Bad Karma
05-27-2001, 11:06 AM
I'm looking for recommendations on intake manifold for a pair of 1986 350 Magnums (260 HP).
The boat is an 86 Chris Craft 260 Stinger, which weighs in a roughly 6000 lbs dry.
Stock, the boat ran high 50s (57, 58 MPH) on a handheld Magellan ColorTrak GPS. Props are Merc Laser II 23P, with RPMS about 4500.
With a few minor upgrades last year, (Corsa equalized exhaust; IMCO Powerflow exhaust manifolds), the boat would hit 60-61 MPH under perfect conditions (all speeds recorded by GPS). RPMs would be roughly 4700.
Over the winter, I've rebuilt both engines, with Holley 750 marine carbs, Edelbrock Torker II intake manifolds, Competition Cams hydraulic valve train (camshaft, lifters etc). Valve train is not rollerized, as 86 blocks wouldn't take it. GM changed this in 1987.
However, best I can get now is 4900 RPM, with the GPS indicating 59.9 MPH. Now, my boat hasn't gained any significant weight over the winter, but why would I turn 200 more RPM, and yet drop 2 MPH in speed? Yes, my prop slip is probably increasing, but how could I go about correcting that?
The engine builder claims 60 - 80 more HP with this rig (per engine), and has the dyno sheets to prove it. Timing is bang on (32 degrees), and since I run nothing but 87 pump gas in it, I'm reluctant to advance it anymore than that.
I'm wondering if the Edelbrock RPM Air-Gap would be a better choice in intake manifolds, since (A) it's dual plane, and (B) it's high rise, as opposed to low-rise. Hatch clearance isn't an issue, and I really don't want to run the engine above 5500 RPM (prefer 5000 RPM MAX anyway). Could I get a similar benefit from using a carb spacing plate of 1/2 inch to 1 inch to improve airflow?
I'm about to try 21P Mirage Plus props, to see if I can get the RPMS above 5000, since single plane, low-rise intakes tend to produce more power above 5000. Does anyone have any suggestions as to which intake manifold (Edelbrock or otherwise) is the correct choice?
Any ideas would be greatly appreciated,
Pat
The boat is an 86 Chris Craft 260 Stinger, which weighs in a roughly 6000 lbs dry.
Stock, the boat ran high 50s (57, 58 MPH) on a handheld Magellan ColorTrak GPS. Props are Merc Laser II 23P, with RPMS about 4500.
With a few minor upgrades last year, (Corsa equalized exhaust; IMCO Powerflow exhaust manifolds), the boat would hit 60-61 MPH under perfect conditions (all speeds recorded by GPS). RPMs would be roughly 4700.
Over the winter, I've rebuilt both engines, with Holley 750 marine carbs, Edelbrock Torker II intake manifolds, Competition Cams hydraulic valve train (camshaft, lifters etc). Valve train is not rollerized, as 86 blocks wouldn't take it. GM changed this in 1987.
However, best I can get now is 4900 RPM, with the GPS indicating 59.9 MPH. Now, my boat hasn't gained any significant weight over the winter, but why would I turn 200 more RPM, and yet drop 2 MPH in speed? Yes, my prop slip is probably increasing, but how could I go about correcting that?
The engine builder claims 60 - 80 more HP with this rig (per engine), and has the dyno sheets to prove it. Timing is bang on (32 degrees), and since I run nothing but 87 pump gas in it, I'm reluctant to advance it anymore than that.
I'm wondering if the Edelbrock RPM Air-Gap would be a better choice in intake manifolds, since (A) it's dual plane, and (B) it's high rise, as opposed to low-rise. Hatch clearance isn't an issue, and I really don't want to run the engine above 5500 RPM (prefer 5000 RPM MAX anyway). Could I get a similar benefit from using a carb spacing plate of 1/2 inch to 1 inch to improve airflow?
I'm about to try 21P Mirage Plus props, to see if I can get the RPMS above 5000, since single plane, low-rise intakes tend to produce more power above 5000. Does anyone have any suggestions as to which intake manifold (Edelbrock or otherwise) is the correct choice?
Any ideas would be greatly appreciated,
Pat