View Full Version : 350 Vortec build help

12-09-2012, 10:49 PM
Hi guys, I am in the process of building a 350 Vortec for My z-21. I need help and opinions with the top end. I will be using a 2000 5.7 4bolt main block with the stock heads. My machine shop looked at the engine and it has such low run time he is just going to freshen it up (re ring, bearing kit, brass plugs, etc) my questions are, what cam, intake, carb, combo? what should I run for an electric fuel pump? any opinions on roller rockers? I Would like somewhere between 300-350hp. I dont want to go too much more then that because of useability, and the Alpha drive. Thanks in advance, Andy.
I did get new non white wall tires for trailer lol.

12-10-2012, 08:01 PM
Comp Cams 264 hr xtreme marine cam . Send the heads to the machine shop and get new springs to handle the extra lift. If you are in fresh water or have freash water cooling the edelbrock performer rpm is a great intake and a dialed in 650 holley I think you will be real happy probably in the 350 hp range

12-10-2012, 09:38 PM
i have a 350 Vortec 4 bolt that i just finished building. i put a ZZ4 cam in it with a Marine Power midrise intake and a 600 vacuum Holley carb, 1.6 roller rockers. i'm figuring around 320 hp. runs good in a 16, last time out 65 at 5500 rpm. needs more prop now.

12-11-2012, 07:44 AM
Excellent! now talking to my machinist, he said we may have an issue with the studs pulling out of the heads with increased spring pressure, and to do a 1.6 don't I have to drill the pushrod holes?

12-11-2012, 07:53 AM
the stock springs only have about 80-90# seat pressure at best, you really only need to increase it to about 125-130# on the seat with a lift safety factor of about .560 total valve lift, I believe the 3927142 GM springs would be fine or even the new beehive GM replacement that a couple company's offer... the seat pressure isn't what lifts the studs, it's the over the nose pressure using dual springs... you may have to have the machine shop cut down the guide bosses and use a positive valve seal, but this is no biggie to do. Depending on what heads you use you MAY have to open up the pushrod holes with a drill bit about 1/8" or so, once again no biggie, and I absolutely like the 1.6 rocker idea, as it makes the cam *think* it's a little bigger, typically it's a 3-4% duration increase going from 1.5 to 1.6 rockers.

just my .02

12-11-2012, 07:58 PM
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Running 1.6's on my set up and as Buzz said you will need to open up the pushrod holes a little, I went with Eldebrock E-tec E-200 alum heads with the larger intake and exhaust valves. Also went with the increased spring pressure in the beehive springs, we went with comp cams don't remeber the part number and went with a .510 lift mid 230's duration cam. Was told that anything over about .490ish would require the beefier springs.

12-11-2012, 10:26 PM
Im running the 1.6 rollers on pressed in studs and beehive springs. The rockers are self aligining due to no guide plates. Ive taken it upstairs to 5500 for an extended time with no ill effects (yet!!). I didnt open up pushrod holes and seem to have no problem with rubbage. It sure does come alive over 3400 when secondaries open up, kinda feels like an Acme rocket strapped to the azz. lol. What do you guys think about high volume oil pumps. Was told that I may have a problem when she's spun up with cavitation due to oil being pumped to the top too fast!

12-11-2012, 11:33 PM
I'm running a HV pump, spin it up to 6,000 and haven't starved it on oil yet, I'm running a 7qt pan with the tall merc/Wix filters that hold just under 1 qt.

12-12-2012, 07:59 AM
Thanks Guys for the advise I will definitely do a high volume oil pump. My heads already have stainless exhaust valves, and are in good shape. what should I do for a timing chain? I was thinking a gear but the noise would probably drive me nuts. also carb question... mechanical or vac secondaries, and edelbrock or holley? and with the 1.6 rockers did you have to get special push rods?

12-12-2012, 08:46 AM
If your going with guide plates you will need hardened pushrods

12-12-2012, 08:50 AM
I havent starved yet either but I dont have a large capacity pan. The guy I work with who's been build engines for 20 years cautioned me on it. They learned the hard way on stockcar small block. Blew it to pieces first time out, oil starved lower end. Come to find out stockcar motors can't run high volume pumps cause your never out of the throttle long enough for the oil to get back to the bottom. But hey I haven't seen a drop in my pressure yet so I'm gonna TRY and not worry about it! Holley!!!! I'm running a vac. 600. Mechanicals are somewhat easier to dial in, but a properly tune vacuum works just as well. Vacuums supposed to be better on fuel, but it that was a major concerned we wouldn't have boats, right? I bought a secondary spring kit to tune how fast they open but havent taken the time to play with it. We have speed limits on the water here so I need a calm gulf day to test and tune. Single or double roller timing chain. Chuck the OEM mesh chain.

