Marine motor
Marine motor
(OP)
We have recently built a 350 chev for a marine application in a rear mount ski boat
The engine has restrictive exhausts but is fitted with a 750 double pumper holley, Super victor manifold,pro action heads with 202 & 190 valves with a cam running the following specs intake 252 dur @50
exh 260 dur@50
.525 valve lift
106 centres
The engine is running on pump gas at 10.5 to 1 and is set at a maximum of 36 degrees timing
Any ideas on what sort of performance we should expect from this combo. The engine sling shots around like a two stroke from 3000 to 5100RPM then hits a wall Any clues?
The engine has restrictive exhausts but is fitted with a 750 double pumper holley, Super victor manifold,pro action heads with 202 & 190 valves with a cam running the following specs intake 252 dur @50
exh 260 dur@50
.525 valve lift
106 centres
The engine is running on pump gas at 10.5 to 1 and is set at a maximum of 36 degrees timing
Any ideas on what sort of performance we should expect from this combo. The engine sling shots around like a two stroke from 3000 to 5100RPM then hits a wall Any clues?





RE: Marine motor
By restrictive exhausts, I presume you mean water cooled log type exhausts, or worse still the out the top and through a "P" trap type.
I run a rear mount 350 SBC with log type exhaust, a 650 double pumper, a Victor Junior, genuine Chev twin point delco distributor, and a little roller cam with .480 lift and about 260 deg at 0.050.
The heads are stock 350 low performance 75 cc chambers with 1.9 something inlets and 1.5 exhausts. I spent a few hours on the ports, and back cut the valves and multi angled the seats. I run so called 11:1 pistons which are yeilding spot on 10:1 comp.
It does 6000 rpm all day, and will barefoot 3 big guys.
Your motor should have more power if all is OK
Have you checked fuel pressure at the carb inlets.
Is the throttle opening all the way.
Does it have a crappy small air filter or flame arrestor.
Is to much water going into the exhaust.
Are you getting points bounce or valve bounce.
Is the sump design OK, or is it real shallow with the crank splashing a lot.
Is the cam in at close to split overlap. I think there are 2 standard positions for SBC crank keys, as some 70's model polution motors had a different key position. Just check that the inlet and exhaust are both within 0.010 of each other in their distance off the seat at TDC on the overlap stroke.
Having said all that, I think it is more likely a boat or prop problem
Regards
pat
RE: Marine motor
Shaft angle to steep, which drives the nose in.
Cav plate down to far. Same result.
Poor design of underwater gear. To much drag. The magnitude of the drag on even a small obstruction is usually greatly underestimated.
To steep a shaft angle also upsets the effective pitch of the prop. The blade traveling up gets less pitch and the one going down gets more.
Another major cause of hitting a wall is a concave on the planing surface.
A convex causes bouncing, but a concave causes suck down. The faster you go, the harder it sucks down. An indication of this is if it goes a lot better on rough water.
The biggest cause of hitting a wall is overgearing or proping. I think my prop is 11.25 by 15 Menkens steel 2 blade with 12% gears
Regards
pat
RE: Marine motor
I think the boat bottom is OK as we recently blue printed it and made sure it was straight and true but it is a solid heavy hull. It is a flat bottom hul and the cav gear is working correctly
We are running 13% gearing with a 11 x 14 three plade prop which I thought should be OK.
The exhausts are the wet log type. Would the fact that the ports in the manifold are still quite small and not matched to the heads at the moment be a governing factor?
Even with this I still thought we would be capable of around 6000rpm. Cam timing etc have all been double checked and the ignition is electronic and seems to be performing
RE: Marine motor
Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)
RE: Marine motor
I don't use gaskets on the exhaust to head seal. I just use a small bead of silicone rubber applied closer to the outside than the inside edge so it only ozzes out, and not into the port. No mufflers, at least 2.5" or prefferably 3" tail pipes, and just enough water to stop heat damage to hull etc, and to keep noise levels reasonable. Water in the exhaust makes steam. Steam creates backpressure and kills HP
Regards
pat
RE: Marine motor
ind set. What works in the norm, hardly ever works here. When you put the two cams in an unrestricted motor, the tables turn.
Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)
RE: Marine motor
Another source of exhaust restriction can be the rubber tail pipes (if fitted) sucking in or delaminating and colapsing.
If it's the engine, it normally fades out, but if it's the hull, it normally hit a wall, like it's a brick wall.
Free up the exhaust flow, consider a wider lobe centre, shorter duration cam.
When I ran a better motor and slightly bigger prop for ski racing, I lost 600 RPM by converting from over the deck tube headers to a thru the transome water cooled log (for class rules). I lost another 400 rpm going from twin 660's on a tunnel ram to a 780 on a Victor Junior.
I picked up 500 of the lost RPM by cutting 1/4 inch of the prop.
90% of boats I drive are over propped, under cammed and under carbied. This is because "car" mechanics work on them, and equate cruise speed as 1500 RPM. Boats cruise at 3000 RPM, and the prop equates to a 3000 stall speed convertor, so your average social ski boat engine really operates normally under the same conditions as a pretty wild street machine.
Is it a steel or bronze prop.
It sounds a bit big for a bronze 3 blader, but props are a black art, and 2 props with the same nominal pitch and diameter, can perform very differently.
Steel props go harder because the blades are thinner.
Regards
pat
RE: Marine motor
Shaun TiedeULTRADYNE/LUNATI Arl,TX(stiede@ev1.net)
RE: Marine motor
We are going back to the water next weekend and will see what happens
We will have the distibutor back and set up for 20degrees advance at idle and a total of 36 degrees all in at 2200RPM to see if we pull out of the hole better.
We also have a smaller prop to try but most of the ski race boats here that are running 350 chevs are using 18 to 20% step up with 11 x 15 three blade props so I believe that we should be able to pull our 11 x 14 with 13% gears
I know the exhausts are restrictive and the cam does not suit this so there is probably 800- 1000 revs there. Would be interesting to slip the headers back on for a trial
I will let you guys know how we get on and maybe we will be in the market for a good hull(certainly hope not)
PS heads are pro action iron heads with 200cc runners and 64cc chambers
RE: Marine motor
Like Pat said, these motors are not for the car man, but I guarantee you a boat motor man can build one hell of a car engine. I get clients that tell me they can't afford to buy me so they will just take the motor from their drag car and put it in their boat. I tell them if it lasts ten seconds in your car, it won't last 2 in your boat nor will it get out of its own way. on the flip side, if it runs hard in the boat, I guarantee it will fly in the car. The boat is the ultimate torture test on a motor.
Shaun TiedeULTRADYNE/LUNATI Arl,TX(stiede@ev1.net)