ad356

whats the difference between the fuel economy of a stock 305 tbi vs. a 350 tbi? only mods being exhaust(headers and catback 3"). my 305 achives fuel economy that is acceptable even with fuel prices being what they are. how much of a loss can i expect when i swap out for a 350. i have a 350 4 bolt main, complete sitting im my garage, my plan is to rebuild it and swap it. im probably going to go with a near stock cam, maybe edlebrock RPM or performer series. im probably going to keep the stock heads, i think getting better heads would ruin fuel economy. can i expect to lose only 1 or 2 mpg or is it much worse, would it be half my current millage. im not looking to build a race car, i just want a daily driver street machine. my car rarely sees the drag strip, and when it does its only for the entertainment value. also, is there anything that can be done when i do my build to maintain or even increase fuel economy. im hoping for a ball park millage of 17 city 23 highway. is that attainable with a 350? my car is not a garage queen, i love driving it. its driven everyday except for the winter, when it goes into "hibernation".

i wish i could say i dont care about millage, but at a possible 4.00/gal i do. im not building anything until spring so i have time to think about it.

KrisW

iTrader: (5)

You're really not going to find much difference between the two in normal, day-to-day driving. Many 350's here get better mileage than the numbers you are giving.

Efficiency is what you want, so you need some good heads. Some late model vortecs or even some 87 vintage LG4 305 heads are a good start. A matching performer type TBI intake or a carb intake that's nearly stock will be good. I like to port match, and I swear it makes a difference in driveability and mileage, but others don't, and port a little. Free flowing exhaust and then end it all with a custom PROM tune.

Everything you need to do it is on this site, so I say go for it.

ad356

first off, why would you want to use 305 heads? could somebody explain this to me. are the stock 350 heads really crap? what about L03 heads, which is what my car is equipped with right now. i thought an LG4 was more anemic than my L03 wouldnt the heads be restrictive and the valves be small? someone at work told me if i was going to swap a 305 for a 350 its going to chew up gas. of course the guy that made the comment about it is one of those "boy ricers", he has a POS "tricked" out honda civic. of course he thinks american v8's are guzzlers, that the stereotype.

five7kid

iTrader: (14)

There's a thread on the FAQ forum that describes the differences between 305's, and gives some ideas about how to improve performance with them. In that thread, you'll learn that LG4 and L69 (and LU5 & early LB9, for that matter) used the same 416 casting heads. The L69 was rated at 190 HP. In 1987, LG4 and LB9 engines got 081 castings, virtually identical to the 416's, except with center bolt valve covers and 72 degree angle intake bolts in the middle. The LO3 uses the famous "swirl port" heads, which do a good job at the factory 170 HP rating, but start losing it after that.

If your purpose in a build is fuel economy and low end torque, swirl ports are the way to go. The work great for that. If you want more performance, go elsewhere.

The LG4/L69/LB9 heads flow better than 90% of the 350 heads the factory used through the years. The 3rd gen 350 heads are in the 10% group. In fact, the only difference between L98 and LG4/L69/LB9 heads is the 1.84" intake valves and 58cc chambers (vs. 1.94" and 64cc for L98). The ports are identical.

The typical used/JY 350 has dished pistons. If you don't rebuild with flat top pistons, 305 heads will help raise the compression to a more reasonable performance level. With 1.94" intake valves installed, they will outperform 90% of factory 350 heads, and if dished pistons, will outperform "good" 350 heads.

ad356

this engine that i bought is a complete 350 assembly that came out of a late 80's chevy pickup truck. are the heads on this engine are boat anchors? i was going to have them rebuilt but i dont know if they are worth it. where would i get a set of LG4 heads? i would think i would be able to get a set for almost nothing and have them redone instead. that will increase compression? then i wont be able to run it on regular pump gas, currently my L03 runs great on regular, no issues. i would like to maintain that ability.

just to clarify things my goal with this project is:
1. durability, i want this engine to go through the paces when i ask it to without failing
2. fuel economy- looking for near 25 MPG highway on regular pump gas if possible
3. performance- hopefully 250-275 hp. want car to feel like it has some power. that 305 is pretty anemic, i just want to add some power. im not looking at building a 450+ hp beast, i just want to have a strong rebuild. the sad truth is that my 96 corsica "winter bomb" with a 3100 v6 puts out 160hp. the corsica is lighter, so it probably would be able to keep up with my camaro. thats unacceptable, my camaro needs to smoke it. remember my stock engine is 170 hp, and at 250 hp it would be a 80 hp increase. if the 305 and the 350 weight the same(they should) that would be a nice increase.

do i think 250-275 hp is possible with a 305? yes i do, however i dont think its worth doing especially on an engine that probably is original to my car which has 165K on it. it costs alot of money to do that to a 305. need a new cam, need new heads, need new TBI unit. i rather start out with a fresh rebuilt engine to begin with.

five7kid

iTrader: (14)

Rebuild the 350. Put an "RV" cam in it. It probably isn't roller, or at least doesn't have the roller provisions machined in the block. If it is machined (it's been known to happen), get a roller cam and use the 305 roller lifters and retaining parts.

250 HP is about all you should expect for 25 MPG with TBI.

86irocguy

just keep in mind the deck height and valve clearance when u go to swapping heads from a 305 to a 350 thats a big issue other wise u might end up with some serious internal problems. also when you go to swapping cams you need to take into consideration what rpm range you want this engine to run as for a daily driver, idle to 5,500 rpm's is where u would base your power range to be if you are lookin for fuel efficiency. and be sure to match all your components such as cam, lifters, springs and rocker arms. building an engine and changing valvetrain parts is something that is really critical and if you dont get that right combination you could damage some parts even you engine. just my opinon!!! good luck with your swap i just done a 305 to 383 swap in mine and i love it !!!!!