12-12-2012, 09:17 AM
IMO, the 600 is borderline too small

if you insist on using it, use the yellow secondary spring that comes in the multi spring kit and put 68 jets and a 6.5 PV in the primaries...

12-12-2012, 09:26 AM
On the 350 Vortec I had in the Minx started w/a 600, and was marginal.
It ran much better w/the larger carb I replaced it with.

12-12-2012, 10:12 AM
wow lots of good info here, I am planning on getting the rollers that dont require guides, my question i guess is the length. And I guess I 'll get at least a 650 holley. I already have the brand new DUI marine hei ign set up that was on it and Ive heard some chatter about the gear needing to be replace to properly mesh with the cam and not wear out, nbd. So if you guys were in my shoes, would you use the 1.6 roller rockers without putting studs in? And I'll order a large oil pan. My only other question is going to be the intake because the quadrabog on the factory one probably only cleared the hatch by 2-3". will a midrise as mentioned be ok?

12-12-2012, 10:59 AM
GM sells the correct distributor gear for the roller cam upgrade..

12-13-2012, 08:11 AM
IMO what a lot of builds miss is a windage tray. Do any of you think your oil sits obediently in the bottom of your pan? Think again!!!!!!!!! Every bump makes the oil splash many times splashing up into the crank shaft where it gets whipped into foam. I would not put any marine or performance engine together without one, pretty cheap insurance!


12-13-2012, 08:32 AM
Thanks Buiz. I went through several different CFM calculators to determine engine size versus carb. They all put a 350 ci engine not needing anymore than something like 580 cfm at 5000. Now those calculators may be for vehicles and not marine applications, I'm not sure. Extended high rpm running is never really seen on four wheels like that of a boat. That being said I would like to throw a 750 on for comparsion. Dont want the engine leaning out because the carb is at the very top end of its limitations. May go ahead an try your tuning suggestions.

12-13-2012, 10:13 AM
the carb/engine size calculator is to determine the MINIMUM cfm the engine needs...

not what it SHOULD have, or NEEDS

with that said, my train of thought is to add 20-25% to the minimum calculation, in this case 700-725 cfm should handle things comfortably..

12-13-2012, 01:14 PM
I gotta 750 Vacuum sitting at the house, although its not a marine Holley, I think it should work for testing? Whatcha think?

12-13-2012, 01:16 PM
:crossfing: :tv: :)

12-13-2012, 02:25 PM
Cross my fingers huh? lol not very promising answer. Think I will wait to get a marine carb!

12-13-2012, 03:59 PM
you have it, you said it was for testing, so use it.. :cool:

12-13-2012, 07:07 PM
My new 325 hp small block crate engine from mercruiser came with a 750 carb if that helps.

12-17-2012, 10:23 PM
OH YEAH! ITHINK SHES GONNA LIKE 750 CFM. Didnt get out to test it but running it on the hose she seems to be alot more snappier off idle. Can't wait til Sat. to dunk her in.

12-17-2012, 10:44 PM
Good to know about the 750, I am currently carb shopping for my 350 Vortec. Going with a Performer RPM and a big stinking flame arrestor, just haven't decided on the carb yet. Please post an update once you test in the water!

Just Say N20
12-18-2012, 08:16 AM
Keith Eickert built a bored out 1972 400 for my Ski-Sporter. I supplied him with an Edlebrock Performer RPM dual plane intake. He tested a few different sized carbs because they were available, and ended up with a Holly 750.

I realize that the carb is just one part of the package. The dyno numbers bear out that this was a good choice. Max hp of 430 @ 5,300. Max torque of 490 @ 4,300. At 3,000 rpms the engine is at 263 hp and 460 ft/lbs. And this was with old school, 202 double hump heads (albeit ported/polished). My understanding is that Vortec heads flow much better right out of the box than these did before being massaged, and maybe even after being messaged.

12-19-2012, 09:12 AM
Ok, so I'm thinking 600 to 750cfm marine square bore carb for my 350 Vortec. Any recommendations on particular models? Vacuum vs. mechanical secondaries? I have had Holleys in the past on hot rods, but not married to them. Honestly I would like something that is pretty close to "turn key" since I don't have an AFR guage on the boat and don't want to get into a tuning nightmare. Seems like the Edelbrock/Carter is easier to set up vs. a Holley with accelerator pumps, jets, squirters, power valves, linkage cams, etc.

