Ominous Z06

I bought a set of AC Delco Rapidfire #9 for my '91 ZR1. What gap should I use?

Thanks,

Eric

Heart of the Beast

Information from the ZR1 net site indicates that Rapid Fire #5 are the plugs used in our Z's. The service manual states .35 is the gap. However, when I went to install the plugs the gap was .40 on some and .41 on the others. Of course I made the stated adjustments. I'm sure some folks can let you know what gap measurement is going to help produce the best results. Have fun with cylinder#8 it was a challenge. This is what I did to make it happen...I took the spark plug socket and let it drop down into the tube, then I put a short extension into the tube and attached it to the plug socket, attached a universal to the short extension and then attached a long extension to the universal and then finally the rachet. Being extremly careful slowly but sooooo slowly started turning the rachet until the plug broke free. Now the reverse process...you'll be able to take down everything pretty easily until you have to fish out the short extension, plug socket and plug out of the tube. I used a heavy duty flexible magnet I bought at sears to bring the said parts to the surface where I could take out each...one at a time. The process is actually harder to explain than it is to do. All of the other plugs came out fairly easy. Good luck!

LT5

I'm using Rapidfire #9's and they work fine. I found out about them in a thread in which Hib Halverson said something about these being superior ZR1 plugs.
:chevy

Heart of the Beast

Can you elaborate on why the #9 would be superior to the #5? I understand that #5 are not the only spark plug you can use. I was just relaying information. Perhaps, I've taken this in the wrong context!

'90 #28 CPC test vehicle

LT5

Ask Hib Halverson. He can explain the reasons to go with #9

Heart of the Beast

Perhaps, you shouldn't talk about performance/superiority issues if you can't back up the information...Mr. Halverson can you kill this issue before the pettiness gets out of hand? Thanks a bunch!

LT5

I was just suggesting RF#9's and never claimed to be an expert on them, so don't get your panties in a wad.....If you insist...RF's are the performance descendant of another plug that was originally used in ZR1's. They stopped using the original plug b/c of fouling problems...RF9's are the replacement for those plugs. YOU HAPPY YET? Have a nice day :jester

Ominous Z06

This is the thread in which Hib recommends RF #9's. I have a lot of respect for his opinion and knowledge. That is why I went with them. I wasn't expecting a major hp boost or anything.
http://forums.corvetteforum.com/zero...398096#1398096

LT5

:yesnod: :chevy :cheers:

Heart of the Beast

Ditto on the respect for Mr. Halverson!! I was just passing information and it seems it was misinterpreted as Biblical information. Now that I know the difference, thanks to Ominous C5, I'm going to purchase me some #9's. No harm, no foul!

8388

I read here where some manuals recommend gapping at .35, and I'm curious, I've got a 93 and the manual calls for .50, did the the later models get a hotter ignition system ? There's a benifit to wider gapping on non supercharged or turbo motors so long as the ignition sytem can produce a good enough spark. Just wondering when it changed from .35 to .50 and why?

ZR1

The 375 motors had a recommended gap of .035 and the 405 motor have a recommended gap of .050. This was done because the recommended plugs changed between the model years from a "normal" plug to a platnum plug. I have always used the .050 gap no matter what plug I use since I change them yearly anyway. There is no performance difference in plugs. If there were don't you think every car maker everywhere would do so?

8388

I just looked at some RP# 9's and gapping them at .50 would be wrong in my opinion, the ground strap to electrode angle seems a bit much for .50.

I wanted to look at the RP# 12 which is supposed to be proper replacements for the 93's and newer but they were out of them, I located some Delco 41913's at a chevy house across town, their are the factory replacement for the 41907 which were discontinued and are what I pulled off my car, they appear to be the originals. With 18,000 miles they looked like they were ready for replacement.

8388

Well I replace all of the spark plugs on my 93 Z the other day, they were the originals # 907's, I was checking the gap on each of them as I pulled them out, some had .60+ and some as high as .80+ gapping, that much of the ground strap and electrode was missing, stock gapping is .50. I hope this explains why I had two sharp falls on the dyno graph just before peak hp and torque. I replaced the 907's with the 913's which are the plugs that replaces the old 907's.

Hib Halverson

I see this thread is pretty lengthy and my name has been mentioned several times...
Uh-oh...:)

Actually, my attendance on the CF is spotty and I apologize for not seeing this thread sooner.

It is true that I have believed for some time that the ACDelco RapidFire #9 is an excellent choice for LT5s that are driven aggressively on the street or are raced occasionally. That is what I have used in my car for several years. The RF9 is equavalent in heat range to the FR1LS, the *original* stock LT5 plug. Shortly before ZR1 production began in late 1989, the stock plug was changed to the FR2LS due to fears GM had about predelivery plug fouling.

