GearGrinder

iTrader: (1)

I am building my 6.0L for a 1970 Vista Cruiser. With the combination of parts I'll basically have a choice of 4 compression ratios. The engine started life as an LQ4, but I'm torn on whether to put dished pistons back in or go with flat-tops.

I did a quick and sloppy calculation on the CRs. I'll measure everything and be a little more exact once I get some free time at work.

It will be bore .030 making it a 370 or 6.1L

So here's my choices:

9.775:1 with dished pistons, and un-cut LS3 heads
10.20:1 with the heads milled down to 64CC, if possible
10.60:1 with stock 68cc heads and flat top pistons
11.12:1 with milled heads and flat tops

This car is not a drag racer. It's an 18ft. long station wagon. I plan to drive it everywhere, so maybe I shouldn't be forced to put premium in it, but I also hate giving up HP...

I am using stock style replacement pistons and the only other factor is that the flat-tops have the nice skirt and top coatings and the thinner ring package.

so for a daily driver, road-trip wagon, would you take the HP or the discount gas?

+badss+

iTrader: (12)

if not racing id shoot for 10.20 and run 87 oct.

IRONFIST

I'd probably go with something with higher compression. Even if you don't like the higher horsepower that comes with increased compression you should keep in mind that the higher efficiency created with higher compression is going to give you better fuel mileage. In the end you won't be saving much.

MikeG

iTrader: (33)

I would think you would want all the torque you could get. I'd go high compression.

NC01TA

Here's another consideration. Can you get 'no ethanol' gas in your area? I have learned that most places that do offer 'no ethanol' gas usually have it in premium only, sometimes the mid grade. I get an easy 2-3 mpg extra with the 'no ethanol' 93 plus you eliminate all the ethanol problems and 'sitting' issues which involve condensation in the fuel tanks.
If you don't know, here is the site to check: www.pure-gas.org

GearGrinder

iTrader: (1)

Good point, I didn't even think about that. Maybe I am obsessing over it too much, I know it will get better gas mileage than the 455/TH400 that was in there haha.

I need to do a search, but I may be restricted by valve to piston clearance when using a flat top w/ no relieves AND the large LS3 valves.

RPM WS6

iTrader: (3)

Are these dynamic or static ratios? Depending on cam size, you might be able to do just fine at 10.20:1 on 87 octane, if that's a static number.

This is true, ethanol-less gas will most certainly provide better gas mileage. However, if you are going to drive the car often as you stated, I wouldn't worry about the "sitting/condensation" issues. If the car does sit, you can still prevent these issues with the use of Stabil. In my area, we don't have any gas stations that sell anything other than E10 (or E85), so we just have to make do. With regular use of Stabil, I've never had any corrosion or fuel system issues in my limited use, ethanol-fed cars ever.

71novaguy

iTrader: (2)

he beat me to it. depending on the cam, you can get away with higher compression with lower octane

redtan

iTrader: (5)

Why on earth would you want that low of a compression ratio (even on a non racer) is beyond me. Below 10:1 on a naturally aspirated car will be a dog and for what, to save 30 cents a gallon?

Plus with a higher compression and more torque all over, you will get better gas mileage than a low compression engine. Thus the difference between low mpg/regular gas and high mpg/premium gas will be even less than 30 cents a gallon which is less than 4 whole dollars a fillup...

I would go with somerthing right along 11:1 and use premium. A few extra bucks every fillup is well worth the torque and driveability of a high compression engine.

GearGrinder

iTrader: (1)

Well my thinking was, the stock 6.0L LQ4 had 9.4:1 CR,It was not a dog, as far as trucks go and again, I'm building a station wagon. I can get those pistons, LQ9 pistons and even flattops with a relief for a reasonable price. So I have some options. Although it is a moot point because I was already leaning towards higher compression and that's what i'm going to go with. With my truck it would be more like $10 a fillup and that's at least once a week as much as I drive.. it adds up.


We're trying out some stuff on my LS3 heads today at work. probably will only get a baseline for flow testing, but I may get around to CCing them as well and determine how much I want to mill off.

redtan

iTrader: (5)

Remember that a 10.9:1 comp ratio LS2 in the GTO and corvette is rated for a minimum of 91 octane. With say a low-mid 10:1 comp ratio and a good tune you should be ok with even 89 or so, which is not much more expensive than 87. The mpg benefit of higher compression will most definitely cover the few cents extra a gallon.

I would not go less than 10 that's for sure.