Blocking heat riser crossover

Building a 429 engine for my truck.
Daily driver.

Block or not block heat riser?

Strictly not a drag racer.

I have heard that if you block off heat riser you will get terrible gas mileage.
Is that correct?

If so, I'll just keep it open
Am also going to run a FiTec fuel injection on it.

Bob
Tucson

75truck wrote:Building a 429 engine for my truck.
Daily driver.

Block or not block heat riser?

Strictly not a drag racer.

I have heard that if you block off heat riser you will get terrible gas mileage.
Is that correct?

If so, I'll just keep it open
Am also going to run a FiTec fuel injection on it.

Bob
Tucson

It all depends upon how you use it and what the weather conditions are, for the area you operate it.

Leaving it open will allow it to run better sooner in cold weather. Once warm, it won't make a big difference either way as the whole thing will be heat soaked. In theory, your EFI will attomize the fuel better than a carb, and may be fine without it being open, but I tend to leave them open on everything except dedicated race applications.


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In warm locale applications we always block the cross over.
For the occasional winter driver we restrict to 5/16" for good low temp drive-ability.

160* or180* heat soaking the intake is one thing. Given aluminum's conductive properties having exhaust constantly running through the intake gets it so hot it kills torque until the latent heat of vaporization cools the intake. I've seem carburetors boiling on non restricted / blocked aluminum intaked applications here in WA state which doesn't break record temps often.

I did an experiment one time to try and quantify how blocked vs. unblocked presented itself on a dyno.

Just a basic 460 hp @ 4,900 ish iron headed deal. I used brass shim to block both exhaust cross overs knowing full well that this would last only a short time. When they finally blew through the dyno showed an immediate loss of 30 lb ft of torque at the bottom of the pull. The top of the pull recovered to with in about 7 hp.

Cool intakes increase VE and detonation resistance.

If operating in Tucson block it.




S

randy hit it right. so did carl. block a heat cross overs where I live and you can forget about driving the car. iced over carb before it even warms up. who cares about hp or torque if you can not even pull out of the driveway.
... I am sure not only ford but Chrysler, chev, international, and other vehicles used cross overs along with exhaust flappers for a reason. mainly so you could drive the car in bad weather, duh. zbob

Block it, unless your operating 100% of the time on the north slope in Alaska

I live in central Florida, so cold weather is almost non-existent here. In trying to get to the root of a vapor-lock/fuel bowl boiling issue I blocked the heat X-over, but I went one step further with installing a valley pan intake manifold gasket spec'd  for a '78 F350 460 w/LPG application(Felp-Pro MS 96044). Installing this valley pan may not be popular but it gives an additional "shield" against heat by blocking the hot engine oil from splashing on the bottom of the intake, helped for my application/needs.

for what it is worth Minnesota/Manitoba is usually colder than most of Alaska....lol

zbob wrote:randy hit it right. so did carl. block a heat cross overs where I live and you can forget about driving the car. iced over carb before it even warms up. who cares about hp or torque if you can not even pull out of the driveway.
... I am sure not only ford but Chrysler, chev, international, and other vehicles used cross overs along with exhaust flappers for a reason. mainly so you could drive the car in bad weather, duh. zbob

Had you more carefully read my post there Zyphyr B you would have noted my suggestion for cold weather operation. "For the occasional winter driver we restrict to 5/16" for good low temp drive-ability."

This actually works quite well for winter drivers in the northern US and Alaska. By allowing oil splash to warm the intake but using restricted heat for immediate plenum heating driveability is maintained in cold weather.   Client feedback backs this up. Duh...

Many Felpro gasket sets come with cross over restricters. I wish ours did.

Intake manifold heating is far more relevant when dealing with lean 70's emissions carburetors and their Effed up cold weather drive ability issues when the cross over plugs with carbon. Mopar 360's of the day come to mind.

Our BBFs are plenty rich at idle and part throttle to avoid drive ability issues even when cold.


Show of hands... How many folks here drive their carbed 460 powered ride when it is 20 freaking degree's below 0???
I have a couple with no complaints.




S

75truck wrote:I have heard that if you block off heat riser you will get terrible gas mileage.
Is that correct?

Clearly this is a hotly debated topic among the interwebs, which generally means it doesn't make much difference either way, but the fuel mileage question is an interesting one, as is part throttle drivability, and throttle response.  Again, with your EFI it may be less of an issue, but in theory the exhaust crossover will help atomize/vaporize fuel which would potentially increase fuel efficiency.  



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sorry scott never read your post, you sir are on my ignore list. only reason I read your more recent post was due to another member mentioning it. one again sorry. zbob

Not trying to pry, cause any ill feelings or stir the pot, but......WTF! Zbob, Why would you put Scotty on an ignore list?? He seems to be one of the most knowledgeable and helpful members here. From what I have seen and/or read, Scotty is always trying to help those in need in one way or another, am I missing something?

I have no idea about the dynamic of this conversation who/who part. I have however removed the crossover mostly on Clevelands and driven in -20 to -30^C conditions with 20/50wt oil as I was young/poor and had to drive my hot rod in the winter to work. This is with a 750DP and a 850DP no choke and parked outside. The start up routine was first disconnect the coil wire and turn over until oil pressure was reached, then reinstall coil wire and pump two times and turn the key to start, then idle in N until operating temp reached and drive comfortably sideways in the snow down the highway until I reached work, I have done this many times.

it is a discussion board, no malice or slander offered. just my experience driving carbed trucks and cars in a dd situation. if others have different experiences, great! zbob