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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
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[This message has been edited by Racing Ray (edited March 12, 2002).]
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Ray,
Cam was changed, to a Competition Cams camshaft. However, I don't have the part #.
I can't tell you what the specifics specs are.
I take it there are no opinions on the Intake manifold then?
Pat
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[This message has been edited by Racing Ray (edited March 12, 2002).]
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Hey Bad Karma,
For what it's worth, I agree with Racing Ray on the manifold choice- torque is king. His comment on HP is also on track- it's not too difficult to add a few ponies higher up in the RPM range, but it's more difficult to improve low-end torque at the same time. Torque is what heavy boats with small blocks (like my boat) need.
This probably doesn't help you, but when I built my pre-1987 small block, I just used a hydraulic roller cam retrofit kit. The only difference is that the lifters are linked to prevent them from turning (newer blocks have a square bore), and there is a bearing in the front to prevent cam walk. It also costs a couple hundred more. The cam I used was this grind (#3):
http://www.cranecams.com/master/apps/chevy39.htm
but with this retrofit kit:
http://www.cranecams.com/master/apps/chevy18.htm
The roller setup saved a little in resistance, but helped big in air flow, as the lobe profile can have steeper ramps. One area you did not mention was your heads. The heads are something that most people don't spend alot of time on, but can be an area of big gains (or losses) on a naturally aspirated motor.
Also, although you will probably not turn that motor enough to use all 750 CFMs (Racing Ray's comment), it's not the size, but how you use it (I use that alot). Very important- fuel curve. Although it's easier to dial in a fuel curve on a correct size carb, that 750 may work.
It's been my experience that most engine builders are not experts in the art of fuel delivery (racing guys are usually on top of it). My engine builder just kept re-jetting my (out of the box) 750 CFM to get the exhaust temps down, but that created other problems. I had my carb re-worked (by an expert) and gained 40 HP and 30 ft-lbs on my 383- just with the fuel and ignition curves.
There was a great book I bought by Dennis Moore on small block marine engines- very, very helpful:
http://www.amazon.com/exec/obidos/AS...351579-2016500
Good luck!
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Actually, it's Dennis' book that gave me most of my suggestions.
As to my engine builder; they are at least experts in that area. My preference was to go 650 on the carb; their dyno proved better numbers on the 750. I couldn't argue with pure empirical data.
The heads have been polished, and the standard 5-angle valve job done on them. (Which, according to Dennis Moore, is a bit of a waste, when a 3-angle valve treatment works just as well).
As to the intake manifold, I saw the numbers on the flow bench. The single plane dropped off quite a bit less than the dual-plane manifold. Hence their reasons for going with the single plane Torker II, as opposed to an Performer RPM. Sticking with Edelbrock, the Victor Jr. would have been better, but was slightly more expensive (not a major consideration at the time; I wasn't offered the option).
As to bottom-end torque, with twins, I've got plenty. It would have concerned me if I had little torque for getting on plane, but loads of top-end speed. As it stands, the boat quite literally jumps out of the water, but the most I can turn is 4900 RPM, regardless of which prop setup I use (Laser II 23P AND Mirage Plus 21P both turn 4900, and both hit 60 MPH on GPS).
23P Mirage Plus took me to 60 MPH as well, but only 4600 RPM, and was slightly slower out of the hole. I'll probably stick with the 21P Mirage Plus.
Pat
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Pat,
Sounds like you've spent alot of time & money (I can sympathize).
I have the Holley Dominator dual plane manifold (similar to the Edelbrock RPM) with the aluminum RPM heads (170cc intake runners) ported to match. This manifold should do up to 7,000 RPM. My 383 pulled peak HP right where Dennis said it would- 5,200 RPM.
With regard to the dyno sheets, where is your peak HP and torque at (RPMs)? Did you change the ignition?
Jeff
