intake manifold

[carguy5068]

I was thinking of swapping out my cast iron intake manifold for a lighter alluminum one since i have a few sitting around. the engine is a 260hp 5.7 chevy in a 79 cobalt. one intake is a edelbrock dual plane performer rpm air gap with powerband between 1,500- 5500 rpm. the other one is a single plane edelbrock torquer2. 2,500- 6,500 rpm. my concearns are operating range am i going to lose to much low end power with the single plane to reap any benefit from the higher rpm range? other concearns are water temp. I dont see many alluminum intakes on boats and was wondering why with weight transfer always being an issue. this motor is not a car/truck motor its the origional engine with low hours. never seen salt water nor will it ever. the carb is an edelbrock rpm. model 1910 850 cfm quadrajet. was also thinking of changing cam to a comp cams262. extreme marine but another boat guy said that cam is probably close to what the 260s were running stock and to go with a 268dur 489" lift that comes in at 1,800 - 6,200 rpm. which puts me running the single plane manifold. if this was a car it would be a no brainer to me but im confused here. another guy told me i can make up the low end with a prop but then would i lose the top speed gained from cam/ intake combo? hoping a few more opinions from you pros would help me decide. I know what everyones thinking its an old boat with worst outdrive built, but ive totally rebuilt upper and lower drive on this. and if you saw the boat you'd think it was new. any help on this would be greatly appreciated. p.s im searching out a mercruiser setup to convert from omc comepletely eventually since they made my boat with both drives available. :facepalm:

 

[Bondo]

Re: intake manifold

powerband between 1,500- 5500 rpm.
2,500- 6,500 rpm.
model 1910 850 cfm quadrajet
at 1,800 - 6,200 rpm

Ayuh,.... If ya wanta build a Race Car motor, use those parts...

You've got a Boat motor...
Boat motors have to be built to run,... Idle, to 4800 rpms....

Btw,.... Aluminum motor parts aren't generally used on boat motors, because of Corrosion...

 

[carguy5068]

Re: intake manifold

not looking to build race car engine and the parts ive listed are far from race. just looking see if modern technology could improve on what i have. lots of dual pattern camshafts out there to spread powerband. the marine cam i listed is good from idle to 5,500 rpm, but your saying shut it off at 4,800? Ayuh... not. thanks for the tip on corrosion. I'll go with a standard edelbrock performer also rated from idle to 5,500 rpm. as far as corrosion i will coat water ports with por.15. its heat resistant and sticks like **** to a bedsheet. I use it on insides of corroded gas tanks. the 850cfm edelbrock quadrajet is getting better mileage than the factory 750 quadrajet that came on it. my buddys 97 bayliner has a 3.8 v6 and i highly doubt hes got more low end torque than my 350 but he gets over 50 miles per hour out of it on glassy water due in my opinion to rpm capability and less weight in *** end of boat. which is what i was trying to acomplish with manifold. outboards also get their speed from rpm capability so i guess thats where my confusion came from. b.tw. my factory cobalt boat tach shows redline between 6 and 7,500rpm. is that just for looks?

 

[joewithaboat]

Re: intake manifold

not looking to build race car engine and the parts ive listed are far from race. just looking see if modern technology could improve on what i have. lots of dual pattern camshafts out there to spread powerband. the marine cam i listed is good from idle to 5,500 rpm, but your saying shut it off at 4,800? Ayuh... not. thanks for the tip on corrosion. I'll go with a standard edelbrock performer also rated from idle to 5,500 rpm. as far as corrosion i will coat water ports with por.15. its heat resistant and sticks like **** to a bedsheet. I use it on insides of corroded gas tanks. the 850cfm edelbrock quadrajet is getting better mileage than the factory 750 quadrajet that came on it. my buddys 97 bayliner has a 3.8 v6 and i highly doubt hes got more low end torque than my 350 but he gets over 50 miles per hour out of it on glassy water due in my opinion to rpm capability and less weight in *** end of boat. which is what i was trying to acomplish with manifold. outboards also get their speed from rpm capability so i guess thats where my confusion came from. b.tw. my factory cobalt boat tach shows redline between 6 and 7,500rpm. is that just for looks?

Don't bash bond-o too hard, he is pretty knowledgeable. Although he tends to stick to the "stock" stuff. Lol

Comp cams and Crane, heck any cam manufacturer are going to have good "marine" cams. Flat tappet or roller. The RPM or the rpm air gap work well on small block chevy's. It is important to match all of your components and make sure the will operate efficiently with each other. Lots of good build threads on here.

