Just a couple of observations:
1. 3700RPM @ 65 MPH w/4.10 gears indicates approx. 24" diameter tires. Seems really small for a full size pickup.
2. "4.10 GM 10 bolt limited slip no posi".....limited slip IS posi..can't be both.
I don't see anything particularly wrong with your current engine, except maybe a little under cammed, but I like a little lope. If I were in your shoes, I'd leave the 4.10 gears in, get some 28"-29" tires and go with a TH700R4. You'd have a lower 1st & 2nd gear for takeoff power, and in OD you'd be cruising at 65MPH @ about 2200RPM, or less.
As far as the 383 option, I like the looks of it better than the current engine, but mostly because of the higher comp ratio. You could get some 58 CC aluminum heads for your 350 and bump the comp ratio up a bit, without having to resort to racing fuel.
Just some ideas tossed out. I'm sure more qualified people may have different ideas.
JA

 

Hi I just built my first engine last year and I have put maybe 3000 miles on it,I think that the way its set up isnt right because it feels slow the other cars in traffic seem like they could spank this chevy mileage and power wise,it gets 10.5mpg and if I let it shift on its own it gets to high gear at 30mph,freeway speed it seems to use alot of gas and its at 3700rpm at 65mph im at the point where im embarrased to tell people I built this motor and it cant even keep up with the toyota next to me
72 C30 longbed
350 .030 over 9:1cr
TH400 trans with stock converter
4.10 gm 10 bolt limited slip no posi
492 camel bump heads 2.02/1.60 pocket ported
1.52 roller rockers
256h xe cam
performer rpm intake
625cfm carter carb
50k hei igntion
champion plugs .045 gap
headers
glasspack mufflers
I was thinking of building another motor
383ci 9.5:1
th700-r4 2200 stall
3.42 gears
powerhouse 190cc heads
1.60 roller rockers
270H magnum comp cam
performer intake
650cfm speed demon carb
denso plugs .060 gap
do u think this would run better?

This is a pretty big truck for a 350. You didn't mention pistons, shape of the crown is very important. Most 350's use a circular dish to control compression, this is not a good design for flame propagation. When using a small chamber head, the piston should use a "D" dish configuration. This maximizes squish and quench performance which reduces the necessary spark lead by speeding the burn and makes the engine innately more detonation resistant.

Given the truck, engine, and gears 10.5 doesn't seem out of line for mileage; but carb tuning is important.

492 heads are obsolete by any standard.

The 256 cam is pretty mild.

I suppose the TH400 hasn't been rebuilt, they get tired about the same rate as the engine they're attached too, if it hasn't been rebuilt by someone that knows what they're doing, it should be. It's easy for automatics to cause power to disappear as heat.

The 383 sounds like a good choice, especially for lugging a long bed around. A big block would be even a better solution.

Bogie

 

you shouldn't be turning 3700rpm to go 65mph, unless you have really small tires on it. I just put a fresh mild 1976 350 in my 68 GMC K25. I put in a Crane powermax "truck power" camshaft, Edelbrock performer intake, Holley 600 square-bore, headers with duals and thrush turbos. The motor's not stock but its nothing fancy.

For a 40 year old truck, its pretty damned quick. Even though the carb and timing still need minor tweaking. its got a 4-speed with 4.10's and 31's on it. The 350 has no problems pushing the truck around. its all stock inside, no fancy heads, no flattop pistons, just a stock 350. It idles lumpy, it sounds great and goes excellent for and old truck. I've surprised a lot of the ricers around town with it.

