Driveability of high compression cars

[Jacsey 70Z]

We have a pump at the local Sunoco station that sell the Turbo Blue 110 leaded, and as long as I can get it I'll use it. The car run great and cool and the performance is well worth the cost, as long as it's a weekend car and not a driver. At 11:1 compression I hate to cut the octane very much at all, Oh Yeah the smell is almost worth the price alone.

[@wot]

Raising the octane from 93 to 102 is very difficult to do with boosters or other additives. The problem is simply volume. 16 or 32 ounces of additive can't chemical change 18 gallons of fuel. Advertising claims are designed to make you buy, but the reality is you'll need 5-10 gallons of race fuel to start raising 93 octane fuel. A couple gallons of toulene is also very effective if you want to expirement.

[budnate]

[ QUOTE ]
Raising the octane from 93 to 102 is very difficult to do with boosters or other additives. The problem is simply volume. 16 or 32 ounces of additive can't chemical change 18 gallons of fuel. Advertising claims are designed to make you buy, but the reality is you'll need 5-10 gallons of race fuel to start raising 93 octane fuel. A couple gallons of toulene is also very effective if you want to expirement.

[/ QUOTE ]

I have heard/read that as well if you really have a 11:1 motor the little bottles dont work enough to get by on..I did the 50/50 deal with prem unleaded and 110 that the Union 76 guy had in a pump off to the side, when he quit I ran the same mix but with Av gas...both seemed to work fine and never heard a ping once...and with drag boats there either off or the pedal is on the floor and running hard across the lake.

[Belair62]

Toluene is nasty stuff !!

[Lynn]

[ QUOTE ]
Toluene is nasty stuff !!

[/ QUOTE ]

Anyone actually use Toluene? If so, what kind of ratio? I can still buy a 5 gallon can at the local paint store. Don't remember the price, but last time I checked seemed very reasonable. I was even thinking of putting it in my old BMW turbo as I have modified the boost levels and thought I might squeeze a few more grins out of it.

When I build my engine for my 69 Z, I plan on having the pistons cut down slightly to lower the compression and cc the heads just a bit bigger, shooting for 10.25:1 compression. Still figure I will need some help on the octane and was planning on about 10% toluene.

Just for the sake of discussion, here is a guy that really likes the stuff.

http://www.elektro.com/~audi/audi/toluene.html

As for using thicker head gaskets, that is a huge losing propostion in my opinion. Anything that kills your quench will only make detonation worse, especially below 3,000 RPM, even though compression will be slightly down. Yeah, I would like to say I was a genius and figured this out by just looking at the physics of the situation. Truth is, about 25 years ago I had a 302 with original 11:1 compression that was pinging ever so slightly. I put on double headgaskets, and it was 10 times worse. Did some research and discovered how critical the quench is to combat pinging. So when building from scratch, go for the lowest piston to head clearance the piston manufacturer recommends.

Just my opinion.

Lynn

[Enoch]

Can some of you engine builders answer a question for me? Reading this post made me think of a statement a friend of mine told me (he has been building motors for 30 years). He said that you could reduce or eliminate detonation by making custom pistons that will give you "zero deck height". Meaning that the piston at it's peak of rotation is perfectly flush with the deck of the block.
What I'd like to know is this, have any of you heard about this and why would it help?
Thanks, Rich.

[Tenney]

http://www.hiperfuels.com/index.cgi?Page...ndedDepts=81048

[Lynn]

[ QUOTE ]
Can some of you engine builders answer a question for me? Reading this post made me think of a statement a friend of mine told me (he has been building motors for 30 years). He said that you could reduce or eliminate detonation by making custom pistons that will give you "zero deck height". Meaning that the piston at it's peak of rotation is perfectly flush with the deck of the block.
What I'd like to know is this, have any of you heard about this and why would it help?
Thanks, Rich.

[/ QUOTE ]

OK Rich, I'll take a stab at this one.
First, I know there are much more knowledgable engine builders out there. I am a (substitute derogatory adjective of your choice) lawyer by trade, but still a mechanic at heart. Worked as a mechanic at several independent shops and a GM dealership, then owned my own shop for a few years before going to law school.

Your friend is on the right track, although, proper quench can't cure detonation all by itself if the static compression is high enough. If it would, car makers would be putting out 14:1 compression engines.

First, it is important to understand detonation. While it has been studied for decades (Gulf introduced lead in the 30's, calling it "nonox" gas to deal with those 7:1 "high compression" engines of the day) the exact mechanics of normal gasoline burn and detonation are not fully understood. It just happens too fast.

