Spark Plugs, TBI Vortec L31, Heat Range

**pmkls1** · 03-20-2014

Originally Posted by Fast355

Having run his exact engine, the OEM plug is TOO HOT for that application....The dynamic compression ratio of a stock L31 is too high to be able to run enough timing to maximize the power potential of the vortec cylinder heads. The hotter heat range is likely contributing to his detonation issue. Not to mention the TBI ignition system is setup to fire copper plugs, not iridium or platinum both of which typically require more voltage to jump the wider gap. A properly gapped (.035") set of R42LTS plugs would work well for him.

I have run the same engine both with the O.E. SCPI induction and with TBI as well. And while I disagree with just about all of that statement, I find it futile to continue to debate the subject as your opinion on the matter isn't going to change and arguing will not contribute anything useful to this thread. Let's just agree to disagree and leave it at that.

**Woods** · 03-21-2014

Thanks for the insight guys, if anyone else has thoughts, I would appreciate them.
Fast355, this is the type of hands on I was hoping to get, thank you

Originally Posted by Fast355

Having run his exact engine, the OEM plug is TOO HOT for that application....The dynamic compression ratio of a stock L31 is too high to be able to run enough timing to maximize the power potential of the vortec cylinder heads. The hotter heat range is likely contributing to his detonation issue. Not to mention the TBI ignition system is setup to fire copper plugs, not iridium or platinum both of which typically require more voltage to jump the wider gap. A properly gapped (.035") set of R42LTS plugs would work well for him.

I was on Jegs and found these engines kits based on the L31 long block

http://www.jegs.com/i/Chevrolet-Perf...oductId=813788

http://www.jegs.com/i/Chevrolet-Perf...oductId=813788

The spark plug they are using is with their kits is the NKG TR55. The AC Delco equivalent is R42LTS6.
I am thinking the R42LTS gapped at .35 will be what I am going to give a try.

If anyone has opinions, and hands on insight, it is appreciated.

Thanks

**1BadAction** · 03-21-2014

R42LTS are great running plugs. Ask if they have CR42LTS, that's a coated commercial version of the same plug.

**Fast355** · 03-21-2014

Originally Posted by 1BadAction

R42LTS are great running plugs. Ask if they have CR42LTS, that's a coated commercial version of the same plug.

He could alternately ask for the M version of the same plug. Many GM crate engines have specified "M" plugs over the years. My boat has MR42LTS

http://www.amazon.com/ACDelco-MR43LT.../dp/B0013WFURM

**1BadAction** · 03-21-2014

Originally Posted by Fast355

He could alternately ask for the M version of the same plug. Many GM crate engines have specified "M" plugs over the years. My boat has MR42LTS

http://www.amazon.com/ACDelco-MR43LT.../dp/B0013WFURM

Is that stainless? If so, that is one helluva good price. Last I bought SS Champions for my OMC 2 stroke (Yes, I hate champions, but OMCs love them) they were close to 10 bucks a plug.

**Woods** · 03-21-2014

Ran across these on Summit, MR43LTS $1.97

http://www.summitracing.com/parts/ado-mr43lts/overview/

**EagleMark** · 03-22-2014

If you look at application they are listed for a lot of 8.1L engines...

Then the mail in rebate...

http://www.rockauto.com/lang/en/promo.html#acdelco

**tayto** · 03-27-2014

Originally Posted by Fast355

Having run his exact engine, the OEM plug is TOO HOT for that application....The dynamic compression ratio of a stock L31 is too high to be able to run enough timing to maximize the power potential of the vortec cylinder heads. The hotter heat range is likely contributing to his detonation issue. Not to mention the TBI ignition system is setup to fire copper plugs, not iridium or platinum both of which typically require more voltage to jump the wider gap. A properly gapped (.035") set of R42LTS plugs would work well for him.

The factory 195*F thermostat is also too hot IMO. I run a 170*F thermostat in my L31 and bring the fans on at 176*F for low and 180*F for high speed. I never get hotter than 182*F even in summer, pulling a load.

