Premium worth the Premium?

[ftpiercecracker1]

My machinst and i are debating on how high a compression ratio we want to run on the stroker. One thing that he brought up is that some vehicles get BETTER fuel mileage with higher octane fuels. My question to yall is. . .

How will my fuelmileage be effected by running or not running premium.
Basically what is the best combination of Compression ratio and Octane to OPTIMIZE fuel mileage and performance.

Obviously as CR goes up so to does the need for a higher octane to prevent pinging, but where does cost begin to out way performance gain?

His intial thought was to have a CR of something like 9.5, 9.6, or 9.7

[Cheromaniac]

All else being equal, an engine with a higher CR running on premium fuel will achieve slightly better fuel economy than a lower CR engine running on 87 octane. The emphasis is on the word "slightly", because the percentage increase might be too small to offset the higher cost of premium fuel.
Build your engine with a higher CR and premium fuel for performance, not fuel economy.

[ftpiercecracker1]

thanks, thats the same general idea i had as well. I just wanted to consult the "been there, done that" guys before i made a final decision.

Baring the cost of premium, how big of a difference would premium make?

Edit: why would anyone have a high CR when they could have the best of both worlds? IE. . .
Build your motor so that it would allow you to run 87 and just fill it with High test when you want that extra kick.

Just how much does compression ratio effect performance?

[Cheromaniac]

Edit: why would anyone have a high CR when they could have the best of both worlds? IE. . .
Build your motor so that it would allow you to run 87 and just fill it with High test when you want that extra kick.

Ah but therein lies the issue. If you build a low compression motor that'll run on 87 octane without pinging, filling up with slower-burning premium will actually hurt performance so it'll be counterproductive.

[W_A_Watson_II]

Yep, most people confuse Octane with the fuels power or hotness. It's all a big chemist experiment going on, and getting the reaction to "fire" just right is the key to power and efficiency (economy).

[Retlaw01XJ]

Just how much does compression ratio effect performance?

Here's a compression/HP calculator:
http://www.wallaceracing.com/hp-cr-chg.php

I plugged in 250 hp @ 9:1. Increasing CR to 10:1 results in 257 hp.... not a big deal.

Compression ratio should be matched to the cam being used. A bigger cam can tolerate more CR, since the intake valve closes later which affects the DYNAMIC compression ratio (DCR). Do a Google search on DCR if the term is new to you. There are some great articles out there on the subject.

[ftpiercecracker1]

I am aware of DYNAMIC compression ratio, but have very little knowledge of it. I have been told when compared to static compresssion, dynamic compression ratio has very little effect on the outcome.

Is there such a thing as an "ideal" compression ratio?
9.1 sounds like a good number to shoot for.

[5-90]

WAY too many factors!

Thumb rule - if the engine has detonation, up the AKI one step and re-check.

There is no "hard-and-fast" relationship between SCR and ONR or DCR and ONR, it doesn't happen. Things that also affect ONR:
- Quench clearance and volume
- Combustion chamber design
- Engine component materials
- Cooling system efficiency
- Cylinder exhaust scavenging
- Heat retention of valve heads (!)
- The "squish ring" where the head gasket bore is larger than the engine bore plays a part (heat is retained there)
- Finish of combustion chamber parts (all sharp edges should be "broken" slightly, the factory does not do this. A sharp edge concentrates heat, becoming a "hot spot" and leading to preignition. Edges may be readily "broken" using a 120-grit roll and a bit of hand pressure on a drill motor)
- And probably a few others that I'm not thinking of at the moment.

DCR is ALWAYS going to be lower than SCR. The SCR is the "geometric" compression ratio - the ratio in cylinder volume between the piston being at BDC and at TDC (you probably already know this.) The DCR is the "trigonometric" compression ratio - the ratio in cylinder volume WHEN THE INTAKE VALVE CLOSES and the piston at TDC. Here's a page that explains how to calculate it - http://cochise.uia.net/pkelley2/DynamicCR.html. (There were a number of calculators that came up, but I wanted to SHOW the math, not process it for you.) Essentially, you recalculate the stroke using the IVC event (and consider the upstroke of the piston before IVC to be "wasted motion," since compression only begins once the cylinder is sealed. Why have the intake open with the piston going up? "Inertial supercharging." Trust me for the moment, okeh? An engine will generally have greater output if you leave the intake open a bit after the piston starts going up than if you slap it shut at BDC...)

DCR plays a FAR greater role in ONR than SCR does.

Also, don't forget AFR and any supplemental cooling (H2O/MeOH injection, NO2 injection...) as a way to lower ONR (the latter up to a point.) Preignition is a function of heat in the cylinder when it is being filled - reducing heat (or reducing hot spots and evening out the heat distribution) is how you reduce ONR.

