Camshaft LSA

[MechRick]

I'm getting ready to pick a camshaft for a 5.0 project that is going to be megasquirted. Stumbled across a concept that confuses me a bit.

Most of the cam manufacturers are stating that cams with wide lobe separation angles reduce dynamic compression, reduce cranking compression, and increase intake manifold vacuum.

http://www.compcams.com/technical/faq/lsaproperties.asp

Seems obvious that picking a cam that increases idle vacuum will make it easier to tune with a speed density system. But megasquirts give you the option to shut off closed loop control below a select RPM, as well as run Alpha-N control at idle. I would think it could handle tight lobe cams, and the only real consideration becomes vacuum for power brakes.

Can someone explain what is happening with lobe separation angles, and the associated tradeoffs?

 

[xctasy]

It depends on EFi injector firing and engine configuration. The traditonal Bosch rule was that peak piston acceleration governs when the pulses of the injector should be tuned.

If its bank fire or semi sequential, you can tollerate different lobe center spacings. Overlap is a function of lobe spacing.

If its fully sequential, the pulse widths have to phase in with the lobe centers.

Above 3000 rpm, everything is constant feed, irrespective of the kind of injector firing you have.

In the old Bosch CIS(K-Jetronic), it was important to put another 5 degrees on the lobe center, ie 115 instead of 109.

If cam timing was moderate, like at lash values of 256 to about 265 degrees, you could stay at 109 degrees.

Where things get screwd up is short 109 spacings with large over laps and at lash durations. Then the injector feed gets blown bakc, and you have major issues retaining the air fuel mix in the intake port, and making it run seamlessly without major power or torque losses.

Emissions wise, longer lobe centers have become manadatory.

Ford backed off to 115 and greater in the OBDII era 5.0 OHV SEFI's, and GM, 117 to even 121 degrees on the 5.3 to 7.0's. When turbo or supercharged, more center was added, like about 4 degrees.

You can make engines sing with narrower lobes, but you have to reatin air fuel mix in the port, and then manage fuel standoff by changing the injector postion. Jenvey and Fox EFi systems do this, but the injector is no longer a tradtional port EFi system.

MS2E and MS3 allows you to tune all that kinda stuff with ease.

LCA change to wider is mostly due to emissions and engine tuning to meet dyno requirements.

If you look at all the graphs on Average LCA histrocially used by David Vizard and Marlon et al, and then check them against cam masters like Ed and Buddy Rawls, they zoom in on other important factors. Generally, due to pulse tuning and atomisation, EFi trails some of the best Independent runner carb systems, and cam design and Intake design for EFi is a new cutting edge art whic cam masters, EFi techies and air flow modeling techncians have been working on. Its hard to provide any other general info.

I got more in 5 minutes with Buddy Rawls than 25 years studying Fords EECIV system

Others might like to chime in...

 

[Soldmy66]

Here is a link to an article re LSAs that I enjoyed reading and that might be helpful:

https://www.hotrod.com/articles/camshaf ... explained/

 

[MechRick]

I've read that article before, but re-reading it shows this nugget... Tight lobe separation cams have an earlier closing intake valve, which builds more low RPM compression.

That makes sense.

 

[pmuller9]

You get the basic idea of how overlap affects combustion at idle and low rpm.
Separately from that the intake valve closing point affects the power band.

The actual response is engine specific.
Larger displacement engine tolerate more overlap and have wider power bands that will allow much later intake valve closing points.

On your 5.0 a good indicator for overlap response is the point where you start to hear the idle lope.
Example: On the Ford 4.9 six we generally don't get idle lope until the .050" duration overlap approaches 0 degrees.
The Comp 268 has -2 degrees overlap and produces a little lope at idle but improves with a header where the scavenging reduces exhaust pressure and less reversion in the intake port.
A 5.0 V8 has smaller cylinders and will be more sensitive to overlap.

In the end (since you are not restricted by emissions) you get the cam that gives you the power band you want and tune the EFI around that.

A tight LSA cam with long duration lobes that has a rough idle and rough low rpm comes into the power band abruptly like someone turned on a switch.
This type of profile offers a very strong midrange.
This would be the case where you disable closed loop on the EFI until the engine is into power and manually tune the fuel maps below that point.
If you are doing sequential injection you can start injecting just as the exhaust valve closes to reduce the unburned fuel going out the header below the power band.
A long tube header will help both ends of the power band in this case.
A stepped header will give additional help with a secondary scavenging pulse.

As the LSA gets wider the transition into the power bands smooths out, the power band gets wider but also flattens out.

It depends on what you need for power.
If you are going for a big cam with a tight LSA, on a 5.0, I would limit the .050" duration overlap to 16 degrees.

Will this be a street engine or some type of racing?

 

[MechRick]

It's going in my bronco 2.

I've got a built C6 to go behind it. Cruise RPM will be @3000. I would like to cam it to make peak torque there.

Of course, if it lopes a bit at idle, I won't complain...

The heads will be ported E7TE. Shorty headers for the exhaust.