 

they are 8 lug 12x16.5 wheels in the back and 9x16.5 in the front,the trans is the original it only had 68,000 miles on it my father was the original owner,but It came with a 402 engine I only pulled it for boredom I just felt like doing something new with it.
I've got reverse domes with 4 valve reliefs,the motor never seems to have gotten the vacuum that comp said it would get it said 20" at 1000 but it pulls 15.5" @ 1000 rpm its not too good at freeway speed its hard to keep up with the traffic,the cam but is what comp suggested but to be honest it seems to come on late and not pull too hard at the top end Im a little dissappointed to be honest for the mileage I get lots of cars that get twice the mileage I get could spank my truck

 

Your current engine combination is set up for low rpm's and cruise speeds.
The Carter and Edelbrock AFB carburetors by design and a little on the rich side on the primaries and lean on the secoundary side. Use should only have the 2 screws on the front of the car about 1- 1 1/2 turns out from the seated posistion.
Also, the ignition curve in the distributor should be set up to bring in total advance around 2000 rpm.
More than likely, your going to have to tune on that carburetor for a while to find the sweet spot for your engine. It can be done, although it will drive most folks crazy trying to do it. You may want to consider a good Q-jet, as they will produce great power and mileage.
Glenn

 

Sounds like you could gain a lot from carb and ignition tuning.

What is the initial and total timing?

Have you read a plug or made any adjustments to the carb?

A 1 ton ton is heavy for a 350. Don't expect too much mpg or acceleration. Are you getting 10.5 mpg in the city, highway, or combined?

What are the cam specs (duration at 0.050")?

Assuming you have a 30 inch tire, 3700 rpms at 65mph means the gear is 5.08:1. Are you sure the tach is right?

Engine should run OK. The intake is a little big. 15 inhg is low and could indicate low timing and improper fuel mixture.

How does it idle? Smooth or with a miss.

oil pressure good when warm?


Glass packs don't flow very well. Install some dynomax super turbo's. Much more flow and much quieter.

 

I agree with what everyone else has said. A 1 ton truck is not going to get very good fuel mileage. How ever I would check the timing and advance curves really close. There is a lot to be gained with the correct timing. You want to run as much advance as possible, but no pinging under load.

 

Take your timing light and put it away unless you have a brand new balancer.
1)Get out there and keep pulling advance till you hear spark knock on acceleration then back it down till it stops.
2)Take it on a full bore run and read a plug,jet the carb accordingly.Bigger jets if its too lean and smaller if its too fat.
3)A distributor advace kit is a good tool too.
I don't know carter carbs but on a holley you can change springs to adjust the secondarie opening point.
My guess is its all in the tuning.
You've got acceptable street compression.Good heads a slightly small cam but that should give it nice lowend and throttle response.You setup seems OK to me.
BTW oppinions may vary,but I've never heard anything good about Champion plugs in a SBC.(honestly I've heard they make um run like crap) I'd get a set of Delcos,R45TS is the stock plug.Might try a R44TS also.

 

im sure on the tire size/rear end ratio the tach is right,
256/268 adv. dur
212/218 dur @.050
.447/.454
110lsa
106 lsa
oil pressure is high its above what my stock gauge reads
i was thinking of switching to a 600cfm holley that my store sells that I work out mainly because of the electric choke my manual choke is a pain to start when its cold
how do u keep pulling advance?do u keep pushing the distributor advancing? the idle lopes a little when cold but is not completly smooth

 

im sure on the tire size/rear end ratio the tach is right,
256/268 adv. dur
212/218 dur @.050
.447/.454
110lsa
106 lsa
oil pressure is high its above what my stock gauge reads
i was thinking of switching to a 600cfm holley that my store sells that I work out mainly because of the electric choke my manual choke is a pain to start when its cold
how do u keep pulling advance?do u keep pushing the distributor advancing? the idle lopes a little when cold but is not completly smooth

The lobe center angles of 110 and 106 computes to an LSA of 108 degrees, that's pretty tight which means even for a "short" duration cam there's a lot of overlap. This will greatly reduce idle vacuum which can really screw with the carb.

Bogie

 

Measure total tire height and get back with us.

Is it a factory tach?

Is the speedo right?


212/218 is a pretty good size cam. It should pull to 5500 rpm easily.

Adjust idle mixture screws for best idle (highest vacuum).

20 degrees initial timing with 40 total would be a good start.

