Which plug for alcohol?

cubicinches

New member
Which plug is the best to run on a flathead with methanol? I'm currently running an Autolite 303 and it runs pretty good with that, but I'm almost positive that's not the correct plug, I just used it because I had one since that's what I run on my truck. I want to get the correct one before I hit the track with my new engine. Also, what jet size would you recommend in a briggs carb for alcohol? I have one in there that runs, I'd like to know what's right though.

Specs:
.380 lift billet Comp Cam
Stock length billet rod
Stock stroke
Stock bore
3hp aluminum flywheel
Comp double valve springs
30 degree timing advance
 
Anybody else have an opinion?

Yeah. Another vote for Autolite 411. The few other plugs used cross reference to the 411's heat range from either another make of plug or another type of plug , such as a shorter reach plug like the 353 (3/8" vice the 411's 1/2"), which pulls the whole plug, including that pesky side electrode, up into the spark plug hole.
 
Yeah. Another vote for Autolite 411. The few other plugs used cross reference to the 411's heat range from either another make of plug or another type of plug , such as a shorter reach plug like the 353 (3/8" vice the 411's 1/2"), which pulls the whole plug, including that pesky side electrode, up into the spark plug hole.

So are the 353 and the 411 in the same heat range?
 
Specs:
.380 lift billet Comp Cam
Stock length billet rod
Stock stroke
Stock bore
3hp aluminum flywheel
Comp double valve springs
30 degree timing advance
the compression ratio can have a lot to do with it. How many CC's in the head? I don't know about Flatheads, but there's no way I would have run a 3/8 thread plug in an engine that calls for a half inch thread plug. If the plug you're using is extended reach, and it's hitting the valves, use a shorter reach plug. I wish I had a plug chart. And
again, I don't know anything about flatheads, but the jetting, both high and low, if that's what you have, should be about twice the area of the Jets you would run with gas, maybe a little bigger. It's the area of that jet that you want to check. For instance; a .056" jet is 106% bigger than a .039" jet. Now that's in area, not diameter. (Pi R sq) And you should use pin gauges to check Them. You can buy cheap sets of pin gauges off of eBay. Buy the .011" through .060"
set.
It's called tuning, and tuning is tough. (Al Nunley)
comments compliments criticisms and questions always welcome.
 
Autolite 411

The 411 is the way to go. The shorter reach plugs become shrouded by the hole threads. Something to watch out for with the 411 though is that you have enough clearance on the valves. I have turned a motor a little harder than I was supposed to and floated the valves causing a valves to nick the spark plug. You can eliminate the chance of this by running a copper plug washer and indexing your plug.
 
the compression ratio can have a lot to do with it. How many CC's in the head? I don't know about Flatheads, but there's no way I would have run a 3/8 thread plug in an engine that calls for a half inch thread plug. If the plug you're using is extended reach, and it's hitting the valves, use a shorter reach plug. I wish I had a plug chart. And
again, I don't know anything about flatheads, but the jetting, both high and low, if that's what you have, should be about twice the area of the Jets you would run with gas, maybe a little bigger. It's the area of that jet that you want to check. For instance; a .056" jet is 106% bigger than a .039" jet. Now that's in area, not diameter. (Pi R sq) And you should use pin gauges to check Them. You can buy cheap sets of pin gauges off of eBay. Buy the .011" through .060"
set.
It's called tuning, and tuning is tough. (Al Nunley)
comments compliments criticisms and questions always welcome.

I'm not sure how many cc's the head is, but it's been clearanced for the valves and I also welded up the plug hole, moved it over, and angled it to get better fuel burn. I just will run whatever plug people have had good experience with, and know will run well. I have a big jet in the carb now, and I also have pin gauges, I'm just not there to measure it at the moment. It runs like it sits, and sounds good, so I guess I'll throw in an Autolite 411 and try it out to see how it runs on Saturday lol. It should be alright. The engine is a bunch of parts I had laying around that I put together to run anyway, because I'm a broke college student, and that's the way it goes haha.
 
If you've moved the plug closer to the cylinder and angled it, you will need a longer reach plug to go through the thicker part of the head. You also will not have to worry about it hitting the intake valve any longer -- that's just part of the reason you move the plug location in the cylinder head of a stock appearing or big flathead. Au 411, ND 24, NGK BU8H, and more will work just fine. A great resource site on spark plugs is www.sparkplugs.com -- lots of great tech info there and cross references galore.
I'm surprised that you went through the effort to angle the plug, but have not went to a high compression piston (Wiseco or other) and a longer rod, which will help the flathead tremendously.

