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Old 06-07-2007, 04:00 PM
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exhaust crossover questions

SBC question for you guys.
will run holley 750 HP no choke on the street, will be driven in winter in pacific NW
have performer RPM intake with exhaust crossover
have AFR 195 heads, and exhaust crossover hole is about 3/8" (much smaller than rectangular hole in intake)

I was told that on cold days it would help alot to run the exhaust through the crossover by having a valve on the passenger collector close, all exhaust would run through the crossover, to drivers side and out. That seems like an awful lot of air to squeeze through a small hole. When the engine got up to operating temp, the valve would close and exhaust wouldnt heat up the crossover anymore

this valve is called a heat riser valve, hedman sells one.

My question is, with the tiny size of my exhaust crossover, will the valve do any good? seems like it would kill the engine if it could seal.

Also, will much exhaust be flowing through the hole with the valve open? and will that decrease performance much?

Thanks,
Jerome

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Old 06-08-2007, 01:04 AM
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How times have changed. Back in the 'old days' all engines were set up like that! The X-over valves were actuated with bi-metallic springs that unwound as the header got hot and opened the valve. Main problem was rusted and sticky valves that froze open or closed. Had to WD40 and pound on them to keep them loose. Will work fine and definitely helps engines warm up quicker. Just can't rev the engine to 10,000rpm w/ valve closed but you wouldn't try that would you?
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Old 06-08-2007, 01:22 AM
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I will be using the valve with a shorty header, whereas I think these were designed for stock manifolds. Will there be a problem with that?

Does anybody know if these valves are closed 100% when cold or if they are just reducing airflow? Obviously if the restriction in the crossover is alot, then the exhaust is going to blow the valve if it is closed up tight. Would be great if these things were just sort of sprung closed so if the pressure was too much, then it would allow it to open.

Thanks,
Jerome
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Old 06-08-2007, 11:38 AM
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Quote:
Originally Posted by ljjjrome
SBC question for you guys.
will run holley 750 HP no choke on the street, will be driven in winter in pacific NW
have performer RPM intake with exhaust crossover
have AFR 195 heads, and exhaust crossover hole is about 3/8" (much smaller than rectangular hole in intake)

I was told that on cold days it would help alot to run the exhaust through the crossover by having a valve on the passenger collector close, all exhaust would run through the crossover, to drivers side and out. That seems like an awful lot of air to squeeze through a small hole. When the engine got up to operating temp, the valve would close and exhaust wouldnt heat up the crossover anymore

this valve is called a heat riser valve, hedman sells one.

My question is, with the tiny size of my exhaust crossover, will the valve do any good? seems like it would kill the engine if it could seal.

Also, will much exhaust be flowing through the hole with the valve open? and will that decrease performance much?

Thanks,
Jerome
Forget the heat riser valve, SeaTac doesn't get cold enough to warrant their use, Fairbanks would. They rust and carbon up inside of a few months to a year and quit working anyway.

A bigger help on cold rainy mornings is heated air going into the intake. Keep a factory type aircleaner housing with a hot air riser and control valve off one side of your headers and the engine will be happy all winter long.

Bogie
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Old 06-08-2007, 06:59 PM
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Yes, rusting closed or open is the main drawback to those valves. That is why Detroit has gone to the intake air heaters instead. And your concerns about them being too restrictive or 'blowing out' are unfounded. There is plenty of flow area in the X-over for normal street driving - you can't tell a difference. And the valve is a 16ga steel plate butterfly valve - totally indestructible.
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Old 06-09-2007, 12:21 AM
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I agree, skip the valve. It was more of an emissions thing to get the engine up to temp faster. Over time the heat will creep in anyway regardless of the valve, so it was more of a time issue than actual peak heat.

I would (personally) skip the valve but leave the passage open. It might cost you 2 hp, but I've done a few bypasses in my life that I regretted. I had a 454 that I would much rather have lost the 2 hp than have to wait until it was fully hot before I could mash the pedal. That particular combo was a pain. Two days of carb tuning with an expert by my side and we couldn't make it not bog until it was fully warm. If I had left that crossover passage open, it would have been much less of an issue... and that was SoCal.
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