p.j

Hi all , just looking for the right gap for the plug in my Evo 10cc engine. Thanks

av8tor1977

My Feedback: (6)

.022", or about the thickness of an Exacto knife blade is what we use for CH and RCEXL ignitions. Curious, but I looked all through the EVO manual and couldn't find the spec. That should work fine for you though.

AV8TOR

p.j

Thanks for the quick reply.

w8ye

My Feedback: (16)

The engines seem to run a little better at .025" rather than .020". So if you error on trying to achieve .022" -- go slightly large. Though too wide on the gap and the spark my miss fire?

ahicks

My Feedback: (2)

Though I'm all for setting the gap in the area of .025-.026 on the CM-6 plugs, to date, I've not seen the advantage with the 1/4-32 types. I've been running those as they come out of the box.

CK1

My Feedback: (60)

For years I tried different gaps on cm6 plugs with no noticeable change in performance on any engine .
1/4-32 plugs have been a different story.
I have run 1/4-32 plugs in many different engines mostly to fit a twin engine in a tight cowl . Brison 6.4 , 3W70 b2 , 3w 80 b2 , roto70 twin and some singles too dle 30 , roto 35 , brison 3.2 , brison 4.2 ..
Anyhow all of the mentioned engines had been run on cm6 plugs although some were converted to cm6 with bushings. All of the needed retuning as
rerequired after changing to a different plug . I set all cm6 plugs to .025" . I initially set the 1/4-32 plugs to the same and couldn't get the performance that was there on the cm6 . Reduced gap to .020 , better but not right . Tried .018" and actually was on par with cm6 performance. Note : These observations with different engines occured over several years but each engine was checked for performance on cm6 within hours of conversion to 1/4-32 .
I don't know the cause or explanation for a lower gap on 14-32 to perform better than a larger gap but I have seen it . Now I'm NOT saying it works on all engines , I'm very sure it won't . But , after growing up around cars and trucks and reading automotive oscilloscopes for 35 plus years . A plug gap change does impact performance .

av8tor1977

My Feedback: (6)

Interesting. What performance differences exactly did you encounter?

Thanks,
AV8TOR

CK1

My Feedback: (60)

In nearly all the engines the higher gaps resulted in initial lower static wot rpms and higher cyl. head temps . After retuning carb the head temps came down some but rpms didnt rise to pre change levels until the gap was lowered . Note that no ignition timing changes where made and all were set to 30 degrees btdc before any changes to the engines or mesurements were made . Bench thrust measurements were in synch with the static wot rpm readings , lower on all engines with larger gaps .
I looked for my charts but after several years of moving they are in a box somewhere , probably with my missing starting gloves and a jug of clear . Y'all know what clear is right?

av8tor1977

My Feedback: (6)

As I said, "interesting".

Hmmm.... North Carolina. Could that be "Everclear". Never had a chance to indulge. (Maybe that's a good thing...)

AV8TOR

CK1

My Feedback: (60)

Yes I thought it to be interesting also . Wasn't really looking for it at first , but it caught my attention so I checked each following engine to compare. My thoughts are it may be relative to a change in the plug resistance or lack of resistor and the effect on the coil output or ramp up . I'm clearly no engineer but I have caused embarrassment for a few at GM over the years ..
BTW , Clear " or " white " is much better than the store bought Everclear

ahicks

My Feedback: (2)

In my case, I was looking for results similar to what happens with rich running mid range CM-6 based engines while sorting out one of the new very rich running 10cc RCGF rear exhaust engines. Opening the gap up on that engine had absolutely no effect.

Completely off topic, but maybe still noteworthy, was the discovery that engine could be run MUCH leaner on the low speed by simply slowing the servo opening speed (-.3 seconds) like you would on a flap or sequencing door servo. The logic we used was that this carb venturi was WAY bigger than necessary (9.3mm) and the slower opening speed allowed the carb to maintain airflow/vacuum, vs. the rapid opening which was causing a nearly complete collapse in vacuum. Kinda like having an 850 Holley carb on a small block, set up with the secondaries to open with the primaries (going to fall flat on it's face). -Al

av8tor1977

My Feedback: (6)

Yep. The air can accelerate much faster than the fuel, the engine goes lean, and hence the bog. And as you say, a too large carb exacerbates the problem. A lot of less savy hot rodders thought the surge of power after the bog meant real good things were happening, when in fact that bog was killing their acceleration.

There are Walbro carbs with an accelerator pump, which does help. What would be really cool though, would be a carb with an accelerator pump AND a midrange adjustable mixture circuit. I keep wanting to see if I could create one, but never get around to it, yet.

AV8TOR

CK1

My Feedback: (60)

I was into restoring late 60's early 70's Chargers and 'Cuda's back in the late 80'and most of the nineties . By the time I got my hands on them most of them had been mugged by kids with little or no knowledge and deep pockets. I had stacks of 750 holleys that I had replaced with 550 and 650's .One guy thought I had rebuilt his engine because it had such better acceleration . All I did was put the correct carb on it and tune it.
In the case of carbs .....
"BIGGER IS NOT ALWAYS BETTER "

av8tor1977

My Feedback: (6)

I always got a grin when I opened the hood and saw a screw inserted in the linkage of the 3310 750/780 cfm Holleys to make them into a mechanical secondary carb. The problem is, a lot of these guys you just can't talk to; they're totally convinced they are right. That's when I just smile and walk away. And LAUGH if I encounter them in a race!

