Just saw this OP January post. Probably too late, but here's my 2 cents... I've had the options the OP is asking about.
Caveat 1: My results are driver impression. Nothing was dyno'd
Caveat 2: All the time, changing the specs of each scenario will have a noticeable effect. Powerplants are a "system". If you want max performance, then improve the entire "system", not just some components. I have not wanted to spend the time & $$$s to do that, but I am happy where I ended up with my Sunday driver.
Caveat 3: I discuss I-6 options below. I am NOT providing feedback on other engine swaps. I can't speak to those -- others can and
will do that... just wait
- Stock 258 with 95 MPI: I had this first. MPI (and I suspect any fuel injection upgrade like TBI) is a HUGE upgrade in performance. It did have little predetonation when I floored it, but usually there was none with 87 octane.
Many (most?) people get a lot of predetonation when putting on MPI. An adjustable fuel pressure regulator didn't help me those times when I floored it.
I don't think TBI has predetonation issues, but some states DMVs won't pass TBI, only MPI (I got the HESCO MPI sticker).
- NOTE: I can't speak to carb upgrades, but after years fiddling with an OEM carb on my AMC 304 1979 CJ-5, I swore off carbs (not the food)... life is short and I'm not a "Carb Whisperer".
- Stock 258 with 95 MPI and 4.0 HO head and stock exhaust and TJ OEM air intake.
Result: No noticeable difference. And the predetonation did NOT go away. And Yes, I had the 7120 HO head.
- Upgrading the exhaust would have helped, but then again upgrading the exhaust without the HO head would have helped.
Fast forward 9 years.... then I cracked the block's water jacket at the weakest headbolt found on #6, so off I was to replace engine... cheapest route for my "around town family Sunday driver CJ-8" was a budget stroker:
- "Budget stroker" 4.0L block, 258 long nose crank, +30 over and decked to 0.013" head ported/polished ($2,296 including core and S&H in 2017) with '95 MPI, stock exhaust and TJ OEM air intake.
Result: An even MORE noticeable difference over 4.2 with 4.0HO head, but I definitely need to run 93 octane.
It runs great and I'm quite satisfied, but it is quite clear the TJ intake holds it back significantly.
If I wanted to spend the time and money to improve it further (I don't have a need to improve as this suits us just fine).... I would in this order:
- improve air intake (multiple options)
- improve exhaust (multiple options)
- use a later 4.0 intake manifold that is curved and not squared off like the '95
If I was going the budget stroker route again, I would deck it to 0.0" or close thereof for the sole reason that I might be able to run regular 87 octane. Maybe even as much as -0.010" (but I'd have to think/research that more). But it runs great decked at 0.013"
If you want to spend more $$$ and more time, get a short nose stroker so you can change over to S-belt, and
maybe install a later FI system and
maybe a later head casting (newer has more advanced engineering... no?).
My 2 cent conclusions:
- If you are keeping the 258 and it's not tired and you don't want to swap engines, choose FI -- NOT 4.0 Head.
- If you have MPI and think 4.0HO head will eliminate predetonation, my experience is that it will not. Save your $$$ and just buy premium.
- If your MPI engine is shot and you are in a hurry to get a new engine so your kids can go to SandBlast (and you are not looking to turn your Sunday driver into a speed demon), then for me it was only $2,296 (including $300 core charge and S&H) to get a stroker.
This is just I-6 options. There are lot's of good guys here with good feedback on other powerplants.
Jim