MILD THUNDER

iTrader: (3)

We have had a thousand cam threads here. Seems like its the area of most interest to guys building a custom marine engine. First, I would like to ask, what makes a cam the "perfect cam". For years, we were told, the best cam, is the cam that makes peak HP, where you plan to prop it too, and peak torque, about 1k rpm below that. WHY?

Lets say for example, you tell your cam guy/engine builder, you want to spin the engine to 6000rpm. He designs/picks a cam, you go to the dyno, bingo, it makes peak hp at 6000, and peak torque at 5000. you then come to the internet, and say how perfect it is, and how he nailed it. Lets say that number is, 650hp for conversation sake, at 6000.

Your buddy builds an identical combo, but has a different camshaft. His made 700hp peak power but, at 6400rpm though. You tell him his cam is too big, and he should see a custom cam guy. However, what you didnt notice, was at 6000RPM, he was making 675hp, 25HP more than your "perfect" cam. You then proceed to tell him, that you need "torque to spin the prop".I hear this every other day. How you need torque, not HP, to turn the prop. HP IS TORQUE! . Its not possible to make 650hp at 6000, and be making more torque there, then your buddy whos making 675HP at 6000.

Of course many of the times, you might run into a scenerio, where a longer duration cam, or simply different style cam, may suffer some torque loss. This is not written in stone that it will in fact lose torque with a longer duration cam. The other thing, is you need to look at , where and how is this trade off in power taking place? Would you give up 10-15ft lbs at 4000, to gain 25, 40, 50 hp at peak?

Lets say your 650hp engine, is making 650 FT lbs at 3800RPM. Your buddys engine, is only making 630FT lbs at 3800RPM. You then tell him, how important cruise speed is to you, and you want that extra torque there. I don't know many guys who cruise at 3800RPM, with the throttle wide open, like it was on the dyno test. Now, all things being equal, which engine do you think would pull a higher pitch propellor? One making 650HP at 6000, or 675HP at 6000? The 675HP one obviously. SO, while the dyno sheet may show more torque on your engine, he may in fact be able to turn a 30 pitch prop at wot, while you , can only turn a 28p prop. He will be the one cruising faster at 3800RPM.

I have seen quite a few custom cammed engines lately on the dyno. One in particular, had a custom spec'd package from a cam guy. He was asking for a 700hp 6000rpm combo. One thing I noticed, was while his setup did in fact make peak power at about 5900, I saw no reason to turn it there. Why? Because when closely looking at his dyno sheet, at 5300rpm, he was only making about 5 or 6hp less, than at 5900!! So, in 600RPM, he picked up 5-6hp. First glance at the dyno sheet, and most would say the cam guy nailed it. I don't feel the same. I have seen this same thing in several of these combos. The power slowly climbs to its peak, and once you get past it, the power falls off a cliff.

We need to look at several things when camming an engine. Obviously power is one, reversion, idle characteristics, and of course, reliability. Reliability comes from core material, lobe design, and so on. Getting a cam that was spintron proven, is a bonus. Many lobes have been designed , but not spintron tested, or spintron tested with typical bbc valvetrain components. A lobe design that was spintron tested for an LS valvetrain, might not be what you want in a bbc valvetrain.

There has been some interesting dyno testing and back to back testing done by a few pro engine builders that I am aware of, where they have swapped custom spec'd camshafts out, and saw some very interesting results. Maybe they will chime in with that info. Many of us argue cam theories all day long, but its hard to argue with legit back to back testing and real world results. The majority of us, cant afford , or have the time, to swap cams out on the dyno.

This isn't a bashing thread, nor intended to be one.

Question. Can anyone tell me WHY, we should make peak torque, 1000RPM below, our peak HP? And what effect that has on the performance of the hull?

Question. While I am all for propping for peak HP, if you have an engine making 623HP at 5300, and 628hp at 5900, wouldn't you rather turn an extra 2-3 inches of pitch, which would give a better cruise rpm, and overall easier on parts/engine life? Or do you run less pitch, for the same top speed, but a much slower cruising speed. ? I am all for turning the rpm, if it nets gains. 5hp though, doesnt change a thing.

phragle

iTrader: (3)

Your question is unanswerble. There is no definitive answer. Everybody's idea of whats perfect is different. Its like ice cream, whats the best flavor? congrats, you have just started a pissing match over chocolate vanilla and strawberry and the guys that like pistachio mint with chocolate chips get really phucking defensive. Then there is the fact that your talking dyno vs real world results which can be different, like using a computer ice cream tester vs. a human tongue..

MILD THUNDER

iTrader: (3)

The definitive answer, can be found by dyno testing cams back to back, and overall boat performance/longevity. But thank you, for offering some good technical insight about ice cream.

