Horse power....

[mike]

This is a great discussion. I've got a 78 Royale with the 403. Already planning on FI once I get all the bugs worked out and want to pull my jeep.
I'm thinking 3.7 gears (which doesn't seem all that low to me, since my jeep is 4.10 and I want 4.88's!). How do these engine respond to headers and
a Cold air style intake? Probably need a cam to really fix the late 70's engine management for economy.
--
Mike and Jessica
West Michigan (Just north of Grand Rapids)
1978 Royale Twin bed, Dry Bath

 

[chris tyler]

Regarding the original question: While I dont have a procharger in my gmc, I have a good bit of experience with turbos and roots style positive
displacement superchargers and some seconary experiance with centrifugal blowers like the pro charger.

First, I think mounting one in a GMC would be an absolute bear as well as making service difficult. Second, Centris work better as RPM increases. PD
blowers work as soon as you crack the throttle, and turbos take load and a bit of time to spool up. I dont think a pro charger is the way to go for
our application.

A PD blower would be nice, but comes at he cost of hatch mods, higher intake air temps and high cost.

To go with boost I think a low boost turbo is the way to go. I will get around to it eventually as I have the parts already. Not a priority in the
flatlands of FL.

Agree with gears being your best bet right now. Years ago, when I lived near Don Garlits in Sefner and my buddy was friends with his daughter he told
me the best and fastest route to performence was without a doubt gears and a cam. Holds true for our GMCs too, I think.
--
76 Glenbrook

 

[Rdenney]

No, everyone buys a final drive as a complete unit.

Another option is the well-regarded planetary final drive used by Cadillac,
with a 3.21 ratio. I have one, but my coach is even lighter than Matt’s. It
climbs well enough but it is not geared for drag racing.

Manny sells a kit that used to be a Caspro product, as I dimly recall. It’s
called power-drive, I think, and it uses a smaller engine sprocket on the
chain drive, with a shortened chain. By itself, it is the equivalent of a
3.5:1 final drive, when used with the stock 3.07 differential. When coupled
with the 3.21, you get the equivalent of a 3.67:1 ratio.

Rick “who doesn’t tow but who does drive up and down lots of hills” Denney

On Tue, Oct 1, 2019 at 5:27 PM tonka6cuda6--- via Gmclist <

> So did everyone do a crown gear and pinion swap or did they change the
> gears and chain?
> --
> Rich Mondor,
>
> Brockville, ON
>
> 77 Hughes 2600
>
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>
--
'73 X-Glacier 230 "Jaws"
Northern Virginia
Offlist email: rick at rickdenney dot com

 

[6cuda6]

Im now convinced that a 3:70 gear ratio would be a bonus....now how to get there will be the key.

As far as forced induction....well i have built many high performance vehicles over the years for myself and customers....as much as some may see it
as difficult all one needs to do is look at the engine bay with the 403 in it and visualize all that open space on the left side....lol....winter
project i guess.
--
Rich Mondor,

Brockville, ON

77 Hughes 2600

 

[wrightjohn 2646]

Mike,
The 3.7 gear with the 403 motor is a good choice. It will give you plenty of pep and allow you to haul around that Jeep. How heavy is your Jeep?
Headers are a plus, as would be going to a 3" system when using the headers. Gearing for us is somewhat different for us as compared to a standard
vehicle. We use the gearing to get the motor up in its sweet spot. Did you see my post from earlier in the day, look for powerjon!

J.R. Wright
GMC Great Laker MHC
GMCGL Tech Editor
GMC Eastern States Charter Member
GMCMI
78 GMC Buskirk 29.5' Stretch
75 GMC Avion (Under Reconstruction)
Michigan

 

[mike]

> Mike,
> The 3.7 gear with the 403 motor is a good choice. It will give you plenty of pep and allow you to haul around that Jeep. How heavy is your Jeep?
> Headers are a plus, as would be going to a 3" system when using the headers. Gearing for us is somewhat different for us as compared to a standard
> vehicle. We use the gearing to get the motor up in its sweet spot. Did you see my post from earlier in the day, look for powerjon!
>
> J.R. Wright
> GMC Great Laker MHC
> GMCGL Tech Editor
> GMC Eastern States Charter Member
> GMCMI
> 78 GMC Buskirk 29.5' Stretch
> 75 GMC Avion (Under Reconstruction)
> Michigan

Thanks John! I did see your post. Sounds like you have a pretty sweet set-up in that Buskirk. To answer your question, my jeep is 4000 lbs, give or
take. I haven't driven it across the scales yet. It is an 04, with the inline 6 and running 33" tires. I'm familiar with gearing up to get the revs
up, its the same for that old 4.0L tractor engine that Jeep used for a bazillion years. Its not terrible with 4.10 gears, but when I start pulling
some of the bigger hills in N Michigan, she definitely benefits from a downshift...the 4.0L is in its sweet spot around 3000 RPM. Here is the Jeep...I
like showing her off too!

Back to the coach, FI, Gears and some tubes will go on the list. Nice thing is that stuff will still work when I have to do the eventual rebuild.
Its not eminent, but I know that sometime in the next few years, my engine will need a rebuild.
--
Mike and Jessica
West Michigan (Just north of Grand Rapids)
1978 Royale Twin bed, Dry Bath

 

[ken & dennis 2013]

I installed the 3.50 chain drive in my 23' in '98. After 30k-40k miles I
moved that 455 & 3.50 to the 26'my son now has. After 82+k miles there's
never been ANY problem. And it's quieter than any higher ratio final drive.

No question the chain mod imposes more torque on the tranny. But the 425
has stood up extremely well to far more than any 455 will ever put out,
much less a 403.

But a final drive is MUCH easier to install unless the engine's out anyway,
as mine was.

Ken H.

On Tue, Oct 1, 2019, 12:06 PM tonka6cuda6--- via Gmclist <

> Can you just change the gears and chain instead of the final drive? Just
> seems much easier in theory.....havent actually looked at it.
> --
> Rich Mondor,
>
> Brockville, ON
>
> 77 Hughes 2600
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[Agoogol]

Browsing only phone so maybe I missed it but I don't see anybody talking price....how much does it cost to have somebody swap out to the 3.70 gears
for you? Ballpark.

Afraid that maybe beyond my capabilities.
--
Kevin Carter
1977 Kingsley
Centennial CO

 

[jerry burt]

Kevin,
This will give you an idea of what a new one will cost. You might get lucky and find a used one but don't hold your breath.
It may be worth picking it up from Applied and save shipping, core charge, and crate.

https://www.gmcrvparts.com/product-p/gm1-1.001.1.htm

COMPLETELY ASSEMBLED UNIT WITHOUT OUTPUT SHAFT.SPEEDOMETER REDUCER REQUIRED **_HIGHLY RECOMMEND LIMITED SLIP DIFFERENTIAL **.
CORE NEEDS TO BE RETURNED IN 45 DAYS TO GET THE FULL REFUND ON THE CORE.

Final*:
3.07 [Add $1,259.95]
3.21 [Add $1,095.00]
3.55 [Add $1,605.00]
3.70 [Add $1,698.80]
4.10 [Add $1,948.40]
Core Charge*:
$350 Core Charge -Rebuildible Core Required For Refund, 45 Day Limit [Add $350.00]
Core Shipping Crate*:
Core Shipping Crate $100 [Add $100.00]

Installation labor would be additional.
--
Patti & Jerry Burt Fresno, CA.
73 Gmc 26' Canyon Lands
77 Palm Beach - Parts Coach
Members: FMCA - GMCMI - GMCWS
A truly happy person is one who can enjoy the scenery on a detour.