[68WarDog]

Recently I was involved in a conversation regarding overlap where the subject of putting a bigger cam in a high compression engine will help ward off detonation, this is a 455 iron headed 12.5:1 engine. I understand that a certain amount of overlap (>85*) will reduce idle and low speed vacuum and compression, BUT at some port the proper amount of octane is necessary to ward off detonation. What am I missing, and can someone explain how putting a big cam in a high compression engine will hold off detonation? I'm not understanding how this works. 😕

[johnta1]

The 1st thing is that 'overlap' means both valves are open at the same time.
(intake and exhaust)

This means the piston moving up is not compressing the mixture as well as it should.
(because some of the mixture is going out the exhaust)

At low RPM this causes the low vacuum and poor idle.
Most racing cams are for high RPM, where the time portion of the exhaust 'leaking' is small in comparison. The mixture in the chamber is also filled more with good fuel, and not burned exhaust.

There is probably whole lot more to it though than this opinion.

[68WarDog]

Originally Posted by johnta1

The 1st thing is that 'overlap' means both valves are open at the same time.
(intake and exhaust)

This means the piston moving up is not compressing the mixture as well as it should.
(because some of the mixture is going out the exhaust)

At low RPM this causes the low vacuum and poor idle.
Most racing cams are for high RPM, where the time portion of the exhaust 'leaking' is small in comparison. The mixture in the chamber is also filled more with good fuel, and not burned exhaust.

There is probably whole lot more to it though than this opinion.

So how does increase overlap make using a lower octane fuel doable in a high compression engine?

[Stan Weiss]

You do know that a bigger cam does not have to have more overlap. Think cranking compression. If a bigger has a later IVC it will reduce cranking compression.

Stan

[FrankieT/A]

Camshaft design is very complicated and a lot of people try to make rules as to what a cam does. You really can't put cam specs in a box per se. But to answer your question overlap is a contributor of changing dynamic compression which helps with detonation. The science of camshafts could take volumes.

[68WarDog]

Originally Posted by Stan Weiss

You do know that a bigger cam does not have to have more overlap. Think cranking compression. If a bigger has a later IVC it will reduce cranking compression.

Stan

Thanks Stan, what I'm trying to understand is how picking a cam with a lot (>80* )of overlap for a 12.5:1 iron headed engine can prevent detonation on pump gas . So my basic question is " is it possible to run 93 octane fuel in a 12.5:1 iron headed engine by picking a cam with a lot of overlap?

[Stan Weiss]

Originally Posted by 68WarDog

Thanks Stan, what I'm trying to understand is how picking a cam with a lot (>80* )of overlap for a 12.5:1 iron headed engine can prevent detonation on pump gas . So my basic question is " is it possible to run 93 octane fuel in a 12.5:1 iron headed engine by picking a cam with a lot of overlap?

Sorry I can not help you. I don't see overlap by its self doing anything to prevent detonation. I look at IVC myself.

Stan

[jonmachota78]

Originally Posted by Stan Weiss

Sorry I can not help you. I don't see overlap by its self doing anything to prevent detonation. I look at IVC myself.

Stan

I agree. Overlap is not even a variable in calculating dynamic CR.

[Steve C.]

Old material, but it might be of interest....

Dynamic Compression Ratio
(Will my engine run on pump gas?)

http://members.uia.net/pkelley2/DynamicCR.html

.

[Half-Inch Stud]

It's complicated, so i will make it dead simple: The EGR effect from ovelap provides the right-sized charge at low rpms for low-loads thru full load.

[PunchT37]

Just because you have a high overlap cam doesn’t mean you can run high compression on lower octane.
Once you “get up on the cam” it will detonate there with crap gas. Now you’re in the cam’s efficient zone.

I had a high overlap cam in a 455. This thing had 6 pounds of vac at 1000 rpm’s.
Going down the interstate at 70, the engine was turning 3000 rpm’s. The vacuum was 18 to almost 20. So, no **** gas in a high compression motor just because of a cam.

They can help some, but don’t get carried away.

[Dragncar]

Originally Posted by 68WarDog

Thanks Stan, what I'm trying to understand is how picking a cam with a lot (>80* )of overlap for a 12.5:1 iron headed engine can prevent detonation on pump gas . So my basic question is " is it possible to run 93 octane fuel in a 12.5:1 iron headed engine by picking a cam with a lot of overlap?

High overlap cams take advantage of the scavenging effect at high RPM. Poor low RPM vacuum . Also exhaust reversion at low RPM. All bad in a street engine, This is the Street Section.
Iron heads seldom mean high RPM unless big bucks are spent on them.
High overlap cams mean high stall converters or stalling at stop signs. Hard brake pedal.
Do not do it. It is a recipe for a dog engine you will not be happy with. A overheating engine. Potential grenade.
If you build a engine with a crutch, that leads to other crutches.
There are people who have done it. But they did not just throw a bunch of parts together, they knew what they were doing.
And a 12.5 CR engine with iron heads on 93 will always be on the edge IMO.
You can build a 9.5 CR 455 with enough power to roast the tires for blocks and run in the low 11s.

