Opti Spark and firing order....4/7 swap theory with the LTX
07-12-2011, 01:23 AM
#1
Opti Spark and firing order....4/7 swap theory with the LTX I came across an article (A QUESTION OF EFFICIENCY) in the September '11 article of Car Craft where Jeff Smith included some insight from retired GM Engineer Don Webb about the firing order and torsional vibration of the LS compared to the SBC. In this article they went into the Opti-Spark and the firing order:
"Because the spark/injector timing is derived from the reluctor wheel, it is located as close as possible to the neutral spot on the crankshaft, which exhibits no torsionals whatsoever. This happens to be just ahead of the centerline of the No.7 cylinder. All torsionals forward of that point go in one direction, while to the rear of it, they are in opposition. This is one reason the Opti-Spark distributor in the Gen II SB (LT1) is not as accurate as hoped, because the crankshaft and the cam are constantly feeding torsionals into the reluctor, causing it to dance around and vary spark timing by several degrees. In spite of the torsionals, it was still a major step forward from the old post-style distributors."
I know it's been brought up before, and the general consensus about the 4/7 swap cams is it may not be worth more power unless you're spinning the motor up high, and it's just for a smoother motor. However, it may benefit the LTX platform just to true up the timing. Who knows, it may make optis last a little longer as well (easier on the optical sensor components through less harmonics?). Just some food for thought that figured the LTX community would like to see.
"Because the spark/injector timing is derived from the reluctor wheel, it is located as close as possible to the neutral spot on the crankshaft, which exhibits no torsionals whatsoever. This happens to be just ahead of the centerline of the No.7 cylinder. All torsionals forward of that point go in one direction, while to the rear of it, they are in opposition. This is one reason the Opti-Spark distributor in the Gen II SB (LT1) is not as accurate as hoped, because the crankshaft and the cam are constantly feeding torsionals into the reluctor, causing it to dance around and vary spark timing by several degrees. In spite of the torsionals, it was still a major step forward from the old post-style distributors."
I know it's been brought up before, and the general consensus about the 4/7 swap cams is it may not be worth more power unless you're spinning the motor up high, and it's just for a smoother motor. However, it may benefit the LTX platform just to true up the timing. Who knows, it may make optis last a little longer as well (easier on the optical sensor components through less harmonics?). Just some food for thought that figured the LTX community would like to see.
07-12-2011, 07:49 AM
#3
TECH Veteran
It's not going to do anything to give the opti longer life, IMHO. While the LSx may have more accuracy, the opti was the best distributor ever used for an SBC before that. For the normal guy, it's not going to make any real difference.
07-12-2011, 10:41 AM
#5
The engine will run more smoothly with the 4/7 swap and have less stresses on the crankshaft. You might notice it running smoother. There is no actual power gain to be had.
Having said this, there are LT1 cars with the Optispark running some pretty quick times. When the Optispark runs good, it's very good.
Unless you're already getting a custom cam grind, it's not worth it.
Having said this, there are LT1 cars with the Optispark running some pretty quick times. When the Optispark runs good, it's very good.
Unless you're already getting a custom cam grind, it's not worth it.
07-12-2011, 11:34 AM
#8
I'd really like to see a side-by-side comparison on an LT1. Identical cams just 4/7 swapped and ran to see what the exact difference would be (smoother operation, more accurate timing). GM did it with the LSX for NVH, but I think there's some power there through refinement.
Gregrob - Your sig is amusing.
Gregrob - Your sig is amusing.
07-12-2011, 08:37 PM
#15
Very cool 
Mine is an LS1 firing order not 4/7 =) Def looks like from the same maker. All i remember is it put a dent in my wallet and it took 6 mo with 3 mo of heat treatment to finish. Can't wait !
Joel what is yours going in?
LT1:
1-8-4-3-6-5-7-2
4/7:
1-8-7-3-6-5-4-2
LS1:
1-8-7-2-6-5-4-3
Mine is an LS1 firing order not 4/7 =) Def looks like from the same maker. All i remember is it put a dent in my wallet and it took 6 mo with 3 mo of heat treatment to finish. Can't wait !Joel what is yours going in?
LT1:
1-8-4-3-6-5-7-2
4/7:
1-8-7-3-6-5-4-2
LS1:
1-8-7-2-6-5-4-3
Last edited by Nastyc4; 07-12-2011 at 08:53 PM.
07-12-2011, 08:50 PM
#16
Mine took 18 days from the time I ordered to at my door, lol. Mine is a .900" base circle. Our noses look a little different, and the boxes they were shipped in are also different. I got mine from lsmeng.com, it was pricey but worth it.
edit: it still baffles me why they insist on putting the old school fuel pump pushrod lobe in there, lol. Our blocks don't even have a provision for it.
edit: it still baffles me why they insist on putting the old school fuel pump pushrod lobe in there, lol. Our blocks don't even have a provision for it.
07-12-2011, 09:10 PM
#17
Mine took 18 days from the time I ordered to at my door, lol. Mine is a .900" base circle. Our noses look a little different, and the boxes they were shipped in are also different. I got mine from lsmeng.com, it was pricey but worth it.
edit: it still baffles me why they insist on putting the old school fuel pump pushrod lobe in there, lol. Our blocks don't even have a provision for it.
edit: it still baffles me why they insist on putting the old school fuel pump pushrod lobe in there, lol. Our blocks don't even have a provision for it.
How much power to gain? I dont care if I lose 5 hp. At 8000+ I'm sure it will sound different than an LT firing order which is all I cared for. From what I've read only I would hope the balance and harmonics are better which is why I chose to go this route.
And obviously to have that little 'special' added to the build instead of it being a cookie cutter 1000 hp LS+ blower build I see every day.
07-12-2011, 09:17 PM
#18
I got mine from (dare I say their name.. shhh) I initially wanted the 4/7 swap. Then i read about how the harmonics are superior on the LS1 firing order but no cam company had blanks for that on an LTX core or something along those lines. It was a one off which is why it took so much time etc. I remember calling comp and asking them for it they had no clue and just said no.
How much power to gain? I dont care if I lose 5 hp. At 8000+ I'm sure it will sound different than an LT firing order which is all I cared for. From what I've read only I would hope the balance and harmonics are better which is why I chose to go this route.
And obviously to have that little 'special' added to the build instead of it being a cookie cutter 1000 hp LS+ blower build I see every day.
How much power to gain? I dont care if I lose 5 hp. At 8000+ I'm sure it will sound different than an LT firing order which is all I cared for. From what I've read only I would hope the balance and harmonics are better which is why I chose to go this route.
And obviously to have that little 'special' added to the build instead of it being a cookie cutter 1000 hp LS+ blower build I see every day.
