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Mild 350 build and TBI conversion... Suggestions?

newyorkin

newyorkin

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I have a pair of early 90's 350's from wrecked Police Caprice's (not LT1's) I hoarded a few years ago, and I think they had over 100k on them, but I can't remember. I planned to do a swap into my K5, but then sold the K5 and just kinda let them sit.

I need to swap one into my '85 K20 now. Since the '85 is carb'd and the '91 is TBI, I'm doing a conversion at the same time. The 85's heads are the problem, so I can't just move the old top end to the newer motor.
I have a harness and ecm from an '89 K5, I may go with a GM sender/pump or just do an external and leave the tank stock.

Since I'm going to open the motor and check the bores, change the bearings, etc, I was thinking I might as well beef it up a little. I haven't even taken off a valve cover in probably 8 years (except on my minivan, but that doesn't count), so I can't remember half of what I should know.

I've never done a cam upgrade, so I never paid attention to the numbers...
I figured I'd do a cam on this motor, but I'm really not sure if I'm going to need to change the prom. Will I? Can I get a mild cam that will give me some more power without having to change the prom?

I think it's a roller motor already, so I'm not going to mess with the lifters or rockers. I'd like to leave the heads as-is, but does anyone know if heads from an '87 TPI Z28 350 will make a positive power difference? I'd think not, but I got an offer of a pair really cheap. I haven't looked at my casting numbers, but I don't see why they be different heads...


Any other suggestions for a little more power? I need to keep this as low budget as possible.
 
newyorkin said:
I have a pair of early 90's 350's from wrecked Police Caprice's (not LT1's) I hoarded a few years ago, and I think they had over 100k on them, but I can't remember. I planned to do a swap into my K5, but then sold the K5 and just kinda let them sit.

I need to swap one into my '85 K20 now. Since the '85 is carb'd and the '91 is TBI, I'm doing a conversion at the same time. The 85's heads are the problem, so I can't just move the old top end to the newer motor.
I have a harness and ecm from an '89 K5, I may go with a GM sender/pump or just do an external and leave the tank stock.

Since I'm going to open the motor and check the bores, change the bearings, etc, I was thinking I might as well beef it up a little. I haven't even taken off a valve cover in probably 8 years (except on my minivan, but that doesn't count), so I can't remember half of what I should know.

I've never done a cam upgrade, so I never paid attention to the numbers...
I figured I'd do a cam on this motor, but I'm really not sure if I'm going to need to change the prom. Will I? Can I get a mild cam that will give me some more power without having to change the prom?

I think it's a roller motor already, so I'm not going to mess with the lifters or rockers. I'd like to leave the heads as-is, but does anyone know if heads from an '87 TPI Z28 350 will make a positive power difference? I'd think not, but I got an offer of a pair really cheap. I haven't looked at my casting numbers, but I don't see why they be different heads...


Any other suggestions for a little more power? I need to keep this as low budget as possible.
Click to expand...
Thunder is the man to talk to on this stuff, him and Dorian, they really know their stuff:bow:

I`ve talked with Brian at TBIChips, and there are cams you can use where PROM changes aren`t necessary, such as the Melling M1 (?). All thats need, according to him, is to bump the timing up 2 degrees from base.

EDIT: Here`s a link I got from him tonight: http://www.cfm-tech.com/GM_tbi_tuning_tips.htm
 
You should have 193 swirl port heads on those engines you have. The TBI system has 65 LBHR injectors Roler cam ect. I can think of no better combo in a truck for torque and MPG. personally i would leave them just as they are if you are not going to do any other mods like headders, exhaust ect. If you change the cam and inprove flow you will need a new chip.
Dont put the old 85 heads on a TBI block. You will be totally disapointed with the results. The 85 heads are 8.2 to 1 compression ratio. TBI heads are 9.2 to one they make a lot more power than thold carb smog heads.
The Z28 heads are a diferent head. They will make more power up on the top end but you will loose torque on the low end and take a hit on gas MPG. IIRC the 87 Z28s never had 350s. Only the IROC-Z got 350s in 87. So make sure those heads are not 305 heads. Either way i would not use them on a truck unles you want performance over low end torque and MPG.
 
That's good enough for me, I want torque over HP, so I'll forget the IROC heads (you're right, my mistake, I checked the email again and he did say IROC).

When you say 65lb injectors, do you mean 65lb for TBI, or TPI?
edit: I was planning to get a standard tbi-vehicle gm stock pump.

So you think I should leave the TBI motor as-is?
That's good for me, but I was hoping to get a little more pulling power than I remember my K5's having with the same basic motor (minus the roller stuff).
 
Talk with brian over at TBIchips.com. He probably has some pretty good cam/chip combo's already figured out, and that would save you some hassle.

Of course, headers wouldn't hurt as well, last numbers I saw on a base 350 were roughly +50ft lbs of torque with just a header swap.

Quoting carcraft and another post I made:

"Even on a stone-stock 350ci small-block Chevy, a simple set of 1 5/8-inch headers are worth as much as 53 lb-ft of torque and 34 hp." Thats entirely useable in the 3000RPM range it occurred.
 
The 65 lb/hr injectors are for the TBI. There are only two injectors...so they have to be big.

I thought they were 55 lb though...but either way.
 
Nope, the interceptors got larger injectors than normal TBI 350 engines. They probably have better heads too.

As mentioned above, the IROC heads actually flow really well for a stock casting, right behind Vortec heads. However, TPI was designed to make a ton of torque, while your TBI isn't. That means you'll suffer in the low end department, but will make a lot more power in the upper RPM range.

If you want to keep a stock chip, I've leave the engine relatively stock, maybe adding some exhaust at the most.
 
Russell said:
Nope, the interceptors got larger injectors than normal TBI 350 engines. They probably have better heads too.
Click to expand...

Well that sucks for me... The throttle body I'm going to use is from an '89 'burb 350. The police 350's already had the injectors scavenged when I got them... Ironically, I got one motor when it was complete, all the way up to the air cleaner, and gave it back because I ended up not needing it. Then I got the chance to pick it back up and get a second one with it during a fire sale, and when I got them, both were missing the injectors.

Hopefully, the burb injectors will be fine. It looks like I can pick up higher flow injectors cheap enough, too.

I'm leaning now toward just headers and a stock freshen-up, maybe an aftermarket intake. The worst part of this project will be making room in the garage to work on the motor...
 
If you change injector size, you will need to reprogram the PROM, just another aspect to consider of course. :)
 
If you are going to reduce injector size, another alternative is to increase the fuel pressure to compensate for the smaller injectors:

New fuel pressure, P2 = P1*sqrt(m2/m1)

Where m2 is the required flow in lb/hr and m1 injector size in lb/hr. So if you were going from 65 lb/hr injectors to 55 lb/hr injectors and the initial fuel pressure (P1) is 13 psi then,

Your required flow, m2 = 65lb/hr
Your injector size, m1 = 55 lb/hr

P2 = 13 * sqrt(65/55) = 14.13 psi



___
 
IIRC, the police car motors had a sportier camshaft than did the trucks. Might want to research the validity of this recollection though. Third Gen might have some info on it.
 
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