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355ci upgrade. Nothing wrong...just want more power..OF COURSE!

2K views 23 replies 10 participants last post by  001mustang 
#1 ·
Looking for some input....although the engine runs great...there is just not enough oomph. Runs and drives great...but I really want to be able to smoke the tires off the line.

Was thinking a cam upgrade (XE274 or XE284) and or a steeper rear-gear. Any advice? Or am I missing something>

350 SBC, bored .030
Stock Crank
Spped pro Flat Top (~9.5:1)
Stock Connecting Rods
Edelbrock Performer RPM Heads (#60899), 64cc, Straight Plugs
Comp Cams XE268H Camshaft (224/230@.050, .477/.480)
Performer EPS Manifold
Edelbrock Carb, 600cfm
Mallory HEI
1 5/8" Heddmen Headers
3.55 Rear with TH2004R Bowtie overdrive Tranny
 
#4 ·
seems like you are using a stock converter? You should be able to spin the tires easily with that setup. A bigger cam will make it worst.

get a 2500 to 2800 stall.

gears will help, 3.55 isn't bad but a 3.73 would be ideal for your cam.


what does the ignition timing setup look like? (initial, total, how much vacuum advance)??

A RPM intake would be better for your setup

I would also put that carb were it belongs, in the trash. Get a 705 holley.
 
#5 ·
I agree with the previous poster.

In my humble opinion, you are a bit mismatched in your combo. Those heads can flow pretty well up top, but your cam & intake are restricting you. A cam along the lines of the XE274 (or similar) along with an RPM air gap intake may be a better choice. Your gears are probably fine, but you need enough stall. You'll want more carb if you let those heads flow.

I have a similar build with a 383, rpm heads, rpm intake, XE274h cam, and 3.73's + a manual OD trans. I, too, am pretty pleased with the combo.

As far as your carb goes, for persepctive I'm switching from a qjet (750 cfm vacuum secondaries) to a blueprinted 750 cfm double pumper with mech secondaries.
 
#6 ·
When I bought the tranny from Bowtie overdrives, they supplied a 2400 stall ("High Performance Stree") converter with the package. It has always seemed like it didn't stall as high as I expected...but thats just feel. Once I get the revs up it pulls good...

Too many variables: cam, intake, carb, converter, rear gears but sounds like the converter may be a good start?
 
#7 ·
Carb

I like your set up, but you can add a little more kick to your step by doing some changes to the metering rods and jets for that elbrock AFB. Get with your parts dealer for number and stats. Have you had it on a Dyno?
I don't think that changeing to a holley will get that much more.
Good Luck
USN
 
#10 ·
set it at 18 initial (without the vacuum advance), then plug the vacuum advance into the the manifold vacuum. total mechanical will be a little high (40) but that will be ok. more timing will make it much more responsive off the line.

could run 20 to 24 degrees of initial timing but you will need to reduce the amount of advance in the distributor.

you have a modified engine, so you also need to modify the fuel and timing.

what timing chain are you using?
 
#12 ·
do you have the cam installed straight up (no advance or retard)?

your mallory hei should an adjustable total mechanical advance. set it to 7.5 degrees (15 degrees crank) and set the timing at 20 initial which would make 35 total mechanical (20 +15 =35). then adjust the vacuum advance to 15 degrees so the engine will run at 50 degrees going down the highway and 35 degrees at idle.

Then adjust your carb to give more gas on the accelerator pump.

should get the tires to spin with that set up.
 
#13 ·
a little higher stall and a steeper gear will amek it feel like you have a lot more power on tap. A cam and intake would complement that well, but you wouldn't feel it until you reached the top end of your power band. a 3,000 stall and a 4.56 gear would make it feel like a new animal. And with your OD its not that steep of a gear.
 
#14 ·
wickster,
454 is correct,,,you timing is way off for the motor combo...
(costing you 30Hp+)

"if" the base is 12, and the all in timing "is" 34,,,then there is the totally typical HEI 22*'s of centrifugal (not 24-26 common in a pickup truck dist and 2*'s is a big difference)

safer to take small steps:
recommended base for your cam is 14-16 base,,,(that's 36-38 total WOT with the 22 cent)...
test drive that WOT and with vac adv plugged in at part throttle on a steep hill for ping...

your car should be able to handle the timing all in at about 2600-2800 due to the 2500 stall/3.55 gears so next try a lighter centrifugal springs change...
do another test drive

