[boatanchors]

on a chevy small block - 383 what is the limit on compression when i have to stop using pump gas and go with the high dollar juice? i know some people might have different opinions im just looking for a rough number so i know my limit on this build.

[Cyberats]

I haven't seen any cars sold with higher than 11:1 so I would say it's the limit
baring improved engine materials, which are costly. Beyond that I doubt you get better performance out of higher octane even. But I don't know anything about race builds, there maybe different setups in those engines.

[boatanchors]

see, im building a street car and with the setup i have on mind ill have about 11:1 maybe a little less. eagle rotating assembly all forged 6" rods flat top pistons and a set of dart iron eagle platinums for the head 2.02 valve 64cc combustion chamber. that makes about 11:1 right there. i just dont want to run that octane fuel in a street truck. but who knows i might.

[F-BIRD'88]

10.5 or less. 91 octane gas = less. Get the 72cc chamber Dart heads for a flat top piston 383 on "pump gas".

[ap72]

13:1, however that is in a superfinely tuned and built engine.

[choirboy]

I agree with F-Bird, 10.5 comp. any higher and you will not always be satisfied. Also, he hit it right with the larger chamber heads... this actually reduces compression ratio. There are lots of articles out there on low compression pumpgas motors that are HOT.
Rick

[ap72]

If you're going with anything over 10:1 make sure you have cnc'd chambers. you can't afford any hot spots due to casting irregularities.

[randyhemi]

it all depends on the build. compression relate to not just pistons, but stroke and cc of heads. if you'r motor is 11 to 1 total with stroke, cc in head, and pistons you will be "ok" with 91 octane.but no higher. thats the max. it will run better with higher octane if its 11 to 1. so better to go with 10 3/4 to 1 total.

[boatanchors]

yeah i know about the larger chamber makes lower comp. and all that stuff i have racked my brain reading and asking questions and thinking. i just wasnt sure on the limit. thanks for the advice i appreciate it



adam

[coldknock]

Mechanical compression isn't the most important factor. Cylinder pressure is. Contrary to popular thinking, the two aren't connected in a linear fashion.

A larger camshaft with higher compression is better can be just as happy with 93 octane as an engine with a smaller cam and lower compression.

Dynamic compression, look it up.



Larry

[ap72]

Originally Posted by coldknock

Mechanical compression isn't the most important factor. Cylinder pressure is. Contrary to popular thinking, the two aren't connected in a linear fashion.

A larger camshaft with higher compression is better can be just as happy with 93 octane as an engine with a smaller cam and lower compression.

Dynamic compression, look it up.



Larry

Static compression is STILL an important factor. most people that learn about dynamic compression think that its the end all when in reality static is just as important if not more so. Sure dynamic compression give you an idea of how you can squeeze a little more out of an engine, but it has its limits.

[F-BIRD'88]

If that was actually true then racing motors with long durations cams would not require racing gas with a high octane. They do...so....

Its just not that simple. You're ignoring the effect of the "heat of compression".
If you compress a volume of gas x amount, it is heated up. Compress it more, it gets hotter. Once it exceeds the temp that allows self ignition all hell breaks loose. (detonation, preignition and piston holes isn't far behind) Its the net temperature of the charge during combustion that determines the auto ignition (spark knock limit) threshold more than the (cylinder) pressure. "Gas Laws" "heat of compression" Look it up.

[ap72]

Originally Posted by F-BIRD'88

If that was actually true then racing motors with long durations cams would not require racing gas with a high octane. They do...so....

Its just not that simple. You're ignoring the effect of the "heat of compression".
If you compress a volume of gas x amount, it is heated up. Compress it more, it gets hotter. Once it exceeds the temp that allows self ignition all hell breaks loose. (detonation, preignition and piston holes isn't far behind) Its the net temperature of the charge during combustion that determines the auto ignition (spark knock limit) threshold more than the (cylinder) pressure. "Gas Laws" "heat of compression" Look it up.

You're right about compression causing heat, and its simply stated in the gas law PV=nRT, or PV/T(1)=PV/T(2), but that kind of proves the dynamic compression theory. the actaul reason the dynamic theory doesn't work is actaully much simpler. dynamic compression is only effective where there is no intake or exhaust scavenging- at REALLY low engine speeds (idle or lower). As soon as you put these effects into the mix the cylinder pressure go up tremendously inspite of the extended cam timing. What extending the cam timing does do is push their effects higher in the RPM range where fuel atomization and distribution is much better. There i still a limit as to the cylinder pressure a fuel can take, but when you improve the delivery of the fuel you delay detonation, to a certain point.

A race engine with a high compression ratio and large cam can run to 2,000 RPM on 87 octane all day- because the cylinder pressure is still low- but as soon as the "cam comes on" (scavenging begins to take effect) it will rattle like a can of beans.

[coldknock]

You're not paying attention to what I said.

When you start playing with cam timing to lower cylinder pressure you have to pay attention to volumetric efficiency so that you can control the point at which detonation would theoretically occur. It's best to have this happen at a higher rpm than it would've otherwise. There's less real time for it to happen and therefore not happen. It's a careful balancing act.

Combustion efficiency is a factor, the longer it takes to complete a burn cycle the greater the chance of detonation occurring. Better efficiency = lower timing = more tolerance for higher cylinder pressures using a given fuel. This is especially important at lower engine speeds.

As for the comment alluding to competition engines......any competitive engine is running on the ragged edge of detonation, for maximum combustion efficiency. That almost always amounts to more power. So the argument is a moot point.

The maximum mechanical compression ratio for a given combination is directly tied to where maximum cylinder pressure occurs in the rpm range, combustion efficiency, volumetric efficiency, the type of fuel used and the application in which the engine will be used. It's not simply a number arbitrarily given from a text book based on another person's experience.

It's a lot more complicated that my simplified explanation, and your rebuttal. My point was that cylinder pressure can be manipulated so that higher static ratios can be used with lower than normal octane ratings. You just have to do your homework to make it happen.


Larry

[F-BIRD'88]

What extending the cam timing does do is push their effects higher in the RPM range where fuel atomization and distribution is much better.

But always at the expense of low end and mid range torque. Think of how you actually use a "street motor". You end up with a motor that is lazy thru most of the rpm band you want to use and then at high rpm (where cylinder pressure is now high) you cannot run optimum spark timing to avoid detonation.

You end up with a motor that has no torque, doesn;t accelerate the car or truck as well and is very heavy on fuel consumption and does not go as fast as it could.
You do not end up ahead of the game or even equal. You end up with a poor, inefficient engine design. It just doesn't work well in the real world.

The most common mistake that people do when designing a "performance motor" is over camming it.

but that kind of proves the dynamic compression theory.

No, it proves the real world limitation and over emphasis that people put on "dynamic compression theory and calc", in fooling them selves that they can compenstate for excessive mechanical compression ratio (with X fuel octane) with (over) extended valve timing. by picking a bigger cam than the motor actually needs.

Higher compression engines (that actually make real power) need higher fuel octane, than lower compression engines do.