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#21
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I went the half/way .... blocked off one side. Left the choke side open (Q-jet), blocked the far side. I'll never be firing this thing up to drive to work on a <40 degree day so no big deal.
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#22
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That's a good way to go and I have thought about doing that many times .
It should work very good especially on the 1974 and earlier heads with there dam massive Exh crossover openings.
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Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
#23
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Even better if you fill the passage in head as it will create a balanced exhaust pulse and even a HP gain. Jeff
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Carburetor building & modification services Servicing the Pontiac community over 20 years |
#24
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I had mine filled when SD Performance did my heads. Worked well on the dyno, don't know about the street yet.
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"I know just enough to keep me here, but not enough to get me out" |
#25
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About all that happens with blocked off crossovers is no way to use a divorced or hot-air choke and the engine will not be as efficient until the engine fully warms up and the intake heat-soaks. You can't stop this from happening, so blocked crossovers on a street driven vehicle considerably increase how much time it takes for the intake to heat up. The entire time the high velocity incoming air is cooling the intake and requiring a richer mixture until it warms up some.
With the way these cars are driven these days it's really not a big deal. If you have a carbureted/TB application and live in a colder region, and want to use the vehicle in sub-freezing temperatures you will need a working exhaust crossover for best results. Coincidentally this time of year I get a couple of calls a week. Just had two this week from folks who modified their SBC truck engines (one was in a 1972 Blazer the other a mid 1980's 4 x 4 truck) by installing later model Vortec heads on them. Vortec heads do NOT have provision for an exhaust crossover. The complaint from both owners was the same. LOOOOONG warm up times and the engine wasn't quite up to par until 15-20 minutes of run time and everything well heat soaked. One of them actually experienced some "icing" when he tried to take off right after start-up. His engine refused to idle well without some "feathering" of the throttle at the first couple of stops, then was fine after it heated up some. He contacted me for tuning advice. I told him it's not a tuning issue since it was fine once the intake got some heat in it. Neither on had the first hint of an issue in the warmer months, so problems in this area are going to be in the Winter and cold outside temperatures. I haven't had a working crossover on any of my 455 engines but did on my 400 and both 428's that were in the Ventura years ago. I never had the first running issue with them open, and even lived in the South and used the Ventura a lot in the hot summer months. So I really see no negatives from using it on a street driven vehicle and for sure you are going to need to leave them open for correct divorced or "hot-air" choke applications........Cliff
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If you can read this, thank a teacher. If you can read this in English, thank a Veteran! https://cliffshighperformance.com/ 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), |
#26
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Cast ceramic just sits there in zero stress at room temp., Lowest stress at hot, highest stress at cold ( least delta-temperature for CTE difference)
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#27
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Living in TN, not the worst of winters, I've seen frost form on the carb body of cars with the heat blocked off, now humidity has to be high, and temps 35ish or below, but car was almost undriveable until it warmed 20-30 min. I usually restrict, instead of block, because of this.
Performance gaskets used to come with block offs, and restrictor?
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1970 amc rebel 4-door, faded blue paint,290 emblems on car, 455 under the hood.Turbo 400,3.15 gear. Best so far 1/8 et,8.90 on street tires, Since that I have added 400 heads. I call it a rat sleeper. |
#28
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[QUOTE=flat-bill;6103584]
Quote:
^^^^^This^^^^^ Just to fill in more detail, normally EFI does not operate in closed loop during cold start up. So the O2 sensor is not correcting the base fuel map when cold. Closed loop control is locked out by the ECU when any fuel modifier table is used- such as cold start enrichment. In addition, most tuners go further and lock out closed loop control until coolant temp reaches 140-150F. This is so the learned compensation tables do not get corrupted by artificially rich conditions during the few minutes when the engine is cold. The system runs off the base fuel map only, modified by the cold start enrichment table which adds (or subtracts) fuel based on coolant temperature. There are other cold temperature tables that control acceleration enrichment based on coolant temp, so you can drive it without a stumble when cold. If you have problems with a cold start/warm up conditions, you can read (datalog) the O2 sensor vs coolant temp once it starts, and see if you need more or less fuel at any given temperature. Then you can adjust the enrichment table until it runs properly.
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1964 Catalina 2+2 4sp, 421 Tri-power 1965 GTO, Roadster Shop chassis, 461, Old Faithful cam, KRE heads 305 CFM, Holley EFI, DIS ignition. 1969 GTO 467, Edelbrock 325 CFM, Terminator EFI 1969 Firebird Convertible |
#29
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This is a very general opinion but what I think....
For wet manifold (carb/TBI) setups, I would say leaving the exhaust crossover open is best for most people. I would do filled stock heads or aluminum heads in port fuel injection applications. |
#30
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Your opinion is pretty much spot-on for "wet-flow" systems.