GLOSSARY:
AFR - Air/Fuel Ratio
AKI - Anti-Knock Index, "pump octane" number
BDC - Bottom Dead Centre
DCR - Dynamic Compression Ratio
IVC - Intake Valve Closing
ONR - Octane Number Requirement
SCR - Static Compression ratio
TDC - Top Dead Centre

(If you already know something I explained, ignore it. Figure I explained it for the next guy - who may not already know that!)

[ftpiercecracker1]

and again just for good measure

That must have taken quite a bit of effort to writeup , and you give me to much credit. While i do understand what TDC, DCR, SCR and a such mean, most of what you said i will have to read over many times before i can wrap my tiny brain around. However, i understand the logic behind the idea of "inertial supercharging".

I will be sure to read over what you wrote here several times so that i can fully absorb, contimplate, and respond accordingly.

i am Very tired. off to bed i go

[ftpiercecracker1]

reducing heat (or reducing hot spots and evening out the heat distribution) is how you reduce ONR.

If i am to understand this correctly it would seem that if you ran a low low T-stat, you could IN THEORY run a higher CR. But that would probably cause problems elsewhere.

The two ways that i know of for decreasing DCR are increasing combustion chamber volume/physical size, and or improved cooling capacity. FWIW, the head and intake will both be recieving a port and polish. Hopefully this will help diminish my chances for hot spots.

For SCR it seems that in order to change this you much look at cam duration and lift.

If its not too much to ask, could you explain to me a little bit about cams and the terminology that goes with them?

thanks

[5-90]

I'll do what I can...

Changing your thermostat (lower op temp) has only a nominal effect on ONR or DCR effects - because it's probably a thirdhand correction (you're working way downstream here...) Ideally, you want to correct the primary problem when it happens - which means trying to remove/absorb heat from the combustion chamber DIRECTLY (scavenging, H2O/MeOH fog, ...)

By the time you're working through the cooling system, you're already looking at a temperature drop by about an order of magnitude (figure coolant jacket operating temperature being 180-215*F, but combustion "flash" temperatures are typically up around 1,800-2,200*F...) See why coolant efficiency isn't much of a help?

You could get rid of more heat by direct conduction/convection with improved airflow, since the capacity of that system for heat absorption is theoretically unlimited, and you don't have to worry about rejecting it somewhere else (liquid cooling systems are a two-stage process with a number of factors - the coolant must first ABSORB the heat from hot parts, then REJECT the head in the forward heat exchanger - "radiator." There are ways to improve the efficiency of the system - a surfactant to reduce surface tension of the cooling liquid being universal and probably most efficient anyhow - but it's still a third-stage cooling boost, by the time it gets there.)

It's a POTENTIALLY useful theory, but not very ACTUALLY useful. Still, every little bit can (and often do) help - I'd class this as being similar to rollerised rocker arms: useful for complementing other modifications, but of minimal use done on its own.

SCR is only changed through machine work (increasing cylinder bore) or parts replacement (cylinder head, piston, crankshaft.) The volume of the combustion chamber in the head, the volume of any dish or dome in the piston, and the stroke of the crankshaft are all players here. Also, changing the head gasket can have a minimal effect on compression ratio (changing the bore and/or compressed thickness of the gasket.) However, it's far more common to change a head gasket to become/increase "quench compliant/ce" than it is as an absolute performance mod.

DCR is strictly changed by changing the camshaft, by altering the time at which the intake valve closed (down near the bottom of the stroke.) Have you read the link I gave earlier? I'd suggest you do, because it explains the idea better than I could in short order. I'll wait...

Back now? The IVC even is the ONLY event that has any effect on DCR. Yes, DCR is still somewhat dependent on the "geometry" of the engine - but the DCR may be changed by changing JUST the camshaft, and changing the engine geometry WITHOUT changing the cam generally only has a nominal effect on DCR anyhow.

The reason DCR<SCR is simple - less of the stroke is used (engine geometry) to calculate the DCR. Read the link again, I'll be here...

Make any more sense now?

Aluminum parts can be used to increase CR, because aluminum has a higher heat transfer ability than iron (in fact, you MUST run a slightly higher CR with Al parts than with Fe, for that very reason.) This is part of what made the Buick V8-215 so reliable - it was 1960's production, but they used an Al block and Al heads (eliminated gasket scrub,) and Land Rover used it for over thirty years after buying the tooling from Buick. In fact, Rover refused to sell the tooling back - and the old "Series" LRs were pretty much the ONLY vehicles you could take off of the showroom floor and drive longwise through the Sahara.