(at about this point you will likely need to change atleast the carb rods to thinner to go further)

for a street tune up,,,I'd call it done....
(vac advance adjusted for approx 50* all in timing total)
I like to stay on the "safer" conservative side for no ping...

here's a good timing article:

http://www.gnetworks.com/v4files/barrygrant/Ign timingwithimages.pdf
 
#19 ·
here's a easy to use gears/tires/mph/rpms calculator:

http://www.tciauto.com/Products/TechInfo/Calculators.asp#6

(LOL, I'll be "Mr. Conservative" on this thread)

flywheel TQ x tranny ratio x rear gears ratio = force on the tires...
so,,,,
deeper gears is more acceleration force at any/all rpms driving normal or messing around
versus,,,
the bigger cam swap helping add some HP at higher rpms and needs more parts changed to make it work and drive decent on the street....

what is excellent on your present cam is you are at the inital HP peak at only 5,000rpms
the 274 doesn't get to the initial HP peak till 6,000!!!
 
#21 ·
only to illustrate what the "real world" result with a cam swap will likely do:

click on the 268 and 274 cam graphs on this comp dyno mule link which is the same build and CR:

http://www.compcams.com/Technical/DynoSheets/

now plug in the "WOW" 27HP 274 cam gain into the TCI weight/HP =et/mph calculator on the link above...

3400lbs/342HP = 12.44ET/108mph
3400lbs/369HP = 12.12ET/111mph

blink your eyes very quickly once,,,that is aprox 1/10th second...
the 274 cam is 3 very fast blinks quicker (whoopee)

bottem line:
do the gears swap first for the most dramatic improvement and then research a cam swap if you do want more...
(the deeper gears=less load on the motor=more timing sooner is OK=more power,,,,the 268 motor may/may not "want" 18base/18cent or 20base/16cent after the gears swap,,,,the Hg is recovering much quicker (motor wind up is quicker) due to less load)

I'd go with 4.56
risk is that will be so much TQ on the tires at launch you will be fighting for traction on a street surface....
illustration:
413 ft/lbs x 3.0(?**) first gear x 3.55 rear gears=4398 ft-lbs of force on the tires
413ft/lbs x 3.0 first gear x "4.56" rear gears=5649 ft-lbs twisting the hides!!!
(?**=do look up your low gear ratio,,,small difference make a big "total" difference)

footnote:
with the present 4400 ft/lbs of force accelerating only 3400lbs,,,your car should "scoot" darn good as is,,,should be no problem lighting them up from a stop sign...

my car, with only 1ft/lb per lb of car weight would burn the tires to the cords if I let it from a stop sign...

?????
check your timing in 500rpms increments with a timing light to verify the centrifugal curve,,,to indicate the cent is working correct,,,not hanging up
 
#22 · (Edited)
Now that the weather is finally good enough to get the car out....I am back to this post.

Car is running great and i have decided to swap the rear gears. Currently running 3.55's with a TH200 OD transmission. Car still has the original (with Eaton POSI) 8.2 10 bolt. Available ratios are 3.73, 3.90 or 4.10.

I am leaning towards the 3.90's but still might be able to run the 4.10's without any issues? I have a 28 inch tire (255/55/17) and at 65 mph would be 2140 rpm in OD?

Am I missing anything?

**MOVED TO TRANSMISSION - REAREND FORUM**
 
#24 ·
wickster said:
Looking for some input....although the engine runs great...there is just not enough oomph. Runs and drives great...but I really want to be able to smoke the tires off the line.

Was thinking a cam upgrade (XE274 or XE284) and or a steeper rear-gear. Any advice? Or am I missing something>

350 SBC, bored .030
Stock Crank
Spped pro Flat Top (~9.5:1)
Stock Connecting Rods
Edelbrock Performer RPM Heads (#60899), 64cc, Straight Plugs
Comp Cams XE268H Camshaft (224/230@.050, .477/.480)
Performer EPS Manifold
Edelbrock Carb, 600cfm
Mallory HEI
1 5/8" Heddmen Headers
3.55 Rear with TH2004R Bowtie overdrive Tranny
Have you verified your carb is tuned correctly by reading plugs in each mode of operation?

I happen to use the Edelbrock 600cfm carb. I get great torque and good milage; I'm sure I loose some ponies above 4 grand. A larger carb could hurt your off the line performance and lower RPM torque.
600 cfm should be plenty of flow for under about 3500-4000 RPM.

If you want to maximize power above 4000-5000 RPM you could use a bigger carb and perhaps a Edelbrock RPM intake, etc.

I like my Edelbrock carb but I disposed of my Hedman headers.
 
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