Fast355

iTrader: (2)

I don't agree with that at all. I was knocking down 17-18 mpg from the 350 TBI in my fullsize van and putting 375 HP to the wheels.

ad356

so what is this swap going to cost me? im thinking 1,500 bucks to rebuild it, get everything ready and installed in the car. i already run a 3" exhuast and headers purchased last year so thats already paid for. 250 hp is probably going to be enough to make my car quite fast. thats more hp than any stock third gen 350. why are the stock rating so low anyways. i think the 350 is something like 230hp. is that because the stock tpi is a major restriction? i have always believed that tbi is actually a superior induction system in the stock form. its much cheaper/simpler to modify as well. to mod a tpi you need 8 injectors, runners, manifold, and other expensive items. with 1 holley 675 cfm unit, and a better flowing intake manifold and away you go. anyways, if i run a "RV" cam and rebuild my existing stock heads would a stock 350 tbi prom be able to support it. can you change the cam without a custom tune? i have an idea how a custom chip is made but i dont know how to make the approiate changes to the tune without throwing it way out of whack.

KrisW

iTrader: (5)

You need to cruise the DIY PROM section to research chip burning. You can swap a cam without tuning, but you'll never really realise your full potential without tuning. Once you start, you are likely to get hooked...

Your heads are a restriction above 300 or so horsepower. I know you say that you want mileage and that's great. The reason people here don't like swirl ports for the most part is that we all feel it's money wasted. Better gas mileage has many, many factors. I would argue that the engine can be more efficient and yield better mileage with the 305 heads due to a higher compression ratio; everyone has their own magic to attaining max mileage...

Bottom line is this: If you get a set of good 305 heads from a TPI, LG4, or crossfire engine, you can put real money in them without feeling that you ever wasted money on them. They will be great for an ultra efficient gas mileage engine and also be a solid part of great performance upgrade in the future.

If you want just mileage, get some good Vortec 350 (L31) heads and run them. They are the best efficiency head you can find in a junkyard and also support great performance.

ad356

i like the idea of vortec heads, i already considered them. however i have to see how much money i have to spend this spring. a set of vortec heads would add 600 bucks to the build, and add a vortec specific intake at 300. so add 900 bucks to the build, i dont know if i can afford that right now. iv figured the stock rebuild to cost roughly $800 to use all stock parts, and rebuild the existing heads. if i could find heads and an intake out of a JY and pay next to nothing for them, i would have them rebuilt instead. would these vortec style heads be found off of a truck engine. were any of the later truck engines tbi? that way i would be able to get a manifold as well.

86transam383

i get 8-9 in the city(lead foot) and i get 22-25 on the highway with a mild-wild 383/700R4.. if i just drive to and from work i can make a tank last a work week(5 days)

ad356

are 305's actually bad for fuel economy? can a 350 actually obtain better millage do to the fact it doesnt have to work as hard? does my L03 actually waste gas because its weak cam, poor flowing heads, and crappy intake design? can a slightly better cam and better flowing heads actually manage better economy? also, do you run TBI on your 383 or is a carb?

InfernalVortex

iTrader: (4)

If L03's were hurting your gas mileage GM would have never made them. The entire reason for their existence was to pad GM's CAFE numbers.

86transam383

i'm running a carb.

KrisW

iTrader: (5)

You can get entire Vortec 350 engines at the junkyard, they came in trucks and vans from the late 90's to 2003, I think... The TBI intake will still have to be purchased, no factory option for that. You might score one from ebay, but most likely you will have to buy that piece new.

I say start junkyard hunting now for the vortec heads and saving for the intake. Run a stock TPI camshaft and your computer will still be pretty happy.

five7kid

iTrader: (14)

1996 to 2000 (I think - not sure off the top of my head whether it's 1999 or 2001).

After that it was the LSx-based engines.

KrisW

iTrader: (5)

My 2001 GMC 3500 Savanna Van has the L31 and another guy here has a 2002 with the L31...

vin code R engine...

Looks like just the vans held out until 2002.

Hope this helps!!

Fast355

iTrader: (2)

Actually there IS a FACTORY intake for TBI and Vortec heads. They came on Export trucks and Boats. They were machined for 2" bore TBI units, but lacked EGR fittings.

KrisW

iTrader: (5)

You got a source for that one? I'd love to put one on my 93 pickup!!

Fast355

iTrader: (2)

I've seen the marine ones on Ebay in the Boat section quite often.

KrisW

iTrader: (5)

Outstanding! Thanks!

Firebird_dave7

hey well let me help you out i have a 350 too and honestly if you put 305 heads on thats a waste cause of the chambers so take your stock heads or some good truck heads shave around .150 thousandths and that will give you higher compression on mid grade fuel(87 octane is good and reasonable)and thats what you should be fueling that Third Gen anyway. Now if thats not in the thought just put a bigger than stock cam and a lil bit bigger intake or i would say carb(Holley) and then exhaust is the next thing, this will help alot!

five7kid

iTrader: (14)

Sorry, I have to disagree. The smaller chambers don't affect anything if compression is otherwise controlled. Shaving the heads to a smaller chamber will have the same effect (while messing up geometry unless angle milled), so what's your point?

It's pretty well established that smaller chambers with a dished piston is a superior approach to larger chambers with flat-top or domed pistons.

Shaving .150", or 150 thousandths? Or did you mean .150 of a thousandths of an inch? Shaving .150" is huge. Perhaps you meant .015". Shaving .015" will drop about 2 cc's, not much of an effect. Little more than surface clean-up, really. Head gasket thickness will have more of an effect.

KITT1983

iTrader: (3)

awesome performance info here