[Speargun]

I've read several old threads about this with the pros & cons for blocking off the exhaust crossover or not, but they all were in regards to carb/choke applications.

I'm curious about what the pros & cons are when dealing with EFI on a street driven engine.

Since there is no choke to deal with and the EFI will adjust the AFR, what advantage is there to adding heat to the intake?

[blueghoast]

Originally Posted by Speargun

I've read several old threads about this with the pros & cons for blocking off the exhaust crossover or not, but they all were in regards to carb/choke applications.

I'm curious about what the pros & cons are when dealing with EFI on a street driven engine.

Since there is no choke to deal with and the EFI will adjust the AFR, what advantage is there to adding heat to the intake?

I would think none.

GT.

[glhs#116]

I would say there is EFI and EFI. The main point of the exhaust crossover for a street engine is that you want the fuel to stay vapourised and not clinging to the inside of the intake like dew on a frosty coke can. There's actually a lot of temperature drop just across the throttle blades from the fuel vaporisation. So..

If we are talking throttle body EFI then we are still in pretty much exactly the same boat as a carb and I would say you do want a functioning crossover. However, if we are talking mutlipoint port fuel injection then we are talking about a dry manifold. In that case you should definitely block it off.

And, again, I want to emphasise "street" here. The crossover is a huge help in the real world of cold starts and idling. For a full race application that goes to the track on a trailer or never sees cold weather then I think most people block it even for carb.

Sam

[Tom Vaught]

The fuel is mixed with the air, not by the cross-over heat, when the engine is first fired up but by the injector atomization of the fuel. Fuel Droplets are created by unstable growth of small waves on the jet surface and caused by interaction between the injector jet and the air around the jet. These waves become detached from the jet surface to form ligaments which disintegrate into drops. Mean drop diameters are much smaller in size vs the initial jet diameter.

Block that sucker off if it is port fuel injection. Agree with Sam if just a electronic "carb".

Tom V.

[Bruce Meyer]

Your in Florida. Block em. A cooler intake = more power.

[glhs#116]

Originally Posted by Bruce Meyer

Your in Florida. Block em. A cooler intake = more power.

Altho.. https://www.theguardian.com/us-news/...-plummet-video

:-O



Sam

[67gtospud]

Block it off. The efi does a better job of atomizing fuel initially since the injectors are spraying out pressurized fuel. Unless you're driving the car in oce cold temps, there really isn't a reason to not have the crossover blocked off in both carburated and efi applications.

[Sirrotica]

FWIW, In the OEM dry FI manifolds there is a water jacket cast into many of the throttle bodies for heat. The hop up guys will tell you to seal off the water supply to the throttle body to raise efficiency for HP applications.

Years ago I had my instructor at the Vo-Tech I attended that built a intake manifold from tubing for a inline 6 that would actually build ice and choke the runners of the manifold when conditions were just right (Dew point, temperature, and humidity). They ended up wrapping the intake runners with heat tape to prevent the icing in the runners when the conditions were right to cause the icing.

Now having a V8 with the intake nestled between the cylinder heads is a little different than an inline engine with the intake hanging off to one side of the engine. There are probably reasons to eliminate heat in an intake, and again with as many reasons to retain heat.

Most likely where you live, and what your attempting to do would govern your actions. There is probably not one hard fast rule to cover all circumstances. Over the years I can think of only a few times I've eliminated the heat crossover on a Stratostreak engine, those engines were all race engines, not street cars.

[JLMounce]

I run a FiTech system on a KRE headed 455. By nature those heads are not cast with a cross-over, so I don't have one.

Even with the FiTech, in very cold start applications, the car will struggle a little bit to achieve a stable idle. What needs to be done is to let the car warm up for about a minute to minute and a half. Just long enough to get a bit of heat into the chambers, that's all. Then it will run/drive as typical.

This occurs in temps where startup water temperature is below 40 degrees.

So it's not a huge deal to not have the cross-over working with these systems. If you lived in the upper midwest and drove the car in sub-zero temps, I'd recommend keeping the cross-over active. Considering you live in Florida and likely don't drive the car in temps below freezing, I'd block it off.

[Speargun]

In my case, I run a Fitech, so a TBI aka fancy carb. I also live in Florida so the norm is hot & humid except for the 8 or 9 days a year it's cold enough for the iguanas to fall out of the trees like earlier this week. Today we're back into the 70's.

