GM LS3 Hot Cam

We have an LS3 6.2 V8 installed in our plane. In stock form, at cruise altitude between 9,000 and 11,000 feet, using 3600 rpm (1890 prop) for power, we will get 180hp. With the Hot Cam our horsepower goes up to 217.

Would the resulting increase in IAS, and the extra climb performance make the new cam worthwhile? The kit is $500. And, the cam in question is a bone stock GM LS3 component.

Hoping that some of you "engine gurus" will have some good input!

Which airframe?

On the RV-7 (easy example, since Van publishes 160, 180 & 200 hp numbers), Vans specs say that a 40 hp increase (25%) results in a ~15mph (~7.5%) increase in top speed. Speed vs hp is a cube function, so extra hp doesn't really get you a lot for the extra fuel burned. Climb is a different story; The -7 goes from 1900fpm to 2550 fpm (34% increase) with a 40 hp (25%) increase in power.
http://www.vansaircraft.com/public/rv7perf.htm

I'm certainly no 'engine guru', but that big a hp increase at that rpm (20.5% increase at a fixed 3600rpm) with only a cam change sounds highly suspect. Who makes the 'Hot Cam', and has the gain been verified on an independent tester's dyno, with no other changes to the induction, exhaust, etc?

Using GM's published torque/hp curves for the LS3, if the stock torque peak is moved down to 3600 rpm the resulting hp increase (at sea level) is ~31 hp. At your target altitude, it would be closer to 20 hp.
http://www.chevrolet.com/performance/crate-engines/ls3.html

So the claim for the cam change implies that it both shifts the peak down in rpm, and radically increases torque, as well.

Please publish more info on the plane; do you have a website/pics etc available?

Charlie

The numbers I have were taken from the GM Performance website, which shows stock hp @3600 = 260, with the Hot Cam @3600 = 305. This is sea level, standard day. The GM sites are a bit conservative on the dyno data. Most of the other performance sites show more impressive numbers.

It is not an RV, as you probably guessed...3/4 scale P-51. But I find myself seeking the wisdom that is found on this forum!

Z 06

The Supercharged Z06 engine would get you 650 HP. Not that you need or want that much, but running at a lower RPM you could easily hold 300 -350 HP from take off past 10,000 Ft. What prop reduction drive do you have?

F1R said:

The Supercharged Z06 engine would get you 650 HP. Not that you need or want that much, but running at a lower RPM you could easily hold 300 -350 HP from take off past 10,000 Ft. What prop reduction drive do you have?

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I already have the LS3. 300 to about 350hp is our limit. The reduction is 1.9 - 1. We are now in discussion with Comp Cams. They are looking at a custom grind for performance from 2800 - 4200 rpm. They claim this is a first for them.

That bump in HP is totally feasible. I put a cam and intake on my LS7 powered vette, and bumped compression ratio to 13:1. Yielded over a 200hp increase (710 from a stock 505).

If they can breathe freely, a cam can produce amazing gains on these LS motors

unitink72 said:

That bump in HP is totally feasible. I put a cam and intake on my LS7 powered vette, and bumped compression ratio to 13:1. Yielded over a 200hp increase (710 from a stock 505).

If they can breathe freely, a cam can produce amazing gains on these LS motors

Click to expand...

You changed the intake, too. [oops, and the compression] And both your and GM's numbers are peak HP at sea level, right? Not at 3600 rpm and 10,000 feet. Whatever gains you get from a mod, you don't get to keep the same number of HP at altitude; you just get to keep the same % of increase (as long as the mod isn't boost, of course...).

Back to the original question; What are the Titan's HP-required numbers for speeds equal to an RV-7? If it takes significantly more HP for the same speed, then the extra HP is likely to buy you even less speed than it does for the -7.

rv7charlie said:

You changed the intake, too. [oops, and the compression] And both your and GM's numbers are peak HP at sea level, right? Not at 3600 rpm and 10,000 feet. Whatever gains you get from a mod, you don't get to keep the same number of HP at altitude; you just get to keep the same % of increase (as long as the mod isn't boost, of course...).

Back to the original question; What are the Titan's HP-required numbers for speeds equal to an RV-7? If it takes significantly more HP for the same speed, then the extra HP is likely to buy you even less speed than it does for the -7.

Click to expand...

The GM LS3 Hot Cam would give us a solid 13% - 14% power increase at all altitudes. I would guess that would be good for 6 to 8 mph increase, but would improve the ROC by a much bigger margin. The standard T-51D (V8) will easily achieve climb rates of 3,000 FPM or greater. The supercharged T-51 (Honda V6) would do the same in climb, and cruise at 210mph IAS @8000'.

The T-51 Mustang will easily do 232 mph IAS at 5500', with the new high speed wing, and the V8 or V12.

A simple cam swap nets 40 to 50 hp with the LS3. There are several performance websites that have experimented with this, and with published dyno lab results. Just a cam swap, nothing else. The LS3 responds well to this as it already has great breathing heads.

"Using just a cam, we managed to increase the power output of the LS3 from 496 hp to 573 hp, while torque was up from 491 lb-ft to 526 lb-ft."

http://www.enginelabs.com/engine-te...ng-three-performance-cams-on-the-engine-dyno/

http://www.superchevy.com/how-to/en...m-tests-on-an-ls3-engine-bumpstick-boomerang/

TX,

I overlooked your earlier post about looking for a custom grind for 2800-4200rpm. A cam for a lower rpm torque peak is what I was about to suggest. Just about all 'performance' mods for cars seek to shift the torque curve *higher* in the rpm range, and/or raise the max rpm; not what you're looking for at all. Bummer that your drive ratio locks you to such a low rpm; that's not far above direct drive territory. If it was just a touch wider, you could operate closer to the engine's torque peak (best bsfc) & it would still be loafing, compared to what it's mechanically capable of handling.

BTW, it's certainly impressive that the airframe can safely handle such a wide range (weight/HP) of powerplants.

Charlie

Charlie - Yes, the gearing for the V8 is 1.9-1. Not sure why. For the V6's, it's 2.2-1.
So we are at 3600rpm max for cruise due to prop tip speed limits of 475mph. Hope that the cam guys can come up with something that would get the power band down into our useable range.
The T-51 was originally designed with a 100hp Rotax. Now it has V8's and V12's.

One more note...a new 96" prop is in the works. This would limit the engine rpm to 3100.

Just heard from Crane Cams, the folks who do custom grinds....."Sorry, we have nothing that will work for you."

That's a bummer.

As a FWIW, I've heard 3rd hand about guys building up LS3's for a/c use, using the truck cyl heads to keep intake velocity (& torque) up at lower rpms. Any chance the truck cam would work, too?

Charlie

thread reopened

After Doug said that this thread was appropriate, he re-opened it. Thanks Doug!

Let the informative dialog begin anew on LS-3 cam shafts, moving the torque peak down to the normal rpm range used in aircraft installations, etc.

One question I had for the OP was why the RPM limitation on the prop is so severe. I think he said tip speed of 475 mph. Thats only Mach 0.62.
It is very common for props to run much faster than that. A 72" prop turning 2700 rpm is Mach 0.77. (Not the helical tip speed that includes the vector addition of the forward flight velocity). For cruise at 2400 rpm, its Mach 0.68. So why does the OP need to turn his prop so slow?

scsmith said:

After Doug said that this thread was appropriate, he re-opened it. Thanks Doug!

Let the informative dialog begin anew on LS-3 cam shafts, moving the torque peak down to the normal rpm range used in aircraft installations, etc.

One question I had for the OP was why the RPM limitation on the prop is so severe. I think he said tip speed of 475 mph. Thats only Mach 0.62.
It is very common for props to run much faster than that. A 72" prop turning 2700 rpm is Mach 0.77. (Not the helical tip speed that includes the vector addition of the forward flight velocity). For cruise at 2400 rpm, its Mach 0.68. So why does the OP need to turn his prop so slow?

Click to expand...

First things first...our propeller is 84", and we may opt for the correct 3/4 scale 96" propeller when it becomes available next year.
The 84" prop hits 475 mph (700 fps) at 3600 engine rpm, which is 1890 prop rpm. This is with a 1.9-1 reduction ratio. 475 is the tip speed cruise limit, where Mach .88 (+/-) would be a normal takeoff limit (red line) for tip speed.

In the RV world, it is likely not an issue as the diameter is normally smaller. But having a slower turning engine should be beneficial, and perhaps more efficient.

We have chosen the GM LS Hot Cam. It will give a minimum of 14% to 15% power increase. So the engine / PSRU / prop can all turn slower for the same power output. The primary benefit is being able to develop more power at altitude. At ten thousand, it is 189hp vs. 224hp. That is a significant increase, not to mention the added rate of climb.

