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How to tune for big cam and low manifold vac

 
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richardparker
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Joined: 23 Jun 2011
Posts: 22
Location: NC
847.94 points



PostPosted: Tue Jan 31, 2012 5:05 pm    Post subject: How to tune for big cam and low manifold vac Reply with quote

I just built a complete motor.
Bored and stroked a 3.2 acura v6 to 3.7
all new everything except block,and bare heads.
I also installed a big cam.

The motor has never been started to a running state.

I have tried starting the motor it turns over fine,it has fuel,it has spark.
Their is about 1/2" of manifold vac when cranking and goes up to 5" when it does try to start.
compression seam decent accross all cylinders.
I've been told that the overlap on the cam is to much and the low manifold vac is causing the map sensor to just dump fuel.

So I took a better look and sure enough the plugs are getting soaked with fuel.

I've taking the cams out and are waiting for cams that are oem cams for a beter model than I have to get the motor running and broken in.

Then I want to get back to the cams. when I get some kind of engine management.
But,I want to know how can I beat the low manifold vac to run the cams.
They make my motor sound like a cammed v8 when it attempts to start.
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clay
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Joined: 24 Nov 2002
Posts: 3209
Location: South Carolina
318129.23 points


1972 Chevrolet Nova

PostPosted: Tue Jan 31, 2012 10:21 pm    Post subject: Reply with quote

O.k. I'm going to throw what are probably some weird ideas out here - I was hoping you had a carburetor and distributor Very Happy . First since the map needs higher vacuum - supply it from another source. Either a vacuum pump of some sort or even from anther vehicle. You could tee in a needle valve to allow you to regulate the vacuum signal. If the map is mounted directly to the manifold you might have to remove it and block that port off - I'm not that familar with your particular engine. Another possibility is to manipulate fuel pressure. Lower should help lean things out but I'm not sure you will be able to drop it low enough to help - I just don't know. I'm not sure what you can do to manipulate timing. If it's a situation where you have to disconnect a jumper to remove computer control to set timing - do it. Then jack the timing up. This should simulate a locked distributor that really helps with a low cylinder pressure / bad mixture situation. These are weird I know but maybe they'll help you or spur other ideas. Clay
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10sec.et
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Joined: 29 Mar 2006
Posts: 3473
Location: Houston,Texas
346658.74 points


1969 Oldsmobile Cutlass

PostPosted: Wed Feb 01, 2012 12:33 am    Post subject: Reply with quote

first thing that comes to mind is the computer is seeing low vacuum, thinking the throttle is open and adding fuel. i could be wrong but some systems rely primarily on the MAP for fuel delivery. had a Dodge truck kick my arse with a bad EGR. would flood the cylinders before it had a chance to start because of low vacuum while cranking.

do you have a distributor or six coils ? if you have a distributor then you can manipulate the timing to some degree (no pun intended). have you tried starting it with your foot to the floor [clear flood mode] ? i would clean the plugs and try that. use a propane torch and heat the ground strap to cherry red. that will burn off all the carbon. ultimately, youre going to need some tuning software. i have been battling a LS7 the past two weeks and had a similar issue the first few times i tried starting it. it needed a lot more idle air (among other things).

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Paul P
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Joined: 15 Aug 2002
Posts: 2404
Location: Townsend, Mass.
81616.60 points


1971 Chevrolet Chevelle

PostPosted: Wed Feb 01, 2012 1:15 am    Post subject: Reply with quote

First thing that comes to mind for me is are the cams dialed in to the manufacturers specs? That is important.

Have you installed "Big" cams per someones recommendation?

Usually the cam manufacturer will recommend a certain grind based on your specific engine combination and vehicle specs.

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2001 Focus 2.0 Zetec
stock cams, bolt-ons and tune
15.63@87 MPH 1/4mi

1971 - Chevelle 408 SBC N/A
6.86@102.5 MPH 1/8mi
10.78@122 MPH 1/4mi


Last edited by Paul P on Thu Feb 02, 2012 4:39 pm; edited 2 times in total
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SMOKEmUP
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Joined: 30 May 2002
Posts: 3169

65097.54 points


1979 Chevrolet Camaro

PostPosted: Wed Feb 01, 2012 1:08 pm    Post subject: Reply with quote

Is this a speed density or mass air system?

Like everyone said above check the mechanicals to ensure you don't have any leaks and/or miss aligned cam timing.

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