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I an in the progress of doing some maintenance/tune up on my 71 350 (stock 8.5 : 1, stock cam, quadrajet, MSD ready to run with blaster 2 coil) and want to replace my spark plugs too. I never dealt around a lot with spark plugs, i just used what Rockauto lists and would apprechiate to learn some more about to get the best (running) result for my 350.
Currently, old Autolites are installed.
I have a set of new NGK XR5 lying around, but im not sure if theyre a good choice (too cold?) for a 8.5:1 comp ratio 350. Maybe XR4 is the better choice? I read somewhere that with NGK, the higher the number, the colder the plug.
71 CSM lists AC Delco R45S / R46S
Rockauto lists NGK XR4 too for year 71 350.
Is it right, higher comp ratio = colder plugs, lower comp ratio = hotter plugs?
I assume, setting gap at 0.035 - 0.040 would be ok with my MSD ignition system.
Thanks for some information/help.
Last edited by 71OldscutlassS; October 31st, 2023 at 01:41 AM.
Yeah, the XR4 are probably more ideal. One thing the XR5 will allow a more aggressive timing setting. More part throttle timing in a 8 to 1 motor helps throttle response.
i know, the 8,5:1 engines like a lot advance in idle/part throttle. I alwas read that its better to use manifold vacuum instead ported, but i get a bit confused, because the MSD chart says to connect it to ported vacuum to prevent from excessive advance at idle:
im pretty sure that i have more than 12 inches of vacuum at idle, so this would mean with vac. advance at manifold vacuum, i would have to shoot for initial at about 6° btdc to get the idle timing around 22° btdc. Or better to say, set the total timing (36° @ 3500 rpm) so, that initial falls at about 6°btdc. This seems kinda wrong to me, but feel free to correct me.
Currently i have my distributor at this curve with the blue stop (~21°):
maybe someone can explain me if a faster curve (lighter springs) would be better for a low comp engine? Im still some kind of a timing/ignition-rookie and would very apprechiate to learn some more about.
Till now, i dont really know how fast/slow the centrifugal advance should come to total for best results on a stockish engine. Or is it really a try and error thing?
So giving the XR5's a try is worth it? Otherwise, ill save them for my future 455 build and purchase a set of XR4's, but maybe others have some expirience with NGK's in 8,5:1 engines.
thanks for every help, i always apprechiate when i can learn some more!
Last edited by 71OldscutlassS; October 31st, 2023 at 09:13 AM.
It is a bit of trial and error. It all depends on the car, driving style, terrain, altitude, etc.
Put it together, and drive it. Theoretically, 8.5:1 should run on 87 octane. Keep an ear out for detonation, then play with the curve.
It is a bit of trial and error. It all depends on the car, driving style, terrain, altitude, etc.
Put it together, and drive it. Theoretically, 8.5:1 should run on 87 octane. Keep an ear out for detonation, then play with the curve.
Yeah this makes sense. so Im gonna try the XR5's. Car is in its winter-break now, but i have to play around/understand/set the ignition timing and curve properly, so i then will see how it runs.
1) Is it right, higher comp ratio = colder plugs, lower comp ratio = hotter plugs?
2) I assume setting gap at 0.035 - 0.040 would be ok with my MSD ignition system.
1) Not necessarily, although often the case. A lot also depends on your ignition. Unfortunately you may have to experiment a little to straddle that fine line between fouling and pre-ignition.
2) With your setup I would go a bit bigger. 0.035 is close to spec for a factory points ignition. I would try 0.045 and go from there.
Last edited by BangScreech4-4-2; October 31st, 2023 at 10:06 AM.
I have to apologize, i currently remebered that i once opened up a thread for MSD Distributor tuning on stock 350, so i whatched this old thread again.
Seems like i need a vacuum advance limiter for the use with Manifold vacuum to avoid exact the problem i described above.
What are youre expieriences with those? Which is a good product?
Yeah, the XR4 are probably more ideal. One thing the XR5 will allow a more aggressive timing setting. More part throttle timing in a 8 to 1 motor helps throttle response.
On this older thread, you gave me this advice:
On your 8 to 1 Olds 350, add about 4 degrees advance for a total of 38. This gives 20 base timing with 18 mechanical. I also would leave the probably 20 degrees of vacuum advance the can puts out. The above numbers are fine if you have a 9 to 1 or better motor, you do not. Your part throttle will be much improved with more part throttle timing. I have ran many 8 to 1 Olds V8's, they love a lot of timing.
So, black stop bushing (18°) and better not to limit the vacuum advance?
And, i guess, with vacuum advance connected to ported vacuum? With 20 degrees base and vac.advance at manifold vacuum would be 40 degrees at idle?
I made a limiter for my original GM HEI vacuum canister to limit it from 24º to around 10º.
My HEI is set to 18º initial without the vacuum advance, so with the advance connected to manifold vacuum it has around 28º at idle. This smooths out the cam's lumpiness, increases the idle vacuum, and keeps the RPM steady when the AC is turned on.
On your 8 to 1 Olds 350, add about 4 degrees advance for a total of 38. This gives 20 base timing with 18 mechanical. I also would leave the probably 20 degrees of vacuum advance the can puts out. The above numbers are fine if you have a 9 to 1 or better motor, you do not. Your part throttle will be much improved with more part throttle timing. I have ran many 8 to 1 Olds V8's, they love a lot of timing.
So, black stop bushing (18°) and better not to limit the vacuum advance?
And, i guess, with vacuum advance connected to ported vacuum? With 20 degrees base and vac.advance at manifold vacuum would be 40 degrees at idle?
Yeah nothing wrong with that especially with the XR5 plugs. I have ran around 50 at idle and nearly 60 degrees part throttle. I ran similar numbers at 9 to 1 compression but experienced low speed bucking. I found less timing made it less responsive even with high compression ratios. Then you run into bucking and pinging.
I made a limiter for my original GM HEI vacuum canister to limit it from 24º to around 10º.
My HEI is set to 18º initial without the vacuum advance, so with the advance connected to manifold vacuum it has around 28º at idle. This smooths out the cam's lumpiness, increases the idle vacuum, and keeps the RPM steady when the AC is turned on.
great info here, that helps a lot! I like DIY solutions like this! If i would have to limit the vac. Advance on my MSD distributor, i have to check if i could fabricate something similar to it.
I also red that there some aftermarket "restriction cans" or something which go in the vacuum line between manifold an distributor too, but i have to do some research.
Originally Posted by olds 307 and 403
Yeah nothing wrong with that especially with the XR5 plugs. I have ran around 50 at idle and nearly 60 degrees part throttle. I ran similar numbers at 9 to 1 compression but experienced low speed bucking. I found less timing made it less responsive even with high compression ratios. Then you run into bucking and pinging.
Ok, I never thought that they really need/like this high advanced timing at idle! So, the lower the compression ratio, the earlier the ignition? I guess this is because with lower comp. ratio, the combustion is "slower" than with higher ratio?
So, you recommend for this engine with XR5 plugs the 36-38° total all in at ~3000rpm with 18° mechanical (black bushing), 18-20° initial + ~16° vacuum advance the MSD gives connected to manifold vacuum, to get around 36° advance at idle?
I think now i get closer to understand it. Thanks a lot!
October 31st, 2023, 12:00 AM
