V6 Tuning Question(s)

I am always trying to get my engine tuned as well as possible - and that includes the combination of tuning/jetting the carb as well as adjusting timing. Overall, the engine has been running fine - really darn well from the best I can tell, but I got a few questions anyway. The main symptom that has my attention is that the engine seems to lug more than I would expect in the 1500rpm range (lets say up to about 1800rpm). When it gets up close to 2000rpm, it just hums away nicely. It cruises at 2500rpm so nicely (to my untrained ear and and seat of the pants feel). 2000-2600rpm is where I DD it mostly - and that seems to be a good sweet spot for a v6, but I wish it would pull a bit better at the lower end. With a 3 speed, its a pretty big rpm drop when shifting up a gear, and it seems I really have to push it up to close to 3000 rpm to keep the next gear close to 2000rpm. Obviously, this is exaggerated here in the mountains, but even a slight uphill incline really seems to require 2000rpm for the engine to avoid lugging. I don't know how much of this is just this engine (and getting accustomed to my modern Toyota v6 that pulls well down to 1000rpm) vs what is due to my tuning, and how much might be due to the comp cam 252 grind - which is slightly higher rpm focused than a stock cam. Additionally, I feel like my gas mileage since the rebuild has just been worse than previously (12mpg these days) and I think it might indicate I am not in the optimal range for the engine's performance. Keep in mind of course that I am running 4.88s and 33s - so that seems reasonably appropriate (although I think 5.38s would be perfect).

I feel like it used to pull quite a bit better at the lower end, but I have messed with the tuning over the last couple years and guessing I have caused this issue myself. Right after the rebuild, I was running more advance on the initial timing (12BTDC), and also running a larger carb jet (49). Over time, I have slowly backed off the initial timing (8BTDC) as I was worried I was getting a pre-detonation misfire and I leaned out the carb (all the way to a 47 jet) as it was fouling plugs at elevation. At the same time, it drove ridiculously strong around town (but smelled rich) with the starting set-up. Given that I go from 5000' to 13,000' in the summer, I realize tuning the carb is a bit of a compromise. As I leaned out the carb it runs better at elevation, but its probably a bit lean around town. I tried running 48 jets as a compromise, but was getting plug fouling and rough running engine when wheeling at 8000-1000' so went back to 47s and never had a problem all the way to 13,000'.

So this is just a long winded way to ask if my concerns around low end/rpm performance is probably due to me leaning it out and under-timing too much. I feel like I can't push the initial timing too far advanced given the lean fuel mixture? Seems like a richer fuel mixture allows for more timing - or is that just my mis-interpretation? And what would be the best way to bring back some of that performance given my conditions ? thanks

 

James - I'm with JP on this. The combo of the 33s and 4.88s (which is what I have too) isn't the best with the 3 speed and no OD. The OD makes a world of difference - I had that before going to the SM465 and kept it. The 252 Grind worked great on the motor with a stock carb, 3 spd, OD, 4.88s and 33s. The gear split was needed for sure.

 

Hey thanks guys...I do have an OD and I use it quite a bit (as in I shift both levers quite a bit for a 'half-gear shift'). But here in the city it is a bit annoying to be shifting so much....You know the general pattern here: accelerate from a red light as fast as possible to 40mph, before slamming on the brakes at the next red light a block or two away. I have been driving my jeep a lot this winter - not wheeling it like in the summer - just DDing as its so much fun (new top with heated seats in the winter are AWESOME!). Maybe I am just getting so used to driving it that I am starting to try and drive it like everyone else. I have a 35mile roundtrip commmute with mix of freeway and city streets. Standard OP is to start in 1st+OD, shift to 2+OD then to 3rd, and then to 3rd+OD (if I get there before slamming on the brakes). Sometimes 2nd+OD will lug a bit if I don't run up the rpms in 1st+OD, and 2nd+OD to 3rd+OD is when it really likes to lug. 3rd+OD really does require highway cruising (65mph) and minimal incline. It drives more 'normal' going through the gears without the OD - but people like to run into the rear bumper when I shift out of 1st to 2nd at under 10mph. I thought only a 4cyl needed a safety triangle on the back - but the 33s and 4.88s are probably maxing out the v6.

I think this is an exhaust issue...it mostly fouls the passenger bank....I have a thread from last year asking about this....it runs manifolds into a y-pipe and I think the way the y-pipe joins is not good - its more of a T where the passenger bank exhaust pipe T's into through going pipe from the drive side to tailpipe.

 

you remember well...I would like to fix the exhaust...5 years ago I got rid of the fenderwell headers and shop work for bending the pipe wasn't cheap...definitely regretting I didn't know better at the time. Wonder how much it would be to cut out the T and replace with a Y...or if I could hack it together myself. Another option is maybe I just run dual exhaust to the rear with two separate pipes..but the passenger side muffler is difficult with D18 and rear gas tank.

 

thanks for the feedback all...I think I have a plan - sometimes I have to write my thoughts out and then re-read it a number of times as I read all yall's comments to help my stagnant brain start functioning.
1) I am going to deliberately and little by little bump my initial timing back up - probably going from 8BTDC (where it is now) up to 10-12BTDC (assuming no pre-detonation issues). Manifold vacuum readings suggests it likes a bit more initial timing (actually it likes a lot more 12-14BTDC, but that is what made me nervous when timing advance went all in).
2) I will begin to address the exhaust issues as best as I can given I don't think I can rebuild the entire system at the moment ($). I will probably start with trying to grind and clean up the manifold ports - I've heard this isn't such a fun job and I am probably going to need some special hard grinding bit for my die-grinder to work on the iron manifolds. I will also get some new manifold gaskets - I don't like the fel-pro ones I can get from my local FLAPS as they are individual for each port and I feel like they don't fit or stay in place all that well. I have seen some recommended on here that look way better - don't need the gasket impeding exhaust flow.
https://www.summitracing.com/parts/rfl-13-004/overview/make/buick
3) I may even be able to cut-out my T-junction and replace with a y-junction....but have to learn a bit about connecting or splicing in a section of exhaust pipe. I see a lot of exhaust junk at FLAPS that slip fit together with an expanded end - but then I assume I need to weld it solid. Probably can't do that in place on the jeep and would need a lift to get the entire system in and out as a single piece.

Another thought - how would changing the mechanical advance weights affect things. I don't really have a good way to map out how much and when the advance comes in (graph of rpm vs advance - I would probably need to get an adjustable timing light), but wondering if a softer spring would allow the mechanical advance to come in a bit earlier would help? Or maybe a softer vac advance canister - the high elevation up here limits how much vacuum can be achieved. Might be worth trying to investigate?

Oh - and somewhere along the way I am hoping to get an eaton locker installed before the Rubicon this summer!

 

I've used a tune kit for HEI to adjust the curve based on an article I have somewhere. I'll see if I can post it, but basically I was looking for a smooth curve as the RMPs went up. I did the weights first, then the vacuum advance with an adjustable can there too.

 

arghhh...was under the jeep looking at a few things with regard to this thread...and found a coolant leak/puddle from the heater core....add it to the list of stuff to be fixed....