[Steve C.]

Jim Hand did a technical article regarding the relationship between cylinder pressure, compression and detonation.

From it... "If you were waiting for me to tell you what CR is optimum but "safe" for you, neither I nor anyone on this planet can tell you what is safe."

The article was intended to provide some knowledge only to help select the optimum CR based on fundamentals.

And further he stated.... "Each case is different in that different quality gas is available, the vehicles are of different weight and have different transmissions and gears, different cams are used, the chambers are prepared differently, and so it goes."

Are today's LS motors capable of sustaining much higher compression on much lower octane fuel apples-to-apples with the design of a older Pontiac ?


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[JLMounce]

Originally Posted by Steve C.

Jim Hand did a technical article regarding the relationship between cylinder pressure, compression and detonation.

From it... "If you were waiting for me to tell you what CR is optimum but "safe" for you, neither I nor anyone on this planet can tell you what is safe."

The article was intended to provide some knowledge only to help select the optimum CR based on fundamentals.

And further he stated.... "Each case is different in that different quality gas is available, the vehicles are of different weight and have different transmissions and gears, different cams are used, the chambers are prepared differently, and so it goes."

Are today's LS motors capable of sustaining much higher compression on much lower octane fuel apples-to-apples with the design of a older Pontiac ?


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Yes and no. The standard LS3 found in your garden variety corvette is advertised at 10.7:1 compression. That's on an all aluminum engine with modern heads. The engine management system is also helping here with the knock sensors, so while the engine does live on pump gas, there may be times that timing is being pulled for various reasons.

If we equate that back to the Pontiac engine, people can and do run those types of compression ratios with the modern head offerings. Some have claimed to be able to do so using factory style iron heads.

The big advantage is in that engine management system. Most of the Pontiac engines found in this section of the forum don't have the ability to pull timing based on a knock sensor. Many (as evidence by the ongoing threads here and in the heating/cooling section) struggle to keep engines cool to begin with. And so it's simply easier for that LS to cope with modern fuels with higher compression.

In the LT line of engines that no employs variable cam timing, you've got the added benefit of being able to retard or advance the cam to build or relieve cylinder pressure to work with higher static compression ratios.

If you look at the Ford Coyote motor as well as many other modern engines, they've also employed piston oil squirters to help cool the pistons in order to cope with higher compression and available pump gas. In the case of the coyote motor it runs an advertised 11:1 compression, to which people also routinely add 10-12 pounds of boost to.

What that tells me at least is that heat management is key. Get the heat out of the cylinder and off the piston. We also know burning the charge as quickly as possible helps.

We already have much of what's needed in the aftermarket to accomplish that. I'm sure an intuitive machinist could probably figure out an oil squirter in the block.

How much will a 455 with at 11:1 compression with a 300CFM head and a cam around 235* @50 make? How efficient would it be with a modern overdrive transmission, proper converter, proper tuning and modest gearing?

[HWYSTR455]

One thing on newer performance engine is that the engine management 'manages' the 'tune' in relation to perceived det by the sensors. Yes, they have higher SCRs, but that doesn't mean they run an aggressive 'tune' to take advantage of available performance. Most of the time, they don't. Some of that is for advertising, the number provided by manufacturers, and means very little in real life situations.

Thing with SCR isn't really that you make more power, but that they operate more efficiently at lower RPMs with higher SCR. That's the big advantage. And that you can use a more efficient cam profile. As an example of the effect of HP on increase of SCR, going from 10:1 to 11:1 gains you a meager 1.5% HP. On a 500HP engine, that 7.5hp. Is that worth the risk of dancing with det? Maybe, because the total gain of HP is only part of the story, but it still is a basis for calculating risk.

General rule of thumb for SCR when adding/using aluminum heads is there is a theoretical loss of SCR by 1 point. It's not a loss of SCR, technically it's a loss of thermal dynamics, you make up for it by going an additional point. So when you look at a new car's engine and say it has 10.7 SCR, that's comparable to an iron head engine of 9.7 (theoretically). So really, there is no 'running higher SCRs' in new engines.

I was planning my current build a few years ago, seriously looked at going the LS route, and wanted to. Couldn't afford to do it the way I wanted, so dropped back and punted with refreshing the engine that was in it (461) and added some stuff like port EFI, etc. Looked at crate LSs, and building one the way I wanted, both were too expensive for my blood. I would have done the LS specific ECU/ECM from Holley, Dominator LS I think it was called at the time, had trans control was the biggest reason.

VVT is cool, but it really limits you to OE programming, and increases the cost of the build in parts. Heat management is a good thing, but the cost of returns makes it difficult to justify in a 'hobby car' engine build.

Thermal coatings on piston tops & skirts, rods, and other areas is not cheap. And what will that provide you? A few HP yes, but better efficiency is the real benefit. To recoup the costs, you would have to drive the car like 50k miles probably.

Anyway, point is that it's a good thing to apply modern tech to 'hobby cars' up to a point, after that, it's really just a wow-factor, and throwing money away.

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