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Comp exhaust lobe comparison


Jon

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I want to keep close the mpg I get currently when I put a cam in. I also road race on track days occasionally. So I want to use a cam with low overlap that will make great midrange torque and not sacrifice too much up top. I'm thinking something with a 231 LSL .617 intake lobe. The exhaust lobe is where I'm tripping myself up. I only do one or two road course events a year. That is usually four to six twenty minute sessions of hard driving. My heads will be kept stock, so stock LS3 compression. I want to do either a 231/235 114 +2 or 231/234 114 +2 Exhaust lobes I'm looking at: 235 LSL at .621 234 XER at .598 234 Xtreme Marine at .612 Edit: Louis G of LG is recommending whatever I chose be a 114 lsa +4, which is in line with what Richard of WCCH recommends also. Richard would like to see my cam on a 108-110 ICL.

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LilJohn on Yellowbullet said this:

They wont all be the same. The .050 duration only tells part of the story. Lets look at the LSL versus the Exm...

.006. .050. .200. Lobe lift

285. 235. 158. .365

290. 234. 152. .360

They look a tad bit different when you pick more than two data points.....6 degrees difference at .200 is pretty substantial.

Which is better.... i dunno... n/a isnt my thing.

But I personally think a more aggressive evo might bring more torque into play in the midrange that would help the car off a corner better...putting it on the +4 would do that... but id also want a tick more duration with a lobe thats not quite as quick as the LSL is.. theres a 237 HUC that isnt in the master catalog that might be worth looking at as well.

292. 237. 155. .359

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What kind of springs do you plan on using? Do you have flow data on the stock head? Ultimately, what is it yo want out of the car?

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What kind of springs do you plan on using? Do you have flow data on the stock head?

Ultimately, what is it yo want out of the car?

I was going to use the Brian Tooley .660 lift dual springs with titanium retainers. Stock heads flow 313 intake and 204 exhaust at .650 lift according to AI. http://advancedinduction.com/LSX/AiLSx280ccGM821Head.php

I want a broad torque curve that will pull like a freight train from 3000-6500. That gets mid 20's mpg highway and can be used for road course duty on track days. Not asking too much eh? :P

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LilJohn on Yellowbullet said this:

They wont all be the same. The .050 duration only tells part of the story. Lets look at the LSL versus the Exm...

.006. .050. .200. Lobe lift

285. 235. 158. .365

290. 234. 152. .360

They look a tad bit different when you pick more than two data points.....6 degrees difference at .200 is pretty substantial.

Which is better.... i dunno... n/a isnt my thing.

But I personally think a more aggressive evo might bring more torque into play in the midrange that would help the car off a corner better...putting it on the +4 would do that... but id also want a tick more duration with a lobe thats not quite as quick as the LSL is.. theres a 237 HUC that isnt in the master catalog that might be worth looking at as well.

292. 237. 155. .359

OK, so this is always where things get sticky. He is right talking about EVO and torque, because that is the most important event to determine how torquey it will be. This is also where the tradeoff is between power and torque production.

Horsepower is essentially the efficiency of the engine, how much work it can perform in a given amount of time. Torque is an actual physical measurement of the amount of work it can produce. Horsepower is never a measurement, and always a calculation, it cannot be felt, but it can be observed in MPH at the dragstrip, or the headlights disappearing behind you (or the tail light disappearing in front of you). The thing that pushes you back in the seat is torque, not horsepower.

After ignition, the piston crosses TDC and begins to be thrust downward by the expanding combusted A/F mixture. We want to time the combustion so that maximum force is exerted on the piston when there is a large mechanical advantage placed on the crankshaft by the connecting rod. As the piston moves past halfway in the bore, this mechanical advantage starts to decrease.

For a torque-based engine, we are going to delay EVO and keep the mixture shoving on the crank pin. While this builds torque, HP suffers because the mixture has to be forced out by the upward motion of the piston on the exhaust stroke, which hurts efficiency. For a HP based application, we are going to have EVO occur sooner, so the expanding mixture can push itself out of the exhaust port. This harms torque production, but makes the engine more efficient because it is not having to force the mixture out. The piston can easily head upwards on the exhaust stroke.

Back to the lobes. Having a faster opening exhaust valve, means I can delay opening of the valve, which assists in torque production, or I can go with a milder lobe and open it just a wee bit sooner. Since the valve does not really start to flow until the valve is off the seat a bit, running a softer exhaust profile saves on wear and tear without really sacrificing performance.

The nuts and bolts of it is a change in lobe design will affect the EVO event.

Your power band is higher than where I thought you wanted it. So, you may want to do some more research before settling on the Spin cam. Something with some more overlap will certainly keep the engine pulling hard at 6.000 RPM. But then, header choice becomes more important as well.

As the exhaust leaves the primary pipe, the pressure wave exiting is reflected by a suction wave returning towards the back of the exhaust valve. By timing this appropriately, the suction wave can help draw air and fuel into the cylinder. Your choice in the tri-Y design will help in this regard, because the exhaust passes two collection points, which decreases the suction signal, but keeps it present for a greater length of time. Header primary length, size, and collector dimensions all play a role in how well the engine uses the energy from the suction wave. This can provide large increases in power if managed appropriately. Purpose-built headers will outperform off the shelf sets by a very large margin.