The Edelbrock 1409 (600cfm) gets a lot of great reviews from small block guys, but based on some of the comments on this thread may not be as optimal as a 750. I understand that in theory, ie you can jet down a 750 but you can't squeeze any more air through a 600. Just wondering if the juice is worth the squeeze or if the 1409 would be less of a headache overall.

Also, any thoughts on phenolic or aluminum carb spacers for the dual-plane RPM intake? I have plenty of room under the hatch and they are inexpensive, so if it's worth a shot I'm all ears.

OP, not trying to derail your thread! Just thought I would ask some similar questions here to avoid starting something very similar to what you're working on.

12-22-2012, 10:43 PM
Well got to go out and try the 750 today and I'm not convinced that its better than the 600. Here's why, lost top end by 4mph and rpm by 400. Only got 60 at 5000, same prop. Was 64 at 5400 with the 600. Now that being said I'm not sure what jets are in the 750, is it running too fat. Was the 600 too lean hence making more power? I can tell you the transition on the 750 from primaries to secondaries was unnoticable, meaning to me that its closer in the ballpark. The 600 had a ledge at transition, felt like a shot of nitrous. Maybe need to jet down the 750 and try again.

12-23-2012, 08:40 AM
you sure the secondaries were opening?

joseph m. hahnl
12-23-2012, 09:23 AM
My understanding is that Vortec heads flow much better right out of the box than these did before being massaged, and maybe even after being messaged.

Two reasons one is the intake ports are taller. But that is not why they flow better.This is probably the only instance you will find that size doesn't matter.

Believe it or not it is port profile. The main reason is, there is a large radius at the bottom of the intake port instead of a 90 degree sharp corner down to the rails. This was a huge learning curve for GM and I think that a stock vortec will out flow most after market pre vortec hi perormance heads just for this one reason alone.

Ok, so I'm thinking 600 to 750cfm marine square bore carb for my 350 Vortec. Any recommendations on particular models? Vacuum vs. mechanical secondaries? square bore carbs are fine, but the intake should be a dual plane spread bore they are much larger and still fit square bores carbs.Rule of thumb is Mechanical secondaries for marine application as manifold vacume can be all over the place as the prop is not allways in a under a load condition such as when you are on top of the chop. Eddlebrock is a very simple designed carb that is easy to get gaskets and service. I've serviced mine right at the dock, Can you say Ethenol?

12-23-2012, 10:37 AM
U know honestly I couldn't tell u. Never checked to see if they were. Had my dad with me when I was testing and didn't pull the arrestor to see.

12-23-2012, 11:11 AM
or the throttle total travel is too short..

12-23-2012, 12:50 PM
Here's a video of it running. http://www.youtube.com/watch?v=QLfSWT10Tlo

12-23-2012, 01:36 PM
After you see if your throttle plates go to full wide open throttle position as Jim suggests ~

It sure sounds like a 750 CFM carb is what you need on your engine based on your build specs. With it you should see improvement on your top end RPM and not a decrease in speed.

You might want to put a bit of actual boat running time on your new 750 carb then "read"(inspect) your sparkplugs to see what their ceramic center insulator near the spark gap looks like.
Very dark, & sooty, looking and you are running way too rich a mixture and you may need to jet-down a bit etc.

Be sure you don't lean out the carb's jets way too much by putting in too small of jet sizes as it could easily result in burning some piston tops, & valves, due to running a too lean carb fuel/air mixture under the very severe constant load conditions of a marine/ boat engine.

joseph m. hahnl
12-24-2012, 08:15 AM
I got a little lost on this one. The RPM and Speed are from the same engine
with the 600CFM ? same cam? Roller cam? Same Intake?

If it's all the same then the secondary and the throttle plates would definatly be a good place to start. I would think that the vacume acuater is pre set and have springs available for different levels of vacume . In other words you will have to run a vacume gage on it to see where your manifold vacume is at. and see if the acuater is in the vacume curve.

12-24-2012, 09:00 AM
Just a carb change. I can tell u that the difference in engine sound between the two carbs is quite noticable. The 600 seemed to wind higher kind of a singing sound. The 750 never got that kind of noise, just seemed like it needed more...... something. Now keep in mind this is not a marine 750 but a street vacuum 750. Holley has completely different specs as far as secondary metering plate and primary jetting.