Sea level numbers for the stock cam vs. Hot Cam are:
3600 rpm = 270hp
3600 rpm = 315hp

Comp Cams has an off the shelf model they recommended for us. It was more aggressive than the GM cam. Both cams were very close in power output from 3000 to 4000rpm.

Crane Cams told me..."Sorry, we have nothing for you."

The LS3 is an excellent choice for a number of reasons. Cost, reliability, and power. My friend has a SBC 327 LC9 in his plane. He uses 3000rpm for takeoff. It has gobs of torque! The acceleration is something else. But the LS3 will blow it's socks off!

Hope this helps shed some light on the process, and hope it is of interest to some of the folks here. I have never seen an RV with anything other than a conventional aircraft engine.

The above engine data came from GM Performance. Prop tip speeds came from: http://www.warpdriveprops.com/propspd2.html

Hi Myron; welcome back.

Actually, they're everywhere.
http://www.beltedair.com/
has been around for decades. Lots of Subaru power, too. And my personal favorite, the Mazda rotary is flying in quite a few, as well.
https://rotarygatheringmay2014.shutterfly.com/

Charlie

Something amiss in your math

If you have a max tip Mach limit of 0.88 (which is very reasonable) on a 96" prop, that is 2300 RPM. (not counting the vector addition of forward flight, which usually is not included in quoted tip speed. It would be a negligible contribution at takeoff anyway).

Your cruise limit makes sense if you are flying high and cold. Although I think it is very conservative. 4000 engine RPM, 2100 prop RPM, would be M=0.75 at 18,000 ft which would work very nicely. It shifts you much closer to the stock torque peak wouldn't it?



RPM/60 * 2PI * radius (ft) = tip speed (ft/sec).
Mach 1 = 1100 ft/sec at sea level.
Mach 1 = 1030 ft/sec at 18,000 std day

scsmith said:

If you have a max tip Mach limit of 0.88 (which is very reasonable) on a 96" prop, that is 2300 RPM. (not counting the vector addition of forward flight, which usually is not included in quoted tip speed. It would be a negligible contribution at takeoff anyway).

Your cruise limit makes sense if you are flying high and cold. Although I think it is very conservative. 4000 engine RPM, 2100 prop RPM, would be M=0.75 at 18,000 ft which would work very nicely. It shifts you much closer to the stock torque peak wouldn't it?



RPM/60 * 2PI * radius (ft) = tip speed (ft/sec).
Mach 1 = 1100 ft/sec at sea level.
Mach 1 = 1030 ft/sec at 18,000 std day

Click to expand...

It would appear that Mach tip speeds are the limiting factor for takeoff power settings, and that transitions to a miles-per-hour figure for cruise power settings.

This may, or may not be a good analogy...the transports I fly are all flown at Mach speeds above FL260, and we transition to IAS below that level. And they both have limits that effect that aircraft.

For example, a typical departure and climb speed for the B-777 would be to maintain 325 knots indicated until intercepting Mach .84. Then continue the climb at .84. We normally cruise between .83 and .86.

Thanks Doug for allowing this.

What happens when prop tip speed goes above .86ish Mach, do they break off, too much wasted energy? new stuff to me

bret said:

What happens when prop tip speed goes above .86ish Mach, do they break off, too much wasted energy? new stuff to me

Click to expand...

If you want the quietest and most efficient thrust propeller system, select a prop configuration (and reduction drive ratio) that will keep the tip speed for your cruise rpm at or below 700 feet per second or 475 mph. Above this speed "compressibility" of the air in front of the prop leading edge begins to occur which begins to rapidly degrade the performance of your propeller.


Over .92 Mach. the airflow begins to detach from the propeller which decreases efficiency and dramatically increases noise. To improve performance and public relations you should consider reducing RPM so your tip speed will be well below 0.92 Mach.

The prop gets vastly less efficient as the tips approach Mach 1. Makes lots of noise too. That's why airshow performers in T6's and 985 powered Stearmans like it. While inefficient the mini sonic booms at the prop tips rattle the windows (and the soul) and make for a dramatic performance. Noise and smoke. That's what the masses want. You will also hear this on seaplanes with Borer or "long climb props". Ever been under a C-185 after takeoff on climb out?

So noise = lost energy. Shock waves = noise. And exceeding the critical Mach number at the tips = loss of useful span

Jim

jliltd said:

The prop gets vastly less efficient as the tips approach Mach 1. Makes lots of noise too. That's why airshow performers in T6's and 985 powered Stearmans like it. While inefficient the mini sonic booms at the prop tips rattle the windows (and the soul) and make for a dramatic performance. Noise and smoke. That's what the masses want. You will also hear this on seaplanes with Borer or "long climb props". Ever been under a C-185 after takeoff on climb out?

So noise = lost energy. Shock waves = noise. And exceeding the critical Mach number at the tips = loss of useful span

Jim

Click to expand...

That's cool if you are flying a T-6 at an airshow, not so cool in an RV!

Here are some numbers for you RV-8 types considering doing this:

rpm.........................Horsepower / Torque

3000...................(stock) 225/370 - (Hot) 225/380

3200...................(stock) 240/375 - (Hot) 245/390

3400...................(stock) 250/380 - (Hot) 270/415

3600...................(stock) 270/390 - (Hot) 310/425

3800...................(stock) 290/390 - (Hot) 315/435

4000...................(stock) 305/405 - (Hot) 350/460

4200...................(stock) 340/420 - (Hot) 370/470


As you can see, the benefit does not kick in until 3400rpm and above. But factor in cruising at high altitudes, say 9,500 to 11,500, and the benefit will be in more power available.

Figuring a 3% hp loss per thousand feet, at 10,000' msl the stock engine would get you (@3600 rpm) 188hp, where the Hot Cam gets 214hp. We figure a 14 to 15% increase in power available at altitude.

The above numbers were taken from GMPP. They are not linear, and there was a bit of interpolation employed. Note that these numbers are extremely conservative, as any performance shop using a dyno will report data that is far in excess of what GM publishes.

Like they say, under promise and over deliver.

I have been flying a 402 cid LS V8 with the GM Hot Cam in my plane, equipped with an 80" diam 3-blade Catto prop - Craig said the helical tip speed limit for this prop was .94M, and WOT runs show that the prop pulls right up to this point. This allows turning the engine at a max 5000 crank rpm with the 1.8:1 drive ratio, making good use of the cam. It is my understanding that prop tip chord/ thickness play a significant role in allowable max tip speed.

Can the RV8 use an 80" prop?

Jeff

TXFlyGuy said:

If you want the quietest and most efficient thrust propeller system, select a prop configuration (and reduction drive ratio) that will keep the tip speed for your cruise rpm at or below 700 feet per second or 475 mph. Above this speed "compressibility" of the air in front of the prop leading edge begins to occur which begins to rapidly degrade the performance of your propeller.


Over .92 Mach. the airflow begins to detach from the propeller which decreases efficiency and dramatically increases noise. To improve performance and public relations you should consider reducing RPM so your tip speed will be well below 0.92 Mach.

Click to expand...

M=0.92 is a pretty good cut-off point where efficiency plummets and noise goes way up. And yes, big Continentals with big props, like C-185's and Bonanzas are turning above that, and make a lot of noise.

But 700 ft/sec is WAY below that, and the efficiency curve barely begins to roll off slowly. The horsepower gain from turning your LS3 faster would outweigh the efficiency loss for a fair bit, easily up to 2200 RPM (4200 engine RPM) at mid-altitudes.

For you RV owners, with the Chevy V8's, this cam does require changing out the stock springs. That is, unless you have the LS3. The springs on the LS3 / LS6 are identical.
The cam kit comes with a set of LS6 springs.

Why? All RVs are at Vne 200 knots, or so. I was running my RV-10 just short of that using an LS1 stock engine. I don't see the need for a faster cam - at least in an RV....
John

Jeffda said:

Can the RV8 use an 80" prop?

Jeff

Click to expand...

Mine Does.

johngoodman said:

Why? All RVs are at Vne 200 knots, or so. I was running my RV-10 just short of that using an LS1 stock engine. I don't see the need for a faster cam - at least in an RV....
John

Click to expand...

The easiest answer is climb, and high/hot performance. And I'm not saying it's the best idea, but there are lots of RV's running at/above VNE with aero cleanups & HP increases.

And the principles can apply in more general terms, in other, smaller alternative engines.

Charlie

rv7charlie said:

The easiest answer is climb, and high/hot performance. And I'm not saying it's the best idea, but there are lots of RV's running at/above VNE with aero cleanups & HP increases.

And the principles can apply in more general terms, in other, smaller alternative engines.

Charlie

Click to expand...

Charlie, you would be correct. The only reason we are doing this is for increased power at our typical cruise altitude of 9500 to 11000 feet. With the other limits we have, such as max prop speed, and max hp to the PSRU, there is no way we will ever use the 500hp that is available.