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Customer Discussions > Automotive forum

1981 gmc k2500, 350 engine questions.

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Showing 1-25 of 45 posts in this discussion
Initial post: Aug 4, 2012 3:15:12 PM PDT
P. Bray says:
Hello, I am restoring a 1981 gmc k2500 with a 350 in it. I am going to do work on the engine, but I don't have a whole lot of experience with engines, only body work. My current issue is the exhaust manifolds appear to have some sort of vacuum lines coming from them. I asked around and the only answer that I got was that they could be something dealing with some sort of anti smog/emissions system. I was told that "sometimes" the whole system can be yanked right out with little to no effect on the engine. This would make it much more easy and affordable to get performance headers on it. On the other hand, I was told that "sometimes" removal of the smog-system causes the engine to run like garbage.

I would imagine this would be dealing with the vacuum of the motor, and I would guess what type of carb the engine had would play a role. Like I said, I don't know a lot about whats going on under the hood, but this vehicle is how I am going to learn. What I do know that it is carbureted and not fuel injected.

Any help on this issue would be awesome!


In reply to an earlier post on Aug 4, 2012 4:50:14 PM PDT
JackV says:
It is an emission device, not a vacuum line. Air gets pumped into this from an air pump. You should also see that (air pump) under the hood connecting to the lines. You can yank the whole system. It does not affect how the engine runs, just emissions.

Posted on Aug 4, 2012 5:34:53 PM PDT
P. Bray says:
Awesome! thats what i was hoping for. I was also wondering about the air intake system. Its got the pre-heater duct for the air intake. Could I put a different style intake on it, such as a cold air or something like that, and eliminate the pre-heater without it effecting anything?

In reply to an earlier post on Aug 4, 2012 5:47:44 PM PDT
Last edited by the author on Aug 4, 2012 5:48:08 PM PDT
JackV says:
If you mean the ducting that takes heat from the exhaust, you can remove it. However, if you live in a cold climate that has "fog"/humidity, this preheat system helps prevent carburetor icing. It doesn't have to be freezing to be a problem. The pressure drop in the venturi causes a temperature drop below freezing and that's where the icing starts. (Airplanes have a special procedure to prevent this.)

There's a vacuum valve that shuts off the heat once the engine warms up. Cold air ducting won't give any noticeable HP difference without also improving the cam+valve springs. Exhaust headers will help a little bit though.

There's a long topic about all this on Amazon. In a nutshell, just a CAI with no other changes is a waste of time and money. But if you like the sound and you have other things planned, go for it. Just keep the cold weather caution in mind.

Here's the topic if you want to read various opinions

Posted on Aug 5, 2012 6:47:32 PM PDT
P. Bray says:
Another question that I have: being that my engine is carbureted, does that limit my options as far as what kind of camshaft, lifter, valve, and timing kit that I can install in the engine? I am looking to do some engine work to the truck, and I already have a new carb and intake manifold picked out. I have been looking on this website,, and they offer kits containing a new cam, lifters, valve, and timing gears and chain all in one. They they have only two of them that say "C.A.R.B. approved". The note in the description says that particular model cam setup is 50 states legal with carburated engines 1987 or older. Does that mean that my options are limited only to kits that are "C.A.R.B. approved"? or could I use any cam setup that matches what I am trying to do with the engine, all-though some of them might not be legal?

In reply to an earlier post on Aug 5, 2012 9:10:01 PM PDT
JackV says:
Depends on where you live. In many states (most?), cars older than 1987 can be modified. It's a 25 year cutoff point. So next year it will be 1988 and so forth. However, for California any vehicle made 1975 and later has to pass emission testing. In that case it has to be a C.A.R.B. (California Air Resources Board) compliant. Check your state to be 100% sure.

So if you pass GO, then any kit can work. Jegs has pretty good deals but not the only source. Be careful not to spend too much though, since you can buy complete engines that might have everything you want with all new parts if you are looking for a big boost. The rest of the engine is still 'aged' with old pistons, rings, bearings, valves, oil pump etc. IOW, I wouldn't do it to an engine with more than ~70,000 miles. Again, your decision. So be sure to add up the price of all the mods and compare to complete engine rebuilds.

Here's a source in my state just to give you an example of cost

There are many others.

Btw, if you happen live in California, then you also can't modify the exhaust air injection system, intake (has to be approved), even though the engine will run just fine.

