350

[33fire]

Ok I have a guestion for anyone, I'm trying to build a 33 chevy rat 2 dr sedan
and do the car under 5000 so I already have a lot of parts and I just picked up a 5.7 1986 truck, block # 14093638 and heads are 14102193, I no it's a low comp, motor but I would like to use it even if I only have to change the heads, ok now I have a plan to use my tunnel ram with 2 550's on it and I would like to use a thumper cam. So does anyone out there think I can make this work. Thanks

 

[skull]

bottom end

it will work, but it'll be a dog on the bottom end, no torque for take off unless ya use really low gears.

l know this 'couse l built almost the same thing and O to 6O you timed with a hourglass.

Later

 

[sgtpontiac]

There is a ton of variables that present themselves

first of all....by low compression do you mean 8.5 or so??? What is the lift and duration of the cam you are looking at... You said thumper cam so I'm assuming that it's a longer duration wider overlap to get the big cam lope?? I would go to the comp cams site, Edelbrock or other engine builder site that would work with you on setting it up....my opinion is that low compression motors don't normally respond well to long duration / overlap cams and multiple carbs.... and I think the twin 500's would be WAY too much for your needs...unless they are forced induction and then you don't need or want a ton of duration or overlap...more lift maybe... as I said, you have a ton of things to consider and we'll need some more specific info to help you...JMHO...maybe others will have another opinion.....

 

[33fire]

it will work, but it'll be a dog on the bottom end, no torque for take off unless ya use really low gears.

l know this 'couse l built almost the same thing and O to 6O you timed with a hourglass.

Later

I like the hour glass, I do have a 10-1 350 on the stand but I was holding off on that one but it looks like It's coming off for the rat I guess I was just hoping it would work but I already new the answer thanks for all the help.

 

[33fire]

first of all....by low compression do you mean 8.5 or so??? What is the lift and duration of the cam you are looking at... You said thumper cam so I'm assuming that it's a longer duration wider overlap to get the big cam lope?? I would go to the comp cams site, Edelbrock or other engine builder site that would work with you on setting it up....my opinion is that low compression motors don't normally respond well to long duration / overlap cams and multiple carbs.... and I think the twin 500's would be WAY too much for your needs...unless they are forced induction and then you don't need or want a ton of duration or overlap...more lift maybe... as I said, you have a ton of things to consider and we'll need some more specific info to help you...JMHO...maybe others will have another opinion.....

Thanks for your help I think it's going to stay on the stand.

 

[33fire]

Ok thanks for your help and yes it's a low comp, motor#14093638 block and the heads are #14102193 alls in real good shape also I do plan on running a thumper cam the small one with twin 550's and I no I'm going to have to rejet them to make it run, now my new question is the valve springs I've been looking and I'm not sure what spring to use because it's not a race motor but I no that I need to change them any help would be great thanks again.

 

[sgtpontiac]

Man........you open a can of worms with each ??

Valve springs should be consistent with the type of cam, the rpm you intend to push it too and generally the parts and pieces you are using...... is the cam you are considering compatible with the type of intake and carbs you are running.....so on and so on...
I've seen so many people buy tons of go fast parts on a motor and be sadly disappointed by how it performs....they assume the parts they buy will work on any motor......not even close to the case...you need a cam and associated parts that will work with the heads (compression) and exhaust, intake and carbs you intent to run.....even the ignition, ign. timing play a role ....... if you want a strong motor....if you are going for looks and not performance......then i guess it doesn't matter....a big tunnel ram sitting there looks great.....but if it won't idle, or bogs so bad you bite the steering wheel if you jump on it or it won't pull until your above 3000 Rpm....to me that isn't fun...so I would sit down and put some things in order...what do you want visually, what do you want performance wise and then get with people who can help you put together a killer power plant that will run good, idle and give you goose bumps when you hit it...so planning is key to a good motor.....parts....not so much....also, you'll save money in the long run... JMHO

 