[25stevem]

Here’s a handy drawing that makes it easy to see how more intake duration changes the IVC and in turn both static and dynamic compression.

[Gach]

Originally Posted by 68WarDog

Recently I was involved in a conversation regarding overlap where the subject of putting a bigger cam in a high compression engine will help ward off detonation, this is a 455 iron headed 12.5:1 engine. I understand that a certain amount of overlap (>85*) will reduce idle and low speed vacuum and compression, BUT at some port the proper amount of octane is necessary to ward off detonation. What am I missing, and can someone explain how putting a big cam in a high compression engine will hold off detonation? I'm not understanding how this works. 😕

There is absolutely no way a 12.5 compression iron head motor will run on pump gas. Don’t care how much overlap you run. That thing will donate itself to death. Compression and rpm usually the determining factor. when it’s comes to how much overlap.

[Jay S]

Adding more overlap is not an asset, it will be working against you. Overlap makes the engine more efficient in it’s powerband, it needs to be less efficient to manage the lower octane. You have to make up for that loss in efficiency with really good head flow or it won’t run well anywhere.

[Cliff R]

Managing pump fuel at higher compression ratios isn't a simple matter of having a camshaft with more overlap.

"Big" cams with more overlap more times than not are also ground on tight LSA's with the ICL advanced for an early intake closing point. If they left those "big" cams on wide LSA's they wouldn't idle like chit, stink so much out the exhaust, impress all your friends with the "menacing" idle quality, and lower engine vacuum at idle speed nearly as much.

You also reach a point of deminishing returns with these things. At some point, based on CID, compression, head flow, etc, you've simply got to start pulling a long duration cam down on a tighter LSA or it will push the power range beyond the capability of the engine parts you've chosen. These big CID long stroke builds aren't very good high RPM designs right to start with, and the heads used on them don't have much more cross section than the average high performance SBC head. So they will like larger cams right to start with.

I've pushed to envelope pretty hard for managing pump gas at higher compression ratios using both iron and aluminum heads.

The "recipe" for success goes well beyond just chosing the right cam for it. Tight quench really helps, plus efficient combustion chamber shapes. You also have to be able to keep the engine from running really hot, or that quickly throws a big monkey wrench right into the middle of your deal. The engine may be OK in the Winter and cooler/cold months but completely IMPOSSIBLE to operate when things really heat up outside. So heat works against you with this sort of thing. You are already in some trouble using Pontaic "open" chamber heads as they have flat chamber floors and not very efficient compared to "modern" designs.

Tuning for higher compression also has to be spot-on, with FULL control of the timing and fuel curves.

Even with all that said a true 12.5 to 1 compression ratio iron headed 455 would be a considerable challenge to set up for low octane pump gas. You'd have to cam it big enough to lower the cranking compression to about 170-185psi and push the LSA out wide enough it pushed peak torque up past 4000rpm's or so, with a very "flat" torque curve.

I didn't sit down and do all the "math" on it but the cam would need to be up near 250 @ .050 and 300 degrees or so @ .006". Might get away with 112LSA with the ICL at 109-110. For sure it would be quite an adventure. I wouldn't be afraid of it, but you are out in territory that very few venture into.

I'd add here that even though you may see some really high compression "pump gas" engines done for Dyno competitions those engines are very carefully built and cammed for making BIG power on dyno runs. They can "kill" the compression at lower RPM's when the events are happenng few times per second and the engine is good at cylinder filling, then fill the cylinders well in the upper mid-range and top end for optimum power without a lot of octane required. Getting one of those to survive a steady diet of pump gas between the fenders of one of these vehicles would be difficult, if not near impossible.......IMHO.........

[PunchT37]

Originally Posted by Cliff R

Managing pump fuel at higher compression ratios isn't a simple matter of having a camshaft with more overlap.

I wouldn't be afraid of it, but you are out in territory that very few venture into.

I did. And it taught me not to go there again on any street engine.

[Stan Weiss]

Bore = 4.21"
Stroke = 4.21"
Rod - 6.625"
CR = 12.5:1

300 Intake duration ICl 110 - 223 PSI Cranking Compression

320 Intake duration ICL 112 - 182 PSI Cranking Compression

Stan

[Skip Fix]

Most big cams and cams with more overlap close the intake valve later so it does not build as much pressure in the cylinder esp at lower rpm. Helps detonation that way like a lower CR.

Also if the exhaust system is not scavenging well it can also increase reversion of exhaust back into cylinder and even intake tract.

[Cliff R]

11.3 to 1 CR, 4.21 bore, 4.21 stroke, rod 6.625, 289 @ .006" cam with 236" @ .050" with the ICL at 109.5 is right at 200psi cranking pressure.....