bolt main unless the pan is dropped..
Pistons are (cast) stock.
Timing chain and sprockets become suspect around 125,000 miles.
Heads are no doubt small (high torqe) peanut ports, Remove the rocker covers and look at the raised number...236 head is "Peanut port" and nothing special about the valve train.
Small cam was the norm (195 intake @0.50)
TH400 pan is shaped like Texas.
You will hafta tear the trannie down and count the clutch packs and a good shift pack (not B&M) could be installed

Jim

Gary - K7GLD · 08-24-2009, 09:09 PM

Thanks Jim - every online forum seems to have at least one in-house expert on a WIDE variety of subjects - and you seem to be the one most on duty here - thanks for your help!

Maybe you can help me with one more - HOWINHECK do you check timing on one of these '88 GM Winnebagos, the fan shroud and dash overhang would seem to make ANY visual observation of the front timing mark virtually impossible - is there some other accepted way to do it?

OH - one more thing, looks like there are *2* different TH-400 pan shapes? One is like you described further above, the other is square, like THIS:

B&M Products Cast Aluminum Deep Transmission Pan - GM All TH-400 20280*-*Automatic Transmission Pans*-*Drivetrain*-*Vic Hubbard Speed & Marine

And THAT is what mine is shaped like - maybe it varies by year model?

sknight · 08-24-2009, 10:07 PM

That pic is wrong. That pan is for a TH700, looks like they screwed up.

The TH400 through the years all used the same pan varying the depth for HD applications. Here's a link to the same part number pan from Summit. B&M 20280 - B&M Automatic Transmission Deep Pans-Overview – SummitRacing.com The very early models used a "Candybar" filter, 65-66 I believe are the years, all following use the flat style.

As for timing it, most likely there is a second timing mark just below the power steering pump on the left side of the engine. It's right where the oil pan meets the timing cover. Hook your timing light normally and don't forget to put it in base timing, then just use the lower mark. Have an assistant move the dist for you.

JimElliott · 08-25-2009, 10:01 AM

Hook your timing light normally and don't forget to put it in base timing, then just use the lower mark.

Snight, The lower timing tab requires one to hook up to #5 plug..
If (I doubt it) the ignition is computer controlled the (tan/black) wire must be removed.....
Base timing (4º) is kind of stinky.....8º would be much better and speaking of better 10º is for me and don't forget to disconnect the vacuum advance.
That would also be a great time to check out the mechanical advance (sans vac advance) 16º is the best I've ever seen (stock ignition).

Jim

Gary - K7GLD · 08-25-2009, 10:44 AM

Quote:

Originally Posted by JimElliott

Hook your timing light normally and don't forget to put it in base timing, then just use the lower mark.

Snight, The lower timing tab requires one to hook up to #5 plug..
If (I doubt it) the ignition is computer controlled the (tan/black) wire must be removed.....
Base timing (4º) is kind of stinky.....8º would be much better and speaking of better 10º is for me and don't forget to disconnect the vacuum advance.
That would also be a great time to check out the mechanical advance (sans vac advance) 16º is the best I've ever seen (stock ignition).

Jim

I had another suggestion as to advancing the timing - only concern, is too MUCH total timing under load, and the danger of heat, melted pistons - unless a mechanical stop limiter was also used? Good thing you mentioned the #5 plug - would have connected to #1 otherwise - and been totally confused!

YUP - looks like mine has the regular shaped tranny TH400 pan outline - the pan itself is just shaped differently, and deeper than the ones commonly displayed on aftermarket sites - and the one I pointed to earlier must have been an error by the vendor...

Nick-B · 08-25-2009, 10:55 AM

Gary. Count the number of bolts in the pan.

13 bolts for the TH400/475 (3 speed) - pan is NOT square
16 bolts for the TH700 (4 speed OD) - pan is nearly square
17 bolts for the TH4L80E (4 speed OD) - pan is rectangular

The 400/475 is a strong tranny and was probably originally in your MH.
If you have the 700, it is not a strong unit for MH use. I would be surprised if any Class A chassis ever came stock with it. (I am often wrong about these things

). If this is what you have, make sure you have a good transcooler and keep it well serviced.
The 4L80E (strong unit) didn't come out until like '91 and was computer controlled. It is possible it has been retrofitted to your chassis and would make for a nice setup.

Gary - K7GLD · 08-25-2009, 12:34 PM

Quote:

Originally Posted by Nick-B

Gary. Count the number of bolts in the pan.

13 bolts for the TH400/475 (3 speed) - pan is NOT square
16 bolts for the TH700 (4 speed OD) - pan is nearly square
17 bolts for the TH4L80E (4 speed OD) - pan is rectangular

The 400/475 is a strong tranny and was probably originally in your MH.

Any external visible differences between that 400 and the 475 a person could look for?

Nick-B · 08-25-2009, 01:20 PM

None that I am aware of but I'm not an actual tranny specialist. I just know enough to get myself into trouble

Here's a quote from an off-road forum message: "The main difference is that the TH475 has straight cut gears in it, and the TH 400 has helically or angle cut gears. The angle cut ones are quieter , but they are not as strong because they can pry apart under a heavy load. The straight cut ones are really strong and they are used in motorhomes and like C60 trucks."
Sounds about right to me.

JimElliott · 08-25-2009, 01:40 PM

Quote:

Originally Posted by Nick-B

None that I am aware of but I'm not an actual tranny specialist. I just know enough to get myself into trouble

Here's a quote from an off-road forum message: "The main difference is that the TH475 has straight cut gears in it, and the TH 400 has helically or angle cut gears. The angle cut ones are quieter , but they are not as strong because they can pry apart under a heavy load. The straight cut ones are really strong and they are used in motorhomes and like C60 trucks."
Sounds about right to me.

Right on Nick
Straight cut gears will have a nice whine in low gear....

Jim