There are tons of opinions & anecdotal evidence all over the net on whether or not to block it, but does anyone know of any actual scientific data pointing one way or the other? I.e. The best fuel atomization occurs between X and Y degrees.
Some will say that it must be needed because it comes on all production cars. But why? The problem with that argument is that car manufacturers do a lot of things that are solely designed to meet some EPA/safety standard or to offset a change somewhere else. The catalytic converter, for example, was designed so that a car could run cheaper (less refined) gas & maintain the same emissions.

Aftermarket heads don't have the crossover. Is that because of empirical evidence or because "That's what we've always done"?

[steve25]

Block them in Fl, but don't let the car idle to warm up, drive it!

[David Jones]

I'd melt mine out if I could.
Warm up time goes up considerably. Once it's up to operating temp, it's all good. Getting there is slowed considerably.

[Lee]

I've ran FAST EZ 2.0 and XFI Sportsman throttle body efi systems in below-freezing temperatures. BOTH systems start instantly, idle smoothly in gear, and can be driven immediately, just like any new car.

[1funride]

With my Fitech EFI I blocked it off and have had no issues. I have started it in 30 degree weather after weeks of sitting and it started right up, rough idle for a few seconds then the EFI smooths it out to a high idle and doing just fine, in fact I could even drive it with very easy throttle advance....never happen with a carb.

[JLMounce]

Here's a more science detailed article on the issue. It's wiki, so take that with a grain of salt. https://en.wikipedia.org/wiki/Carburetor_heat

Theoretically, with the TBI system, you would still want the cross-over based on the scientific principles at work. This would create the best drivability in cold weather operation and especially after initial startup.

That said, if you look at how the FiTech calculates fueling requirements, the fuel load package is designed partially around cylinder temp. Since the system doesn't have an instrument in the cylinder, it derives that number from the intake air temp sensor against the coolant temp. I don't know the entire calculation, but those are the two sensor inputs that are used to derive that figure. The key here is the IAT sensor and where it's located. That sensor is located in the throttle bores, above the annular discharge rings.

Again, not knowing the exact cylinder temp calculation, but assuming it's taking air temp at the IAT sensor and the calculation accounts for temperature drop from the fuel atomization, having the heat cross-over active, represents a temperature variable that the FiTech software can't account for since the air charge is being actively heated after the air temp reading is taken.

Yes, the system can compensate fuel delivery requirements based on the oxygen sensor that is acting as a corrective device for issues with the fuel load calculation. If the system is setup close to what the engine needs, this is probably not an issue. If the VE table is far off however, you may be at the limits of the fuel trims and the system may not be able to compensate enough.

All of this to say that in my opinion, given your location I would not run the cross-over. If you have a day or two a year that the car is being driven in temps at or below about 40 degrees. Just let it idle for a minute or two before setting off and you'll be fine.

I had my car out just yesterday in 40 degree weather. started the car and coolant temp was below 80 degrees. I drove off without issue after letting it idle for a total of about 30 seconds.

[flat-bill]

[QUOTE=JLMounce;6103553]I run a FiTech system

Even with the FiTech, in very cold start applications, the car will struggle a little bit to achieve a stable idle. What needs to be done is to let the car warm up for about a minute to minute and a half. Just long enough to get a bit of heat into the chambers, that's all. Then it will run/drive as typical.



You just need to fatten up your warm up enrichment. No need to put up with that. Take advantage of the tunability of the FItech. Billk

[Dragncar]

Do it, no downside. Everything still heats up. And it is not just about heat. The main difference is what happens behind the vales on stock heads, it all negative.
It sends pulses up the wrong way on the adjoining cylinder. And that is not god for performance. Also, you exhaust note will sound a lot snappier.

[Half-Inch Stud]

I enjoy the combo with blocked Crossovers. Id fill every head i own with the ceramic stuff i used & posted before. Aluminum as a second choice. Sheetmetal plates as a no-sir.

[Speargun]

Originally Posted by glhs#116

Altho.. https://www.theguardian.com/us-news/...-plummet-video

:-O



Sam

Welcome to Florida!

[Dragncar]

Whittmore welded mine up with1/4" steel plates. I welded up a set of 96s myself doing the same thing. What you need is the nickle rod with flux that does not conduct so you can weld 'down in a hole" and not worry about hitting your seats. Its really not that big a deal. Just clean your head very well slowly pre heat it and slowly weld it and let it cool slowly. You can wrap it in a heat blanket if you like.
Pontiac iron has a lot of nickle in it so it welds nicely with nickle rod.