Thread: Cam and CR question

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Hi Folks,
I'm having slight problems in planning my engine project. I have a 5000 lbs vehicle with a 350 SBC, bored .060 over, so it's a 360. I took it apart because the chap who built it screwed everything up, put on bent and cracked heads and so on... The block is a good 4 bolt, bores are still clean because the engine didn't run long enough to even establish any sort of ridge at all. I want an engine that will deliver good mpg and have nice bottom end torque and a smooth idle, but with more power than stock. Enough blabla, here is my problem:
I read that...
... you should aim for the highest CR possible without domed pistons, because that will increase the efficiency of the engine (more mpg, better combustion)
... short duration cam with tight LCA and little overlap if you have a heavy vehicle, want bottom end torque and a smooth idle
... go for tight squish by zero-decking the block or using thinner gaskets if the engine is perfectly flat
... use a low lift cam and high lift rockers to get good lift (to get more mixture into the cylinder in the restricted time a short cam opens the valves
Surely the first and second statements contradict each other, as I.M.O a high CR needs a longer cam to work without detonation, because the dynamic CR will have to drop (which a longer cam does) to eliminate detonation.
So what CR and cam duration should I aim for? I'm planning to get good heads (Dart, World, 041, 186, that style) with good swirl, so I need to go for tight squish to make the most of them. I computed that with 64cc heads and zero deck I could top 10.5:1 static CR even on pump gas, which is better here in Germany (about 96 or more octane). But then i need a longer cam than I want. I've checked out Comp and Crane and got basically two different opinions: either long cam, high CR or short cam, low CR. Isn't short cam, high CR going to work? What will that give me? How would the engine respond to that combination? Would really like to here as many different opinions as I can get. Or where am I going wrong here? Thanks again for all your help,
Mad

You're not necessarily going wrong anywhere. The fact that you're questioning all this shows you're on the right track.

Yes, you want a higher c.r. to improve efficiency, but like you said, that requires a longer cam with a delayed intake closing point. If you run high c.r. with a short cam, you will build excessive cylinder pressure and nothing short of race gas will prevent detonation.

I just don't build high c.r. motors for the street and try to discourage it when talking to others. For a daily driver or one that needs to lug for towing or other mundane duties, the best plan is maximum 9.0:1 c.r. and a cam that matches to keep the operating range down low where you can use it.

Yeah that's what I'll be probably go for anyway, seeing as also Crane and Comp say that's maybe the best direction to head for. But neither Crane nor Comp would give me maximum CRs for their cams. They just say "not under 8.4:1" and the longer the cam the higher the CR they give.
Now here's another thing to consider. Here in Germany you can easily get 100 Octane fuel for marginally more money than normal gas. And I will be installing a Butane-system, Butane has about 105 octane. What's the highest CR you think I could run on 100 octane? I only need the gas (meaning petrol fuel, not the Butane, which is a real gas) to start the engine, but once it's warm i can switch to Butane and run on 105 octane (costs half the price of regular gas here). So I guess I could go quite a bit higher on the CR? The setup I am aiming for is:

rotating assembly, converter and block stock
pistons flat top (2 relief) hyper
thin head gasket
58cc high swirl heads with good chamber (polished to oppose detonation, will make it 60cc)
under 170cc intake runner for high velocity
Edlebrock Performer (NOT RPM) intake
Edelbrock 600cfm spread bore carb
headers 1 5/8 tubing into 2.5 inch dual exhaust with H-tube
1.6 roller rockers for higher lift on a probably short cam

This combination should be good for low end torque, shouldn't it? But there's always the question: What can I do to make it even better? I need a cam that will give me optimum performance for a daily driver, I have an OD transmission (TH700-R4, B&M reinforced) and will put in longer gears (probably 3.0:1, got 3.73 at the moment). That's as close a guess I can make at the moment and it's the first motor i'm going to really plan and build myself. I've done some rebuilds, but that's not the same, this time I want to build an engine to fit the car it's going to go in. Many people just want as much power as possible, but that's not my plan on this project, this time it's going to be a custom fit power cell to shift that heavy load of Detroit wonder metal
Are there any cams you could specifically recommend? Or has anyone found a combination that works really well in his heavy vehicle?
Would appreciate any info on "Do's" as well as "Dont's"
THX!

