Higher compression?

[lukecp]

I'm thinking of swapping a set of Vortec heads onto my truck, if i can afford them. With the pistons i have in it now, the switch to 64cc combustion chambers would give me about 9.7:1 compression. What octane gas would i have to run with that high of compression? Are there any negative side-effects of bumping my compression up that much (i have about 8.5:1 now).

[c-10 racer]

you should be able to run ok on 93 octane gas at 9.7 to one as long as you ignition timing isn't radically advanced or something. keep in mind that your starter may complain a bit going to the higher compression, you may want to upgrade it.

[slam33]

Don't see a problem with it, run the high octane fuel to be safe.

[lukecp]

Well...the highest octane gas i can get is 91 where i live...would 91 and a bottle of octane boost cut it? I think i read somewhere that Vortecs would survive just fine on 87 octane with 9:1 comp...
I really dont want to swap out my pistons for lower compression

[Longhorn Man]

You should be able to get 92 there...about teh only places not required to have it is in the upper elevations. (Denver)
An intertesting tidbit, Tx requires the gas stations to go up a notch...94 is widely available.

[lukecp]

I dont think i've seen 92 for sale in my town...most places only carry 87/89. I will go double check tomorrow, there are a couple stations i don't go to (the expensive ones, lol)

[Suicide-D]

I have that high of compression and yes I made the mistake of trying to run the medium grade.... boy did I regret that tank of gas I dumped two bottles of 104 in it and still pinged my a@@ off..(34 gallon tank)

[Low69CST]

How good do you think your rings are?? Your not going up to a very high compression, but you are increasing it quite a bit. If your rings are weak you'll be blowing smoke.

[jku72]

I realize I am bucking the trend, but I run 10.5 compression ratio. I love it, but I do not "hotrod" either. If you stay out of the throttle, and set the timing by ear, you can get a pretty fuel efficient engine. I have a regular 20 dollar NAPA starter, and it turns it just fine. I am such a cheapskate, I only buy the cheap gas, but by setting the timing to "just start" to knock under full throttle, the truck runs great. I index the spark plugs, and runa slightly cooler thermostat, a 175 F.
As a result of this, I get 19 mpg at 65 MPH, out of a 350 with a 4 speed, and 3.07 rear. I feel there is a lot of room for improvement, as I still have a single exhaust, with a carlike muffler.
Higher compression is okay, but you have to tune for it.

[Randy70C-10]

You may have to back off the timing a little, but you should be OK with 91 octane. I like to run 94 in all my V8's.

[71402BB]

I would have to agree with jku72 on this one. I'm having my 402 built so that I end up with 10.5:1 and was told by the machinist thats doing the work that I would be able to run on 87 octane.....

[mikep]

10.5:1 compression and 87 octane ? Your machinist is not being truthfull. Maybe if you are running 20 degtrees total advance you could get by with it. Otherwise you'll be pinging all over the place. 20 degrees and it will run pretty poorly. Better with 9:1 and 35 degrees. There are no "tricks" that will do the trick no matter what your machinist says.

[Longhorn Man]

I think your machinist has been near the solvent tank too long.
Only way youy could do that would be with alluminum heads and multi port fuel injection.
There is a chance of street gas and a cam with lots of overlap surviving at about 10:1...but I would highly doubt you'll be at the local gas station much.
AV gas ain't cheap either.

[shortbed70]

I think youll be ok but if your cam is bigger than stock be carefull because a motor with a stock cam can run fine and then put a bigger cam in and you drive ritty chitty bang bang. It's all about timing, tuning, and how you drive with what you got.

[lukecp]

I forgot to check on the gas....working 12 hrs a day dosen't give you a lot of free time .

Well, my rings SHOULD be fine, the motor has about 12xxx miles on a complete rebuild, with Sealed Power rings, and it dosen't smoke. Also...my cam has quite a bit of duration....270/280 degrees of it. I think i heard somewhere that a cam with alot of duration will help bleed off some excess compression. So, the big question: Would the upgrade to Vortec heads be good for my daily driver? Will i see any difference in gas mileage if i switch over? Will the jump in compression give my motor a shorter lifespan?
BTW, im hoping to get about 330 HP out of my engine. I have 290 Desktop-Dynoed horses out of it now, and 355ft. lbs. Think the Vortecs will give me that?

