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EMI Thunder exhaust and Comp xm270 cam

Old 09-29-2014, 03:01 PM
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Default EMI Thunder exhaust and Comp xm270 cam

I am changing the set up on my 7.4 365 Mag motor with a Comp Cam XM270 226/236 @.50 (what comp recommended) Victor Jr. intake, new Comp valve springs, Edelbrock 750 Marine carb and some type of new header/manifold. I was looking at the EMI manifolds and they say dont not use with an aftermarket cam. Any ideas why??? I cant not afford 3500.00 x 2 for CMI E-top headers and I liked the 1100.00 price a pair for the EMI's. Is there anything else out there that is within reason for exhaust?
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Old 09-29-2014, 03:18 PM
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I prefer the imcos or the stainless marines by far to eddie marines.

They say not to use with aftermarket cams because of the concern about reversion- which will wreck your build real quick.

I wouldn't trust what comp says when it comes to boating. Setting up a boat is nothing like setting up an auto when it comes to top end selection.

Bob Madera/ RMbuilder Marine Kinetics - seems to be this boards go to guy re cam selection.

Alexi and Tegaue have always given me good advice.

Good luck.

Uncle Dave
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Old 09-29-2014, 03:55 PM
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what size riser are you using.if your using the short one that work with the silent choise.beware.I have a comp marine cam in my build.comp 11-306-4.its 222@50 on the intake,and 226 on the ex.i have the short emi risers on my6 502 and this is about as big as I want to go.if I idle down to say about 600 rpm the motor will start to injest alittle water.if I idle around 800-900 rpm I am ok.i know this for a fact and have seen first hand the damage it can do.if you have longer risers you will probley be ok.if your worred go dry,,,,,,,,,but you might not like the attention you get.
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Old 09-29-2014, 04:01 PM
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I will be using the short risers as I want stock location on everything for my silent choice. There is not a lot of room between my stock risers ends to the hull wall (typical Fountain set up where the engines are pushed way back since I have the Bravo drive set up).

Would headers help get rid of the potential reversion problems or is this what you get with bigger cams?
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Old 09-30-2014, 03:55 PM
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Originally Posted by blownsvt1
I will be using the short risers as I want stock location on everything for my silent choice. There is not a lot of room between my stock risers ends to the hull wall (typical Fountain set up where the engines are pushed way back since I have the Bravo drive set up).

Would headers help get rid of the potential reversion problems or is this what you get with bigger cams?
I think in general its cam size.i cannot say about headers.my emi set up is on my 27fever.so I know what your saying about fountain.unless your going real big h/p and got a lot of coin.i would stick to manafolds.
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Old 10-16-2014, 10:31 PM
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I have the CMI's with the long tube risors and cannot run the silent choice in my 27 fever. I run a full roller 600 cubic inch motor. I am puling the motor this week to see what cam I have because I plan on a larger lift and some duration. I have not had any reversion problems
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