#41  
Old 01-26-2020, 07:05 PM
Dragncar Dragncar is offline
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Originally Posted by "QUICK-SILVER" View Post
With factory firing order that would take one REALLY BIG cam. There's 180 crank degrees difference between exhaust events on center cylinders.

Cross talk from bank to bank can and has been noted. Plugging a single cylinder on each head will stop that. Then you have one cylinder from each side heating the crossover. With cylinder firing further apart you don't get the cross talk.

Clay
Exactly, PMD should have used one cylinder per side to heat the carb.

  #42  
Old 01-26-2020, 09:58 PM
jonathonar89 jonathonar89 is offline
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Originally Posted by Cliff R View Post
There are many myths that abound in this hobby associated with improving engine performance and efficiency and blocking off heat crossovers is one of them. Setting up a distributor for a super-fast advance curve and eliminating the Vacuum Advance is another.
Not going to name anyone but this reminds me of a certain guy who restores Firebirds out East who likes to show off his performance street builds on Facebook. His recommendations to people are to lock out timing at full advance and get a ”performance” starter so the engine has no problem starting.

My thought is wondering why he recommends starting/idling engines at a full +30 advance. I don’t think he realizes he’s making it harder to start the engine trying to fire it so far out. I’ve told him this but says it works because of cam overlap. I’m not in agreement there but who am I to argue....I surely am not building expensive engines and doing restoration myself.

  #43  
Old 01-27-2020, 08:12 AM
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Last time I looked it was 2020, so we should be a lot smarter these days. I still know several engine "builders" that eliminate the vacuum advance and install light distributor springs on their engine builds. Those engines are still missing 15-20 degrees or so timing at light engine load, so will NEVER be nearly as efficient as they should be. Doesn't mean they woln't run OK and live a long life, but you'd think with all the good info out there these days folks would be tuning these engines for best efficiency in the "normal" driving range instead of having them use more fuel and not be as responsive as they could be.

As far as locking out timing, I don't even do that with full "Race" applications as it makes them difficult to crank especially if there is a good amount of heat in them. I will shorten up the mechanical advance curve some and don't use VA on them, but for sure they will crank and start easier without all the timing in at start-up........Cliff

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  #44  
Old 01-27-2020, 03:35 PM
Will Will is offline
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Originally Posted by geeteeohguy View Post
...The engineers figured it out decades ago, guys.
In general I agree with you, I'm just being persnickity here...

I hate that "the engineers knew what was best" saying as a blanket statement. The engineers were constrained by many things us hobbyists aren't, which is why we modify things to begin with. If that saying was true, no one would ever change anything from stock.

Okay, that's my stupid rant for the day, carry on.


And on a related note, Cliff mentioned a hand choke. The car I learned to drive on, my brother's '70 SS 350 Camaro had a hand choke. not sure why, probably because it was easier to install that than to try to figure out how to get the factory choke to work on the Holley carb on the aftermarket intake it had. I think every kid should learn to drive on a car like that. Manual choke, manual transmission with some kind of super heavy clutch, manual steering with one of those small diameter foam grip three spoke steering wheels and a modified and quite powerful engine. Once I had that beast mastered there wasn't anything I couldn't drive.

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  #45  
Old 01-27-2020, 06:52 PM
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dataway dataway is offline
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I know it varies engine to engine. But I wonder, considering the volatility of new fuels, specially those containing alcohol, in general they will vaporize at a lower temp than the fuels pre-1980. Which would seem to indicate they would require less heat in the manifold to prevent fuel drop out, however the same factors would make new fuels more likely to ice the carb ... correct?

I figured running with the choke side passage open, driver side closed, and with an fully functioning factory intake pre heater might allow for a good compromise. A bit slow on warm-up but while outside air temps are cold it will be drawing through the pre-heater until underhood temps go up. By the time that scenario has played out the manifold will have heat soaked enough to alleviate the need for much manifold/carb heat.

Not a perfect setup, I can still imagine a few scenarios where for five minutes there might be some pedal massage required. But I think it would work for anything but extreme temp situations.

  #46  
Old 01-28-2020, 11:01 AM
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For the most part increasing the percentage of ethanol makes the fuel harder to burn, and requires more of it. Heat becomes your friend with these things.

I know a few folks who run higher compression and dabbled with E-85. Most had pretty good results, however several mentioned a learning curve involved with making sure the engine was heated up and well heat soaked for best results. Somewhat of a contrast into using race gas with high compression as most racers I know cool them down considerably between runs vs making they are up to temps and well heat-soaked.......FWIW.......

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73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile),
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Old 01-28-2020, 11:41 AM
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Originally Posted by dataway View Post
I know it varies engine to engine. But I wonder, considering the volatility of new fuels, specially those containing alcohol, in general they will vaporize at a lower temp than the fuels pre-1980. Which would seem to indicate they would require less heat in the manifold to prevent fuel drop out, however the same factors would make new fuels more likely to ice the carb ... correct?

I figured running with the choke side passage open, driver side closed, and with an fully functioning factory intake pre heater might allow for a good compromise. A bit slow on warm-up but while outside air temps are cold it will be drawing through the pre-heater until underhood temps go up. By the time that scenario has played out the manifold will have heat soaked enough to alleviate the need for much manifold/carb heat.

