Exhaust Crossovers in the Intake

[Rockfish39]

I could use guidance from the older and wiser on this one...

I did the math last night and came to the conclusion that Phil Reed has been wrenching on cars for 16 years, 2 months, 14 days, 6 hours, and 23 minutes longer than I have...
(+/- .005 std dev) :p :p :p

ANYWAYS, Im upgrading to 2 fours on my car and I have the Offy low profile intake in-hand. The center ports are WIDE open, just as they are on a stock intake.

In the past, when I have built SBC engines I have always blocked those off with the stainless slugs that come with performance Fel-Pro intake gaskets. All that heat burns paint right off and I always thaught that cool air density charge inside the intake it the best way to go. If you have ever seen Edelbrock AIR GAP manifolds, they work on the same principle.

I dont live in Minnesota or Nebraska, so starting the car at -22deg F, I dont have to worry about...

QUESTION: should I block the ports on the 2-4 intake, or leave them open.

To aid in my continuing education on "W" motors, if the answer is leave them open, please explain why....

Thanks so much!!!

Rock:cool:

[59elcooldsuv]

At the machine shop last week I watched a Poncho fanatic working on some heads that he was prepping for a racer.

He had poured molten aluminum down the exhaust ports to block the heat riser ports, then ground down the rough side in the port.

I asked him about using stainless plugs or sheet and he said that it'll never last - burn out quickly he says.

[Fran Preve]

I've stuffed heavy duty aluminum foil crumpled up in the passages THEN used the intake with block off. Drawbacks to blocking heat cross-over?. Not much with an aluminum intake, they conduct heat pretty well, but you WILL lose some drivability and gas mileage, heat promotes atomization. If your interested more in performance, block, if more in drivability, open (or partially blocked. Different strokes................................

[SSpev]

Yes, you can loose drivability, until it warms up. Never heard anyone say a hot intake improves atomization. Funny. (how much is hot? 100, 200) It will cause fuel to evaporate (different) Evaporated fuel is getting closer to the flash point. Bad. Evaporated fuel is hard to control. The rate of heat release. A cool intake produces more power. More power from same engine means efficiency when up in turn fuel mileage goes up. Heat is for emissions control and startup. A cold intake combined with low tubulance and/or poor carb setup can cause fuel to fall out of the are streem. My 348 and 396 have the passages blocked. No problem driving them. Even in the cold.

[fatride]

For performance , Block'em! I use .015" stainless plates cut a little bigger than the port, use a little High tack to keep them in place and your good to go.
;)

[Southtowns27]

I'd block 'em off too...We do on our SBC circle track engines. I just take a small piece of aluminum flashing, cut it to fit over the port and high tack it in place. Works good, never had one burn out or melt

[jester]

So, just so I know how badly I screwed up. How will leaving them open effect the performance of this beast hot and cold?
And second, should I take it apart to block them off?
Will I notice the difference on a street driven 425hp motor?

[fatride]

I wouldn't take it apart right away, if your like the rest of us you'll have it apart soon enough for one or another reason. I could feel the difference in throttle response and power especially on really hot days! Put a phelonic spacer under your carb it will help keep heat off the carb itself. You could probably get away with reusing your intake manifold gaskets if you dont damage them in removing the intake!
:cool:

[Rockfish39]

Originally posted by fatride
For performance , Block'em! I use .015" stainless plates cut a little bigger than the port, use a little High tack to keep them in place and your good to go.
;)

Fatride,
Ill do EXACTLY that... except that I have a roll of tempered blue .015 spring steel. Its easier on the grinder than stainless is...

THANKS FOR THE INPUT EVERYONE!!!:cool:

[hotshoe]

rockfish,
Could you post a picture of yours when complete? I'd be interested in this myself.
thanks,
hotshoe

[348NUT]

I'd block them. I did on my Bronco 302 and you have to have the choke set pretty tight so it won't back fire when driving it cold. Even in the summer but once it warms up a few minutes even it doesn't hesitate a bit. My 2 cents. (sometimes I think it's the only sense I have left!) :D NUT

[Rockfish39]

Originally posted by hotshoe
rockfish,
Could you post a picture of yours when complete? I'd be interested in this myself.
thanks,
hotshoe

No problem Hotshoe... Ill take the pics then post them after Curt sends me my new carbs....

Rock:cool:

[bowtieollie]

Originally posted by Rockfish39
...except that I have a roll of tempered blue .015 spring steel. ...


Now, you are wasting your time.

That tempered steel will corrode so fast in the exhaust enviroment - you won't know what happened to it.

Stainless is the ONLY way to go.

