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Old 06-02-2012, 09:23 AM   #1
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7.4 omc....

Hi all.

I'm hoping some of you clever type on here can help me out...

I'm looking to get a little more out of my 7.4 omc king cobra.

Is there any advantage to putting a pair of decent carbs on? What inlet manifolds will fit?

Is there any advantage to a nice set of bling bling stainless manifolds and risers etc?

Thanks in advance!
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Old 06-02-2012, 09:31 AM   #2
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Engine mods

Something else to brake then Steff LOL
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Old 06-02-2012, 10:02 AM   #3
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Is the Volvo exhaust manifold the same fitting as mine?

Cheers.
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Old 06-02-2012, 10:09 AM   #4
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Get yourself a copy of desktop dyno and you can try out all your changes to see what effect they have. Unfortunately, you'll also be a little disappointed with the effect of the changes.

The massively limiting factor on any old tech cast iron motor is the heads.
Anything else you change, you are still limited by what the heads will flow.
However, to that end, don't bother port matching the inlet to the heads, as long as the manifold ports are smaller than the heads, port matching them you won't see the slightest bit of difference. That I recall, the valve throat area and short side radius on the inlet side are the most critical if you are inclined to start grinding, but with the lovely cnc finished manifolds available these days, I certainly wouldn't bother hand grinding them myself.

Switching to a roller cam you can have much more aggressive opening & closing ramps, but without necessitating you to increase the amount of overlap excessively (overlap roughly meaning rough idle and susceptibility to reversion). With a cam, some small tube headers and a better inlet manifold (although probably still staying with a dual plane one), you'd see something - but also be in quite deep financially since headers actually worth having at all are not cheap.

You could probably just get away with advancing the ignition slightly. They are timed very conservatively as stock to take account of potentially rubbish fuel - but I'd recommend you read up a fair amount on how to set up the timing since if you overdo it, that can get very expensive.
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Old 06-02-2012, 10:10 AM   #5
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Double pumper carbs...or holly's. hi lift cams custom teague risers, new heads old school tunning........ NOS!! but big money. If its a gm block there must be loads of goodies..
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Old 06-02-2012, 10:16 AM   #6
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Quote:
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Get yourself a copy of desktop dyno and you can try out all your changes to see what effect they have. Unfortunately, you'll also be a little disappointed with the effect of the changes.

The massively limiting factor on any old tech cast iron motor is the heads.
Anything else you change, you are still limited by what the heads will flow.
However, to that end, don't bother port matching the inlet to the heads, as long as the manifold ports are smaller than the heads, port matching them you won't see the slightest bit of difference. That I recall, the valve throat area and short side radius on the inlet side are the most critical if you are inclined to start grinding, but with the lovely cnc finished manifolds available these days, I certainly wouldn't bother hand grinding them myself.

Switching to a roller cam you can have much more aggressive opening & closing ramps, but without necessitating you to increase the amount of overlap excessively (overlap roughly meaning rough idle and susceptibility to reversion). With a cam, some small tube headers and a better inlet manifold (although probably still staying with a dual plane one), you'd see something - but also be in quite deep financially since headers actually worth having at all are not cheap.

You could probably just get away with advancing the ignition slightly. They are timed very conservatively as stock to take account of potentially rubbish fuel - but I'd recommend you read up a fair amount on how to set up the timing since if you overdo it, that can get very expensive.

Theory is one way to go, but I'd get a bigger motor!
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Old 06-02-2012, 10:20 AM   #7
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I agree. So I did, and then made it a bit bigger again.

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Theory is one way to go, but I'd get a bigger motor!
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Old 06-02-2012, 04:29 PM   #8
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No replacement for displacement
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Old 06-02-2012, 07:59 PM   #9
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What do you get out of it at the moment? Have you tried a few different props?
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Old 06-02-2012, 11:19 PM   #10
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Buy a decent mercruiser!!! lol, check you pm's found another set of risers you might be interested in.
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Old 07-02-2012, 09:42 AM   #11
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Quote:
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What do you get out of it at the moment? Have you tried a few different props?
I'm getting around the 60ish mark at the moment. The prop I'm using at the moment is about as good as a chocolate tea cup. I've got a better one to try once the weather warms up! Problem is that it's an omc drive and merc props don't fit.
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Old 07-02-2012, 10:19 AM   #12
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http://www.offshoreonly.com/forums/g...ing-cobra.html
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Old 07-02-2012, 10:32 AM   #13
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Cheers Matt. I'll have more of a look on their site. I've got the flow torque hub kit, but need to trim down the Allen bolts on the gearbox.
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Old 07-02-2012, 10:54 AM   #14
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Not so much of a problem then, eh.

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Problem is that it's an omc drive and merc props don't fit.
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Old 07-02-2012, 11:25 AM   #15
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Not so much of a problem then, eh.
Hmmm. You've never seen me using a grinder!
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Old 07-02-2012, 11:36 AM   #16
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Hmmm. You've never seen me using a grinder!
Im dab handed with an angle grinder if you need help!
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Old 07-02-2012, 05:17 PM   #17
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Im dab handed with an angle grinder if you need help!
I'm saving the fuel tank mods for you.... Hehehe
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Old 08-02-2012, 11:09 PM   #18
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Ok folks. At the risk of sounding 'special'.... Are 7.4 exhaust manifolds/stud pattern etc the same on Volvos and the omc?
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Old 08-02-2012, 11:14 PM   #19
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Come on Steff be realistic! You are too tight to go any faster lol!!
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Old 08-02-2012, 11:19 PM   #20
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Come on Steff be realistic! You are too tight to go any faster lol!!
I just don't want to show you lot up with my Łber efficient cdi tdi blue motion Eco gas guzzler!
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