Marine motor

[SB671]

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?

 

[patprimmer]

Pro Action make a large range of heads, so that does not tell me a lot.

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

 

[patprimmer]

Boat problems can be:-
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

 

[SB671]

Thanks Pat

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

 

[ShaunT]

This sure sounds like one of our UltraStreet Cams. We have a 285/295 with the exact same .050, Lift, and LSA. Where do you have the intake C/L at? Also It sounds like it is running out of Gas. I think we have discussed this before, but it was several months ago. The motor should pull very hard from 3000 but still rock up to 6000 in a boat. I still don't have any ideas about what exhaust you are running and how restrictive it is. If it is that restrictive, you need to run a cam with a 108LSA and pull out about 10+Degrees of Exhaust cam. The wall comes from the exhaust, and the more exhaust cam and tighter LSA you have, the earlier the wall happens, and the less bottom end torque you give up for absolutely no gain on top.

Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)

 

[patprimmer]

I would certainly open the exhaust to head flange to slightly larger than the port, so there is no posibility of the flange overhanging the port even with a little missalignment on assembly.

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

 

[ShaunT]

With a restricted log exhaust, it won't make a bit of difference what you do untill you get rid of some exhaust cam and LSA. I have been fighting this for years in Marine and in Oval Track with stock exhaust manifolds and combined with both 4 and 2 barrel carbs. Trust me, I may be arrogant, but I know what I am talking about here. We have seen it over and over when we take our little 272 cam with 240 at .050 with a 108 or even a 110LSA and put it in place of a cam having tighter than a 108LSA and more than 280 advertised/250@.050 the gains in RPM are shocking. Case in point: My Dad has a stock 260HP 350 Mercruiser with through prop hub exhaust. It made 3200lbs thrust with the stock cam. It made 5000lbs of thrust with my little 264/264F10 solid flat tappet. This cam is a net 450 lift with 232@.050. It rocks even from a dead idle. It will pull quite a bit more prop then my 270/270H11 Hydraulic that is 219 @.050 with 460lift. However, due to the increased high lift area of the solid, there is over 500RPM's between the two cams in the same restricted exhaust motor. While it only happens seldomly, a Hydraulic can out power a solid in the limited case of a restricted motor that gets taxed so much more by the air flow improvements of fast ramp solids. While I don't propose a Hydraulic to you, I am just making a point which is: if you can't get it out, you can't get it in. Restricted (constipated) motors require a different m
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)

 

[patprimmer]

I have to agree with Shaun, in that there is no point in having a lot of cam overlap if you have a lot of back pressure and no pulse tuneing. It only means the inlet is facing a lot of exhaust gas pushing back against it, often resulting in reversion rather than scavanging.

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

 

[ShaunT]

Very true. Accept I feel that you can get the slip factor pretty high on a prop if you are not careful. In essence, you could get more RPM that way too, but you wouldn't go any faster. Prop/paddle diamater can also help couple up a prop and make it bite. The cam is the simplest thing to change here, in my opinion. In the last motor I built for one of my own boats, well let me just give you some specs: 350 with 6 inch rod, flat top's, stock valve dia. Vortec head, zero deck(10.3:1Comp) PerformerRPM and a 750carb, and our tight lash cam featuring 271/279@.020, 244/252@.050, 520/540lift, and a 108LSA. I also had a set of REX Marine water jacketed headders running through the Transom. The boat is 3400+lbs, has a 1.5 ratio stern drive, and a 14.5Dia 23 pitch 3 blade prop. I have a hard time getting more than 5300RPM's out of it. Not too shabby for that big of a boat and a huge prop considering the small motor. Incedentially, it's a BAJA and has a hook in the rear and runs fastest in chop. It sound much more radical than it is, but it cruises all day at 2500RPM.

Shaun TiedeULTRADYNE/LUNATI Arl,TX(stiede@ev1.net)

 

[SB671]

Thanks Pat and Shaun

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

 

[ShaunT]

Great, keep us up to date. While we are on the subject of car mechanice working on boat motors and average social ski boat is run hard with 3000+RPM's. For a case in point about cams, when it comes to the customer who want's a gain in a stock 350, and will never maintain it including never change the oil, I sell them our 270Hyd. It is great with through hub exhaust. It idles stock and it is a 1000RPM gain over stock even with a totally stock eng/valve train.
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)