What's new
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.

planning a 350 rebuild focusing on bottom end tq.

L

Lawrence0485

1/2 ton status
Joined
Nov 9, 2011
Posts
118
Reaction score
1
Location
Arkansas
Well the 350 im running is pretty well worn out and using a lot of oil lol. Since this truck is my daily, im going to build a complete long block and do the swap on a weekend. My truck is an 86 k10, sm465, low gears in the axles and running 33's and speedo is still 11% fast according to gps.

I just re-did the exhaust...1.625" long tube headers into 1.25" dual exhaust into 44 flows exiting out behind the rear tires. H pipe going to happen soon when I get a few hrs free.

Ive got a good .60 over 4 bolt main block, with flat top valve reliefed pistons and 5.7" rods. For budget its going to stay a 350 instead of stroking it. Im going to use the old 882 truck heads and just do a casting clean up on the ports and round off any sharp edges in the chambers.

Im thinking of using this cam, its power range is 600rpm-4600rpm so it should still have enough breath to run 75-80 tops on the interstate.
http://www.jegs.com/webapp/wcs/stor...uctId=29408&storeId=10001&sType=&pgGrp=search

Im trying to pick an intake. I have a few stock cast iron intakes but this intake looks pretty apealing.
http://www.jegs.com/webapp/wcs/stor...uctId=69876&storeId=10001&sType=&pgGrp=search

Ive got several carbs to play with but they will have to be jetted down
Holley 750 vac sec with mech choke...this is prob what im going to try first although its to big
Holley 750 double pumper, mech sec...no choke
Q-jet that came off 305-350....this will be my sec option but ill need to go to school to learn how to set it up.

Any ideas on pulling max bottom end tq out of a 350? Or thoughts on this combo?
 
I think you are throwing away a lot of power by sticking with the old heads.

Vortecs are a bonafide 40HP over the older heads, and to properly have a set of old heads reworked will be in the $300 range at least. That was my cost probably 10 years ago.

New vortecs can be had for ~$350/head, and with flat tops and good quench (.015" shim gasket) should put you right at 9.5:1 compression on 87 octane.

I think most will agree that for an on-road vehicle that isn't raced, vacuum secondaries are where they are at. Note that the q-jet you have is either a 750 or 795/800CFM. The whole X CFM for Y displacement has pretty much been disproven throughout the years, only now does it seem like people are listening. Carb design has much more to do with how it works in a given application, than does CFM. A carb will only flow what the engine pulls (normally aspirated), especially on vacuum secondaries.

That said, you will have a hard time finding someone that can fault a properly tuned Q-jet in any application.

With an extremely small cam (right around .450 max lift as I recall) and the very flow-limited TPI runners, running 100MPH+ is not a problem.
 
See Ive thought about the vortec heads but from what I understand they will shine higher up in the rpm range were the old truck heads will pull harder on off idle tq..I could very well be wrong. Id like to find a dyno sheet of a simular motor as Im thinking about and then one with vortec heads see which makes more tq.


This is the best example I can find..in this link, you have 260hp/350tq cast iron head 350
http://www.jegs.com/webapp/wcs/stor...10002&langId=-1&productId=85884&storeId=10001

Here is the vortec motor..Wikipedia
"The Vortec 5700 produces 255*hp (190*kW) to 350*hp (261*kW) at 4600 rpm and 330*lb·ft (447*N·m) to 350*lb·ft (475*N·m) of torque at 2800 rpm."

Well I thought I had an argument lol but it looks like both heads make the same tq just the vortecs will pull better in the upper rpms

I just want to make sure I make the right decision lol
 
Well I've done more reading and I've seen it both ways for towing..some say vortec some say old 882.. The heads cost the same new at about 300each, intakes cost roughly the same. Vortec I'd have to get new rockers, valve covers, and dished pistons to get the compression back in the 9's...

I'm reading that the old style heads will make better off idle tq but the vortec will make more power..so while towing and rpms are 2500-3000rpms what are you gonna use more tq or Hp?
 
So 50% of the time the truck will have 1200lbs in the bed or be pulling a heavy 18' trailer with a heavy stock car. The other 50% it will just be an empty truck daily driver.

On one hand if I go vortec ill have the better chamber that might make better mpg..I'm sure it will make better mpg on the interstate just because it will breath better and be more efficient.

On the other hand if I go iron head it will prob have better mpg than the vortec because it will make better off idle tq which will be more efficient around town. Also this way will be cheaper because I already have the pistons...if I go vortec with the smaller chambers ill have to get dished pistons to get the cr down In the 9's

I'm in the middle of the road because either is going to be a serious upgrade to my worn out 350, huge single plane intake, and mid-high range cam...this motor was in the truck when I got it. The vortec head motor will be more fun to drive daily I'm sure with the extra Hp.

Decisions decisions lol
 
You'd have to compare a 1988 TBI motor to the Vortec motor. The Vortecs have a TINY camshaft stock (although the one you linked appears to be as wel), but you can only really compare stock to stock. FWIW I think people here tend to advise against the motor you listed, instead going with the 290HP variant(?)