I understand the broke college student part -- get used to it -- student loans took me 15 years to pay off. :)

52 jet will get you close on fuel, but there's so much more to consider - pipe, airflow through the carb, fuel vacuum draw and more. If the carb has been opened up to a true stock appearing, you should have around a 59 jet -- this is something that an experienced engine builder / carb guy should do.

Hope that helps.

--
Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cuts
www.CarlsonMotorsports.com
Celebrating 25 years of service to the karting industry
765-339-4407
bcarlson@CarlsonMotorsports.com
 
If you've moved the plug closer to the cylinder and angled it, you will need a longer reach plug to go through the thicker part of the head. You also will not have to worry about it hitting the intake valve any longer -- that's just part of the reason you move the plug location in the cylinder head of a stock appearing or big flathead. Au 411, ND 24, NGK BU8H, and more will work just fine. A great resource site on spark plugs is www.sparkplugs.com -- lots of great tech info there and cross references galore.
I'm surprised that you went through the effort to angle the plug, but have not went to a high compression piston (Wiseco or other) and a longer rod, which will help the flathead tremendously.

I understand the broke college student part -- get used to it -- student loans took me 15 years to pay off. :)

52 jet will get you close on fuel, but there's so much more to consider - pipe, airflow through the carb, fuel vacuum draw and more. If the carb has been opened up to a true stock appearing, you should have around a 59 jet -- this is something that an experienced engine builder / carb guy should do.

Hope that helps.

--
Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cuts
www.CarlsonMotorsports.com
Celebrating 25 years of service to the karting industry
765-339-4407
bcarlson@CarlsonMotorsports.com

I had a block bored .080 over with a long rod...but wore the piston and the cylinder wall out to where it fouled the plug in like 5 minutes just dicking around in the yard as a kid (wow young self). Apparently no one has chromed Wiseco XLS pistons anymore, and I needed a .100 over size of that to make the other engine work. And I had a stock Raptor engine with a perfect block and steel sleeve intact, so I just used that with the same piston. I literally have not bought anything for this engine besides a gasket kit, I just used the parts I had out of the other engine along with others. That's why I have the stock piston. Lol.
 
So are the 353 and the 411 in the same heat range?

No, but they're similar. I'd be more worried about shrouding the shorter plug up in the threads than the heat range in that particular switch, but then I'd never run a short plug like a 353. I always ran a 411 or one of a different make that crossed to a 411, including both the others Brian listed in his post #11.
 
I'm not sure how many cc's the head is, but it's been clearanced for the valves and I also welded up the plug hole, moved it over, and angled it to better fuel burn.
compression is so important, I'm surprised to hear you don't know how many CC's you have in the combustion chamber.
Choosing the right plug is highly dependent on the compression number. I'm no expert on the subject, that's for sure, but I played around a lot with them. I used to have a book, from the champion spark plugs, that listed all their plugs and their equivalent from other makers. That book got a lot use.
With 2 cycles, which I'm most familiar with, the higher the compression, the colder the plug. With my K 78, while not a really high compression engine, it was high enough to need a retracted gap plug. Of course, the 2 cycles, have an expansion chamber and that raises the compression pressure in the engine quite a bit.
It would be interesting to see the difference, in cranking pressure, between a stock and an open 4 cycle. I see people talking about really high compression engines, compared to a stock engine, but no mention of a change in spark plugs. I wonder why that is? It's been my experience that matching the spark plug to the compression pressure is important.
 
compression is so important, I'm surprised to hear you don't know how many CC's you have in the combustion chamber.
Choosing the right plug is highly dependent on the compression number. I'm no expert on the subject, that's for sure, but I played around a lot with them. I used to have a book, from the champion spark plugs, that listed all their plugs and their equivalent from other makers. That book got a lot use.
With 2 cycles, which I'm most familiar with, the higher the compression, the colder the plug. With my K 78, while not a really high compression engine, it was high enough to need a retracted gap plug. Of course, the 2 cycles, have an expansion chamber and that raises the compression pressure in the engine quite a bit.
It would be interesting to see the difference, in cranking pressure, between a stock and an open 4 cycle. I see people talking about really high compression engines, compared to a stock engine, but no mention of a change in spark plugs. I wonder why that is? It's been my experience that matching the spark plug to the compression pressure is important.

I don't know the cc's because it's just a head I had laying around that I've had for probably had for 10 years. At one time, I probably knew the head size, but as of now, I don't remember lol. I just know it's a head that runs good.
 
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