Overlap vs. Wider LSA - Detonation Resistance?

[mopowers]

I recently spoke with a gentleman at a prominent cam company who said increased cam overlap has a greater effect at reducing detonation potential than a wider LSA (specifically a later intake valve closing angle). I've always thought the opposite was true - that a wider LSA which closes the intake valve later would lead to lower effective compression, reduced cylinder pressure, and improved detonation resistance.

So, which one is it? For a given intake and exhaust duration, would a wider or narrower LSA be less prone to detonate?

 

[Illahe]

I recently spoke with a gentleman at a prominent cam company who said increased cam overlap has a greater effect at reducing detonation potential than a wider LSA (specifically a later intake valve closing angle). I've always thought the opposite was true - that a wider LSA which closes the intake valve later would lead to lower effective compression, reduced cylinder pressure, and improved detonation resistance.

So, which one is it? For a given intake and exhaust duration, would a wider or narrower LSA be less prone to detonate?

I'd call Kevin at

I recently spoke with a gentleman at a prominent cam company who said increased cam overlap has a greater effect at reducing detonation potential than a wider LSA (specifically a later intake valve closing angle). I've always thought the opposite was true - that a wider LSA which closes the intake valve later would lead to lower effective compression, reduced cylinder pressure, and improved detonation resistance.

So, which one is it? For a given intake and exhaust duration, would a wider or narrower LSA be less prone to detonate?

I'd call Kevin at Schneider Racing Cams . They seem to have a lot of Chrysler valve timing experience. He has created some effective profiles for me on more than one occasion.

Good luck.

 

[TT5.9mag]

You have to ask yourself, what does changing the LSA do to all valve events and what effect does it have on overlap.

 

[69conv]

Schneider Racing Cams . These are the people who developed all the purple shaft cam family and sold the profiles to Mopar. They got it going on.

 

[mopowers]

You have to ask yourself, what does changing the LSA do to all valve events and what effect does it have on overlap.

I think I have a decent understanding of how changing LSA impacts valve events and overlap. That's why I was surprised when a prominent cam grinder told me that overlap improves detonation resistance as opposed to widening the LSA. I just got off the phone with another company who said that's bullshit. Lot's of differing opinions out there, that's for sure.

 

[TT5.9mag]

I think I have a decent understanding of how changing LSA impacts valve events and overlap. That's why I was surprised when a prominent cam grinder told me that overlap improves detonation resistance as opposed to widening the LSA. I just got off the phone with another company who said that's bullshit. Lot's of differing opinions out there, that's for sure.

It can be contradictory because overlap can change low rpm cylinder pressure which could reduce detonation or increase detonation tollerance. But on the other side of the coin, a tighter LSA cam makes more low rpm torque which can be a contributor to detonation. The takeaway? Don’t use cam timing to solve detonation problems. At least that’s how I see it. Things like chamber finish, shape of the piston, plug location, fuel, and ultimately the most important factor, the timing curve, are what you should be focused on to have the largest impact.

 

[RustyRatRod]

A wider LAS reduces cam overlap, so he's probably correct.

 

[Illahe]

I think I have a decent understanding of how changing LSA impacts valve events and overlap. That's why I was surprised when a prominent cam grinder told me that overlap improves detonation resistance as opposed to widening the LSA. I just got off the phone with another company who said that's bullshit. Lot's of differing opinions out there, that's for sure.

The difference between the to phone calls may be:

Prominent cam grinders “tech” is cranking the numbers on his computer.

Cam grinder answering the phone is referencing his experience developing patterns for Chryslers.

As far as LSA goes… I haven’t seen a thread on here yet complaining about Iskendarian's “favorite LSA” some will get it…

My take: I’ve been to the drag strip dozens of times. Yet I’ve never seen a Dyno cell lined up against the dynamic compression calculating laptop.

It’s always two savvy wrenches hammering out their best effort.

 

[fishmens67]

Don’t use cam timing to solve detonation problems

Yep, because detonation will always catch up and chances are it won't be heard.

 

[Illahe]

I think I have a decent understanding of how changing LSA impacts valve events and overlap. That's why I was surprised when a prominent cam grinder told me that overlap improves detonation resistance as opposed to widening the LSA. I just got off the phone with another company who said that's bullshit. Lot's of differing opinions out there, that's for sure.

Are you running deep gears or too tall of a set for your combination?

TT5.9Mag has a good point about looking at the advance curve in your combination.

