Yup another Cam question

[Lyle29464]

However. This is a bit different. I need a new cam for a 350 GM .

The engine is a 1979 350. has Edelbrock pro heads. 202-166 valves 76CC Compression is 10.5 please give lots of thoughts to my priorities.

Number 1 No water intrusion from incorect LSA ( engine sets low.) very willing to sacrifice performance to keep this risk as low as practical.

Number 2 needs to idle low enough for BW transmission. 600 rpm range.

Number 3 looking for low end torque. 600 to 3000 rpm. NO interest above that. Boat will only be run faster than that to get correct prop and check systems. Want to turn as much prop as possible at 3000.

Number 4. would like to make use of the larger valves with more lift if practical.

Number 5 . No interest in the rough idle or lop stuff.

Please give your recommendations on numbers or even a part number and brand of cam.

Thanks for you thoughts on this. Would like to order the cam soon.

Lyle

The boat will be about 2200 lbs inboard deep V maybe 500 lb load BW 1 to 1 with 14X17 prop or can use 1:28 to 1 if needed. Not sure about that prop. Oops forgot carb info. 750 Edelbrock and manifold

 

[Scott Danforth]

Re: Yup another Cam question

I would suggest a Comp Cams Marine cam. however my first question is why are you limiting the RPM to only 3000?

 

[Lyle29464]

Re: Yup another Cam question

Thought maybe giving up the top might help me where I run.

 

[Scott Danforth]

Re: Yup another Cam question

suggest a 256 duration cam, however the power band still goes up to 5500 RPM, even if you only want to give it partial throttle. think of it like driving your car. do you only keep that at 3000 RPM?

http://www.summitracing.com/parts/CCA-CL12-236-3/

 

[Lyle29464]

Re: Yup another Cam question

Thanks for the info. I will check it out. It says operating range 1300 to 5500. Any way to lower the 1300? really want low end torque.

 

[Scott Danforth]

Re: Yup another Cam question

then you need custom grinds. however if you only want 1000 - 3000 go Diesel vs the 350

you can also increase your intake tract length with a custom built intake to help move the torque peak lower, however doubt you can get it as low as you want.

 

[Lyle29464]

Re: Yup another Cam question

Thanks for the additional info. I have the engine and everything but the cam. Might check the custom idea.
need to keep weight down it's a inboard. Don't think it will handle the weight of a diesel in the center of the boat. My woman does not like the smell. I see some cams advertise idle to 5500. I guess I will research cams and see if I can figure out how to get close to what I want. thanks again. Lyle

 

[Tahorover]

Re: Yup another Cam question

You want low end and a 750cfm Carb?

Great throttle response and low end torque would come from a 350-500cfm carb! You will need small valves and small intake exhaust ports as well . Big valves and ports have **** poor velocity at low RPMs Small ports give you a snappy throttle and quick response just off idle.

Think school bus, dump truck engine specs, things that come with a granny gear! It's all about low end torque to get things moving!

 

[Tahorover]

Re: Yup another Cam question

Thanks for the info. I will check it out. It says operating range 1300 to 5500. Any way to lower the 1300? really want low end torque.

advance the cam four degrees!

 

[dan t.]

Re: Yup another Cam question

First of all if you want bottom end you have the wrong heads,the valve sizes you list are for high rpm 3000-7000. The stock 260 hp cam idles smooth and has good bottom end, Comp cams 252H has excellent bottom end and throttle response, as has been said a 500-600 cfm carb is lots, a 350 only flows 600 cfm at 6000 rpm. 10.6 compression ratio? plan on using premium fuel and octain boost to keep detonation under controle. I realy think you need to reconsider those heads.

 

[StevNimrod]

Re: Yup another Cam question

Am I reading right - 10.5:1 compression? I would think you'd need to moderately large cam to work with that SCR, while simultaneously wanting to avoid reversion with a low sitting engine.

It seems like this is a mixture of wrong parts and conflicting goals.

 

[Tail_Gunner]

Re: Yup another Cam question

Are the heads e-tech alum heads??

