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Discussion Starter · #1 ·
Hi let start off by saying im a new member to this site, okay

so I have a all stock 1991 chevy ext cab short bed pickup, 350 (tbi) with a 700r4 with 207k miles. Im looking into building a sort of sleeper truck, but this is my daily driver. I just graduated and am working at lowes , I have some bills so I cant just dump a grand in at a time, I wanna start off with my cam selection I spose. What kind of cam will give me the meaniest roughest idel possible? and add some horsepower? I don't really wanna get new heads yet but I wanna hear opinions!! ive heard good things about comp cams and lunati cams, and im gonna run long tube headers with dual glass packs dumped. Also will getting rid of that air box and getting a edelbrock air clean improve hp or just look better?
 

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More machine than man
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We all understand the thin wallet. It is a problem we all have. But before you jump in, know what you are getting into. Throwing in a new cam isn't as simple as trying new spark plugs. The cam is an important engine part and is critical to working with the overall design to produce a matched set of parts — SYNERGY!

The problem is you cannot just throw a big cam in there without attention to the heads. A big cam is going to want a higher compression ratio.
http://www.crankshaftcoalition.com/wiki/Cam_and_compression_ratio_compatibility

A cam with more duration=less torque, more horsepower in upper RPM's, lopping idle, less engine vacuum (impacts power brakes), requires lower rear end gears (ie 3.73:1, or 4.11:1 or higher), requires a higher stall torque converter, not good with heavy vehicles.

Also with more duration comes more lift. The factory heads and springs cannot handle a lot of lift. You will need to verify, but I would assume your max lift from the factory is going to be around .450". So you will need to machine the valve guides down.

You factory cam is probably around 195º @.050" duration. I would suggest getting a cam around the 220º @ .050". This is an LT motor too, right? So it should have a roller cam, which is a good thing.

Of course a new cam means: new lifters, new distributor gear, new gaskets, new timing set, new valve springs. Which is more cost.

You are going to have to get a computer re-tune as well. With you truck this might mean installing a new chip...not sure if hand helds can work on your model. Which is more cost. Not to mention if you start adding more HP and fuel requirements the existing TBI/injectors/fuel pump may not be able to support that kind of power.

Your factory cast aluminum dished pistons suck too. You will want flat top hypereutectic pistons. So now you need to bore, hone, new rings, more money.

And with 207k miles you are due for a rebuild I suspect. I think you would be best to find another block (make sure its a roller cam) and rebuild it, then swap motors. Take your time. As money rolls in pay for the machine work. Get more money, get parts, etc.
 

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Discussion Starter · #3 ·
Will I have to do head work if I just put like an RV cam in? And I think its hydraulic flat tappet but idk for sure what else could I do here and there to get more HP out of my truck? Just gave her new plugs, wires (originals were still in there) and I can tell the difference plus an oil change along with air and fuel filter its running good but I want more power!!!
 

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Wrench Turner
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A cam swap alone isn't gonna net you much power increase.It could make it worse.1st you need to get it ready for a cam swap w/ headers & a good exhaust.An adjustable fuel psi regulator, possibly larger injectors to support the changes.You probably have a flat tappet cam.Your block may be roller ready or not.Even with an RV cam,your gonna @ the very least,have to replace lifters & timing set.With 207K miles,I'm sure your approaching rebuild time.TBI & your ECM are not very tolerant of cam changes w/o a chip upgrade & even then you wanna stay below 218 on @ .050 duration & 112 or above on LSA.
 

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More machine than man
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Will I have to do head work if I just put like an RV cam in?
You will have to inspect your current head to find out how much lift it can tolerate (spring retainer to valve guide). It goes without saying that you will buy the correct springs for the cam, so coil bind shouldn't be an issue.

Read this for more info.
http://www.crankshaftcoalition.com/wiki/Valve_train_points_to_check


I agree with jokerZ71. Do the other bolt on things first. They are cheaper, will result in more power, and gives you a foundation to step up to the cam swap later. Long tube headers. Nice free flowing exhaust/mufflers. New intake manifold.

But again at 207k miles, more than likely you are getting a lot of power loss and current disappointment through blowby on the rings and valves that are not sealing as well as they used to. Which brings us back to the fact that you are beating a dead horse. Either do a complete rebuild or buy a 5.7 Vortec roller block and rebuild that and do a swap.
 

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Hi let start off by saying im a new member to this site, okay

so I have a all stock 1991 chevy ext cab short bed pickup, 350 (tbi) with a 700r4 with 207k miles. Im looking into building a sort of sleeper truck, but this is my daily driver. I just graduated and am working at lowes , I have some bills so I cant just dump a grand in at a time, I wanna start off with my cam selection I spose. What kind of cam will give me the meaniest roughest idel possible? and add some horsepower? I don't really wanna get new heads yet but I wanna hear opinions!! ive heard good things about comp cams and lunati cams, and im gonna run long tube headers with dual glass packs dumped. Also will getting rid of that air box and getting a edelbrock air clean improve hp or just look better?
This is a Throttle Body Injected (TBI), Swirl Port headed engine. You cannot just go change cams and certainly there ain't gonna be any bad *** idle cam going into this as it just will not work with the TBI. The Swirl Port heads have a large swirl inducing vane in the valve pocket. It essentially shuts off flow to half the port. It can be reduced in size and effect but not eliminated. The port walls are very thin so any attempt to modify the vane's shape and size must be done on the vane not the adjacent wall or you'll hit water.