 

ive got the screws set at 1 3/4 out and it seems to be doing a little better I was wondering if another cam might be a little better than this one like
E921P
Elgin
292/288 adv dur.
224/224 dur @.050
.450/.460 lift
114lsa
108 intake centerline
with maybe a smaller performer intake,and a 600cfm holley or edelbrock electric choke vac secondary carb?

 

how do u keep pulling advance?do u keep pushing the distributor advancing?

Turn the distributor counterclockwise(vacuum advance away from firewall) a hair at a time and take it up the road. Keep doing that till you hear spark knock on hard exceleration then turn it back till it stops the spark knock.
Also seriously get a set of delco plugs.........
Plugs seem minor but when i swapped from R45 to R44(one heat range)it made a world of differance in my car.

 

rreruption

did I read your post right....the oil pressure reads "pegged" to the guage max???....what does it read at idle????
if yes....then immediately go get another guage to confirm the guage has died...otherwise you "will be" needing another motor

as most are saying (I agree) it's your tune up (a wrong for the camshaft tune up can cost you 30HP on a 350) or tranny and possibly both if it is behaving really poor....

asking again...what is the rear tires diameter?...to determine if the tranny is sound

your camshaft has 350ft/lbs or TQ at just 2,000rpms (and 411ft/lbs at the 3600rpm peak)...with 4.10 gears that should accelerate plenty decent in town even at 4,000lbs truck weight

256 cam dyno results and 350 build details:
http://www.compcams.com/technical/DynoSheets/XE256H-10_001.asp

350ft/lbs x 4.10 gears = 1435ft/lbs twisting the hides from a stop sign
411ft/lbs x 4.10 gears = 1685 ft/lbs twisting the hides at 3600rpms for passing a truck

1435ftlbs/4000lbs=2.78 lbs or wt per ft/lb of force

key to making TQ is high HG for high velocity....your 800rpm idle Hg should be 18.5 and 20Hg just off idle as comp says...and the needle should be steady....14-16* of base timing (no vacuum advance hooked up) and minor idle screw adjust with a vacuum guage "should" do it with that cam...if not 18"...retorque the intake and check for vacuum leaks....if still not 18" do a compression test and post results

 

I think an RPM intake is a bit too much for a 256 cam. With its cut down center divider, the idle vacuum will be low, and low rpm throttle response will be sluggish. I bet a regular performer intake would really wake it up, be easier to tune, get better economy, and make for a more enjoyable truck engine.
Just MHO
ssmonty

 

ssmonty....you are correct...(I missed the intake info)...the intake is wrong for the cam rpm power range....and wrong for making low end TQ

and I was wrong....he won't be able to get 18Hg with that intake...15Hg is about it....

the lack of street rpms air velocity with the RPM intake is killing his TQ which is why it is a dog

 

that sounds right also I noticed when you try to open the secondaries there is a slight hesitation could that mean a faulty vac advance?I think I agree with the rpm being too big because I think a truck with a small cam should have a little better response than this

 

the rear tires r 31" tall and 10" wide

 

rr,

according to this gears to tire size calculator:

http://wahiduddin.net/calc/calc_speed_rpm.htm

3700rpms=84.6mph with 31.5" tires and 4.10 gears....

your only getting 65mph out of your tranny at 3700rpms with 31.5" tires....

you should be close to 2850 rpms at 65mph with 31.5" tires (they all slip some)

= either your tach and/or tranny speedometer gear is really messed up or your tranny needs attention(most likely).....

follow a friend in a new car...have him signal you while maintaining different speeds 30/40/50/etc so you can chart the tach/speedo/mph

 

I've made some adjustments set the timing up a little and lean the carb a little it will get 17.5" now at 1000rpm and the throttle response is better
but i still think the intake is too big I was thinking about still changing cams intake to a performer which I have an old one sitting in my storage,changing to a holley 600cfm,and I think the vac advance is bad because it still doesnt advance,I also have a set of super 40s I might stick on there they should be much nicer than those old glasspaCKS :evil: