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AQ225D with Pertronix HEI Dpark plug gap

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Walleye4Days

Contributing Member
Looking for the spark plug gap, and trying to better understand spark plus in general, because I've gotten recommendations from .025 all the way to .050! That seems like a HUGE gap in knowledge amongst thre dozen or so people I talked to.

So, I have an AQ225D engine, but no longer have the old points distributor/coil. I have:

- Pertronix Flamethrower High Voltage HEI (marine)
- 8mm United Delphi plug wires (but upgrading to better quality 8mm wires)
- AC Delco CR43TS

I am at a loss as to what would be the correct gap for this setup. Stock VP manual suggests .028 on Bosch HR8A plugs, but that's with both the old style distributor and decades old parts that didn't have the new tech and info we've discovered and obtained since then.

So, what exactly is it that determines the correct spark plug gap, and what would be the correct gap for my setup? (Sorry for any confusion, I just am not grasping the full understanding of ignition to gap relation).
 
- AC Delco CR43TS
Click to expand...

Ayuh,....... If yer motor is pre-'96, the right plugs are MR43T, gapped at .035",......

If yer motor is '96 or newer, it's got Vortec heads, which use the longer MR43LTS plugs at .035",.....

The C prefix plugs are steel bodies, which rust quickly,......
The M prefix plugs have an alloy body to slow, or stop the rust,.....
 
I just ordered the MR43T's so those will definitely be used. Thank you very much for the gap information.

Why is it that so many people have given me ranges of gaps from all over the map, on thew same plug and engine setup? Is there some kind of benefit to smaller/larger gapping?

(I've never been good at undersranding ignition thoroughly. Hell, I'm still trying to figure out which flyweights to use for my pertronix Flamethrower HEI to get the right curve, lol. It came with 3 sets and not sure which one will give me the right advance, hehe).
 
The heavier set of weights will give you more advance. Marine engines normally use a less aggressive advance curve. Stay as close to factory stock as possible. Lacking a "sun distributor" machine or equivalent, it will be a trial and error. Start with the lightest set of weights. Using a timing light, set the specified initial advance ( @ idle) and then check for total advance per spec.
 
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sandkicker said:
The heavier set of weights will give you more advance. Marine engines normally use a less aggressive advance curve. Stay as close to factory stock as possible. Lacking a "sun distributor" machine or equivalent, it will be a trial and error. Start with the lightest set of weights. Using a timing light, set the specified initial advance ( @ idle) and then check for total advance per spec.
Click to expand...

That makes sense. I know I'm supposed to be at 8-BDTC at base, but I'm not sure the curved spec is for an 84' GM-305 H.O. (aka: Volvo Penta AQ225).

When checking advance curve and plotting it on paper, am I just looking for a 28° overall at a certain RPM? The reason I ask, is because my VP manual doesn't have the RPM ranged specs in it. All it has is:

- 8° BTDC @ 600rpm
- IDLE @ 700-750rpm
- WOT @ 4000-4400rpm

I'm thinking they don't include the rpm/advance range specs because they just assume you'll have a VP stock points and coil setup and in that case you'd only need the initial timing info above. So, just trying to figure it out and play it safe so I don't destroy the stock VP dished pistons with too much or too little advance, amongst the other issues it'd bring. Anyone know the desired curve for an 84' marine SBC 305?
 
I think if you have 26-28 degrees total (8 degrees base timing plus 18-20 distributor advance) by 2800-3000 rpm you should be good.

too little advance will only make less power or not allow engine to make full rpm, but no damage will result. Too much detonation...

A good rule of thumb is stay around 26 degrees. I think that’s what the delco est system that is sold as an upgrade kit as well as some newer OEM gives. when I got a new base engine a couple years back that’s what the ‘manual’ said to stay below to keep warranty intact.

jpeg attached is a older mercruiser 5.0 has 30 total but shows typical curve and rpm it comes in at
 

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Walleye4Days said:
.....................

So, what exactly is it that determines the correct spark plug gap, and what would be the correct gap for my setup? (Sorry for any confusion, I just am not grasping the full understanding of ignition to gap relation).
Click to expand...


Let the output voltage of the ignition coil help you determine the gap dimension.
The more energy (i.e., Voltage), the greater the gap the spark will be able to jump and ignite the fuel/air mixture.
Since you now have a HEI system, your gap can exceed the OEM suggested gap of .028", .030" or .032".

In your case, you may be OK with a .040" gap.

As a general rule, never exceed .055".
 
RicardoMarine said:
Let the output voltage of the ignition coil help you determine the gap dimension.
The more energy (i.e., Voltage), the greater the gap the spark will be able to jump and ignite the fuel/air mixture.
Since you now have a HEI system, your gap can exceed the OEM suggested gap of .028", .030" or .032".

In your case, you may be OK with a .040" gap.

As a general rule, never exceed .055".
Click to expand...


Ok, great, thanks Ricardo! I ended up putting them at .039 prior to you posting this, judging off of my undersranding of it all and assuming that'd be safe to start.

But, how will I know whether to gap them a little more or tighten them in some after running them a few hours? Am I looking for anything specific while underway? Am I going to just periodically check the plugs every couple hours of runtime and figure then out that way? Or is it something I'll most certainly realize just from noticing some lots of potential power or some other significant symptom? And how often do I need to be checking those little guys out for issues or etc?
 
Walleye4Days said:
Ok, great, thanks Ricardo! I ended up putting them at .039 prior to you posting this, judging off of my undersranding of it all and assuming that'd be safe to start.

1.... But, how will I know whether to gap them a little more or tighten them in some after running them a few hours?
Am I looking for anything specific while underway?
Am I going to just periodically check the plugs every couple hours of runtime and figure then out that way? Or is it something I'll most certainly realize just from noticing some loss of potential power or some other significant symptom?



2.... And how often do I need to be checking those little guys out for issues or etc?
Click to expand...


1.... If you do not encounter any missing or other issues at idle and/or low speed, and if it performs well at higher speeds, you should be good-to-go.

2.... No more or no less than usual.
 
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