Is this a flat tappet cam... roller cam??

 

[Lyle29464]

Re: Yup another Cam question

Thanks a lot for your time and thought. I have ordered the cam you posted and have changed to a head that will work with it. I really appreciate your type of answer. I asked the question because I did not understand cams and the fine points of setting up a new engine.I knew only what I wanted to do. Not how to get there. It is so refreshing not to read "SO YOU WANT TO!!!" or I must have miss read your post.

suggest a 256 duration cam, however the power band still goes up to 5500 RPM, even if you only want to give it partial throttle. think of it like driving your car. do you only keep that at 3000 RPM?

http://www.summitracing.com/parts/CCA-CL12-236-3/

 

[Tahorover]

Re: Yup another Cam question

Build your engine on Desktop Dyno, You will get what you want with out any guess work or the wrong parts.

The last engine I built I plugged in over 60 different cams until I got the right combo.
http://proracingsim.com/desktopdyno.htm

 

[billbayliner]

Re: Yup another Cam question

Lyle, you mention that you want to turn as much prop as possible at 3,000 rpm. I have to agree with Scott. I think that this is the wrong approach and that it's unrealistic to stay under 3,000 rpm and expect high torque numbers without creating other issues.
Your transmission ratio will be determined by the engine, then you'll prop accordingly to reach the specified wot rpm.
You need to reach your wot rpm with the right ratio and right prop or you'll over-work the engine.

The thing that's in your favor is the small size and weight of this boat.
Unfavorable would be the Deep V.... but great for stability.

As for the camshaft profile, I too would contact someone like Comp Cams. You'll want to tell them exactly what your build is, info about the boat, and how you intend to use the boat.


Lyle, anything that you change to create low rpm torque, does not necessarily let you off the hook for detonation concerns. Even if you advance the cam, you'll still need to be concerned with detonation.
Also, I don't think that you'll find a build that suits both low rpm torque needs and will give you high torque at the higher rpm range. So you'll want to choose your build carefully.

Dan T makes a good point about the carburetor CFM requirement.
Here is an on-line CFM calculator.
http://www.carburetion.com/calc.asp
Just plug in your specs and rpm. I think that you'll find that much over 500 cfm is unnecessary and possibly counter productive unless you want to run at high rpm.

Be careful if changing cylinder heads if your lower end is already assembled. Your piston selection should determine the combustion chamber volume. Or visa-versa if you have not purchased pistons yet.

Here is a cut and paste that may interest you.

Short of overheating, detonation is one of the SBC Marine Engine's worst enemies.
The SBC w/ the wedge area and full dished pistons are more prone to detonation under lets say 3,000 rpm, which is where some think that they want to be. Typically, this is not an RPM where we are fully up on plane. This is not good for fuel economy nor good for planing.

To get a good torque rpm range, every component must be correctly chosen and work well without creating excessive cylinder temperatures under these marine loads.
Several causes of marine load detonation would be poor combustion chamber design (the full dished piston for example), excessive ignition advance (i.e., ignition induced detonation which perpetuates high cylinder temperatures, etc.) all of which lead to excessive high cylinder temperatures.

To avoid detonation, or at least minimize the potential, we can do several things:

Build a squish/quench into the combustion chamber, keeping the squish to around .038" or so.
Don't load the engine excessivley at lower RPM (much under 3k rpm). IOW, stay out of the more detonation prone rpm range for any duration.
Use a higher octane rated fuel.
Don't run excessively lean F/A ratios.
Be very conservative on ignition total advance if you do use the full dished pistons.

The goal is to create a PCP at near 12-14 degrees ATDC and to do so without detonating.
The better build will use a piston that offers a squish or quench effect, so steer clear of the full dished pistons.

 

[Lyle29464]

Re: Yup another Cam question

Thanks a lot for the new info. I have installed many inboard engines just never had the chance to build my own. Think I am on the right track now. Need to read a bit on the squish/quench thingee. Will post results in a month or so. Have lots of changes to make to the boat.