Re detonation: my 360 has 10.5 cc dome pistons with 308 casting Iron heads and it’s backed with a 4spd with 3.91:1 pig. The cam is Kevin’s grind 242/242 .510 on a 110LSA. It runs well with pump 93.

FWIW: At the strip it will detonate on 93 when slicks hook side stepping the clutch. I didn’t hear it, but she blew a head gasket under load…

 

[middleagecrisis]

A lower numerically (e.g. 104) will increase cranking compression over a higher LSA (e.g. 114). Seems contrary to me, but that's what I've always heard and been my personal experience.

 

[Illahe]

Schneider Racing Cams . These are the people who developed all the purple shaft cam family and sold the profiles to Mopar. They got it going on.

A lower numerically (e.g. 104) will increase cranking compression over a higher LSA (e.g. 114). Seems contrary to me, but that's what I've always heard and been my personal experience.

I reached out to Kevin, @Schnider Cams, last month and inquired why the "snotty 106 lSA" in my 340 combination runs faster than the 110 LSA in my 360 combination.

"what is the effect on performance between the two LSA's of my combinations," he said; "that the wider LSA is effectively moving the exhaust lobe away from the intake for a more desirable 'valve timing event' in a given application..."

 

[TT5.9mag]

Yep, because detonation will always catch up and chances are it won't be heard.

And likely isn’t caused by bad cam timing.

 

[Newbomb Turk]

Schneider Racing Cams . These are the people who developed all the purple shaft cam family and sold the profiles to Mopar. They got it going on.

No, those lobes were developed by Racer Brown.

 

[Newbomb Turk]

A lower numerically (e.g. 104) will increase cranking compression over a higher LSA (e.g. 114). Seems contrary to me, but that's what I've always heard and been my personal experience.

Thats because cranking compression has ZERO and I mean Z E R O correlation to detonation. None.

 

[Illahe]

No, those lobes were developed by Racer Brown.

"If only" R.B. still carried the mushroom lifters...

 

[remytherat]

This question is ridiculous and unanswerable, because the LSA and overlap are not independent. End of story.

If you want to talk about valve timing, there are good questions

 

[MOPAROFFICIAL]

I think I have a decent understanding of how changing LSA impacts valve events and overlap. That's why I was surprised when a prominent cam grinder told me that overlap improves detonation resistance as opposed to widening the LSA. I just got off the phone with another company who said that's bullshit. Lot's of differing opinions out there, that's for sure.

Context

 

[mopowers]

I reached out to Kevin, @Schnider Cams, last month and inquired why the "snotty 106 lSA" in my 340 combination runs faster than the 110 LSA in my 360 combination.

"what is the effect on performance between the two LSA's of my combinations," he said; "that the wider LSA is effectively moving the exhaust lobe away from the intake for a more desirable 'valve timing event' in a given application..."

I talked to Kevin today actually. What a great guy with so much knowledge. He's the one who steered me toward a slightly wider LSA (less overlap) and later ICL and said the idea that narrowing the LSA to increase overlap to help with detonation resistance is a somewhat common misconception and complete bullshit.

Thats because cranking compression has ZERO and I mean Z E R O correlation to detonation. None.

Sorry to belabor the topic, but just to clarify... Are you saying that - All else being equal except for the LSA (which impacts cranking compression and ultimately cylinder pressure?), that widening or narrowing lobe separation angle has no impact on detonation resistance? If so, that's the third different thing I've heard (or in this case - read) today. Son of a *****.

 

[mopowers]

This question is ridiculous and unanswerable, because the LSA and overlap are not independent. End of story.

If you want to talk about valve timing, there are good questions

I'll get away from those terms (LSA, overlap) then. Which one of these sets of seat-to-seat valve events is more or less prone to detonation for a given compression ratio?

#1 intake/exhaust

#2 intake/exhaust

 

[Illahe]

I talked to Kevin today actually. What a great guy with so much knowledge. He's the one who steered me toward a slightly wider LSA (less overlap) and later ICL and said the idea that narrowing the LSA to increase overlap to help with detonation resistance is a somewhat common misconception and complete bullshit.


Sorry to belabor the topic, but just to clarify... Are you saying that - All else being equal except for the LSA (which impacts cranking compression and ultimately cylinder pressure?), that widening or narrowing lobe separation angle has no impact on detonation resistance? If so, that's the third different thing I've heard (or in this case - read) today. Son of a *****.