Take a look at older carbureted engines, especially pre-1969. Many even routed additional heat up thru the intake and put it directly on the bottom of the carburetor using a steel-shim gasket to seal things off. In theory it's an excellent idea, but in practical use one can run into scenarios where the heat is just too much and can cause issues with the fuel inside the bowl of the carburetor. All of the older tractors we have here use combination intake/exhaust manifolds where exhaust heat is used to heat the incoming air charge, no exceptions. They are all FLAWLESS in any weather, even the hottest summer days working them really hard. A working exhaust crossover for sure is not required for most of these vehicles as very few folks do much more than get them out on nice summer days and very seldom mess with them much in the Winter months, especially as far North as I am here. IF they were being used for daily transportation, year round, and one was looking for quickest warm-ups, best efficiency/fuel economy, correct choke operation, the crossover needs to be open and working. Even with that said when you get out of the Northern regions where temperature seldom if ever fall below freezing, and you only get a couple of months of cooler weather each year, you woln't miss an open crossover nearly as much......Cliff
__________________
If you can read this, thank a teacher. If you can read this in English, thank a Veteran! https://cliffshighperformance.com/ 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), |
#31
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Here's an idea that I talked about with Dave Bisschop when he did my heads for me. Had the exhaust crossovers filled with aluminum to gain the port tuning benefits and he said that if cold starting was too much of an issue what he's done is drill a single 3/8" hole through the aluminum plug from the crossover into one of the exhaust ports on both heads.
This keeps the ports effectively isolated from each other, preserves (except for that one small hole which is fairly inconsquential) the re-shaping of the port, and allows some heat into the intake to help with cold starting. With an electric choke and the Q-jet I got from Cliff I never really had any issues. I drove the car on some pretty cold mornings and on the coldest mornings I'd just start it and let it idle for a minute before driving and no problems, so never saw any reason to drill through the crossover like that, but it could be done if someone was having issues.
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---------------------------- '72 Formula 400 Lucerne Blue, Blue Deluxe interior - My first car! '73 Firebird 350/4-speed Black on Black, mix & match. |
#32
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Chevy was so notorious that the aftermarket made a tool to roto root out their intake passages, although it really didn't work very well for the purpose. I believe Pontiac had the huge passages engineered for a reason, I've never seen the Pontiac ones plug, but if any were prone to it, the 67 4 bbl Q Jet W/stainless shield under the carb would be the most likely to plug up, IMO. |
#33
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One of the good things about welding your crossovers up with 1/4" steel is that if you want to drill a small hole in one for heating your carb its is a very simple operation. And welding it back up if you change your mind is also very simple.
Welding/filling your crossover is not simply about keeping heat away from your carb, a gasket can do that. Its also not only about the heat and your exhaust functioning more efficiently. Its about your cylinders properly filling too. When ex valves can be open at the same time for moments on the center 2 cylinders. And big overlap performance cams the problems get worse. A pulse of exhaust can go from one piston going up on the ex stroke while 2 adjacent valves are open and the one next to it is trying to suck air in while there is a little pulse of air going up a intake runner. It all happens "behind" those 2 valves, there is a pathway. On Buick heads you can put a soft plug in. Olds have a even larger home than Pontiacs and they need filled/welded too. One hole is enough if you live in cold weather and want the heat. |
The Following User Says Thank You to Dragncar For This Useful Post: | ||
#34
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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 |
#35
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X2 on the above.
__________________
Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
#36
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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.
Filling the crossovers completely with molten aluminum (common practice) is different than blocking them off at the intake gasket. There is a performance advantage when you make all the exhaust ports the same apprx size/shape. Combine that with some effective head porting and good camshaft selection and your new engine build will be making more power than it would have if you had left everything "stock". When making these sort of decisions folks really need to look at the goals for the engine, vehicle, and factor in the negatives involved with such modifications. Since we are in the Street section one should be looking harder at the negatives and make the decision accordingly. For the most part it is a complete waste of time to block off the heat crossovers at the intake gasket. The intake is going to fully heat soak anyhow, so all you are really doing is prolonging the time it takes to heat up. This can be a pretty big deal for some as you also loose the ability to use a factory divorced or hot-air choke system. I suppose one could put a hand choke on an early set-up that was originally divorced, but personally I'm not overly fond of them nor do I like the look of something like that inside the vehicle. This topic comes up often enough and some good info posted on it every time. Since I do this for a living I'll say one more time that I get quite a few complaints from folks who have blocked them off. The complains are usually either that the divorced or hot-air choke system isn't working (they didn't think about that deal) or the engine doesn't run fully up to par until they have driven the vehicle a considerable distance to heat soak all the parts. This time of year I also get quite a few complaints about things not working well in sub-freezing temperatures. Once it warms up outside I typically woln't hear much about this topic until next Winter......Cliff
__________________
If you can read this, thank a teacher. If you can read this in English, thank a Veteran! https://cliffshighperformance.com/ 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), |
#37
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Pontiac engineers made probably the best solution with the RA IV/HO with the divorced crossover to keep the choke operation functional, and avoid some of the heat transfer to the intake manifold, however we all don't have access to the parts to make use of this engineering feat.
The aftermarket never saw that it was feasible to tool up for separate manifold and crossover so the RA IV is the best there is. Pontiac put a lot of money into the 455 SD program, they however drew the line at a rehash of the RA IV/HO divorced crossover, and aluminum intake. Maybe the budget was too small to do it, or the benefits just wasn't as great as the engineers first thought it was going to be.................... |
#38
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It has been a bit better in that regard than the original iron intake was. Very happy with it. |
#39
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My Holley has a hand choke setup but has no air horn . Instead I use it as a hand throttle to keep the engine running when cold. As the engine warms up I bump the idle down. I don't run this thing under 60 degrees.
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#40
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Jeff |
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