LG4/L69 vs LO3 w/ TBI

[Stomis]

Im mulling over the idea of just running a 305 in the s10 project. I was going to go with a hopped up 4.3 but I figured if a stock 305 comes in at the same power whats the point if it fits. Ive already got the TBI harness in the truck from the 4.3 which IIRC will run a v8 with a dizzy swap and ecu change.

Im shopping for a used motor right now and seriously plan on keeping it stock other than an aluminum intake to ditch a few pounds. I was wondering how the TBI would act on a gbody stock 305 w/ the different heads/cam. I could always throw an LO3 cam in said motor, will the swirlport vs 305 HO heads really make a difference with the tune?

 

[dyeager535]

I'd say just run it and see. Since the 305's were "high compression" with terrible chambers, they are prone to detonation, but no way to really know until you throw it in there. If you found a proper PROM (shouldn't require ECM change that I can think of, unless you are talking electronic or non-e trans) for a 305 you'd probably be pretty close.

I assume swapping heads after it's installed would be pretty painful in that body? I really don't know the various 305 engines well, but I can't imagine a swirl port to non or vice versa would make a ton of difference. The HO 305 was the most powerful made IIRC, and I don't think it was a massive increase over the non-HO variant 305's power.

 

[Stomis]

I'd say just run it and see. Since the 305's were "high compression" with terrible chambers, they are prone to detonation, but no way to really know until you throw it in there. If you found a proper PROM (shouldn't require ECM change that I can think of, unless you are talking electronic or non-e trans) for a 305 you'd probably be pretty close.

I assume swapping heads after it's installed would be pretty painful in that body? I really don't know the various 305 engines well, but I can't imagine a swirl port to non or vice versa would make a ton of difference. The HO 305 was the most powerful made IIRC, and I don't think it was a massive increase over the non-HO variant 305's power.

I had an HO 305 in my monte ss and to be quite honest I was pretty impressed with that thing when I juiced up the timing.

Im no TBI expert but I think its the cam thats most important. And yes I think you're right I just need a dizzy and the prom changed.

 

[dyeager535]

Thing for the Monte SS was that they got 3.73's later on (think maybe 3.42 early) so the 305 could really spin up pretty fast and be wound out well. Same engine in a K5 with 3.08's was pretty bad. Could just go a lot faster in 1st gear lol. Put an SS in my buddies C30 with 4.10's and it pulls fairly well. Still running at least!

Cam can certainly change enough aspects to need a tune no matter what to get best performance, but with the 305 I'd be real leery of timing and fueling. IIRC the S10's were pretty much about the same weight as the G-bodys, and if that plus the gearing were the same (or close, although S10s were what, 3.42, 3.73, and less commonly 4.10?) as what the PROM was tuned for, it would be pretty close.

Most of the 350's were relatively low compression, so much more tolerant of fueling and timing being a bit off, which is why you hear people say they swap cams in TBI 350's and they "run fine". GM was obviously concerned enough about the 305 that they came up with ESC/EST solely to combat the potential for detonation on the early high compression 305's, which the 350's did not get.

In the GM OBD1, even auto to manual trans change alone resulted in fairly significant changes to the way GM programmed timing in TBI. Here is a comparison I came up with some time ago (RPM on left obviously, load increases left to right across the top)

It's not a massive difference, but those timing differences at various load/RPM combinations can cause issues, or help. I've never looked at the 305 stuff. Again, my only concern is the tendency to detonate that the 305 has, but without running it as-is, no way to know for sure if it will matter.

 

[Chevy305]

I had the L69 swapped in my K10 when I first bought it. With just headers, aluminum intake, and a Holley that thing would get it. At least I mean it would pull past stock truck 350s which still isn't much.
The biggest difference between the car and truck 305s are the compression. I think the cams might have been slightly different too. Sorry I'm no help on the injection front.

 

[dyeager535]

According to Wikipedia by 1985 all 305's were "high compression". Went to flat top pistons across the board. Not that Wikipedia can't be wrong, but it sounds about right, IIRC my '83 K5/305 did not have ESC.

 

[livid4dirt]

My 83 K-5 with a 305 did have ESC.