For cars that are not driven hard, the best choice, if you want Rapid Fires, is the RF #5 which is the heat range equivalent to the FR2LS and the Rapid Fire plug ACDelco recommends for all LT5s. Also, for show cars that see starts without warm-ups and/or infrequent use, the RF5 or the FR2LS or, in the case of 93-95, the 41-913 should be used.

The reason that my suggestion and ACDelco's suggestions as to Rapid Fires differ is I do not consider starts without warm ups but I do consider aggressive driving. I think the RF5 is too hot for LT5s that are driven hard and five years of experience with the RF9 (I've never had one foul) prove that out.

If you want to use a double-platinum spark plug as installed in the 93-95 engines, the OE choice is the 41-913 but an alternative suggested by Marc Haibeck is the 41-919. I should add that both these are AC "2" heat range plugs and that there is no ACDelco double-platinum-tipped plug that is the equivalent heat range to the RF9 or the FR1LS.

For LT5s that are race only, the RF9 will work in some cases but is probably too hot a plug in most racing applications. I suggest either NGK or Denso racing plugs. Unfortunately, I'm on a road trip right now without access to my NGK and Denso racing heat range data so I can't post any specific recomendations other than saying that I've used the NGK 8 heat range on the track and that is two ranges colder than stock.

Ok--now, for getting the #8 plug with ease:
You need, stacked in order, a 3/8ths-drive 5/8ths plug socket with integral u-joint, a one-inch extension, a second 3/8th-drive u-joint and a 12-in or longer extension. This arrangement allows you to snake the plug socket down into the #8 plug well. You can remove and replace the #8 plug with no difficulty at all.

On the gap:
All LT5 ignitions systems are the same except for the plugs which, as stated previously were the FR1LS at first, then the FR2LS for MY90-92 production, the 41-907 for MY93-94 and the 41-913 for MY95. The 41-907 has been discontinued for some time with the 913 being the OE plug for 93-95 engines. The reason for the different gaps was the FR1LS and FR2LS are not platinum tipped plugs and thus the electrodes wear quicker. To have acceptable plug life, the early engines had plugs gapped at .035. GM figured during the life of the plug the gap would open to as much as .050 with no problems with misfire. When the LT5 was switched to platinum plugs for 1993, the gap was increased because platinum tipped plugs have electrodes that do not wear as quick and would hold a gap near .050 for a longer period of time. Opening up the gap to .050 was simply a move towards increasing idle stability. You can run .050 gapped non-platinum-tipped plugs but you have to change them, or regap them, more often. Finally on platinum plugs, ACDelco's early offerings (92-93) had some quality problems with the bond between the platinum pads and the electrodes. This is why a lot of 92-93 engines (both LT1 and LT5) with original plugs will often have trouble with the pads debonding. The current Delco manufacturing uses a much better bond. I've never seen plugs made after 94 loose pads. Anyone out there with a 93-94 with the original 907s in your engine ought to change to new 913s or a Rapid Fire.

Lastly, on the mechanical property of LT5 plugs and plugs seats: all LT5 heads were machined with what GM called a "universal spark plug seat". You can retrofit early heads with taper seat, platinum-tipped plugs if you want. If you choose that, you must do a one time, "overtorque" of the spark plugs to form the taper seat in the head. This is done the first time you install taper seat plugs in 90-92 heads by tightening each plug to 30 Nm, then loosening them and finally, tightening them to 20Nm. Once that is done you can use either flat-seat or taper-seat plugs interchangeably.

This is perhaps more than most people want to know about spark plugs for LT5s but, hopefully, it will answer some of the questions I've read here.



[Modified by Hib Halverson, 1:15 PM 2/24/2002]

Heart of the Beast

Thanks for the information, Mr. Halverson. I will stick with my RF #5's...since I don't race my Z. Just a daily driver!


1990 #28 CPC Test Vehicle

8388

Matt (Ultra Slow) hasn't posted here yet, but he did experience a problem where a spark plug on his last black 90 ZR1 which were the tapered style plugs had backed out due to probably not being torqued properly ( not by him, but by the previous owner) and on one of those nights we went out, and running the car hard as we sometimes do, caused a lean condition on that cylinder, which he later found out had caused one of the head gaskets to fail. The torque sequence as described by Mr. Halverson is very important and should not be taken for granted especially if using the tapered style plugs.



[Modified by 8388, 8:19 AM 2/25/2002]

Hib Halverson

8388 is correct in emphasizing the overtorque procedure to form the taper seat in the head.

I forgot to add to my previous post that the 30Nm, back off then 20Nm procedure was given to me by a spark plug engineer at Delphi, the supplier of spark plugs to ACDelco.

vettl83

I run the Bosh Platinum + 4's in both of my vetts and they seem to work excellent. I was turned on to them by TPIS Specialties they run them in their racing vetts, besides they solve the plug gap question because you don't have to gap them. If anybody else out there has tried them I would be interested in your feedback. vettl83 1991 ZR1 Red/Saddle #2039