We often see dyno results on engine packages, but almost never see people swap cams out, with no other changes, and witness the results. Without doing so, it is really hard to tell, if in fact, your cam is "spot on", or , can be improved upon.

Boatally Insane

iTrader: (1)

I would think any camshaft from most ANYONE could likely be improved upon if you're willing to do back to back dyno testing..
But like you stated, not many of us have the resources to do so, so in my case, I relied on the expertise of others...

...and yeah.. mostly impossible to pin point the "proper" choice as every build is different..

I wanted to make reasonable power, through manifolds, without sacrificing valve-train longevity..


Many felt the camshaft Marine Kinetics spec'd for me was too small.

Yet it far exceeded my HP goals.

Do I think it could be improved upon with continued testing? Absolutely... but why bother ?



Doug

phragle

iTrader: (3)

Its 90+ degrees out with 1000% humidity, Im trying to finish up hydraaulics on the bot... right now just about ANY ice cream is a good choice.....

Crude Intentions

iTrader: (1)

I think most of us want the same. A motor to make good power with basic turn key reliability for a couple hundred hours. I agree you would need to constantly swap cams on the dyno to find perfection. I would not spin an extra 5-600 rpm for 5 or 6 hp. That's stupid.

We've talked about what I want and I like the lunati cam. Hopefully in the coming weeks I'll have another dyno sheet to compare.

sutphen 30

pick a hp/tq number and build it 1 of the dozens ways to get the result.

getrdunn

iTrader: (7)

Interesting thread and without a doubt will undoubtably have multiple difference of opinions with no real right or wrong rather preference of ones likes or dislikes. Boating styles etc. I've never been able to achieve peak hp 1,000 rpm higher than peak torque. Regardless - to your second question I would agree with proping for 5,300 rather than 5,900 however where is the torque at 53 and 59. As you stated the 5 or even 10 hp diff wouldn't warrant the spinning an extra 600 rpm. However with that said I travel with two sets of props that top speed is the same with each set. My only problem is with the lets say lower rpm vacation props suck if I have to tuck everything in and roll in rougher water conditions. It never fails when I have them on which is rare there's always that one boat you didn't really want to see comes up quickly behind you. So I guess in a nutshell I'd rather choose the good guy race day props that not only pull much harder in midrange they don't get that laziness in rough water conditions when by chance all is tucked in although rare for me however like to maintain ideal control especially with others in the boat. After I achieve ideal AFR's, EGT's and BSFC I rarely reference back to the results as long as I'm satisfied with hp and torque. Don't get me wrong I always work hard to achieve better results with different combos etc with ample dyno time available. Then it's water time. What works in one boat will respond entirely different in another. Slippery hull vs nonslippery hull. You get the idea. So to rap up my opinion regarding what to prop for is based mostly on my boating style. I don't like being caught off guard with more pitch props less rpm. Reentry, midrange pull and response with optimum stability is my vote.

I appoligize MT if I got off the thread some but you do have some valid debates and hope you get the opinions your looking for.

Back to your initial comment regarding the two different rpm scenarios with only 5 hp gain I would think everyone would agree what they'd prop for however if the peak torque was as you mentioned 1k rpm less than peak hp at 6k I'd almost leant toward that in which case boating in rougher water - windy Lake Michigan days. Drives tucked in and tabs down (reasonably, not maxed) seems like you'd be right at or close to peak torque, Where as proping at 5,300 would perhaps drop to far below peak torque. Sometimes I review dyno sheets and am amazed at peak torque and hp numbers with the said cam for the cubic inch build. When I see a respectable lift cam with say 265 plus duration in a 502-540 inch engine I have to question the results when peak hp comes in under 5,800.

HP is torque but torque calculates HP,

I reread your thread and admit I got off track some however to answer another question/opinion I believe you were pertaining to and that is I'd definitely cam to the rpm ranges engine will be run plan and simple. I see what what your saying and has crossed my mind more than once also however I think a lot of the larger cam combos have something to do with dyno smiles. Would be fun to do not only back to back dyno comparisons with same cubic inch combos with exception of cams that reference to your thread and then have two identacle boats to compare the said engines.

buck35

Unless you have dry exhaust, you have to be choosy or have a plan in place or you will be heading down a long and painfully expensive road. I had big dreams when I bought my boat and soon realized the folly. Speed and power cost money and one thing quickly leads to another.
Type of boat, how you boat and where probably biggest determining factors if you're checkbook says so.

buck35

Hope I didn't kill this thread, seemed like a good one with a lot of opinions to be shared. Great to see you back Mt.