If you are working with a 1979 boat that has a ton of hours on it don't be surprised if when you change the cam, intake,fuel, ignition system and carb...
All of this could move the operational rpm limit up, make more power, require a new prop. Which would all be lots of fun... but it would probably not last long before coming apart. Probably drop a vavle, burn a piston or spin a bearing. Not to say i wouldn't do it and have fun along the way.

 

[carguy5068]

Re: intake manifold

Im not bashing him I apreciate his knowlege. but cobalts were far from stock brand new. my boat came new with a mallory dual point distributer, lumpy cam, and a 750 cfm carb. the hours are very low and was garage kept no mice took two boxes of moth balls out of it. 1st owner worked 12 hours a day 7 days a week till retirement and bought new boat with 496 magnum and sold the cobalt to son in law. thats where things went wrong. wish i could have bought first cause son in law was an idiot and tore up lower unit imediately. he rebuilt lower end but did nothing to upper which was probly what he tore up in the first place. I had a hopped up camaro on craigslist and this guy called on it offering boat for straight up trade. told me that he screwed up the shift cable while rebuilding lower and showed me every reciept the boat accumulated since brand new. even had shipping reciept for boat and outdrive to dealer it was ordered through so i jumped on it not knowing what i was getting into. turns out gears in upper were totaly shot bearings froze so i bought a rebuilt one on ebay replaced ball gears, seals, bearings, and shift cable. the boat runs great compression test out at over 130psi on all cylynders 45 lbs oil pressure. im not looking to thrash this thing but can't help but tinker with it. bring it up to date. I'll post some pics the teak woodwork the interior period is imaculant. far from played out. just wish it was mercruiser. what was omc thinking with those ball gears???

 

[Scott Danforth]

Re: intake manifold

stock 260 cam is about 220 degree duration, .394 lift

unless your running thru-hull exhaust, stick with the 262 cam.

 

[joewithaboat]

Re: intake manifold

Im not bashing him I apreciate his knowlege. but cobalts were far from stock brand new. my boat came new with a mallory dual point distributer, lumpy cam, and a 750 cfm carb. the hours are very low and was garage kept no mice took two boxes of moth balls out of it. 1st owner worked 12 hours a day 7 days a week till retirement and bought new boat with 496 magnum and sold the cobalt to son in law. thats where things went wrong. wish i could have bought first cause son in law was an idiot and tore up lower unit imediately. he rebuilt lower end but did nothing to upper which was probly what he tore up in the first place. I had a hopped up camaro on craigslist and this guy called on it offering boat for straight up trade. told me that he screwed up the shift cable while rebuilding lower and showed me every reciept the boat accumulated since brand new. even had shipping reciept for boat and outdrive to dealer it was ordered through so i jumped on it not knowing what i was getting into. turns out gears in upper were totaly shot bearings froze so i bought a rebuilt one on ebay replaced ball gears, seals, bearings, and shift cable. the boat runs great compression test out at over 130psi on all cylynders 45 lbs oil pressure. im not looking to thrash this thing but can't help but tinker with it. bring it up to date. I'll post some pics the teak woodwork the interior period is imaculant. far from played out. just wish it was mercruiser. what was omc thinking with those ball gears???

I know about the ball gears, my Dad had one... their weird. Look for a mercruiser drive.
If you are a gear-head, like me, you will like this.
http://www.rlcperformance.com/images/projects/tripple_digits/project_trippledigits.html

 

[Bondo]

Re: intake manifold

what was omc thinking with those ball gears???

Ayuh,.... oh,... boy,.... a Hot-Rod Stringer drive,.... Does it have trim,..??
Those ball gears will Really be singin' at 5500rpms,.... for awhile...

The problem with Big open plenum intakes, Big intake tracts, 'n Big intake valves is,...

The exhaust manifolds are like a concrete Wall....

Ya end up with No low end, 'n ya can't reach top end, because it can't Exhale....

A 600cfm carb, hi-rise dual-plane intake, 'n 180cc intake ports will make a much Better boat motor...

 

[Scott Danforth]

Re: intake manifold

agree, dual plane intake on a boat. dont think race motor, think tow truck motor.

 

[sam60]

Re: intake manifold

My dad was much like Bond-o. He bought a new Cobalt in 72. They offered 2 models. 16' & 18' open bow tri-hull runabouts. It had a 302 or 305 2bbl with a Merc 888 stern. A friend wanted him to hop it up after he had it a couple of years. He responded by saying so, you think I'll be better off if I change from what the engineers that designed this engine thought was a match for the drive? He didn't change a thing.

A few years later as I was installing headers on my 63 chevy impala, he reminded me that a horse that craps fast doesn't crap for long.

I understand the performance wants, but just want mine to get me out and back. Don't want anything with an OMC part number to break.