[Rockfish39]

Bowtie,
I did not know that tempered spring steel will corrode in that environment... Why is that??? I thaught that spring steel is heat treated in the first place and can resist the exhaust temp with no problem at all....

Since Im changing manifolds, Ill take your advice and use .015 stainless. FOR SURE there will be no problem with that material...

Thanks!!!

Rock


:cool:

[bowtieollie]

Although the steel is tempered, it does not possess the anti corrosion abilities of stainless steel.

The exhaust gases are acidic and corrosive in nature and will erode the tempered steel in a short time.


Its not a temperature issue, its a corrosive atmosphere issue.

[Fran Preve]

Something I should bring up, just thought of it, do NOT block the cross over if your using a heat riser at the exhaust outlet!. The heat riser caused some exhaust gas to cross over to the opposite side while it was opening. Most guys dump the heat riser, but if it's there THINK!.

[59elcooldsuv]

Aha! Yeah, now that you mention it - that IS stupidly obvious, but for some reason it had never occured to me before. Thanks for pointing that out.

[tripowerguy]

I had a part of a milk tank lid in which I always cut out pieces of stainless to block off heat in manifolds. I have always blocked off the heat because heat is your enemy for preformance. I took my 58 out today and it was a little nippy this morning and it took about 10 minutes before it began to run really good,blocking them off hurts drivability for about 10 to 15 minutes then I think it helps a great deal. Roy

[Fran Preve]

Another point. Have you ever pulled an intake and had a coke/grungy oil build up on either side of the valley chamber, and a build up under the intake in the center?. Well this is caused by the HEAT caused by the exhaust crossing from side to side. I see this often, sometimes really nasty!. Blocking the cross-over stops the heat build up which in turn "cokes" the oil. Another reason to block. This won't happen in a few thousand miles, the higher the mileage the bigger the build up. Also, if you change the oil every 3000 miles it slows it down, if you used synthetic it slows it down even more (I change synthetic about 7-8000 miles or as long as it stays clear).

I've been bracket racing for years, I see what things do to improve performance, big things and little things. When I make a change I see it in hundreths if not tenths of a second. In MY experience blocking the heat cross over doesn't do squat, no matter WHAT you may think!. You MIGHT pick up a tenth in the 1/8th, and if you can tell the difference between a 10.50 and a 10.60 brudder your a better man then me!. If you blocked your cross over, and I put a 1/2 inch PLYWOOD spacer under my carb and left it open there wouldn't be a noticable difference. Plywood is the BEST insulator to keep heat from the carb, and the heat transfer betwen the intake and carb is where power is lost moreso than after the fuel has left the carb and entered the intake. Think about that last statement a minute.

Block or not block?. Blocking won't hurt anything EXCEPT cold running and slightly worse gas mileage, you shouldn't with a heat riser valve (and you should NOT have one of THOSE anyway!). Blocking can help stop oil sludge/coking in the tappet cavity, not a problem if you change your oil frequently and/or use synthetic.

One final thought. My 1985 Corvette has a digital dash (which I LOVE!) and both water temp and oil temp read outs. The oil temp is ALWAYS 20 degrees HOTTER than the water temp. Whatever the water temp is when I flip to the oil temp it's 20 degrees hotter. What that means is the oil splashing around under your intake is 200 degrees. Food for thought........................................... ....................

[fatride]

Fran, fist you say blocking the crossover doesn't do squat then you say you may pick up a tenth in the 1/8. Which is it? In my case blocking the crossover along with increasing the pump shot inproved my low end response and helped get rid of a dead spot when I turned up the wick! By the way, I'd be tickled if I could pick up a tenth, and it does matter what I think, after all it's my car!
:rolleyes:

[Fran Preve]

Fatride: calm down, do what YOU think is best!. Now which was it, blocking the crossover?, or adjusting the pump shot?. Only saying maybe the carb adjustment did more than the blocking did. And your talking at the launch where blocked or unblocked isn't going to make a lot of difference, AFTER 60' it will!. As far as a tenth is concerned, in a car that I race every week, or a LOT, then a tenth may be important. But first I have to have a car that runs consistant, as in within a tenth. Then I'll dick with it. And if you can tell me what the car turned faster or slower before you look at the time slip your a better man than me. But I bracket race, I have to dial within a a couple of hundreths. I'm only saying, I bracket race, I race weekly, I build/race for consistancy, I've been doing it for years. If you race occasionally or not at all you can't feel a tenth. But your ABSOLUTELY right, getting rid of a bog or flat spot at launch is something you CAN feel, and WILL make a difference!. But that's a carb pump shot problem, not a crossover one.