GMC Shorty

Hi all.
Newbie here looking for some solid advice on building a strong low end 455 for my 8500# FWD GMC 23' motorhome.
I have a set of C heads in stock (J heads on the MH) but don't know if they'd be really suitable for and engine that lives at around 3000rpm when cruising.
TH425 has a 3.07 final drive.
Have a complete spare running 455 from a parts coach with "freshly rebuilt" heads.
Maybe all I really need is a little port work and a cam to make the stock J heads breath a bit better?
I'd hate to "waste" and original set of Toronado hi-perf C heads...
Any thoughts?
Also trying to find out what those '69 C heads flow at...

cutlassefi

Leave it alone and just clean up the J heads.
You said it yourself, it lives at 3000 rpm and below I would think.
Theres a Motorhome guy here in central Florida that actually coverts J heads for these things.

Note: I’m doing a 422ci Stroker small block for the same application. It has Edelbrock heads, 9.0:1, multiport EFI and a roller cam. BUT the owner is taking it in a different direction. It’ll be more efficient than ANY 455 yet provide similar tq.

69CSHC

If you could figure out how to incorporate true dual exhaust with performance mufflers it should give you a nice pop in acceleration. Single exhaust is a killer on big blocks.



If your feeling the need for speed the video below should get your juices flowing.

Killian_Mörder

It looks like there's enough room for stacking one tailpipe over the other, after both mufflers get separated from the Y

joe_padavano

The number of pipes is irrelevant. It's the cross sectional area and equivalent flow rate that matters. A single 3.5" pipe has as much cross sectional area as a pair of 2.5" pipes, and frankly, probably flows slightly better because there's less surface area for the same volume and thus less boundary layer drag inside the single larger pipe.

VC455

Another possibility is enlarging the single exhaust. For example, combining 2 1/2" exhausts from both banks into a single 3 1/2" has lower back pressure than two 2 1/2" all the way back.

GMC Shorty

Hi Joe;
A common upgrade is headers and 3" exhaust all the way back from the Y...the pipe goes through the passenger side of the frame crossmembers...lots of folks put some kind of torquer cam in it.
Kinda figured the C Head would be too much for the application, but they were perfect and came with manifolds from the Toro so I couldn't NOT buy them

joe_padavano

A friend of mine got one of these last summer and put on that 3" stainless system with the headers.

cutlassefi

The main bottleneck is the intake, not the heads.

redbaron442455

I posted a similar question earlier this year:

https://classicoldsmobile.com/forums/big-blocks-2/motorhome-head-cam-advice-146403/

redbaron442455

My cam guy suggested his rv cam:

• Advertised Duration :264
• Duration at .050 : 214
• Lobe Center : 110
• 1.6 .480

Along with my
9:1 compression
ported c heads
Headers and 3" exhaust
air gap manifold and efi (raised floor)

cutlassefi

I’m wondering how you’re going to fit an air gap Intake under that engine cover.

redbaron442455

it doesn't fit flat covers, some people build a raised box into the cover with cup holders or whatever you like. It's something like 3.5" taller.

cutlassefi

Then I wouldn’t use an RPM, I’d use a regular Performer.

joe_padavano

^^^THIS. Build the engine using components that match your intended RPM range. The latest fashionable parts for a drag car are probably not the best choices for a 7000 lb motorhome.

redbaron442455

I believe the air gap design manifold with no exhaust crossover was suggested to me to try and reduce heat to the efi ecu.
I agree a performer manifold seems like a better fit.

joe_padavano

Probably 99% of those aftermarket EFI systems are bolted to manifolds that DO have a heat crossover. I'm not aware of a rash of heat-related failures as a result. More to the point, these throttle body systems still need manifold heat to avoid fuel puddling in the runners - there's no difference from a carb in that respect. The motorhome is a big, heavy vehicle, not a hot rod. The engine needs to be designed for midrange torque and good low speed performance. Ultimate HP is irrelevant.

69CSHC

I like that.

Equivalent sounds great, as long as the Y pipes exhaust pulse collision disappears.

69CSHC

I wondering if that's much of an improvement on an early GMC 455. I'm coming up with 2 ratings for the 455 GMC, 1973 and 1976s with nothing in between.

1973 GMC 455 = 265 HP @ 4300 375 LB FT @ 2800
1976 GMC 455 = 215 HP @ 3600 370 LB FT @ 2400

Not sure for 1974 but 1973s ratings are 350 HP by old school method (gross). Plenty powerful, adding the performer as CutlassEFI suggested and true dual exhaust or equivalent maybe your looking at 300 HP and 400 LB FT net.

Killian_Mörder

Is or was?

Schurkey

And then you get to worry about front brakes and front wheel bearings.

I'm told folks swap the Olds engines for 500 Cadillacs; more torque and the lack of RPM isn't a handicap.

A switch-pitch TH425 might also be an advantage.

There's various ratios available for the converter-to-transmission sprocket-and-chain system, too.

GMC Shorty

Great video for those of us going to Bonneyville for MoHo world speed records

CuttyShark

Mine came with C heads. I pulled them out and put in a new engine with rebuilt Ka heads which are same spec as C heads. It ran great and got 10-12mpg while towing in ideal conditions and around town with a lot of stop and go and hills 6-7mpg.

till i had a problem with the roller tip rocker arms I installed on the Ka heads.

i had the engine rebuilt and had a new Crank installed along with Aluminum heads and new cam reused the block, pistons and rocker tips. Upgraded to new rocker arms studs.

now I get 9- 11mpg mixed around town and highway driving. It also feels more powerful and has a lot of torque. I used the original concave Toronado type manifold. I have used a MSD Atomic with timing control. Ive no doubt a bigger taller manifold will Improve things but for what it is I am fine with it all fitting under my floor.

if You downgrade to the 403 you can use a Edelbrock performer and get away with upgrading the manifold but you looks Cubic inches.

lukestmz

i am building the 455 in my 26 footer, changing everything except the block crank and rods, efi and aluminum top end , full roller , i have mountains to climb places to be!

RetroRanger

that looks like its in nice shape

Killian_Mörder

Those were mega cool. I wonder as to how many are still left. The one in the following video could use a set of radial tires.
They should have been standard, by the time this vehicle went into production: