I'm looking at building up a 6.0 from an LQ4 block and I live at 5,000 feet of elevation. I'm wondering what kind of static compression and cams people are running at my altitude. This is for a street car, it'll end up with a TKX and 3.73 rear gears. While of course I want great torque/power it also needs to be street friendly. Our temperatures range from the high 90's in summer to below 0 in winter but it's doubtful I'll be taking it out if it's below 20 degrees Fahrenheit. Something that would idle reasonably well in the cold would be nice.

From what I've read LS engines, with aluminum heads and a better combustion chamber, can handle more compression than older SBCs, but I'm not sure how much more, especially when you add altitude (and cam options) to the mix. The highest octane gas from the station is 91.

I don't need to be on the ragged edge of performance but I don't want to build an engine with a 10:1 static compression ratio if 11:1 would be totally fine and help make up for our relative lack of air. I realize cam choice and DCR can also play a part but I think I've read conflicting ideas on how a cam that would lower DCR might not be a great cam for altitude but that's pretty unclear to me. I'd welcome any input but especially real-world examples.

Thanks!
Kris

 

I would use the Wallace Racing DCR calculator, that has a place to enter the altitude the engine will be running at.

If it was my engine, I’d just build it for sea level use. If you build it for high altitude, it will be knock prone if you try to take any road trips to the beach.

 

1. Pick a starting compression ratio and camshaft intake valve closing @ .006" lift
2. Use the Wallace Racing DCR calc tool for your elevation and shoot for 8.0-8.3 DCR @ altitude. If it's a heavy car or truck (>4000lbs), 8.2 max.
   • Note: This will be the line that says... Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of PSI is X.XX. It will likely be around 1 point < the DCR @ 0ft (Your dynamic compression ratio is X.XX).
3. Fill the vehicle up on 87 octane.
4. Pull about 3* out of the high-load region of the low octane table from 3000rpm up.
5. Do all your initial tuning (timing, fuel, etc.) using the low octane table running 87 first (@ 5000ft). Including cold starts, hot starts, idle, driveability, WOT, etc.
6. Copy the newly-tuned low octane table to the high octane table.
7. Burn the residual 87 down to 1/8th or less of a tank, and fill up on 91.
8. Now tune the high octane table.
9. After all that's all complete, put in fresh one step colder-than-stock spark plugs.

As long as your running 91, you should be fine regardless of elevation.

 

Hi, I assembled a race engine FOR Jeff McPherson.
He entered the Pikes Peak event.
My ITB Intake, 14:1 compression, Inlet Duration GREATER than Exhaust.
What is your local air pressure?

 

The local DA appears to be about 8,200 this morning.

We were spectators at Pikes Peak this year, unfortunately the shortened course made our Devil's Playground seats not so great.

 

It's more likely to see 13,000 feet than 0. I don't think it's been at less than 4,500 feet for maybe 40 years. I realize the ultimate solution is some type of FI but that's too much extra for now.

 

What is the vehicle?
What is your town and state ?
I use KPA for measurement. what is your pressure ?
Lance

 

Buy a Turbocharger.
BOOST is cheaper than Pistons.
Boost is cheaper than cylinder heads.
Boost is cheaper than stroker crankshafts.
BOOST is cheaper than selling everything and moving to sea level.
Buy a Turbocharger.

 

It's a 1968 Camaro, Colorado Springs. CO

 

I know, but I'm already in over my head. ;-) The plan is to add compression via cylinder heads and if the future holds, swap heads to lower compression and add boost in the future. But that's a learning curve I can't tackle right now.

 

I've looked at the DCR calculators before and they kind of send me around the bend a bit. The Wallace page doesn't seem to indicate where the close number is taken but you mention .006. If I grab a random cam card, say the sum-8714 cam, that happens to have both .006 and .050 events. I can enter the .006 numbers and get a pretty favorable answer (ignoring altitude for now) from the Wallace calculator. If I take the .050 numbers, and enter those numbers in a calculator like https://uempistons.com/p-28-effectiv...tio-calculator which takes the .050 numbers + 15, I get a much different DCR number.

Why are the resulting DCR numbers so different and how do I know which one is accurate?

I've also seen formulas that use advertised duration to get the close event and if I use that I get close to the .006 numbers on the cam card. I've also seen references to taking the .050 close event and adding 25-28 degrees which actually also gets me close to the .006 measured value.

All of that is why I was hoping someone had some real life examples of what they're running that I could compare against.

 

Because one is right (or nearly so) and the other is wrong.

The one obtained by following the instructions is MUCH more likely to more nearly correct.

That's because the .006" numbers ARE the ones that are usually "advertised" as the seat-to-seat duration.

That's the "rule of thumb" kind of method for estimating the numbers you need for that calculation if you don't have the right ones and all you have is the .050" ones. Usually will put you at least somewhat in the ballpark, even if not absolutely accurate.

Follow the instructions, which call for the "seat to seat", "advertised", ".006" ", etc. numbers. NOT the .050" ones.

In the end, the engine DOES NOT CARE what the "SCR", DCR", etc. is. All it cares, is how many air & fuel molecules are inside the cyls, and how much/little space they're packed into at their tightest. Their density IOW. Which is why a boosted application usually has less compression than N/A; there's more molecules in there, therefore they reach the practical limit of their density, which is the point where detonation may occur, at some lower mechanical compression. You kinda have the opposite situation, "anti boost" if you will: since your air is less dense, you can pack em tighter and get away with it. If you know the ratio of the air density where you are to the sea level density, you can figure out what ratio of compression you can get away with at your location to the compression you could get away with at sea level. The DA is no help here, you need a number either for its weight or mass per volume, something more like lbs per cu ft; or pressure like the psi or KPA, which since air is pretty much an "ideal" gas, the density changes in direct proportion to the pressure. At a guess, a 15 - 20% difference seems reasonable, meaning, your compression can probably be something around 1.15 to 1.20 times what would be tolerable at sea level for an otherwise identical engine. Don't take my word for the numbers though, only the concept.

Lance's suggestion of greater intake duration than you'd otherwise use is also extremely sensible. Since the air is thinner AND the ambient pressure is lower, intake flow is going to be "lazier" all the way around, than at sea level; and exhaust flow will work better, since the pressure difference between the ambient and a cyl as it "blows down" out the valve, is greater. Therefore it will benefit from more intake duration compared to exhaust than it otherwise would, and probably a significantly advanced ICL as well. That combination would produce the same int valve closing point more or less, but earlier opening.

 

Hi, My friend Eddie Sturman is in Woodland Park.
He drove a truck with an inline six (NO Cams and poppet valves) to the top of Pikes Peak.
This was a Camless engine with ECU-controlled Valve Events.
That method could solve your cam choice problem, and he is in your local area.
Lance

 

Just perusing the cams on Summit I don't see a lot of cams that have a lower exhaust duration than intake duration. Or should I just be looking for a closer ratio of intake to exhaust? This TrickFlow one lists a greater advertised intake than exhaust but the numbers flip at .050. Are the advertised numbers the best ones to go by in this scenario?
TFS-30602002 Advertised Duration:286 int./282 exh. Duration at 050 inch Lift:220 int./224 exh.

This particular cam also shows a negative overlap when I put the numbers into a valve events calculator. Is that a good or bad thing for my scenario?

On overlap I'm a little confused as to how it relates to DCR. The Wallace DCR calculator site notes:

"The more "overlap" your cam has, the lower your "actual" as opposed to your static compression ratio will be. "

Other places I've read say that overlap doesn't matter for DCR because it's based solely on the IVC value.

I appreciate all the help, there's a lot to learn and unfortunately lots of conflicting information out there.

 

Low port density (at the Peak) requires a LOW C/L 103* AND more IVD, less EVD (Less Back Pressure)
USE the Lung Test in/out.
Lance

 

DCR as defined is only affected by IVC, and it's more related to WOT throughout the power range.

Where overlap comes in is peak rpm @ WOT, and part throttle characteristics. High overlap will help exhaust scavenging @ high rpm. But the tradeoff will be part throttle characteristics (cruise and moderate tip-in). Overlap is a combination of IVC relative to TDC and EVO relative to TDC.

For your scenario, you want an early IVC and conservative overlap. To get the increase in duration on the intake side, while keeping an early IVC, you'll end up with the IVO @ .050 a couple degrees ATD (or -XX* ATDC) So to avoid excessive overlap, you need to be careful on your EVC.

The Summit SUM-8728R1 would probably great for your combination with the right static compression ratio. You should be right on the money with 11:1 SCR @ 5k ft.

 

That's because AHELLUVALOT more people live at altitudes near sea level, than up in the clouds. I seriously doubt ANY off-the-shelf cam will be The Best for your situation. For the duration split reason as well as others.

The TF cam isn't what you need. It's specific to TF heads, which are quite different in lots of ways, from stock ones.

Any of the cam companies will custom grind you a cam based on your needs. Good chance they'll make that adaptation without you even having to tell em to do it, just based on them knowing the elevation it'll be running at.

The "calculators" for DCR (which incidentally, Wallace was the first and original one of those that was widely available) have a REAL hard time taking "overlap" into account, directly. It, in and of itself, doesn't affect the dynamic compression, butt can be a REAL GOOD proxy for some of the other factors that DO. "Overlap" is the period near TDC when both intake and exhaust valves are open; at low RPMs this allows however much spent exhaust gases, which are at relatively high pressure, to revert back into the intake, and thereby both raise the pressure in the intake a bit, and corrupt the air/fuel mixture with exhaust. At high RPMs it makes the air flow out the exhaust - remember, the gases have inertia - help pull fresh intake charge into the cyl as the exhaust gases are speeding their way outta there, at the expense of possibly pulling some fresh air/fuel all the way through into the exhaust. Spent exhaust gases contain very little fuel or oxygen, thus don't strongly affect the tendency toward detonation. For that reason its direct effect on ... tendency to detonate ... which is what you're generally trying to estimate how close you are by using such a calculator, is ambiguous and uncertain, butt usually not large.

 

To calculate DCR you must use the .006 numbers.
.050 numbers will not work for true DCR.