[lukecp]

ttt

[Randy70C-10]

The vortec heads should flow better, giving you more power. You'll have to use an intake that will bolt up to vortec heads.

[Low69CST]

Don't vortec heads use centerbolt Valve covers?

[Randy70C-10]

Yes, they also use an intake that bolts "straight down".

[Longhorn Man]

Don't forget the guided rockers too.

[jku72]

An interesting article to read is an old Hot Rod Magazine article called "The 350 Chevrolet should have built". They built a 350 c.i.d. engine that ran on 87 octane, and had somthing like 11:1 compression ratio. The major engine features include a 400 cid block, and a 327 crank, with ford 300 I-6 rods. I am thinking they were belting out 400 horsepower or whatever. The secret was long rods yielding longer dwell time and thus less tendency to detonate.
My point is, you CAN run high compression, if you plan for it.
I had a 67 pontiac 400 that broke itself up on 11:1 using 93 octane, but that is an example of bad planning. Of course, before it broke, I was getting 21 mpg out of a giant catalina.
The higher the compression ratio, the greater the engines efficientcy, and thus greater bang for for the buck. (why diesels are so efficient) The limit is of course detonation, but there are ways to avoid this. Trick ignition systems, such as those by "Jacobs" and others work well. Picking the right spark plugs makes a big difference. Aluminum heads are another way to reduce detonation.
I have discovered that there are a lot of secrets to power and mileage, but it is an avenue not many have persued. Try a set of Rhoads lifters, on a meduim power cam, gives you a dorky but efficient cam at low cruise rpm, but gives you the power when you are "on it". Variable ignition timing is the best yet.
My 68 suburban has 10.5:1, and does not rattle, and makes "good" power. (whatever that means) It is a rebuilt 1970 engine, with iron heads, from, I believe, a higher performance engine, the builder claimed "off a camaro", if that means anything. It has a Qjet, and an edelbrock performer, and the geeky original style exhaust. I also have an HEI ignition, with plug gaps set at .055. The timing was tuned to whatever the engine liked best. I advanced, then drove it, if it rattled hard, I retarded, and vice versa until I found the engine's "perfect" timing. I have not touched it in a year and a half. The engine seems otherwise stock.(?) I run 87 octane, and pull in decent mileage, about 19 on the highway, at 65 mph. To be fair, I almost never rev over 4,000 rpm, so you might have to make alterations if you are a drag racer, but IMHO, daily driving is happy with 87 octane.

[MikeB]

Static compression ratio is only part of the story. Other factors include the intake lobe closing point. Performance cams have more duration, so the intake valve closes later during the compression stroke. This is why the radical Chevy cams in the 60s required 11:1 compression. With only 9:1 static compresion, the cylinder pressure would have been very low. A stock cam closes the valve a lot earlier on the compression stroke, so you get too much cylinder ressure with high static compression, leading to heat and detonation. Another factor is quench height (deck height + head gasket thickensss), but that's another story. If it's .045"- .050", you will have more efficient combustion and be less prone to detonation. Finally, combustion chamber efficiency plays a large role. The more swirl, the better the combustion and the less chance for detonation. Those Vortec heads have a very efficient chamber design, which should let you get by with 9.7:1 on regular or mid-grade gas. If not, do what c-10 racer says and play with the timing. For MikeP -- check this out: http://www.speedomotive.com/Building%20Tips.htm Go down a third of the way to the "Compression Ratio" article.

[lukecp]

Thanks alot for the help . I searched around town today, and found 91 at 4 stations, for 1.40-1.46. With a full tank of gas, it would be about $2 more than 87 octane, so i'm not worried about the extra expense. I found a package deal for the Vortec heads: http://www.desertperformance.com/heads.htm
Looks like it comes with everything i need except valve covers. Think the $800 price tag is about right?

[Danny69']

We've got a 1930 Ford Coupe that's running a 383 stroker, and where I live the gas has so much crap in it that you can't run high compression. We usually run aircraft fuel, and it runs good, very good. The only problem with it is that the headers get red-hot and the tips of the spark plugs burn off. Never a good thing.

[imdarren]

Check out this article on Gasoline and Compression. It is on the Idaho Corvette Page. It's good information. Really!
http://www.idavette.net/hib/fuel/index.htm

Darren