 

[carguy5068]

Re: intake manifold

I hear ya on those ball gears and yes has power trim. im going with the edelbrock performer dual plane air gap idle to 5,500 powerband. not that I,ll be cranking 5,500 rpm all the time but want the option there. I smoothed out the risers a little but manifolds im not messing with im constantly looking for mecruiser parts to make conversion. after i rebuilt this outdrive i found comeplete running 496 magnum plus outdrive and e.c.m. for $1,800. just didn't have the money at the time and the low price scared me. after talking more with the guy he went with a stinkin diesle yuck!! but all aside ive got under $1,500 in this boat counting the repairs getting it moving so $1,800 496 upgrade would have made this one hell of a boat. to be absolutly honest im burned out on cars and right now theirs no market for them. so this boat has been a good hobby for me. I tried a new 600cfm holley on the cast intake with adapter more power with factory quadrajet. then i tried one of those 650cfm holley quadrajet replacement. better mileage but quadrajet had more top speed and snap out of the hole. but a friend had this edelbrock rpm quadrajet that he knew nothing about but looked brand new he told me to try it and if it worked he'd take one hundred dollars 4 it. wrote down the numbers and slapped it on. fired and ran perfect still haven't touched it with a screwdriver. numbers say its a 850 cfm model 1910. electric choke. this carb runs perfect no load up. oils smells good no gas. and gets better mileage than anything ive put on it so far.I would reccomend this carb to anyone with a modified 350 on up to a street driven big block chevelle. Im also using a bigger spark arrestor/ air breather so that might be helping with the fuel consumption. once i get my mercruiser outdrive im converting to thru hull exhaust. but like the johnny cash song goes one piece at a time. How about the volvo penta outdrive ? whats your take on them? would one hold up to my combination? honestly do you think im wasting my time trying to modernize this older boat cause i look at newer ones all the time and don't see the quality. and from what everyone tells me cobalts were way ahead of there time hullwise and overall design. the teak woodwork is remarkable.

 

[Bondo]

Re: intake manifold

but a friend had this edelbrock rpm quadrajet that he knew nothing about but looked brand new he told me to try it and if it worked he'd take one hundred dollars 4 it. wrote down the numbers and slapped it on. fired and ran perfect still haven't touched it with a screwdriver. numbers say its a 850 cfm model 1910. electric choke. this carb runs perfect no load up.

Ayuh,.... You've built a BOMB, just waitin' to Explode...

Ya absolutely Have to run a Marine rated carb...


Btw,.... I ain't yer Dad,... I'm runnin' a V6 powered tinbarge that Regularly blows V8 powered boats Out of the water....

Not bad for a Full Dress Fishin' barge....


'n 1 more thing,... Put some danm Paragraphs into yer posts, so's they're Readable....

 

[carguy5068]

Re: intake manifold

who said you were my dad? for your information since you probably didnt run the part number my edelbrock quadrajet is marine rated. bushed shafts special vent tubes and i ran the numbers on the origional quadrajet and it was an 800cfm. so the extra 50 cfm probably make a real bomb. as far as your tin barge goes the key word here is tin. my 19ft fiberglass boat weighs 3,000 + lbs. my dads 18 ft jon boat with an 80 horse short shaft merc. passed alot of boats too. 55+ mph but how safe was that? AYUH.... your definately not my dad. He built everything from hydroplanes in the sixties to AA/Altered and BA/gassers to his daily driver hemi charger that ruled the streets from 1968 to 1972. right down to pull trucks in the 80s. so if he were alive today I wouldn't need your advice. He also didn't act like an arrogant know it all either. his philosophy was the only dumb questions are the ones not asked. so like the old saying goes opinions are like A holes everyones got one. mine is v6s are the biggest waste of metal created car or otherwise. my 350 powered 88 Iroc camaro gets better mileage than my daughters 99 grand prix with 3.8 series two engine. my buddy took a 4.3 v6 out of his s10 pickup and put a tbi 350 out of a 95 caprice. he gets 22 mpg highway and outruns new mustangs. and it has over 100,000 miles on it. so unless your v6 is a big 2 stroke outboard I wouldnt be racing any boats with cobalt written on the side.

 

[Bondo]

Re: intake manifold

who said you were my dad?

Ayuh,... My bad, it was mzahn that said that....

'n nope, I didn't look up the part #s of the carb... a 750cfm is too big for a small block...
At 5000 rpms, it's only movin' less than 550cfms...
Too big of a carb is as bad as to big of an intake tract...

so unless your v6 is a big 2 stroke outboard I wouldnt be racing any boats with cobalt written on the side.