Not a perfect setup, I can still imagine a few scenarios where for five minutes there might be some pedal massage required. But I think it would work for anything but extreme temp situations.
This is one of the reasons I wanted to try the factory RAIV intake. This ethanol fuel does seem to be a bit more sensitive to heat soaking issues. Not noticed with higher fuel pressures in FI situations, but it can wreak havoc on us carb guys if everything isn't setup near perfectly.
I've been pleased with the factory intake and separate heat cross over. Our low temps this winter have hit the upper teens and the car has started and run flawlessly. I can drive away from a cold start without a hiccup. I still think this setup sees enough heat to be happy, intake does get pretty warm to the touch, but it's shown no heat soak issues during the summer when our temps are well over 100.
With that said, our other daily driver, also a carb with divorce choke, and a factory aluminum intake, has full heat crossover function, runs the same fuel, and hasn't exhibited any heat soak issues either. I do take other precautions, such as a good thick base gasket that helps absorb heat, a good mechanical fuel pump with solid fuel pressure, and fuel lines routed properly, in this case, factory fuel lines and routing, and I keep engine temps themselves at bay to keep under hood temps to a minimum, helps with the heat soak issues when you come out 20 minutes later to start the car.
In a nut shell they both drive flawless and don't give me or my wife any issues what so ever.

  #48  
Old 01-28-2020, 11:52 AM
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We also have cars here on the other side of that spectrum. My chevelle for instance, with AFR heads, has no provision for heat cross overs, as many aftermarket heads today don't. Plus I run an RPM air gap intake on it to keep the carb even further away from heat. The carb is a 950hp, so it has no choke horn at all.

It'll start right up on cold days, and I only have to feather the throttle for about 30 seconds before it idles on it's own. From there it's as happy as can be, doesn't stumble, and will drive away from a cold start fine. It has a 242/248 cam in it so it's a little rambunctious but with proper tuning it's actually a very happy engine and easy to drive. 4 corner idle circuit helps, and enough compression for the camshaft helps. I keep the idle AFR's around 13:1 as that's where the engine seems happy in all conditions and gives me a strong idle in and out of gear, and actually returns decent gas mileage for what it is believe it or not.

Dad's car is setup very similar. No heat cross over, same carb, much bigger engine, bigger camshaft, and even more compression. His car on a cold start will idle on it's own within 30 seconds and thump along without complaining. So it's really not a big deal and just part of the game when you want to hotrod stuff like this.

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Old 01-28-2020, 12:03 PM
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I have run several different cars with timing locked at 36/38° for racing you have to use a start retard control have never had a issue starting or cranking.

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Old 01-28-2020, 12:17 PM
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I would guess that an aluminum intake equalizes manifold temps way better and sooner than cast iron, wouldn't cast iron localize heat under the carb more so than aluminum? Aluminum would transfer the heat from exhaust quicker, but also disperse to the rest of the manifold and to outside air quicker ... you'd think anyway, maybe not.

I thought alcohol was harder to burn in the combustion chamber, but vaporized at a lower temp ... meaning contact with cool intake runners would cause less of it to condense on the runner walls, so easier starting/running on a cold engine. As usual I could have things backwards.

  #51  
Old 01-28-2020, 12:31 PM
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Originally Posted by dataway View Post
I would guess that an aluminum intake equalizes manifold temps way better and sooner than cast iron, wouldn't cast iron localize heat under the carb more so than aluminum? Aluminum would transfer the heat from exhaust quicker, but also disperse to the rest of the manifold and to outside air quicker ... you'd think anyway, maybe not.

I thought alcohol was harder to burn in the combustion chamber, but vaporized at a lower temp ... meaning contact with cool intake runners would cause less of it to condense on the runner walls, so easier starting/running on a cold engine. As usual I could have things backwards.
Correct, aluminum dissipates heat better, so when I assembled the DZ engine for our Z, with a factory aluminum intake, I left the heat cross overs fully functional knowing for one, I wanted the divorce choke to work, but I also knew the aluminum intake wouldn't pose as much of an issue with heat soak anyway. I still run a 1/4" thick gasket under the base of the carb though, just for good measure.

I ran the factory iron intake on my RAIII Pontiac engine for years, both with and without the heat cross over blocked. With the heat cross over blocked and driving on very cold mornings, the choke would actually kick back on, lol. If I idled in town for a while it would generate enough heat to kick back off. I later opened the heat cross over. It did at that point exhibit a slight heat soak issue on the hottest of days, If I shut the car off and it sat for 20-30 minutes, I had to make sure I had the throttle cracked open as I cranked. It would fire immediately but run rough for a couple seconds and then be fine. It never flooded or became impossible to start, but you could tell there was a pinch of heat soak going on. Drove it like that for years and was never really an issue, as you became used to it after parking the car it just became normal practice. However once running the car never acted like it would vapor lock under any conditions.
Since I've installed the RAIV intake however all that hot starting stuff has completely stopped. It doesn't mind being heat soaked and will fire with a bunt of the key with no monkey business. The separate heat cross over is a pretty slick deal.

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Old 01-29-2020, 07:58 AM
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I've never once experience "heat soak" but in the hottest summer months we will have a half dozen or so complaints on the website or calls to the shop about it. They almost always come from folks with Chevy engines that still used the exhaust channel in the intake that puts hot exhaust gasses directly on the carburetor. Pontiac did the same thing in 1967 only. Even WAY back then they found out that although heating the carburetor allows for leaner settings and improved efficiency with less emissions, there is still a point of no-return with that sort of thing. Even so I still know quite a few folks who have 1967 Pontiac's that use the original intakes and Q-jets with the heat channels open that don't have any issues.

I actually restored a 1967 Impala SS with a 327 and the original iron intake/carb with the heat channels and stainless plate under the carb. I put a 4L60 OD trans in in and put over 35,000 miles on the car before selling it. It was a joy to drive and got mid 20's for fuel economy on the highway and it never grumbled once in the hottest weather with that set-up. That was before they started putting so much ethanol in the fuel so maybe why I didn't have any issues. This day and age I'd for sure block off the heat channel under the carb at each end but still leave the crossovers open if I had one of those early intakes.......Cliff

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