Compression on pump gas

[lukecp]

I am still thinking of adding some new heads on the rebuilt 350 in my truck this summer. It was a stock rebuild, with a RV cam and cast flattop pistons. So, it should have around 8.5:1 compression right now. I have narrowed down the head choices to either a set of World Products S/R Tourqer heads, or a set of Vortec heads. The S/R Torquer heads have 67cc combustion chambers, and doing a little math, that would give me around 9.5:1 compression. The Vortecs have 64cc chambers, which would put me in the 10:1 range. So, could i run the 9.5:1 engine on 89 octane gas? And would the 10:1 engine require anything higher than 91 octane? I have a RV cam, a 270/280 cam, so would that help bleed off a little extra compression? Would my stock pistons be able to handle that much compression? The motor was rebuilt about 14k ago. According to Desktop Dyno 2000, i will be making in the neighborhood of 350 HP and 410 TQ with either set of heads.

[Hooter]

If you retard the timing you could run on 89 octane gas. But your power will be pooey. I have 10.5:1 on my 454 and I can't run anything under 94 octane. Luckily chevron has 94 octane.

[crazy longhorn]

I think you might need a little more cam with the comp bump, to stay with midgrade pump fuel. I set up a buds Nova ,355 flattop pistons & 492 straight plug heads(z28/LT1). he is right in the 9.5 range, & runs a Crane energizer 278 (.467 lift, & 222 dur @ .050). Being an old "poor boy", he runs 89-90 octain fuel & doesnt have any problems. that cam would need a looser than stock converter, & do better with 3.40- 3.73 gears. good luck.....crazy AL

[Longhorn Man]

Bummer Chevron started the whole car crushing thing to keep from fixing there own polution problems though huh?
I refuse to buy from them.

[lukecp]

Well, i guess that rules out the Vortecs. I am looking to spend less than a grand on my truck, and get it into the 330-350 HP range. I really dont want to go through swapping camshafts. The truck is used for highway trips every now and then, but next year i am gonna have to make a 160 mile round trip with it every 2 or 3 weeks. So, the 3.07 gears have got to stay. Would the S/R Torquer heads with the 76cc chambers be a better option? What is everyone's opion of the S/R Torquer heads?

[bigjimzlll]

the vortecs effeicent chamber design allow you to run higher C/R than a standard design..If your stock pistons if true fllat tops with 5cc valve reliefs will give you 10.2-1 with 64cc heads(and a proper quench of .045)

[crazy longhorn]

Check out this link on dyno combos, it should give an idea on what you need for the power levels you are looking for. There are a few examples of the sr torker heads on 355s, & comp ratios are listed.... http://www.ryanscarpage.50megs.com/ all of that hp search gets to be expensive, & I think if your 355 runs good, a little gearing will get you some "seat of the pants". I have run a mid 3s gear in a couple trucks, & feel that its the best of both worlds.......more power down low,& not a big dent in milage. The 3.40 gears I had in the longhorn would let me cruise @72-73 mph @ 3000 rpms........just a thought

[tom hand]

I am lost????? Why would you try to build a high output motor to run on crappy gas. To me anything over 9 to 1 compression with iron heads is pushing it on 92 octain unless you are kick a$$ at tuning.

[bigjimzlll]

Tom has a point...but if you want a set HP goal, sometimes you have to run a higher C/R, so the cam and heads will work..anything over 220@.050 duration will require around 10-1 to run well..its that dam dynamic C/R(cylinder pressure) that really matters

[mikep]

Theory and reality once again clash...

I run S/R Torquers with 76 cc chambers. The flow numbers on those heads are roughly equivalent to the vortecs and I couldnt be happier with their performance. Its within 5% on all the numbers at just about all the valve lifts. I wanted to keep the higher compression so I am running .125 " domes hypereutectics. I also run a lot of timing. Upwards of 38 degrees . With 87 octane gas I ping. With 89 octane gas I dont ping unless I am really lugging the motor. With 92 octane I never ping. If I decrease total timing to 36 degrees I can run 89 with no pinging ever. I run a very mild cam and with that setup and my heads I get around 400 ft lbs of torque and 340 HP according to the desktop dyno.

[lukecp]

I dont think that 330-350 HP is too high output, i just wanted some opinions on if i could get that much power on 91 or less octane. I might look into some 3.42ish gears instead, or the 76cc S/R torquer heads. Idealy, the truck would run high 14's, or low 15's at the track, and still get 12mpg, but i may be setting my sights a little high. Thanks for all the help!

[mikep]

with 3.54 gearing and those heads and basically the same cam you are running now and a Edelbrock eps intake and 750 edelbrock I get 10 mpg pretty consistently . Thats with the way I drive which is sideways as often as possible. Your mileage would probably be higher. Its a good torquey combo and certainly has better performance than the 396 that was in the truck when I got it.
Even with the 9.5:1 compression I dont have any problem using 89 octane as long as I keep the timing under 36 degrees total which is probably a bit on the high side for what those heads like anyway.

[crazy longhorn]

Another area to look at is quench. if that block hasnt been decked, the pistons can be .030-.035 in to hole, + the thickness of the gasket. As the compression goes up, & the quench is over .055-.060, you will find detonation problems that you didnt see in an 8.5 or 9 to1 motor.........

[lukecp]

The front casting numbers on the engine, the one with the letters in them, i still visibible infront of the passenger side head on my truck. If the motor was decked, they would have been shaved off, right?

[Beast388]

I have the "67"cc S/R Torquers on my 388. In actuality, they cc'd at 72cc's. So obviously they are a little bigger than 67cc's. These heads are great, I am very happy with them and I've had them for about 10 years.

I was very religious about calculating and determining what my comp. ratio would be. I had the block decked to that the pistions were 0.004" in the hole. I used a Fel-Pro head gasket with a compressed thickness of 0.041" so that my quench distance was the magical 0.045". I used a 12cc dished KB piston and with the measured 72cc heads, my compression ratio is 9.46:1. Keep in mind that this is with a bore of 4.06" and a stroke of 3.75". I run 10 degrees initial and 36 degrees total timing. I usually put nothing but 93 octane or the occasional 91 octane gas in her. I have never heard pinging whatsoever.

With valve lift of 0.477I/0.493E and duration @ 0.050" of 210/214, my engine makes PLENTY of power. I think staying concervate with the cam and compression ratio made for a good combination.

IIRC.....Desktop Dyno rated my combo at 350 hp and 425 ft/lbs of torque. So yes, you can get 330-350 hp on 91-93 octane.

[Beast388]

Yes....usually if the block has been decked, then the stamped numbers are gone. Unless someone has put them back, but I have never heard of someone doing that unless they were doing a full numbers matching resto. The original stamped numbers are gone off of my block.