[skull]

motor planning

the 327 l have in the dodge truck is a great example.

the build on it is a stock stroke steel crank, 194 open chamber heads, 10:1 pistons, edelbrock torker duel plane intake, high out put HEI, duel roller timing chain and this cam:


* Advertised Duration (Int/Exh): 260/268
* Duration @ .050 (Int/Exh): 194/202
* Gross Valve Lift (Int/Exh): .383/.401
* LSA/ICL: 112

l had it in a work truck, 1/2 ton 78 chevy and it was a turd. the the 3:73's were the wrong gears , weighed 55OO pounds, turbo 350 tranny, never could find the right timing and l tried several holley carbs that were was not jetted correctly or to big or to small.

l took the motor out of the chevy truck changed a few things. just putting a 600 edelbrock carb, 2OOR4 trans, 3:36 gears, 4950 pounds, played with the timing & put it in the dodge truck and it runs like a monster.

parts are vital, choose what you want it do, drive or show, it's your'e choice in the end.

Later

 

[33fire]

Ok yes a low comp, 8.5 to 9 and I'm useing a thumper cam but it's a small one that caps out at about 5500 to 5800, but I'm looking for a spring # for the vortec heads.

 

[sgtpontiac]

I would use the factory ....5000 to 5500rpm see below

Ok yes a low comp, 8.5 to 9 and I'm useing a thumper cam but it's a small one that caps out at about 5500 to 5800, but I'm looking for a spring # for the vortec heads.

you can get a good set for not so much....if you contact comp they can direct you to the proper set.....although I'd have to say that a set of factory springs will handle 5000 to 5500 without too much problem....(unless very well used) ....if they are old, change them go to comp or other source for aftermarket ones....JMHO....I ran a set of factory springs on small blocks without a problem under 6000...even with a powerglide and 373 gears...5500 without a problem.....all day long... didn't like 6000

 

[33fire]

Valve springs should be consistent with the type of cam, the rpm you intend to push it too and generally the parts and pieces you are using...... is the cam you are considering compatible with the type of intake and carbs you are running.....so on and so on...
I've seen so many people buy tons of go fast parts on a motor and be sadly disappointed by how it performs....they assume the parts they buy will work on any motor......not even close to the case...you need a cam and associated parts that will work with the heads (compression) and exhaust, intake and carbs you intent to run.....even the ignition, ign. timing play a role ....... if you want a strong motor....if you are going for looks and not performance......then i guess it doesn't matter....a big tunnel ram sitting there looks great.....but if it won't idle, or bogs so bad you bite the steering wheel if you jump on it or it won't pull until your above 3000 Rpm....to me that isn't fun...so I would sit down and put some things in order...what do you want visually, what do you want performance wise and then get with people who can help you put together a killer power plant that will run good, idle and give you goose bumps when you hit it...so planning is key to a good motor.....parts....not so much....also, you'll save money in the long run... JMHO

Ok thanks again ok first the car is more for drive close to home and it's going to have open headers it's not for show or race I have other cars that get up and go and I do understand about the performance parts and the money part this one's more about the look and that's it, I didn't want speed. Thanks again for your help as soon as I can figure out how to install photos on this site I'll do it.

 

[donsrods]

A quick way to bump up the compression on a 350 is to run 305 heads, they have smaller combustion chambers. You will gain about 1 point, but they do not flow as well on top end. I ran 305 heads on the 350 (1999 Vortec) in my 23 and had no issues with the extra compression.