Fast, do you recommend a .035" gap on a 10:1 vortec headed TBI 355? I was told to gap them around .045-.055"... Have a brand new AC Delco coil, was told not to go with a MSD. Got the engine running on R44LTS and have a set of R42LTS that will be going in.

**pmkls1** · 03-27-2014

Originally Posted by tayto

Fast, do you recommend a .035" gap on a 10:1 vortec headed TBI 355? I was told to gap them around .045-.055"... Have a brand new AC Delco coil, was told not to go with a MSD. Got the engine running on R44LTS and have a set of R42LTS that will be going in.

.035" would be the ideal gap on that engine. I prefer not to go much wider than that on most applications. I've heard both ways on aftermarket "performance" coils, but I've never seen any hard evidence for either argument. I've run MSD and Accel coils on TBI vehicles with no problems, but I couldn't say that they performed any different than the OE coil. That being said, the OE coil is just fine for your engine.

**Fast355** · 03-27-2014

Originally Posted by pmkls1

.035" would be the ideal gap on that engine. I prefer not to go much wider than that on most applications. I've heard both ways on aftermarket "performance" coils, but I've never seen any hard evidence for either argument. I've run MSD and Accel coils on TBI vehicles with no problems, but I couldn't say that they performed any different than the OE coil. That being said, the OE coil is just fine for your engine.

I agree .035" is where I would run on that setup. A quality aftermarket coil never hurts, however the real thing holding back the small cap HEI is the module. I would use a Davis Unified Module and matching coil if he elects to upgrade. Keep in mind there are two different original EST module part numbers. One (369) that advances the timing up top and one (048) that retards timing. I will say that a MSD 6A box is a very noticeable different on a TBI truck. The idle was smoother, I could run leaner mixtures without misfire and it had sharper throttle response. I also run a MSD 6A box on my large cap HEI in my Corvette. However I use the HEI pickup coil to directly trigger the MSD box and run without a module. That engine (vortec head 305 with 216/216 @ .050 cam, single plane intake, holley 680, and 1 3/4" primary, 2.5" collector, 2.5" dual exhaust) has accidentally seen 6,500 rpm in high gear on the highway a few times and was still pulling hard when it ran up on the RPM limiter. I was out in front of the LS1 T/A when the governor kicked in, 140 mph speedo long burried.

**tayto** · 03-28-2014

I bought a new AC Delco module for a late 80s F-body. However they are no longer printing the "369" or "048" on the module like they did with the factory ones that I have pulled at the wrecker in the past. I remote mounted it on the passenger fender with a homemade aluminum heat sink. I would like to get the MSD 6A box, but I have spent WAY to much on this car already. Wish I would have know about the DUI stuff 2 years ago when I started this project. Live and learn (AND SPEND!).

**Woods** · 04-17-2014

Thought i would follow up on the thread.

I ended up installing a set of ACDelco MR43LTS plugs, gapped at .40. I was a little leery of going 2 heat ranges down (R42LTS). This plug has worked well for me. It has helped (almost eliminated) my knock retard, due to running the colder plug. The plugs I replaced where 3 month old R44LTS, so it is not an issue of needing a tuneup.

A colder plug for me, definitely helped.

**pmkls1** · 04-17-2014

All personal preferences aside, I'm glad that you found a solution to your problem. On a related note, I feel compelled to state that although I prefer to use iridium plugs and that they are the specified replacement plug for these engines, I never meant to imply that these engines wouldn't perform properly with standard copper plugs installed.

**tayto** · 08-01-2014

I finally got around to putting in a set of R42LTS into my Caprice. First off the R44s I pulled out were all black (carbon). Now my car is no where near tuned but it seemed to run alright. I also use to get knock counts like crazy before. Just did a trip from Vancouver, BC to Kent, WA to get my exhaust sorted out and over the 3 hour trip I think I had under 100 counts. Not great but compared to 1000+ within a 30 minute drive before. Have done a few learn runs with EBL and it was pulling fuel like crazy across the board so it seems to be a lot happier now.