View attachment 1716489506

Every camshaft I’ve got from Schneider has performed well.

 

[Illahe]

I talked to Kevin today actually. What a great guy with so much knowledge. He's the one who steered me toward a slightly wider LSA (less overlap) and later ICL and said the idea that narrowing the LSA to increase overlap to help with detonation resistance is a somewhat common misconception and complete bullshit.


Sorry to belabor the topic, but just to clarify... Are you saying that - All else being equal except for the LSA (which impacts cranking compression and ultimately cylinder pressure?), that widening or narrowing lobe separation angle has no impact on detonation resistance? If so, that's the third different thing I've heard (or in this case - read) today. Son of a *****.

View attachment 1716489506

Sounds like you’re getting some resolution.

 

[fishmens67]

Thats because cranking compression has ZERO and I mean Z E R O correlation to detonation. None.

Zero correlation to hp.

 

[Newbomb Turk]

I talked to Kevin today actually. What a great guy with so much knowledge. He's the one who steered me toward a slightly wider LSA (less overlap) and later ICL and said the idea that narrowing the LSA to increase overlap to help with detonation resistance is a somewhat common misconception and complete bullshit.


Sorry to belabor the topic, but just to clarify... Are you saying that - All else being equal except for the LSA (which impacts cranking compression and ultimately cylinder pressure?), that widening or narrowing lobe separation angle has no impact on detonation resistance? If so, that's the third different thing I've heard (or in this case - read) today. Son of a *****.

View attachment 1716489506

Let me say it this way.

I’ve seen absolutely no correlation between cranking compression and detonation or more accurately detonation resistance.

I have seen engines bumping 155 on the gauge rattle like crazy and 200 not rattle at all. I haven’t checked my engine yet with a calculator (it’s sitting on the dyno as I type this, but I had a PAYING job come in so my junk comes off tomorrow so I can do that then mine goes back on and I suspect I will make 75-80 pulls minimum unless it craps itself) but I will here in a bit get on Wallace and see what mine should be, but it better be 195. 200-205 is where I think I want to be. Over 210 I may wet myself a bit.

What I see related to cranking compression is the higher it is the smaller the tune up window is.

That means that you have less margin for error. Things like your timing curve, engine coolant temperature, spark plug heat range and much less obvious things like your overall first gear ratio, rear axle ratio converter stall speed or clutch tune up and most importantly how the driver drive the car become critical.

So if the engine temp climbs 15 degrees (talking sticks here or manually shifted slush boxes) and the driver doesnt down shift and is running a gear high you will rattle the engine where it wouldn’t if the coolant temperature was where it should be.

Another example is spark plug heat range. If it’s a drag car the plug can be a bit hot (it can never be a glow plug but it’s criminal how many guys run a plug thats too hot) but a car that is on a road course you can’t be any too hot.

That’s why I call unorthodox pump gas compression ratio engines (I suppose that would be depending on who you ask anything over 10:1 but I consider anything over 11.5:1 on pump gas unorthodox) some of the most difficult to teach the end user how to use it. I’ve been teaching it for better than 35 years and guys still refuse to believe it can be done.

The short answer is I look at cranking compression as how wide or big the tune up window is. So I suppose in that sense cranking compression can be related to detonation sensitivity but as I said before, I’ve seen pretty low cranking compressions rattle bad.

I almost forgot. RING SEAL and OIL CONTROL is beyond critical in unorthodox pump gas compression ratio engines. If the engine is using a quart per 1k miles and you are hitting the gauge at 180 it’s going to be hard to keep it from rattling. At 195 you just about cant do it. You’ll be running plugs so cold they fouls at low loads and the timing will be so retarded the exhaust gas temperature are screaming and it will still rattle.

Oil control is absolutely critical if you want to bump more than 185 on the gauge. You really need at a minimum a fully functioning PVC system. If you can use a Panevac over that all the better. Or if you are a bucks up dude you can use a vacuum pump. But you have to control the oil getting to the cylinders.

To that end, if your machinist isnt using a profilometer to measure the source geometry of the bores get a new machinist. If he still thinks a ductile iron moly filled top ring is the ****, take your parts and run.

I’ll go run my numbers on Wallace and see what it says my junk should do and post it and then when I get my trash checked I’ll post that follow up here.

 

[RustyRatRod]

Detonation usually boils down to chamber shape, size, whether there are sharp edges that can cause hot spots, including the piston top, and also things like fuel mixture and not the least of which, engine temperatures.