1. "Normal" combustion.
After ignition, the burn progresses at a speed of some hundreds of feet per second, delivering power smoothly and relatively slowly. This "slow" burn pushes the piston down the bore creating the smooth power delivery we all want.

2. Detonation
Detonation occurs when, during the "slow" burn process, the unburned portion of the air fuel mixture is compressed by the expanding gases and warmed by radiation. When the pressure and temperature in the unburned portion pass a critical point, detnonation commences and progresses at a rate of some MILES per second (instead of the hundreds of FEET per second of the "slow" burn). Boom! The heat energy is released much more rapidly, and its conversiion to mechanical energy is less efficient than when "slow" burning occurs. Even mild detonation causes some damage and loss of power. More severe detonation can destroy an engine. That peculiar sound we call knock or ping is actually the cylinder walls ringing. You may as well hit the top of the piston with a hammer.

3. Contributing causes.

High compression. Engines with high compression are inherently more susceptible to detonation because the "slow" burn gases don't have to expand as far to put undue pressure on the unburned portion of the mixture.

Carbon build up. Not just because it obviously raises compression, but usually forms little peaks that get so hot they contribute to the radiant heat that pre-ignites the unburned mixture.

"Dead" cylinder head and piston design, meaning not enough turbulence in the combustion chamber.

Crappy gas, or low octane gas. The lower the octane, the less resistant to detonation. Hey, isn't that what started this thread?

4. Ways to combat detonation. Yeah, I am getting there.

Higher octane. Been discussed.

Make sure chambers and piston tops are clean.

Lower compression. Hurts performance.

Retard timing. Really hurts performance. And mileage.

Move spark closer to center of combustion chamber. Not much of an option on vintage cars.

Aluminum heads. Dissipate heat from the combustion chamber lowering the radiation factor and reduces the chance of detonation.

Increase turbulence. Aha!!! This is where quench comes in to play. Of course, combustion chamber science has progressed incredibly in the last 35 years. Notice all the great heads have heart shaped chambers? Look at the chambers on the 186 head or the 461 head (virtually identical). While they were great for their day, the straight line delineating the quench area doesn't create near the turbulence as the newer heads. However, that doesn't mean it can't create greater turbulence. By decreasing the clearance from piston to head, it creates greater turbulence.

Vintage small block Chevys have a factory deck height of 9.025", assuming you actually have a square block, which you don't. But on the average, that is the deck height from crank centerline. The top of the flat part of the piston (not including any dome, dish or reliefs) is 9", meaning the piston at TDC, is down the hole .025". Head gasket thickness varied, depending upon application, but even with a .030" thick (compressed) head gasket, that leaves a .055" piston to head clearance, or quench. Many of the Chevys got .040" head gaskets, meaning the quench clearance was .065".

Most guys when referring to "zero deck height" are talking about having the block surface decked to a true 9" height from crank centerline, which also makes the block perfectly square. This obviates the need for custom pistons. However, custom pistons accomplish the same thing, but only if the block is perfectly square to start with. Pretty rare. If the block is out of square, you could have a piston at one end a few thousanths below deck, and one at the other end a few thousanths above the deck. Hence the reason GM used a 9.025" deck height and a 9" piston height, to allow for tolerances and not have to worry about a piston hitting the head.

So, what is the "ideal" piston to valve clearance? Most manufactures warn against going less than .010" PER INCH OF BORE SIZE. That would be .040 on a 4 inch bore. Personally, if things are set up properly, I have no problem going .035" on a 4" bore. However, all pistons rock in the bore, and forged pistons rock even more when cold, because they take awhile to expand as the engine warms. That is why in my original post I stated you should go with the minimum clearance recommended by the piston manufacturer.

Sorry this is so long, but hope it is helpful.

Lynn

[@wot]

Most AV gas sold is 100ll (low lead) and will work in a high compression engine. I have not bought any in a while, but it is probably around $3.00 - $3.50 a gal. A relative bargain, considering racing fuel is over $6.00 per gallon. It is formulated to run at 10,000 feet of elevation, not sea level. So if you're going to keep usinf it, especially without mixing, you may want to re-jet the carb.

[Paul D]

Also, I've found that AV gas seems to have a much longer shelf life. It must have some "Stabil" like additives to keep those Piper Cubs from falling out of the sky after sitting for six months.
Paul