The more efficient combustion chambers of the Vortecs alone are worth it, the higher compression is "free" since the octane requirements are the same. What kind of pistons are in it now? Need to know the "dish volume" (valve reliefs count, I see there are new to me piston designs without traditional valve reliefs!) of the pistons to calc the compression. Compression ratio can be all over the place based on head gasket, etc.

I don't know where I came up with my figures, but I was certain I had come up with 9.5:1, which is fine with 87 octane if fuel and timing is right on the Vortecs or similar fast burn chamber. Ideally you want the distance from top of piston at TDC ("deck height", stock is .025") to the cylinder head at .040" with gasket figured in, which realistically is the .015" shim gasket as it is sometimes called. This is why dish pistons are garbage. And why I'm confused as to why GM kept using them, even on the Vortec-headed motors that would have been better suited with good quench.

The Vortec's often measure 67CC's (vs advertised 64CC's) so that would drive compression down a bit as well. Based on their flow numbers they are not exceedingly high flowing heads (unlike the large runner heads that you can buy), they just flow good.

If the price is similar, I would probably go with Vortecs if my compression was reasonable. If the price isn't similar, the older heads will probably do what you want. With the shim gasket compression with 76CC chambers (is that what yours are?) would be near 9:1, which I am unsure if that is acceptable with the old chamber design and 87 octane gas. Probably pushing it at best.

One thing, depending on price to find them, would be to find another set of the same heads, and have those rebuilt, so your down time was minimal. I also know some machine shops have heads on the shelf already done, normally from customers who changed their minds, or never paid.

Valve covers should be a dime a dozen, and take-off crate motor Vortec rockers are probably available pretty reasonably as well, but you are right, it's additional cost.
 
Well im prob gonna buy everything new so that it will just be an engine swap. The graph I listed is very apealing and the cost will be the same vs the old style heads. It seems I was wrong on my calc for compression on the vortec. I don't know the cc value on the flat tops I have yet but they are 4 valve reliefs. The 290 hp motor is a fair comparison for me because thats about what id build if I used the old style heads..I think since the cost will comparable for me since in building from scratch, so why not go vortec..Im pretty much set on vortec at this pt.
 
Only problem I see is getting compression to a tolerable level without using dished pistons, plus keep quench right. Bad quench helps induce detonation, I'd be leery of running higher compression and the "standard" .039 or .043" head gasket with .025" deck height.
 
Ok I see were I messed up...having never built a sbc I figured that it would have a zero deck ie: pistons flush with deck on the block. If the norm is around .02 then..4.06/3.48/76cc/.06 head gasket gets me 8.9..thats ignoring the volume of the notches in the pistons.

.04 gasket gives me 9.1 ignoring valve notches again..

So really just have to wait till I put the bottom end together to see what the deck is going to be.

Also ill just be assisting in the build. Ive got a really good engine builder friend local to me that will take charge on this one..all my experience is in mild to wild air-cooled vw motors
 
Don't forget bore diameter of the head gasket. Many are 4.1". Where are you coming up with a .06" head gasket? That's a really thick one.

In any case, pretty tough to get to 9.5:1 compression. Whatever I'm running is flat tops, 4 valve relief pistons, .039 head gasket, and stock Vortecs, so theoretically I'm around 11.29:1, and I run 87 octane. Took some tuning, but no knock. Something ain't right. :) I've gotta see if I have pictures of the build that show my pistons. I am certain they were not dish, but thats the only way I see getting there unless I'm figuring something wrong, since way back when I came up with 9.2:1...

One alternative, depending on your budget, is to go aftermarket, which would cost more (No idea what the new trick flow super 23's are for quality, but under $1000/assembled pair), but would allow you to reuse anything you wanted to from the current motor. Better performance to boot.
 
Ya so im not buying the .06 thick ones 100+ each lol.
If the piston is around .02 below the deck on the block, ill have no prob getting compression in check.
 
Yes, with big chambers you can get low compression with no effort. :)

FWIW, I didn't cover it before, but my opinion is that given the same truck and same displacement, without doing some crazy modifications, fuel economy isn't going to change drastically no matter what you do. Injection, better heads, roller cam, roller rockers, headers, tuning, you name it.

Our mileage is horrid based on vehicle weight and aerodynamics. Adding or subtracting 100 cubic inches is big enough to affect economy obviously, but all the "efficiency" modifications I've done don't net me any better economy than when the truck was a plain-jane 70's motor with a carburetor.

I wouldn't regret not going Vortec by thinking I was giving up MPG in your case.
 
Ya I made the mpg statement before I saw the dyno charts. Now I bet mpg will actually come up some (after break-in) just from having a fresh engine that actually is built for a truck. I don't expect great mpg..now its getting about 10 but its pig rich and a very crappy combo for a heavy truck
 
Ya I'm super excited..heck I could build a bone stock motor, leave two spark plugs out and it would prob still pull harder than the tired motor in my truck.
 
This one is a 383 but i still like it. Stock vortec heads and a performer style intake. Start watching at the 5:00 minute mark.

[YOUTUBE]Bmj48i_v6-8[/YOUTUBE]
 
You must log in or register to reply here.

Latest Posts

Top Bottom