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Thread: Keeping the low end HP

  1. #1
    Fired Up
    I'm trying to keep the low end HP on a BBC. I am looking at 550-600 hp on a 454, single 850 with the Merlin large oval port heads and headers. Trying to keep the compression between 10:1 and 12:1 and a top RPM at 6500 max. I have not decided on cam specs yet. I have read the BBC marine performance book and it says that the oval merlins will breath great to that range. An engine builder that I spoke with said go with the rectangle port heads, but I'm afraid to lose the bottom end. His experience is building car motors and not performance boat motors (not that it should make a difference). You can read and read, but until someone gives you true experienced information you don't really know until its too late or you are broke. Does it really just come down to the cam? Thanks for all your replies.

  2. #2
    F.U. Cam is a big part but many other items can kill bottom end. Square ports will not help unless you turn big RPM's or use a blower. They kill the velocity into the cyls.
    Exactly what do you mean by low end HP?

  3. #3
    Fired Up
    Let me clarify myself. I am referring to the torque created by the use of oval port heads at low rpm's (during acceleration) versus the hp created by rectangle port heads at high rpm's. Rectangle port heads do not breath to their best volumetric efficiency at low rpms, thus power is lost. Oval port heads do breath better at low rpm's so the torque is greater. Did I explain myself?

  4. #4
    F.U. Just wanted to make sure.
    The more compression you can run the more torque it will make. Compression has a huge effect on bottom end torque. Just depends on what fuel you want to run.
    Cam is definately a killer of low end and Guys over do it here a lot.
    Here is how I choose a cam: The duration determines the RPM range. 230 - 245 @ .050 is what you should be looking for.
    As far as lift is concerned. Let the cyl heads tell you this. As much lift as the head will take before it becomes turbulent is the lift you want. Flow bench required unless the head mfg. can supply you the info. Too much is a waste of HP. It is usually less than you might think.
    A dual plane intake will help retain some bottom end also. They flow well to 6,500

  5. #5
    F.U.,I agree with infomaniac, It just depends on what fuel you want to run. I have a 454 bbc with the oval merlin heads, 10:1 comp,EMI thunder, thru transom exhaust ($1200.00) good exhaust, edelbrock perf. intake 800cfm holley d. pump w/mech. secondaries. I never dynoed engine but builder said around 450-500. I am going to change intake to a victor or victor jr. for more top end performance. I run a comp cam #11-244-4, 244-254 @.50 w a total of 570-575 w/1.7 rockers. In your case more compression equals more horsepower and torque but with anything over 11.1 on pump gas 92 octane youre asking for trouble. Also bigger is not always better for camshafts. My buddy builds drag and sand car VW motors with his own dyno and always tells me stories of guys that think bigger is better but it actually can hurt your HP. If you build an engine over 10.5 or 11.1 comp. you will need to run good fuel all the time. GOOD LUCK.

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