You don't happen to have an 850 or 950 Holley lying around that you would sell cheap would you?

AV8TOR

Truckracer

My Feedback: (19)

Back in the day I built several street engines using large Holley or Rochester 2 bbl carbs on 4bbl manifolds. Fantastic throttle response and hard to beat in a stop light to stop light race. I often used the 550 / 650 4bbl Holleys converted to center hinged float bowles and these were often throw away carbs from other people. I loved them.

CK1

My Feedback: (60)

2 bbl carbs were the ticket for a long time . Great throttle responce . The scariest ride I had built at the the age of 18 used a chevy straight 6 235 with 3 - 2 bbl carbs . It was heavily ported and the head was shaved but . The torque and throttle response was insane . Yeah it whipped most of the big 8 engines in 60 ft and the 1/8th but the 1/4 was 2 long for its short legs . I constantly replaced snapped shafts and yokes on that one . I wish I still had some of those carbs and other parts I took off but at the time they usualy were traded for other parts or sold to pay for tires and fuel .
What i realy wish I had back was soge of the Chargers , Roadrunners and 'Cudas that I've had . The last Charger I sold almost 20 years ago was re sold at an Auction in Charlotte NC for close to 100,000.00 .

ahicks

My Feedback: (2)

My issue, from the time I was a teen working at a Chevy dealership, was I never had a place to put some of the really cool stuff I would likely still have. Imagine being an all ready confirmed gear head at18 working at a dealership with a policy of selling you anything they had for dealer cost! After a while, there were occasional bets placed among the sales staff regarding the outcome of street races between our guys and some other local dealers. The Chevy reps even got involved..... Many Saturdays spent working on govt. projects.... Sleeper cars absolutely ruled!

CK1

My Feedback: (60)

I was a farm kid so Trucks Tractors and implement repairs started early for me . I put together my first car at 15 .I didn't start the Dealer gig until I was 22 but I went down that same path for following 30 + years until I destroyed my spine and a few other necessary body parts . Now an airplane is a struggle to lift . No more power lifting cylinder heads and blocks for me . A sachs 5.8 is the heaviest engine lifting I do now .

Truckracer

My Feedback: (19)

As with so many things in life, had we only known then what we know now, what could have been. My dad had a huge barn that was largely unused after about 1970 and it would have been a great warehouse for self made barn find cars. I'm thankful for what I had and did when I did it though. I was fortunate to own many of todays classic cars long before they were classics. Dang, this forum is turning into an old folks convention!

Truckracer

My Feedback: (19)

Ck1, much of what you reported about the small plugs was typical when they were used with an adapter insert in place of a larger plug. This reduced chamber volume a bit along with a bit less than optimal plug location in the cylinder. I believe the small plugs act pretty much the same as a CM6 when used in a cylinder designed and machined for them. In a few cases, I've even heard of improved performance when the small plugs were used in certain engines.

In all my years around engines of all types, I've never heard of reduced performance with a larger plug gap (within reason) if the ignition was strong enough to fire them at desired rpm.

av8tor1977

My Feedback: (6)

Oh my god. The money I could have made..... I knew a used car dealer that specialized in hot rods and gave me great deals. I would buy one from him, fix it up perfect, drive it for six months, then sell it at a good profit and do it again. I had all of them. (Except Fords, I don't go there.) Cudas, Chargers, AMX's, Camaros, Trans Ams, etc. When I see what they sell for now it just makes me sick. One time a friend had a 1960 Thunderbird sitting near his house. Perfectly straight and intact; just needed the engine rebuilt. Wasn't deemed worth it back then. So we took a tractor and rolled it over upside down, and I torched the 9" rear end out of it to use in my race car!! Would be a 100k car now.... Another friend DID have a barn full of classic Hot Rods that he had fully restored in his spare time. He got bucks down one time, and sold them, cheap. Now that collection would be worth well over a million bucks.

If a fellow could afford to do so, it would still work now. Buy some of these fantastic new hot rods out now, and store them. Get them out in the future when everyone is driving a Prius, and sell them for huge bucks. It would probably be illegal to drive them by that point, but you could perhaps make yourself some gasoline or alcohol and start them up to hear "the good old days"!!

AV8TOR

av8tor1977

My Feedback: (6)

Speaking of raw torque.... I presently have a 1980 full size Blazer with a particularly nasty 454 I built for it. I never hole shot it out of respect for the drive train, but still, everytime I hit second gear at 7000 rpms, it blows the drive shaft right out from under it! Breaks the u-joints right in half. And this is in high range, using high quality solid trunion u-joints, and with an automatic trans!! It's crazy fun to drive...

AV8TOR