Just as it says, what do short runners (like the LT1 injection manifold) do to LSA requirements. I'm pretty sure I know but am curious to hear from the experts on this one. Yes, I know there are MANY things that affect LSA choice but I'm interested in knowing for sure what it does to a cam requirement. I know what an undervalved engine needs, and know what a low rod ratio does, what port velocity does, and what a tight vs. wide LSA does to a power curve. I really just want to know what someone who knows has to say about it.

 

Why be coy - why not post your opinion? The issue is intake tuning and rpm range. In general, short runners favor high rpm operation as does a slightly wider LSA. Widen the LSA too much though and it kills high rpm TINSTAAFL.

Rich

 

 

There Is No Such Thing As A Free Lunch

TINSTAAFL - Google it!

 

What kind of LSA numbers are high/low? The highest I've heard of is 112, lowest is about 105?

 

Cam specs:

1987 TPI system:
305 5spd 350 AT
(LB9-L98)
202 207, .404/.415 115 LSA

1995 LT1:
203 208 .450 /.460 116 LSA

A difference of about 16" in runner length didn't seem to affect that variable too much.

 

Just so I know, the LSA is the Lobe Separation Angle and having a smaller angle means the valves overlap more? So my V6 at 108 would have more overlap than a stock V8 as mentioned above.

If the LSA has little to do with port length, is it mainly a compromise between emissions and top end?

 

Does runner length change the LSA, yep. Is there a hard fast rule on the length and LSA connection, nope.

What you are dealing with is pressure waves and how to open and close the valves for a RPM range that works to get to the best cylinder filling for your given motor. Basically you need to have the most pressure difference between intake port and cylinder from TDC to IVC, for the entire RPM range you want to make power. That's a lot easier said than done.

Bret

 

Here's my take on it. . .

As we know, changing LSA moves around 4 different aspects of cam timing (EVO, EVC, IVO, IVC), so you can't just look at LSA alone. In fact, let's assume for just a second that the exhaust lobe stays in one spot since the runner length won't have much affect on exhaust lobe timing.

So, we're left with IVO and IVC.

IVO - in general, you want IVO to be as soon as possible. In other words, you want the intake valve to follow the piston as it leaves TDC. If you were to actually do this, the cam would have alot of overlap and not be streetable for most. The bottom line is that IVO will generally be chosen by P/V clearance or desired idle quality. Runner length will play a role in what pressure wave hits the port, when, and IVO can be chosen to optimize this, but the amount of analysis required to determine this exactly is beyond the scope of most.

IVC - I believe this is most affected by the short runner length. You see, stock IVC's are in the 30 deg ABDC range, while stout street cams will be in the upper 30's, lower 40's. Race cams get into the upper 40's, lower 50's. I've heard of numbers as high as 70, but that's in very high rpm engines beyond our scope. The point is that, when the piston is rising on the compression stroke, the intake valve remains open because the momentum of the air in the runner continues to fill the cylinder even though the piston is going in the opposite direction. With the LT1, there isn't much air in the short runner, so there's not much momentum to carry it in when the piston is rising. It could easily start going the wrong direction. Therefore, you want to close the intake valve directionally earlier than you would with a long runner intake manifold to achieve the same rpm range. For example, a while back, a reputable company swapped an LS1 over to a Victor intake manifold with shorter runners than stock. The rpm range went through the roof but it didn't gain any power. The closed the intake valve sooner. The result was more peak power at a lower rpm.

Closing the intake valve sooner results in tighter LSA, less intake duration, or more advance.

Mike

 

Mike,

IVO as soon as possible? Most performance cams always have the IVO BTDC. The key here is the pressure difference between intake port and cylinder at TDC and opening the valve earlier or later than optimum is going to lessen that difference.

IVC is related to runner length because longer runners working in the higher orders produce a larger pressure in the intake port, which allows the intake port to remain at a higher pressure for longer in the cycle to overcome the higher and higher pressure in the cylinder as the piston moves up the bore.

In the end it has to do with valve centerlines not LSA, because the exhaust event is tied to how well you fill the cylinder and how much to need to clean out of it. That can have a totally different need than the intake so you can't determine the LSA from runner length.

AGAIN THERE ARE NO HARD SET RULES HERE!!!!

Bret

 

I think all cams open the intake valves BTDC. Did I say otherwise?

In every case I've seen, opening the intake valve earlier only improved power up until the point that P/V clearance became an issue. In one instance, they actually opened the intake valve so far before TDC that a special cam lobe had to be used to close it again to make room for the piston as it passed TDC. All this aside, pretty much all street cars wouldn't be happy with this much overlap due to idle quality, so it's a moot point anyway. Use as much overlap as you can stand.

That's what I said, in so many words. Longer runners can take advantage of a later IVC. Conversely, shorter runners will require an earlier IVC.

That's why I started out by saying, "let's assume for just a second that the exhaust lobe stays in one spot since the runner length won't have much affect on exhaust lobe timing."

I'm having a hard time figuring out if some of your points agree or disagree with mine.

Mike

 

I'm starting to remember why I don't visit forums as much anymore.

All cams don't open the valve BTDC, lots of small cams do but in this instance who gives a rats ***, just some more dumb **** to argue about that is moot.

I've had a lot of instances when a later IVO improved power throughout the RPM range of the motor, so that is a very bad rule to go by and people are not looking at the whole equation correctly.

Does runner length play a part in cylinder filling? (Easy answer is yes here)

So "let's assume for just a second that the exhaust lobe stays in one spot since the runner length won't have much affect on exhaust lobe timing."

That statment assumes that the parts of the motor are not interconnected, which is false or more appropritately, incomplete. If you can put more into the cylinder it's safe to say that you have to get more out, so the exhaust lobe timing WILL change when you are looking for max power.

"Use as much overlap as you can stand." I don't agree with this, use the overlap you need to get the most cylinder filling you can get for the RPM range you are working in. Again it has to do with pressure management around TDC, not a simple concept to explain or teach to a point anyone can do anything with it.

I've seen very little out there in terms of anything to do with getting valve events right by working on pressure differentials in a motor and until you look at it that way there are way to many gross instances where "rules" are very far off and unuseable. Not to mention most of the "examples" that guys use to draw conclusions on valve events are so skewed and the WRONG reasoning is behind the conclusions it's almost laughable.

BUT this is accurate "Longer runners can take advantage of a later IVC. Conversely, shorter runners will require an earlier IVC." I just explained the reason to WHY.

Bret

 

Why does it always have to be an argument, Brett?

As far as I can tell, you agreed with 3 out of 4 of my points, but got offended when I asked for clarification.

Most folks will only tolerate about 5 deg overlap @ .050 for street use. Lets assume for a second that it's a single pattern cam that's 4 deg advanced. That puts IVO at 6.5 deg BTDC. You think that's early enough to cause problems?

Mike

 

How would changing from a LT intake to a victor change things?

Would this call for a new camshaft to compensate? and why?

 

Oh you had to go and complicate the issue. . .

You see, the Victor has longer runners than the LTx intake. So, as Brett and I agree, could close the intake valve later to take advantage of the greater momentum of the charge to fill the cylinder ABDC. HOWEVER, dyno tests have shown that the Victor actually moves the rpm range up when compared to an LT4 intake. Soo. . . it depends on your goals. If you want to move the rpm range back down to where it was, you'd need to moved IVC earlier. If you want to take full advantage of the longer runners and make more peak hp, then move the IVC later, but you'll lose even more low-end than the manifold did itself.

Mike