750 carb. vs. 700 CFM

[tiger9297]

Have a '90 K5 and looking to install new engine. Would like good HP and LOTS of TQ seeing as how I tow a camper alot. I am faced with the choice of fuel systems and there is just not alot out there that compares with a carb. when it comes to hp/$. My question is this: I have seen the Holley 700?? CFM TBI which supposedly will support approximately 275 hp. I also see 750 CFM carbs. on 383 making 500+ hp. I realize that there is a 50CFM advantage for the carb., but does 50 CFM make that much difference or will a carb. just make that much more power than TBI? What is the best injection system to make the kind of power I want? I'd like to have 400 lb. ft. TQ.

 

[dyeager535]

Really need to know what kind of engine combo you plan to run first.

Engine size and cam selection are going to play a big part in what you can run, how difficult it will be, and what will work best given your hardware choices.

 

[tiger9297]

I was thinking of going with a 383 or just sticking w/ the 350. I think I'd get more TQ from the 383 though. I have considered a BBC but I think it would cost significantly more to build. I am a rookie at this so let me know if I need to provide more info.

 

[dyeager535]

Well, 400 ft lbs of torque should be fairly easy to achieve with a 350, even easier with a 383 of course.

You aren't going to find a whole lot of people that advocate yanking off TBI in favor of a carb. Theoretical HP limit for the stock 350 TBI is right around 275 IIRC, if you can get lucky in a search, you might find a few posts talking about CFM of the stock throttle body, power potential, and so on.

TBI *is* going to take more work to extract all of an engines power potential, but if your truck runs good now, do you like the driveability short of the power? The major factor influencing someone that doesn't offroad as to injection or carb is the driveability IMO: do you like not messing with a choke, do you like the economy, etc.?

To throw some more info out there, what's a new carb going to cost you, and how important is cost to you? Are you willing to learn to burn your own chips? Very time intensive to learn, but the cost of the actual burning stuff, short of the laptop, is going to be less than a new carb.

 

[tiger9297]

I like the reliability and driveability of the TBI and really don't want to go with a carb. I realize that 400 lb. ft. of TQ out of a 383 is, as you say, fairly easy to achieve, BUT can I achieve that with a TBI? I can tow my camper at about 65+ mph, but I feel like I'm really having to "push" the 350 to do it. I want an engine that can tow at that speed without having to work so hard.

 

[dyeager535]

Well, an obvious first question could be, what gears are in your truck now?

'90 should have your SPID or RPO label in the glove box, any 3 digit codes starting with "G" are what you are after. If you've got 4 digit codes, I believe it will be the 2nd digit as a G.

You are running 33's, if you've got a 700, I'd almost bet that your gears are holding you back.

Gears are a very simple way to get more performance for what you've already got. Considered that at all?

 

[tiger9297]

I have not really considered gears. The engine has 256K original miles and its pretty tired. I need to have a new engine put in anyway so that's a given. Maybe I need to look at gears too. I am not where I can look at the truck right now. All I know is that is came stock with the towing package. I will check the numbers ASAP.

 

[dyeager535]

So nothing numerically higher than 3.73, guaranteed.

What you could be looking at, is a bone stock replacement engine, with some new gears.

I'm sorry I can't comment on the gear situation any better, but assuming your combo as 350/700R4/241/3.73/33" tires, at least a few people on this board will have some input as to how 4.10's or maybe 4.56's would work in your situation.

When towing, do you leave it in OD? Not even getting into the heat/OD while towing argument, just wondering if that is playing into your perceived or actual lack of power. Not saying you don't know, more power is always better lol, but if you tow in OD, lack of towing power until it downshifts would be obvious.

Edit: I did however go back and play around with combo's here http://www.4lo.com/calc/gearratio.htm Personally, as long as you don't want to exceed the speed limit all the time on the freeway, 4.56's are completely do-able. You could probably leave it in OD unloaded, for all but the steepest hills. Loaded at 60MPH in 3rd gear, your engine RPM's wouldn't be insanely high, but enough that you probably won't be lugging the engine nearly as much as with the (assumed) 3.73's.

You'll still want input from people that have done a gear swap with a combo similar to yours, but I don't think you could go wrong with it. With a heavy enough load, no matter what the gears or engine, you'll be going slow, but gears can still help.

 

[ryoken]

the other thing that MAY be a consideration for you, is legality... don't know if you have inspections down there, but swapping a carb onto a 90 up here is a big no-no...

i'm an oldschooler and diehard carb guy, i'm usually fighting technology at every turn... but once i went FI, i'd never go back to a carb for street application..

it may seem overwhelmingly complicated at first, but its really not... you could get a 383 set up with FI and have it be an awesome motor.. just gonna take a little research, coin, etc, but imho, worth it in the long run... thats my plan, FI'd 383... good torque for a sb and drivability...

BB throttle body, some chip, injector and pump work, etc and you can definitely get the numbers your looking for... and if ya don't mind dropping some coin, you could always go with a sweet MPFI setup and really have a nice potent setup..

Dorians got you headed on the right track... with that 700r4 in there, you could prolly definitely use some gear too.. i'd bet you've got 3.73's, not a bad gear. but with bigger tires and OD you could benefit from some more gear prolly at some point...

 

[tiger9297]

There are no inspections down here. You can pretty much do what you want BUT I don't want to go with a carb. I would much rather stick with FI. My question though, is can I get those numbers with a TBI? Do I need to go with the MPFI setup? Seems like by the time I buy a new TBI and manifold I'd be close to the cost of a MPFI system.

Also, now that I think about it I'm pretty sure I'm running 3.73 gears.

 

[ryoken]

well, you can go low buck on the tbi.. modded boneyard throttle body, some nice injectors, chip work, etc and you'd prolly come in at 1/2 of MPFI.. your wiring is already set up, so thats a big plus.

but yeah, throw in an edelbrock intake and a few other goodies and it could get up there.. only one aftermarket tbi intake that i know of, very nice, but a little pricey.. you could definitely get the power numbers...

going out and plopping 1500 or so on multiport would be less research/setup intensive, but costly initially... a nice alternative if you can swing it financially.. i have a Merc marine MPFI setup i'll be running...

honestly, been a while since i researched much on this so i'm not gonna be your best source of info for this.. others with more experience will chime in i'm sure..