In my opinion a good camshaft grind you are selecting for mileage and torque i have a couple in mind and i will list the grind specs and reasons why and i will not complicate it as some people like to do.......
1) 212-216@.050 with a valve lift with 1.5 rockers .440-465" 112 LSA. This cam will have a rpm range from about 1500 to 5000 .....keeping a LSA at 112 will increase idle vacuum and smoother idle....good heavy duty towing torque cam......now you say you want to run 1.6 rocker arm ratio....the only thing i can suggest in not running a longer ratio is that fact that you may need to alter the push rod holes in the head and run guide plates cause of the change in geometry.
2) 200-215@.050 lift at around 420" and anwide LSA of around 122-115 degree....by keeping the lsa's wide you are decreasing the overlap period wich increases vacuum at idle and smoother idle....running a tight LSA 106-110 will move the power band down to the middle but will result in a poor idle quality and low vacuum......I hope this help because camshaft selection is a critical one especially when you have 5000 pounds to move. Both these grinds can be custom ground by any cam grinder...and fine tuning for your specific needs....I buy all my camshafts from a custom grinder who is local to me.....for both my race car and my towing vehicle so the top cam is what i have in a 1980 1 tonne dually 400 sbc.....pulls hard pretty smooth idle and tonnes of vacuum for power brakes.......its a real stump puller and i get good economy out of it as well.......hope this helps and i think its pretty friendly worded........

MadMax check your private messages.

Max, with 738 cc's in the cylinder, 60 cc's in the chamber, 8 cc's of head gasket (0.039" compressed), 5 cc's of valve relief and a zero deck, the static c.r. of your motor will be 11.11:1.

Call your favorite cam grinder and give him this info along with the fact that you will run the motor on 105 octane fuel and tell him you want the operating range down low for towing. The grinder will be able to suggest the proper cam to maximize cylinder pressure with the available fuel.

A motor like this will make a lot of heat, so plan on an adequate cooling system along with engine oil cooler and trans cooler.

Hey there (MadMax) I've got a pretty good set up i believe, mine is a 350sbc. It has KB120 .030 hyper flats, sitting .010 in the hole. The heads are World S/R torquers, with 170cc intake runners, on a set of GMPerformance .028 head gaskets. which yeld a .038 qwench. For a cam i chose a Retro-Roller from Comp (EX270HR) (.495 in - .502ex)(218in - 224ex @50)(110 L/S)(1600 to 5400). For an intake i chose an Edelbrock EPS performer @ 600cfm #1405 carb. This combo is right at 9.0:1 c/r, and makes close to 1hp per cubic in. If you cant affored a roller, then you can go with a EX262 or a EX268 flat tappet instead.

I just built a motor for my truck which would also work well for you. It's a 350 with Vortec heads and -12cc dish KB 193 hypereutectic pistons. They are set .015 below the deck with a .039 head gasket for a net compression ratio of 9.25:1. The cam is a Comp Cams XE256H cam. I used Comp's Magnum roller tip rockers. On top, it has a Edelbrock Performer intake and QJet carb. Out back, I have a th350 and 3.07 gears. Anyways, it got my truck moving through the quarter in 15.37 seconds at 88 mph. The motor has a lot more in it, too. I should be able to pick up another 700-1000 rpms of useable power by tuning the carb and ignition system as well as replacing the valve springs. Along with a change of gears, I should have no problem getting the truck into the mid 14's. All of this is on 89 octane and the truck gets the same milage if not better than a stock 350.

Thanks for all the input, here is what Comp and Crane said:
Comp says to go with the XE250H, Crane says to go with the H-256-2 which now replaces the H-260-2 or the Z-268-2. Now they all say "smooth idle, good mileage, good low end torque, for towing and heavy duty". Fine. Now I know that. But there are differences in durations and differences in LCA and differences in lift, so surely that must make a difference somehwhere or do you think the difference netween these three cams won't show up in an engine? Then why should there be so many to choose from?
I think for a start I'm going to get a Desktop dyno (I have time at them moment) and test a few little changes. I don't have the possibility to test the final combination on a real dyno, so the work will be in planning it right first time (very easy job )
But please keep me informed on any new combinations or cam choices you've made, here in Germany there aren't so many people who really know very much about SBCs
Keep up the good work,
Max