Posted on Aug 6, 2012 5:11:25 PM PDT
P. Bray says:
Thank you so much for your help! You have been a very big help so far! I lucked out with the truck as far as condition. It's an Alaska truck (i live in Michigan) so it has very very little rust anywhere including the frame. The truck its self was used on an Air Force base so it has very little miles on it (not sure how many hours). But it runs very strong, and only has 64k on it. I'm not looking for a big boost in power. I want to increase the performance yes, but I'm not gonna go overboard with it. Its still going to essentially be my every-day driver (where I live, I rarely travel more than 15 miles one way). Depending on time, and where I am with my life, I might do a complete rebuild on the motor. We will see when that time comes! I know I will have many more questions as I do more research

Posted on Aug 7, 2012 4:18:18 PM PDT
P. Bray says:
I got another question. I dont know if this is what i am going to do for sure yet, but im looking at getting blank cylinder heads. I dont want to get 3mpg, but i want to boost my engines performance. What would be a good size as far as combustion chamber. most range from 67cc to the high 70's. I have already picked out the carb and intake manifold I want to get, and I am going to put performance headers on it. I was thinking of putting a different cam in it, something to boost the torque. I was looking at a cam, valve, lifter, and timing package as i mentioned before. Is there anything I should be looking for as far as cylinder heads, in junction with the cam, valve, lifter, and timing kit. Im not trying to turn my pickup into a drag racer, I just want it to be able to sling some mud. If its not practical then I will probably just stick with the intake manifold, carb, and the headers. Any input on this would be awesome! thanks

In reply to an earlier post on Aug 8, 2012 4:47:42 AM PDT
SpiderWebb says:
Personally, I'm sold on turbos. Build a mild 9.0:1 engine with a turbo cam that will get good mileage and run on pump gas. When you mash it, all hell breaks loose! There are cars with fairly good street manners that have over 1000 HP!

My '87 Buick GN gets about 24 MPG but runs the quarter in the high 11's @118.

Posted on Aug 8, 2012 5:24:30 PM PDT
P. Bray says:
I certainly like the sound of that! Are you talking about strictly a turbo camshaft, or an actual turbo kit?

Posted on Aug 8, 2012 5:27:26 PM PDT
P. Bray says:
also, with that kind of power, would I need to reinforce the frame at all? Keeping in mind the frame on this truck is in nearly pristine shape (for its age. zero rust, the factory coating is present, but peeling off). I've heard of people putting too much power into their cars, mashing on it for the first time, then twisting the hell out of the frame. Would I have to worry about that with a 3/4 ton truck?

In reply to an earlier post on Aug 8, 2012 9:40:21 PM PDT
Last edited by the author on Aug 9, 2012 9:26:41 AM PDT
JackV says:
What is your HP goal and what do you plan to spend? It's very easy to exceed the price you paid for the truck and you'll never get it back, except in enjoyment. Are you manual or automatic?

Turbo's are a way to get way more HP out of a given displacement, but for sure not cheap in your case. Gas mileage really is determined by the size of the engine. A smaller engine that is boosted can match the HP of a larger engine, thus can give good mileage, but that's not really because of the turbo, but more because it's a smaller engine when there is no demand placed.

IMO, the only economical way to do turbo stuff is if a vehicle already has a turbo. Truck diesels are easily increased by 50-100HP by increasing boost and reprogramming. But that's a whole different animal.

I'm guessing you want to stick with what you have and do modest changes with about 350HP max. None of those require frame modifications, except maybe some control arms to control wheel hop. Leaf rear springs on a 3/4 ton pickup should handle it so that very good staggered shocks can control. Frame no sweat. You also need at least a 3.75 rearend gear ratio and even higher numerically if you are interested in acceleration as well as hauling stuff. Even 4.11+ could be a minimum if you don't mind the drop in gas mileage.

A guide to cam driveability is looking at the vacuum at idle and the rpm range. A cam that has less than 16" of vacuum at 1000 rpm is not going to be street friendly see It can also cause power brake issues that require additional pumps to fix.

Much of this revolves around personal choice/preferences. If you have an automatic, you really can't run too much cam without going to a higher stall converter. Cams with idle of around 750 rev to 6500 are about where you want to be. The heads, bore, rockers, etc all interact, so you need to have it all figured out before you buy.

That's why usually if you want more than 350HP it's much better to buy it all done as in the links I gave or Jegs, SummitRacing, etc (see 383 strokers) and get all new parts. Or convert to at least an LT engine or if you want to spend even more an LS engine. It's all done for you.

The relatively easy part is the intake, carb and exhaust. The tricky part is how to get the engine to actually use the easy mods.

What I'm trying to convey is that it's easy to build something that you will not be happy with if this is your first voyage.

Posted on Aug 9, 2012 5:03:31 PM PDT
P. Bray says:
Well this is, in fact, my first voyage. Currently my truck is stock, but in good shape. It has 64k miles on it. Its a 5.7L 350 with a manual tranny. The frame and body are solid. My plans are (after the body work is 100% done and ready to paint) to put a 4 inch lift in it. Im not going to increase rim size, but i am going to put larger tires on it. As far as the engine, I want to increase the HP as much as possible with out completely murdering the gas mileage. As far as mpg, I would be very happy to keep it from 10-15mpg (I rarely, rarely will be doing any serious traveling). I know that no matter what I am going to put headers on it and probably strait pipe it to resonator tips. I was planning on changing the gearing in the axles to be able to turn the larger tires better (but I also don't want to kill the mpg that way either). As far as a budget for what I am putting into the motor of this truck, I dont really have one. Im not going to blow 10k on it, but the truck is a project that I plan on working on for a couple of years, so I plan to acquire the engine parts over a length of time, and starting the engine work when I get all the parts needed.