Related six content...now I have to wonder if the Comp 268H cammed 4.9 I built would have pinged less if the lobe separation were say, 112-114 degrees with the same lobe profile...

 

[pmuller9]

I forgot to ask earlier. Will it be a flat tappet or roller cam?

 

[MechRick]

Roller.

Going to reuse the stock lifters, they look fine.

Probably will be upgrading springs. Might step up the valve size while I'm at it...

 

[pmuller9]

Related six content...now I have to wonder if the Comp 268H cammed 4.9 I built would have pinged less if the lobe separation were say, 112-114 degrees with the same lobe profile...

Yes, a wider LSA would have helped a lot.

I believe the stock cam has a 115 LSA and is the reason why the stock EFI 302 has peak torque at 3200 rpm.
The stock .050" duration is 210* and the .006" duration is 266*
If you want to keep the peak at 3200 and increase the torque, the cam will need a tighter LSA angle.
I'm also assuming you're not worried about power above 5000 rpm.

I would look at a cam that has a 110 LSA with a .050" duration not more than 220*
For low end torque stay with a single pattern cam and advance it 4 degrees.

I was looking at the Howards 222725-10
The high lift will really wake things up.

The E7TE head needs all the help it can get.

 

[MechRick]

You are talking about the HO cam? The full size Bronco engine has, I assume, the dual pattern truck cam that from what I can dig up, torque peaks at 2800.

I like how you are thinking. Take the HO cam and narrow the lobe separation. It will make more torque earlier.

That Howard's cam you mentioned has exactly the duration numbers and lobe angle of Comp's 268H. I liked that cam except for the ping while towing (something I won't do with a B2).

The single pattern cam would make me pay attention to the exhaust ports. They are horrendous.

I don't want to stray too far from information that applies to sixes, but thank you for your advice (you too, xctasy, I'm going to run bank injection, I'll have to watch the overlap numbers).

 

[pmuller9]

Yes, I was talking about the 5.0 HO with peak torque at 3200 rpm.
Which 5.0 do you have?

What is the compression ratio?

There are 2 reason for considering the single pattern Howards cam.
To reduce the overlap over a split pattern cam and to focus more on low to midrange torque rather than higher rpm horsepower.
What I expect to see with the Howards cam is an overall increase in torque.
There will be some lope at idle.

For six cylinder comparison, the 302 V8 is equivalent to a 226 cu in six as far as cylinder size so it will be more sensitive to cam specs than a 300 six.

 

[MechRick]

It's a F1SE block with an E4OD attached to it (that I won't be using). Had to be out of a '91 up truck.

Not sure on the compression, pistons have a slight dish. I just went out to the garage and verified the pistons will need to go oversize. I'll go hyper flattops unless you suggest otherwise.

5.0 engines like to lope. Knew a coworker with a 290 duration cam in a 302 Falcon that sounded like a pro stocker.

 

[pmuller9]

The F1SE-BB block was made for 1991 and later cars and possibly some went into trucks.
They are listed as 9:1 compression ratio.

This will be the same old story. Since we are looking at low end power by decreasing the LSA, the compression ratio should not exceed 8.8 with the Howards cam. The 272* duration set 2 degrees advanced has a dynamic compression ratio of 7 which will work with 87 octane pump gas.

The E7TE head reportedly can have a chamber volume from 60.6 to 63.6 cc so you will need to cc the chambers.

The Speed-Pro H606P pistons have a small 6cc dish and 4 valve reliefs and use the 1.5mm rings.
The Speed-Pro H273CP are flat top with 4 large valve reliefs that have 8cc and use the 5/64" rings
Either piston can yield an 8.8 compression ratio with work to the head.

If you go with larger valves the chamber can be opened up which will help.

 

[MechRick]

How does advancing the cam affect dynamic compression?

I'll CC a chamber in the morning to get a starting point. I'll be grinding on the heads for a while. I've done small block Ford heads in the past, I know what I'm up against.

P heads would have been a much better starting point.

 

[pmuller9]

Advancing the cam closes the intake valve sooner and increases the DCR.

Using the Howards cam with 272* of advertised duration as an example in a 302 V8 with an 8.8 static compression ratio.

If the cam is installed straight up with the intake lobe center at 110* ATDC the intake valve will close 66* ABDC and the piston will be 2.298" from TDC. The DCR will be 6.98

With the cam 2 degrees advanced the intake lobe center will be at 108* ATDC, intake valve closing at 64* ABDC, the piston will be 2.339" from TDC and the DCR will be 7.09.

4* advanced, Intake lobe center at 106* ATDC, piston is 2.380" from TDC and the DCR is 7.19

I'm assuming you want to be able to run on 87 octane pump gas?

The P heads have a smaller chamber but they do flow better.

 

[MechRick]

63cc.

Going to browse the junkyards on Sunday for gt40's ('97 Explorers). They supposedly have a bit larger chamber than that.

Then in the future, I may spring for aluminum heads with a 60cc chamber.

Thanks for the help guys.