There is not a lot that can be done to these engines that is inexpensive. Improvements can easily get you into a new computer chip. Given that this year truck uses the 700R4 transmission which is not computer controlled like the 1993 and up models, therefore, you can change the engine to a carb and ditch the injection if you don't need to pass an inspection to license it. Changing to a carb isn't cheap to do either but it eliminates the restriction on cam timing imposed by the TBI.

With TBI and not getting into an expensive custom chip you have to do changes that don't drop the idle vacuum below 16 inches at about 600-750 RPM hot. While this proviso restricts cam timing in the range of 200 degrees intake and 220 on the exhaust with an LSA not less than 112 degrees it does allow for some space to improve cylinder head and exhaust efficiency.

- You can convert to headers and duals, including dual cats if you have to maintain the exhaust system in some modicum of legal configuration. Although long tube headers are advisable, the heads and cam really won't allow them to capitalize on the overlap strengths of equal length long tube headers, but you gain a lot of flow efficiency that reduces back pressure to where a 10 to 20 horse gain is common. This doesn't really change the engine's rated power, it lets you gain on the difference between rated and installed power.

- Usually an intake manifold is an early improvement but in the case of TBI the only performance manifold is made by Edlebrock. I'm of the position that for a TBI engine this purchase should be delayed till other more substantive things in the heads and cam are done first, as this intake shows good power improvements on the top end, but without changes to the heads and cam the engine cannot wind into that zone.

- Check your block casting numbers some of these truck engines have the provisions for a roller cam. Given the problems seen with modern oils and flat tappet cams it is well worth your while to use a roller if possible. Although this is not an inexpensive swap it saves the cost and disappointment of the increasingly common flat tappet failures we currently see.

These block castings are roller cam provisioned to some degree but used in trucks most likely have flat tappet cams from the factory:
(10243880 1995 and up), 14011148, 14088526, and 14093638.

These block casting are flat tappet without roller provisions:
10054727, 14079287, 14088548, and 14101148.

Be careful reading the underlined casting numbers they are similar looking but are not the same block, one is provisioned, one is not.

For a roller block the GM cams that will work without reprogramming are:
Durations measured at .050 lift. For lift measured with 1.5 rockers add 7% for 1.6.
- 10241264 1996 Vortec 190/.414 intake; 195/.412 exhaust; LSA 116. (The Vortec cam will run with 1.6 rockers without problems)
- 14093643 350 TPI 87-94 roller 202/.403 intake; 206/.415 exhaust; LSA 115
- 12551705 350 LT1 93-95 roller 201/.447 intake; 208/.459 exhaust; LSA 117 (TBI loves this cam because of the 117 LSA it will also accept 1.6 rockers without issue)
- 12551142 350 LT4 95-97 LT4 203/.446 intake, 210/.450 exhaust; LSA 115 (with more duration and lift with less LSA this cam is a bit edgy with the stock programming, especially if mixed with a 1.6 rocker)

Stay away from the temptation to use the Ramjet cam, although the timing and lift is in the ball park for TBI at 196/.431 and 206/.451, the 109 degree LSA makes it unsuitable for TBI without a new program in the chip.

There are other roller cams for full size passenger and cop cars but they are not of sufficiently better timing than the TBI, Swirl Port flat tappet, truck cam you have of 166/.381 intake and 175/.401 exhaust with an LSA of 112.

OEM roller cams into a provisioned block that uses a flat tappet cam from the factory will require these parts and efforts:
- Provisioned bosses might require drilling and or tapping, not all are completely finished in this regard but many are.
- An installation kit consisting of the thrust plate, spider and "dog bone" lifter alignment bars with fasteners, these can be found at Summit, Jegs and other places.
- One set of 16 OEM roller lifters available from the catalog stores. Skip White has a good price on these.
- One set of OEM roller lifter push rods 7.195 inch compared to flat tappet push rods at 7.8 inches these are shorter to make up for the taller roller lifter.
- I happen to like to use the SA Gear double row roller timing set SA314A in these roller block engines, this does require a little trimming of the main oil galley boss that extends into the timing case (not a big deal). The single row roller timing set from Cloyes will fit without any grinding.

This is about a 500 to 600 dollar exercise to replace the flat tappet cam and improve the cam timing by nearly 30 degrees, these cams time pretty close to the 300 horse 327 flat tappet cam but use the longer duration LSA needed by TBI. They will, however, only take the engine to about 250 horses with a better head because the injectors used by the TBI you have cannot supply enough fuel for more than that. That is also fixable but it's another thousand bucks give or take a couple hundred.

