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DOHC and SOHC differences!!!!

32186 Views 7 Replies 7 Participants Last post by  FordEscortLX
Just to clear things up a little!!!!

DOHC (double overhead cam)
Uses two cams instead of so it has a dedicated intake side and a dedicated exhaust side.

SPI (split-port injection)
AKA: CVH or canted-valve head
SOHC (single overhead cam)
Uses one cam that controls all intake and exhaust responsibilites.
----The split port refers to the fuel injection method the engine uses.

What you will find is that it does not necessarily matter how many cams you have but how many valves per cylinder you have on an engine. The Zetec has 4 valves per cylinder and thus is able to flow more air and exhaust than the SPI which has only 2 valves per cylinder. I have seen SOHC w/4-5 valves per cylinder and the same with DOHC. The DOHC advantage is that it has the dedicated cam for each side of intake and exhaust.

Also, having driven a Zetec (in a ZX2 my mom used to have) and having my SPI engine I can say that the powerband in the DOHC is much more level than that of the SOHC. The SOHC seems to make most of its power at higher speeds and RPMs.

Anyone feel free to chime in!!!!!

Gabe Greene
2000 Ford Focus SE
Not so stock anymore!!!!
1st SPI w/a stress bar!!!
Fraternal Order of the SPI
4-DOA 4 Door Owners Association
TEAM Lonestar
"At my signal, unleash hell."
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Sounds good so far
here's a random question for ya. Is the block for the SPI the same as for the Zetec?

On Saturns for example, their SOHC and DOHC motors are built on the exact same short block, with the same crank, pistons, con-rods, etc. Only the cylinder head and valve-train are different.

Is the same true for the Zetec and SPI? THis would go a long way towards explaining the seemingly miniscule price difference on Focus models that give you a choice of motors...

It would be intriguing to do an SPI to Zetec conversion starting with an SPI engine... 30 years from now when the car's a collector piece you can say, "Well, it's not factory original, but all the numbers match!"

2k ZX3, Infra-red, auto, air, cruise, tilt, pw/pl/pm, abs, var. int. wiper conversion, Sylvania XV headlights
IM: JonBoy470
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I'm not a mechanic but just by looking at them they are completely different.
I'd have to agree, only 20 HP difference, but two completely different engines. I'd say they have little in common, best bet would be to drop a Zetec in inplace of the spi. I was going to do this until I heard the new Duratec was coming out, I'd love to see my car around with one of those daddys, yikes.

I use to have a SPI escort and the engines are are different, and the SPI tranny will not hold the extra power.... I modded that engine and blew the tranny, then after the rebuild I took it easy and still blew the second one. I traded it in for the focus and finally screwed the dealer while they were screwing me because I saw the car in their bay with the tranny in pieces. I bet that they still got the better end of the deal though. Oh well I got the car I wanted anyway

2000 SE Sony Limited Edition w/Esslinger UDP, AEM UDP, AEM intake, Borla Exhaust, JBA header, Hella Headlight Conversion, Progress Springs, Eibach sways f/r, energy suspension bushings, 17 ICW's, FS Strut brace, FS wires
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you can swap the heads, but unless you do machine work, do a total swap. it takes alot of work to do it, and you can usually find an entire engine for the cost of what it would take just to machine your block.

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