12-24-2012, 09:19 AM
Now I have to admit this is my first v8 boat, always ran black eggbeaters. The correct prop choice is critical for these boats. My question here is would a heavier pitch prop make the engine develop torque because of load and letting the secondaries work better? As suggested yesterday I checked the travel of the throttle and its getting full movement. I pulled the vacuum pod and checked the diaphargm, its good! The spring seemed a little lite. If the secondaries are opening too soonthen it won't build the torque correct? So we are now at jetting, its not jetted like a marine carb. I have to decide wether I want to spend $450 for a 750 marine, wondering if its really worth it. BTW not done a spark plug check yet.

Phil S
12-24-2012, 03:02 PM
I would definitely check on what Buizilla is suggesting about the secondaries not opening. You might just need a lighter spring. As you probably know, Holley sells a "quick-change" kit, and a spring kit that comes with multiple color-coded springs (probably less than 30 bucks for both). On my last boat, it took me several changes until I found the sweet spot for the secondaries to open. When they opened, you could definitely hear and feel the difference.

Phil S.

12-24-2012, 03:49 PM
Vacuum diaphram operated secondary throttle valve plates are a bit tricky as there usually is not a whole lot of vacuum left to operate them in a boat engine application which is under a constant heavy load condition all the time as compared to an auto application.
You did say that this old 750 carb was originally from an auto application did you not ?

For this very reason I like a mechanical linkage style secondary valve plates better in a marine application.

As Phill points out, along with Jim, you. may need a far far weaker vacuum diaphram bias spring for the 750 carb to work properly in a boat.

With these vacuum diaphram & spring operated units it's all about trial & error until you find just the best diaphram bias spring for your specific application in order to allow the secondary throttle plates to fully actuate.

As Phill suggests~~ Buy the spring kit from Holley. The additional Quick-change kit also makes it far easier to experient with various springs quickly & easily .

Phil S
12-25-2012, 07:25 AM
"If the secondaries are opening too soonthen it won't build the torque correct?"

In my experience, when the secondaries opened too soon, the engine "bogged-down" for lack of better words. That was telling me I had too "light" of a spring. I changed it for the next heavier spring rate, which wound up being the correct one for my application. This was on a V-6, with a smaller CFM Holley, but it should react the same on your application.

Phil S.

01-03-2013, 02:22 PM
Just a carb change. I can tell u that the difference in engine sound between the two carbs is quite noticable. The 600 seemed to wind higher kind of a singing sound. The 750 never got that kind of noise, just seemed like it needed more...... something. Now keep in mind this is not a marine 750 but a street vacuum 750. Holley has completely different specs as far as secondary metering plate and primary jetting.

On my current 350 2-barrel Vortec setup, with a low-rise intake and maybe 1" of flame arrestor, it sounds like a shop vac at anything over 3.5k rpm. At WOT it is unbelievably loud with a lot of resonance. The 2 barrel Merc carb flows something like 475cfm, which is marginal to feed a 350 (at 100% VE a 350ci @ 5k can move 506cfm). I definitely can't turn that kind of rpm right now, more like 4400.

I tried cleaning the flame arrestor and it didn't have much of an effect. I think the corroded aluminum fins combined with minimal surface area is just very restrictive. Ran better with the arrestor removed entirely (picked up maybe 150rpm). At the end of the day though I just don't have enough carb and intake to feed the motor at the top of the powerband.

On another thread I'm documenting my conversion to a 600cfm Edelbrock 1409, high rise intake, big flame arrestor, etc. Finally decided on the 600 after reading some comments on other boards and here, particularly this thread and input from Randy "Big Grizzly" C: http://www.donzi.net/forums/showthread.php?28245-carb-suggestions He liked the Holley version and gave some convincing info on the perils of over-carburetion.

If you stick with the 750 it definitely sounds like some tuning is in order. I remember nightmares trying to find the exact right combo for Holley carbs (vacuum and DP) on various hot rods over the years. There are just a LOT of potential combinations of parts to find the ideal setup, and without a dedicated AFR gauge and some experience it can be frustrating. Going with the Edelbrock seemed to be much more "plug and play" for my combo, even if I leave a little bit of power on the table vs. a perfectly tuned Holley. The 1409 and 1410 (600 and 750cfm respectively) are available both new and rebuilt for under four bills all over the place, maybe an option before you go really nuts and spend the same money trying to marinize your 750.

CP Performance has some good deals going on flame arrestors as well FYI. I like the K&N with a "funnel" base but it was too tall to fit under my hatch. Found a standard 5" tall flame arrestor at CP for short money.