Don

 

[Torch]

The problem with low compression coupled with high rpm cams is not the compression but the inability of the high speed cam to fill the chamber at lower rpm's

pressure changes by the square of the change in volume or something like that. I can't remember the law but take an air compressor for example...
Constant volume in the tank, constant volume of the compressor
You would think at a constant speed the pressure rise would be constant but no it is exponential

By increasing the pressure in the cylinder of an engine at tdc a small amount you can realize more work to the crank by the small increase in compressed air resulting in exponentially increased pressures under combustion.
a small amount of squeeze results in many times more energy harvested out of the same amount of fuel when it burns
so raising compression increases the efficiency at which the burn is converted into pressure against the crank
same thing with rod-stroke ratio
putting a longer rod in an engine changes the angular momentum of the piton as it moves in realtion to the crank

a shorter rod pulls the piston away from tdc earlier but the piston moves slower mid travel
A longer rod pulls the piston away from tdc later allowing the maximum pressure from the burn to push against the crank for more degrees of rotation before the piston drops below lets say the top 25% of travel. The results in more time the pressure pushes when it's at maximum.
The longer rod has slower piston speed in the top and bottom 25%'s of travel but faster piston speed mid travel. This gives the engine more time to burn the fuel at high RPM's...
The speed at which the fuel burns is constant so the faster the engine turns the more the fuel is still burning when it heads out the exhaust. slowing the piston speed down in relation to crank speed increases efficiency at high rpm's

The short rod engine on the other hand more easily converts piston movement into crankshaft rotation because the shorter rod goes off plumb faster and throws the piston force into the crank at a steeper angle

both of these short rod and long rod engines I'm describing have the same displacement and run the same crank. I'm just describing the difference in physics related to the piston speed and rod angle.

The shorter rod engine will pop the vacuum signal to the carb earlier and stronger but peter out mid travel
comparatively the longer rod engine will have a more gradual rise in vacuum signal due to the slower piston speed at first but then will pull vacuum harder in the middle 50% of travel down the bore than the shorter rod engine

The short rodder will need larger valves and intake ports to supply intake momentum to that strong early signal and will be more sensitive to valve overlap and timing.

The long rodder will not begin to breath as soon but will have more of it's breathing focused in the middle 50% of piston travel making the long rod engine less crippled by the high overlap.

Running longer rods gets more flow out of your intake ports, it allows the burn to remain at maximum pressure longer and gives it more time to burn.

You would think low compression would be a disability but not necessarily
The change in volume from the top of the deck to the piston increases the same in a low or high compression version of the same engine
BUT that engine doesn't run without what? a combustion chamber!
so you have to factor that in
and when you do
The high compression engine from tdc to lets say 30 degrees degrees of crank rotation experiences a greater change in volume than the lower compression engine does
greater change in volume means more pressure drop
Also there's the fact that a larger combustion chamber holds more air/fuel to burn.
dropping compression ratio gains volume

As long as you're valve timing and other specs are not screwed up and you can fill that chamber it will hold more mixture than the high compression engine will
larger engines are less sensitive to overlap. the overlap bleed-by becomes a smaller and smaller percentage of chamber volume as engine size increases

All this physics is why a Pontiac with a longer rod than a BB Chevy can breath fine on an intake port that would choke a BB Chevy
It's probably why I could run 302 degrees of overlap and 512 lift with 8 or 8.5:1 compression and still have bottom end...it wouldn't work like that on a stock chevy.

Another reason I could run that much duration is the cam runs a 113.5 degree "lobe separation angle" where a BB Chevy cam would be ground on a 104 or 108 degree spread.

You will best maintain your torque staying close to the factories 114 or so degree LSA.
When you tighten up the LSA it increases overlap and moves torque from the top and bottom of your powerband and focuses it into the targeted range of your cam.
A cam with a 102 degree LSA will have a narrower power-band than one with a 114 or 118 degree LSA but it's peak in the band will be stronger
102 good fr the track if yoru setup to run the mill in a tight rpm range.
112-118 better for the street
The part about duration that hurts bottom end torque is the overlap that comes from the exhaust closing later and the intake opening earlier, not the duration itself.

it's the change from a 118 degree LSA to a 103 degree LSA that kills your bottom end not necessarily how long or how far the valves are opened.