My tin barge is 4.3l, V6, Mercruiser... Which is Very Well set up...
'n yer right,... It's pretty hard to read the word Cobalt in my mirror....


You build whatever ya want, any way ya want,....
I'm outa here,... You already know all the answers anyways....
Have a Nice day......

 

[joewithaboat]

Re: intake manifold

LMAO.... Bond-o you deserved that one.

 

[sam60]

Re: intake manifold

Ayuh,.... You've built a BOMB, just waitin' to Explode...

Ya absolutely Have to run a Marine rated carb...


Btw,.... I ain't yer Dad,... I'm runnin' a V6 powered tinbarge that Regularly blows V8 powered boats Out of the water....

Not bad for a Full Dress Fishin' barge....


'n 1 more thing,... Put some danm Paragraphs into yer posts, so's they're Readable....

Yep, That's definitely not my Father's Oldsmobile! EDIT-All meant as positive and I love the Bond-o comments.

 

[carguy5068]

Re: intake manifold

LMAO.... Bond-o you deserved that one.

yes he did! like to see him put that 4.3 in a boat that weighs more than 1000 lbs with him in it. at least he's got a boat anchor ready. he's running an engine that wheigh's almost as much as a 350 that produces less power and is less economical. then to say a 750 is too much carb for a 350 running 5000 rpms. when the origional carb on from my 350 powered boat was an 800 cfm quadrajet. guess he dosen't realize quadrajets are spreadbore and vaccum secondary to boot. guess i should call my brother and tell him the 780 holley that came from factory on his DZ302 powered 68 camaro is a bomb ready to blow before he putts it through another parade. I also looked up some new marine quadrajets all rated between 750 and 800 cfm. imagine that! guess they didn't consult with bondo.lol remember those turbo charged 4.3 v6 typhoons, cyclones, that barely mustered 13s through the 1/4 with all wheel drive? my buddys rusted out s10 extended cab 2 wheel drive that runs high 12s with 100.000 mile tbi 350 with single exhaust and street tires that he drives to work everyday. that gets far better mileage than the 4.3 he pulled out.

 

[joewithaboat]

Re: intake manifold

yes he did! like to see him put that 4.3 in a boat that weighs more than 1000 lbs with him in it. at least he's got a boat anchor ready. he's running an engine that wheigh's almost as much as a 350 that produces less power and is less economical. then to say a 750 is too much carb for a 350 running 5000 rpms. when the origional carb on from my 350 powered boat was an 800 cfm quadrajet. guess he dosen't realize quadrajets are spreadbore and vaccum secondary to boot. guess i should call my brother and tell him the 780 holley that came from factory on his DZ302 powered 68 camaro is a bomb ready to blow before he putts it through another parade. I also looked up some new marine quadrajets all rated between 750 and 800 cfm. imagine that! guess they didn't consult with bondo.lol remember those turbo charged 4.3 v6 typhoons, cyclones, that barely mustered 13s through the 1/4 with all wheel drive? my buddys rusted out s10 extended cab 2 wheel drive that runs high 12s with 100.000 mile tbi 350 with single exhaust and street tires that he drives to work everyday. that gets far better mileage than the 4.3 he pulled out.

easy now....
I have a 4brl, 4.3 in a 19 foot bow-rider that runs 50mph and sips fuel. 4.3 is a great little engine. Not to say i wouldn't rather it be a 5.7, but it would use more fuel.

 

[Bondo]

Re: intake manifold

easy now....
I have a 4brl, 4.3 in a 19 foot bow-rider that runs 50mph and sips fuel. 4.3 is a great little engine. Not to say i wouldn't rather it be a 5.7, but it would use more fuel.

Ayuh,..... At 150lbs Lighter than a SBC, yet kickin' out the horsepower of a 2bbl fed 350,.....

Ya, the 4.3l is a Real Junker motor.....

 

[carguy5068]

Re: intake manifold

now your talking out of both sides of your mouth and thats being polite. the mods im making to this motor aren't going to make it a racer or a bomb as you put it. the cam i chose is a marine profile designed to give more power at lower rpm but still have pull up to 5,500 rpm. the manifold i chose matches the cam perfectly idle to 5,500 rpm. and takes almost 50 lbs off my boat the carb is 50 cfm more than what the boat was born with and marine calibrated. in a boat with ride and comfort of a cadillac. I plan to pull the kids on the tubes and cruise. and i bet I'll be able to do so more economical than before. Im constantly searching salvage yards and boat adds looking for mercruiser set up that will fit my hull. until then im babying my omc. If you have a problem with that i guess its your problem. in other words save your breath you might need it to blow up your dingy.