Heads that make a flow improvement and increase combustion efficiency will add power. Where improved combustion efficiency is used the power gain doesn't require reworking the TBI or the computer. So keep in mind that a head change while very important to this engine cannot be fully capitalized on without a computer chip and larger injectors. The L31 Vortec head is a popular swap on these engines and bring from 20 to 60 horsepower depending upon the cam and compression used, the higher power number requires a chip. Chips range from 150 to 350 dollars and don't alwys work te first time requring another chip, The L98 heads in iron or aluminum are not quite as good as the Vortec but they will bring a sold 15-20 hp improvement. The ProComp 190 cc port or the Flow Tec 170cc port aluminum heads can be had for about the price of L31 Vortecs and will mate to your existing TBI or the Edlebrock intake with minor modification of the center bolt holes where the L31 Vortec bolt pattern will not and must use an expensive GMPP intake or modify a carb based aftermarket Vortec intake to accept the TBI with a lot of messing around to get a functional EGR.

Flat tappet cams similar to the timing chacteristics of the rollers above can be used for a lot less up front money:

-Edlebrock 3702 flat tappet uses 194/.398 intake and 214/.442 on the exhaust with an LSA of 112. It will carry about 16 inches of manifold vacuum versus 19 to 20 for the Vortec roller (just a data point. The long exhaust duration seems built in for stock exhaust manifolds and single exhaust and stock converter and muffler. With headers and duals this long duration high lift exhaust is not so necessary. This cam is a bit edgy with its long duration, higher lift exhaust combined with a shorter LSA so it can get you into a chip which Edlebrock will supply if necessary.

- Elgin E860 of 202/.404 intake and 207/.414 exhaust with an LSA of 112 degrees.

- Elgin E274SM of 194/.390 intake and 203/.410 with an LSA of 116 degrees would be very compatible with the TBI and is quite similar to the GM L31 Vortec roller timing.

- Howards 112001 of 202/.404 intake and 208/.414 exhaust with an LSA of 114 degrees would also be a strong contender for 20-30 more HP and torque without needing a new chip in the computer.

None of the above cams is going to provide an aggressive sounding idle. To run a cam like that with TBI is a difficult and expesnive operation on the engine and computer.

The next big problem is the TBI, the unit you have flows about 500 CFM with 1-11/16th diameter throttles. The 454 uses a unit that flows about 600 CFM with 1-7/8ths diameter throttles, Holley makes a 2bbl TBI that looks the same as the Rochester’s but flows 670 CFM using a 2 inch throttle and has a different connector end on the throttle position sensor so requires an adapter to work with the GM wire harness. In all these cases the problem of getting power out of a TBI engine is less of how much air flow and more of how much fuel flow. For these TBI injector systems there are injectors that are rated at 60, 80, and 90 pounds of fuel per hour at about 10 psi fuel pressure with an "ON" duty cycle of 80% of the time. They will deliver more fuel by raising the fuel pressure and or increasing the ON time percent of the Duty Cycle but this eats into their life span. To figure how much fuel an engine needs from the injectors a simple formula is .5 pound of fuel per hour per max horsepower. So a 300 horse 350 would require about 150 pounds of fuel per hour which then requires 2 injectors of 75 pound per hour capacity. Given your current TBI uses 60 pound per hour injectors the power limitation they can feed fuel for is 120 pounds per hour divided by .5 pound per horsepower which is 240 horsepower without an injector change. To get around this problem Holley and others offer 4 bbl TBIs which use 4 injectors.

None of this stuff is cheap and that includes the used parts on EBay. So as much as anything you're limited by how much money you can divert to this activity. Unfortunate a thousand dollars isn't going to go far with these computer controlled EFI engines and this only gets more complicated and expensive as the year of the vehicle and engine converges on current the time. TBI is as simple and cheap as these systems get. If you can convert to a carb the cost doesn't change a lot simply because the limitations of the Swirl Port heads requires they be replaced very early in any hop-up activity.

The factory air cleaner system is one of the better things GM ever hung on these engines. It is effective and efficient; a big advantage to these engines is the ability to draw cool air from outside the engine compartment, which the GM system achieves. The Edlebrock air-cleaner may look cool, but it is ineffective at cleaning air or sourcing it from a cooler place other than under the hood. Every 10 degree drop in intake air temp is worth about 1.0 percent improvement in horsepower; so at 100 degrees outside and 180 under the hood would show an 80 degree intake air temp drop. 80 degrees divided by 10nets a factor of 8 times 1%per 10 degrees nets an 8% power increase which for a 200 horse engine would be a free 16 horses improvement over sucking the hotter under hood air. TBI keeps track of the intake temp and will adjust for the density increase of cooler induction air.

Bogie
 
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