I'm running 288/298 (adv) degrees duration and 520 lift on my ford 460 and it's powerband is from idle to 4,500 because it's on 112 degree LSA and the other timing how the cam is indexed to the crank is tuned for lower end power..the valves being open longer and farther is not detrimental at all in this case. it idles stone cold dead in the water like a Cadillac...not a ripple. but it has 395hp at 4500 rpm and 520 ft-lbs at 4,000 rpm through stock manifolds and motorcraft spreadbore (and 10.75:1 compression XD)

For your 350 stock bottom end I'd recommend a dual pattern cam like a comp cams 268H or maybe 280-290 degrees duration on a 112-114 degree lobe center and a powerband from 1500 or 1800 to 5,000 rpm to make use of that low compression and help out with that intake and stick with 1.75" tube headers.
For economy you want to be under the powerband at cruise figgure out if your cruising along at 65 and downshift...that rpm you hit the next gear down from top gear should be right at the bottom of your powerband

Like with my Pontiac 400 with the 302 duration cam... (2000-6000rpm) it cruised the highway around 1200 -1400 rpm in drive with the highway gears and got 18 mpg in a 73 grand prix
And when You'd downshift at that speed it would flash to 1800/2,000 rpm right at the bottom of the cam's range.and it would lay scratch and in about 4-5 secs you were going 80-90 mph
all on 8:1 compression

That 460 I built idles like a caddy, sounds like a school bus and I would guess is close to being as fast as that grand prix was...in a 77 f150 full time 4x4.
but the truck is geared a little lower and it's power tops out at 4500 rpm and that's right where you can feel her run out of wind.
That grand prix... I let my freind drive because I was drunk...
From the 9th street onramp in Lincoln ne to the L street offramp on I80 about 45 miles it took about 18- 20 minutes ( I fell asleep in lincoln and woke up in omaha 15 minutes later and said why we still in lincoln)
He was cruising her along somewhere around 130-140 mph for 45 miles so low compression is not necessarily a handicap
I put my hands around his neck as good friends do and shook him vigorously yelling "you have never been under my car...you have not seen the horror...it is not allowed to go that fast"

he called me up one day "Torch can you come look at my car something's wrong"
"What's wrong Tony?"
"i just have to show you"
so we goes for this little ride around the block see?
and after we climb the hill he says:
"My brakes haven't been working right...this pedel doesnt do SNUFF and I've been using this little pedal over here and its.... uh oh... it's not working now"
we crest the hill... cars parked on both sides of the street... headlights from an approaching car illuminating windows...
About the time we see the oncoming car he finds 50' of curb and swerves around the one oncoming car and rides it out to the bottom the parking brake barely grabbing
we pop the hood so I can take a peek at the fluid and the engine is COVERED with iron from the rotors.

So he's not done yet...wait there is more

he calls me up a couple nights later...im in my grand prix he's in this buick century
he had driven down the hill around the corner to climb the hill and ran out of gas right i the middle of the street and needs to get the car back to the flats so it will run long enough to get it back in the driveway before the tow truck comes.
I back up to it... bumpers match...
Tony this is what we are going to do...
Take your car out of park keep the wheels straight just like they are and I will use my brakes to very carefully ease you down the hill (he still has NO BRAKES)
so we start to move then after about 5 feet I hear the parking pawl trying to click into place, his car is slamming into mine which I just painted, I had cherried out the body and painted it the glossiest of black and polished the chrome..1973 GP with cragar ss it was hot. nice and shiny

So I'm like (smash bang cringe) sorry pal...(crash crunch)...your on your own
And I punched it and got the hell out of the way...he rolled backwards up onto someone's bank
The funny part about it was all the broken tail lights and plastic in the street was from his car and my bumper not the other way around like I thought
I was like OOOOH boy AHHHHH "Stuff" whooo hoooo bwaaaaaaaa ha ha ha ha ha awww look at this mess i told you ...i told you awwww never mind ahhhhhhh haha ha haha this is going down n history!

 

[Old Iron]

You can advance the cam 3 or 4 degrees and you'll have more bottom end torque