If I find that it would be worth it, I would be willing to do a full engine rebuild. Once I finish this truck, its going to be buried with me, so me getting back anything for it besides pure enjoyment and pride is out of the question. Im doing a full restoration of this truck, from top to bottom. It will be my every day driver that i take on trails and off road. I am looking to get as much performance out of the motor as possible.

Posted on Aug 20, 2012 5:33:41 PM PDT
P. Bray says:
I have a question about getting the right components to assemble into a blank head. The head says it needs 2.02" Intake Valve / 1.6" Exhaust Valve. I guess my question would be what lifters would I be looking for? do I need specific ones to match to the intake/exhaust valve, or do I just have to match the lifters to the cam?

Posted on Aug 20, 2012 5:36:02 PM PDT
P. Bray says:
I am also going to assume that the cam and lifters should match the RPM range of the intake manifold's RPM range for optimum performance.

Posted on Aug 21, 2012 8:58:48 PM PDT
Bray, can you give links to the products you are talking about?

Posted on Aug 22, 2012 3:32:58 AM PDT
P. Bray says:
absolutely, basically what im looking to figure out, is what would i need to build this head - . I was gonna purchase the parts, assemble it, then take it to a pro to get tuned and checked before i install it to the engine.

In reply to an earlier post on Aug 23, 2012 9:13:39 AM PDT
Last edited by the author on Aug 23, 2012 9:14:41 AM PDT
JackV says:
Bad link. Is this a project for learning? All this stuff is not something that is easy to explain. It also requires machine tools and the knowledge to measure piston-head clearances for valves and so forth if one deviates from stock stuff.

For practical purposes I'd get a crate motor since it will be more than sufficient for your needs for close to the same cost. Actually less if you consider all the new stuff you get in a crate motor.

Posted on Aug 29, 2012 1:42:33 PM PDT
Dean says:
I think you're getting way ahead of yourself. You need to tear that old engine down and see if it needs a rebuild. I'd bet anything it's got scored cylinders, a worn cam, sticky lifters, and burned valves. There is no way you should be adding any kind of performance gear on a 30-year-old engine without making sure it's in very good shape...and trust me, there's no such thing as a 30-year-old Chevy v-8 that's in good shape. These were the worst years for quality, and the inside of that engine is probably junk.

In reply to an earlier post on Aug 29, 2012 6:34:23 PM PDT
P. Bray says:
L. Carr, that is an excellent point, and with the more I find out about I think I am going to go with jackV's advise and go with a crate motor. I started pricing parts and whatnot, I figured out I'd be spending almost as much in parts as I would to get a brand new crate motor. That being said, where would be a good place to start looking for crate motors? Links would be awesome. Thank you all for your input! Been a big help so far.

Posted on Aug 29, 2012 6:35:07 PM PDT
Carl G says:
I have a 21 year-old 350 that's in good shape. It has 252K miles, doesn't burn or leak oil and it gets crappy mileage in the vehicle it's in.

In reply to an earlier post on Aug 29, 2012 8:04:50 PM PDT
Last edited by the author on Aug 29, 2012 8:11:00 PM PDT
Dean says:
Summit Racing has tons of crate motors. And of course there's tons on eBay. Don't straight pipe the truck, you'll get less power and worse mileage than if you just put some good performance mufflers on it. A street vehicle needs backpressure to work properly.

In reply to an earlier post on Aug 29, 2012 8:05:41 PM PDT
Dean says:
You've got one with a roller cam, which he doesn't. The later engines with roller cams were MUCH more reliable.

In reply to an earlier post on Aug 29, 2012 9:19:44 PM PDT
JackV says:
An engine does not require back pressure to work properly. I have no idea how this got started again. Here's a link to save me writing it up

Notice that he's already referencing SummitRacing and I gave him links and other info. There are some shady outfits on Ebay, so it's a good idea to get some background info on the outfit and references. Does no good to have a "good deal" with no practical way to enforce a warranty (know a few that had "florida" outfit issues).

In reply to an earlier post on Aug 29, 2012 9:37:02 PM PDT
Last edited by the author on Aug 29, 2012 10:02:58 PM PDT
JackV says:
I don't recall if Carl's truck uses an LT engine, however, roller cams also existed in latest regular chev vortec 5700 motors. These are very reliable engines, but not because they had a roller cam.

The GM Goodwrench engine is such a motor. It is an excellent choice in this case. They (GM/dealers) sometimes have really good deals. Roller cams per se, don't make for a more reliable engine, however, current oils have decreased ZDDP which makes flat tappet cams wear a problem. One can buy ZDDP or stick to oils with high ZDDP (motorcycle and boat oils have way more). You can put roller cams in just about any common OHV engine.

His engine with 64k is most likely in good to excellent condition. The only reason any of this comes up is the desire to increase HP. Considering cost and experience, it's best to buy a crate motor, either new or remanufactured and long or short block depending on budget.

Here's a link to ZZ4 350-355HP new GM engine where you don't have to worry about what you are getting. I think this drops right in.

Minor mods and this engine can make over 500HP.
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Discussion in:  Automotive forum
Participants:  6
Total posts:  45
Initial post:  Aug 4, 2012
Latest post:  Sep 5, 2012

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