That's why I always say, the exhaust should be the last thing you do. By knowing cylinder head outflow, EVO, and intended power range, the exhaust selection can be optimized to enhance power production. Plus, Joe and I both know the guy that can build you a set tailor-made to what you want to accomplish. But the tri-Y is a good compromise for the most part.

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Awesome info! 3000-6000 would be more realistic I think. I don't want to have a car that only gets 15mpg because I put something with too much overlap in.

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2000 to 5000 would not require as much overlap, and is probably a more usable band.

Speaking of overlap, how does it affect mpg while cruising in 6th at say 1800rpm? The Spin cam is 4 degrees of overlap. What if you toss in a cam that is 10 degrees of overlap. How much will that affect the mpg?

Here's one of the guys from Tick Performance talking on LSL vs XER vs Extreme Marine 234 or 235 lobes:

I really like the LSL intake/Extreme RPM High lift exhaust combination. It's what I use on the V.2 Polluter. I beleive the added overlap at TDC that the slightly slower ERPM lobe has creates a stronger signal on the intake port via the exhaust port in the meat of the overlap period and that is at TDC.

231/238 .617"/.585" 112+3

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Overlap is only part of the equation, lobe design (especially duration) and LSA are the other parts. They are all interdependent. I will type a more in depth explanation tomorrow. I had a writing assignment tonight and I just want to take a break, LOL.

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Overlap is only part of the equation, lobe design (especially duration) and LSA are the other parts. They are all interdependent. I will type a more in depth explanation tomorrow. I had a writing assignment tonight and I just want to take a break, LOL.

Thanks. I appreciate the help. I'm trying to understand stuff and not just blindly pick something. I would like to understand the 'why' behind different choices.

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So I'm looking at a cam on LSL lobes. 227/235 110+3. That has 11 degrees of overlap and I was wondering how it would do on a 112+4? Because that would only be 7 degrees. How much difference would 4 degrees of overlap make to my MPG?

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Martin at Tick spec'd me a 225/238 .612/.585 112+3. I also paid Geoff at EPS to spec me a cam. I should have the info in a couple days from him.

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I am trying to get my stuff finished up. If you can post up the valve events (EVO, EVC, IVO and IVC), it would help.

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I am trying to get my stuff finished up. If you can post up the valve events (EVO, EVC, IVO and IVC), it would help.

I'll try to get that info.

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Here's what I have so far from Martin: 275/291@.006" 225/238@.050" 149/154@.200" 112LSA 109ICL 115ECL 225/238 .612"/.585" 112+3

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So I've been playing around with the Wallace racing valve events calculator.

How about this grind?

LSL intake and exhaust

227/235 .614/.621 113+3

IVO 3.5

IVC 43.5

EVO 53.5

EVC 1.5

5 degrees of overlap

231/235 114+3

IVO 4.5

IVC 46.5

EV0 54.5

EVC .5

5 degrees overlap

231/235 113+3

IVO 5.5

IVC 45.5

EVO 53.5

EVC 1.5

7 degrees overlap

Here's the Spinmonster cam

230/234 114+2

IVO 3

IVC 47

EVO 53

EVC 1

4 degrees overlap

Posted Image

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I haven't forgotten about you, just been a crazy week. What made you decide on LSL lobes? Not saying it is good or bad, just wondering.

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I haven't forgotten about you, just been a crazy week.

What made you decide on LSL lobes? Not saying it is good or bad, just wondering.

Thanks!

I'm definitely not dead set on them. They seem like a good compromise of lift, quickness of ramp, and longevity.

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Gotcha. They are about the craziest thing you could put in an engine and still have it streetable, i.e., not putting valvesprings in it every other oil change, LOL. As far as overlap, it is beneficial, but can be overdone, just like most good things. By keeping both of the valves open a little at TDC Exhaust, we can use the suction wave created by exhaust outflow to assist in filling the cylinder. And while it is easy to say that (or type it, LOL), we have to recall the dynamics of the engine at this point. The piston is moving at a slow rate of speed. It is also very close to the top of the cylinder, so the benefits we will see from a very small overlap, means we are only filling a very small space (around the size of the combustion chamber). By having a longer overlap event, the piston uncovers more cylinder volume, which allows for even greater support from the suction wave created by the exiting exhaust charge. This is groovy, because, the slow moving piston is not doing very much on its own at this point to fill the cylinder. But, this event is only the most beneficial when the suction waves are timed with intake velocity and piston motion, the rest of the time, exhaust is being shoved into the intake, or air and fuel is being sucked out of the exhaust. This is why cams with a lot of overlap are powerful, but, man, are they peaky. They blow their wad in a very short span of the RPM band. Perfect if you are shifting five times in a quarter of a mile where RPM can stay in that RPM band, or on a racetrack where you are in one gear the majority o the time is a particular RPM band. For street applications we really just want the combustion chamber filled with something besides exhaust gas. By having a little overlap, and a good exhaust, we want to have this occur near torque peak. There may be a little mileage loss and a little intake dilution at other times, but because we are not holding the valves open for very long, it is minimized. Overlap also does not entirely rely on LSA, because when the valve events are timed, and how fast we open the intake and close the exhaust have an effect on what the effective overlap event will be. Well, I gotta go, I'll be back to check on you later.

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