(I wouldn't mind the idea of an aluminum head for our beloved AMC sixes, if they'd come out with a proper aluminum block to go with. I'm thinking dry-sleeve - I've worked on too many wet-sleeve engines to like them.)

There are a few key data for camshafts:
- IVO/IVC. Intake Valve Open/Close
- EVO/EVC - Exhaust Valve Open/Close
- Overlap - This is the time (in camshaft degrees) that BOTH valves are open
- ID/ED - Intake/Exhaust Duration. This is the time (camshaft degrees) that EACH valve is open. May either be measured nominally as "seat-to-seat" (actually 0.006" valve lift) or "duration @0.050"," which is the time the valve is open 0.050" or more (Crane came up with this in the 1970's for a more "apples to apples" comparison of camshafts.) Both numbers are valid, both are now standards.

The first four are timing events, governing WHEN something happens.
The last three are duration events, governing HOW LONG something happens.

Things are going to start getting sticky (bear in mind that I've been doing engine work for about 35 years, and studying engine theory for 25 years. No formal schooling, I sometimes get things wrong, but I've also got the math to understand it. If I do get something wrong, someone please point up AND EXPLAIN my mistake, so we can all learn from it - including me!)

When looking at airflow in relation to camshaft data, you're going to be looking at "area under the curve" as a relative indicator. The more area enclosed by a particular valve opening curve, the more power potential you have (the IC engine is little more than a self-powered air pump, it merely MAKES more power than it NEEDS to operate. This excess power is what moves you down the road.) This may be derived through direct measurement, or may be provided by the camshaft manufacturer. Granted, the camshaft is not the only potential bottleneck presented to airflow - the primary factor is port flow - but it's important.

Why is valve timing important? Let's look at the cycles of operation of a given cylinder (there aren't four, there are actually SEVEN.
- Intake. This is when the air is pushed into the engine (either with fuel mixed by the carburettor or mixed at the port by the fuel injector. If DFI takes hold, the fuel will be shot directly into the cylinder just before it is to be ignited - there are advantages to this.
- Inertial Fill. This is the period where the intake valve is open AFTER the piston is on its way back up. This is the period of "inertial supercharging" I mentioned before, and proper intake design (cf: "Helmholtz Tuning") can increase this. Yes, you lose some charge as the piston comes up, but you usually GAIN more than you lose, so it's advantageous.
- Compression. Takes place between the intake valve closing and TDC.
- Combustion/Power. The fuel is ignited, and the sudden increase in heat energy and cylinder pressure pushes the piston down.
- Blowdown. The exhaust valve opens before BDC, and the increased pressure begins to force combustion gasses out of the cylinder. This sets up motion of the exhaust gases to assist with the exhaust stage and with cylinder scavenging.
- Exhaust. Exhaust valve open, piston moving up to force combustion gasses out of the cylinder.
- Overlap. This is the period around TDC where BOTH valves are open. The incoming air charge helps to force exhaust gas out of the cylinder, and the incoming cool air helps transfer heat out of the cylinder (this is another point where DFI would have an advantage - as it stands now, you lose fuel during overlap, which is part of what makes the catalytic converter necessary.)

Then, the exhaust valve closes, and the cycle starts anew. This should help to show why camshaft design isn't totally a science - it's also part dark art. (That's why no-one was allowed in Ed Winfield's shop - he'd hand over a cam by opening the door about six inches and handing the cam out vertically. He didn't want anyone to know what he was doing in there...)

Jumping back - "What's quench?"

Speaking simply, "quench" is the band around the perimeter of the cylinder that is opposed by the flat surfaces of the piston and head. The "quench band" extends inward from the perimeter of the cylinder bore (running between 10%-40% or so, usually, before the surface isn't flat anymore,) and the nominal clearance is typically 0.040"-0.060" with the piston at static TDC (this is usually about 0.005"-0.010" LESS during actual operation, due to the elasticity of metal under stress. It decreases more as crankshaft speed increases, due to increased stresses.)

Why is quench important? It causes an eversion of the charge gas before it gets ignited - which mixes it up and reduces hot spots (the hotter perimeter gas is forced to change places with the cooler gas at the centre of the charge, which evens out the overall heat in the system.) Improving quench compliance will reduce ONR - even on higher-compression engines (theoretically, a high-compression stroker would require 91 octane or better. I know guys running 4.7-4.8L strokers on 87AKI fuel, with NO PING, primarily due to quench compliance. They're running down around 0.040"-0.045" of quench, which does a more vigorous job of charge reversion.)

Gah. It wasn't any trouble to come up with, but my fingers are tired from typing it all out (it only took me 22 years to learn to type.) Read it over, absorb it, and let me know when you're ready for the next bit and what you want in it!