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21 Daytona
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Discussion Starter #1
My nephew is in the final phase of his procharged 496 project.
he has a 496 with a blow thru carb intercooled M1 procharger.
a MSD distributor and a 6BTM ignition which has an adjustable
boost retard from 1-3 degree of retard with each psi of boost.
currently he has the 5psi pulleys.and running pump super which here at 3500ft is 90 octane.
Should he lock out his distributor? and how much retard should he start with?
what about total timing?
 

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Cantard
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He needs to lock the distributor so he can do the timing curve with his lap top. I will check my notes on the timing settings for you.
 

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Cantard
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Mix up on the ignition box. (Was thinking digital) Run it at 34-36 then subtract 2 for every lbs of boost. Still lock the distributor or you are going to have 2 things happening at once. (mec advance and ignition retard)
 

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We just fininshed dyno tuning a 496 with a vortec making 7lbs of boost.

496
flat tops
Edelbrock rectangle port marine heads 119cc chamber 2.25 intake valves
9.5:1 Comp
Victor Intake
850 Demon blow through carb
Iski Cam 112 lobe sep

724HP @ 5700rpm
34 deg total
BTM set to 1 deg per pound so 27 total deg at full boost.
12.1 afr
1400 egt
When we went to 36 deg total it lost Hp
 

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Cantard
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Nice #s thmpr. I think with a 114 split it might like the 36 but I hear ya on the 34. Did you try it with less timing?
 

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We started at 30 deg and went up from there. We also started at 3 deg per pound of boost on the BTM and worked up from there also. As soon as we went to 36 we lost HP so 34 deg it was.
 

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21 Daytona
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Discussion Starter #7
thanks for the replies, we are going to start out very conservative.
28 total and pull out 2 per PSI, then creep up on it
 

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thanks for the replies, we are going to start out very conservative.
28 total and pull out 2 per PSI, then creep up on it
Thats pretty much where we started. We also jetted the secondary jets two sizes up on the Demon 850. I will have to look at my notes but the jet change made a nice improvement on the HP numbers.

Are the heads on your nephews motor aluminum? If they are iron I would be very consevative. By the time you hear pinging with iron heads you pretty much can bet you popped a head gasket. Iron heads will require more boost retard. Aluminum is a little more forgiving. We started increasing the boost retard first then moved up with the timing. We did this because if you get a bad batch of gas you can turn the boost retard down. If you leave it at 3 deg per you have no room for bad gas. We had no detonation with our motor package at the numbers I had listed above previously. 91 octane fuel.
 

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21 Daytona
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Discussion Starter #10
Yes Iron heads, thats why i figured starting with 2 per psi, so he would be able to go to 3 if needed on the water. maybe we should start out at 26 just to be safe.

his carb is from Lance Patton, it should be here any day now. so not sure on jet size yet.

he also has a autometer wideband AFR, so we should be able to tune it fine.
 

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Definitely start out at three deg per pound of boost and at 28 deg total. We made the mistake with iron heads at 36 total with 3 deg per pound and popped the head gaskets. This was with 7 pounds of boost. The metaphorical fuse is very small with iron heads. Better safe than sorry.
 

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Locked out at 28 deg. then 2 deg. per pound of boost. 5 lbs.= 18 degrees total. The EGTs will be through the roof 1600+ easy. If the engine is 8 to 1 with iron heads at 3000 ft above sea level. I think 32-33deg. locked out and 1 deg. per pound would be way safe. This would be safe at sea level!
 

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he also has a autometer wideband AFR, so we should be able to tune it fine.
Be very careful as to putting total faith into this. Assuming you have it in the collector you are getting an average reading of 4 cylinders. Many blow thru set-ups (especially ones with typical hat )have very large disparities cylinder to cylinder.

You need to look into every cylinder.

Be careful.
 

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21 Daytona
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Discussion Starter #15
Be very careful as to putting total faith into this. Assuming you have it in the collector you are getting an average reading of 4 cylinders. Many blow thru set-ups (especially ones with typical hat )have very large disparities cylinder to cylinder.

You need to look into every cylinder.

Be careful.

Great info guys, who makes the best blow thru hat ?
 
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