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How high of compression can I go?

B

Blazer_Boy

1/2 ton status
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Building the dream motor and trying to decide on a compression ratio. I'll be using the cast-iron L31 vortec heads and 91 octane gas.

<a target="_blank" href=http://www.geocities.com/bigkern76>http://www.geocities.com/bigkern76</a>
 
I'd say 9:1 would be about right. Possibly 9.5:1. I would stay at 9:1 for reliability

John

<font color=blue>"When all else fails, read the instructions, or get a bigger hammer"</font color=blue>
 
For a truck/offroad motor I wouldn't go above 9.5:1. I am no motor guru by any means, but that is what has been recommended to me. I would only go higher if you run strictly mud or sand, and you need high HP. Offroad motors are all about torque.

My Chevy isn't broken, its just out of gas.
 
Right now on the computer dyno, I'm putting out 335 h.p at 5150 RPM and 375 pounds of torque at 3,900 RPM.

<a target="_blank" href=http://www.geocities.com/bigkern76>http://www.geocities.com/bigkern76</a>
 
Personally I would try and get some more torque. But thats just me. My dream motor I will have 400+ lb/ft.

My Chevy isn't broken, its just out of gas.
 
I'd like a little more too, but don't what to do. Its staying a 355 so the 383 and the 400 are out.

-'89 350 one piece rear seal lock
-L31 cast iron vortec heads
-flat top hyperuetetic pistons
-edelbrock performer intake
-stainless steel header/manifold (just like a header only with a manifold shape)
2 1/2" exhaust that dumps into a high flow cat and then turns into duals again (can't find a shop to let me cheat)
comp cams 4x4 xtreme cam @ .050" 210 int duration 218 exhaust .447 int ift .462 exhaust lift w/ 1.5 ratio rockers
maybe an MSD 6al after i get out of the poorhouse

<a target="_blank" href=http://www.geocities.com/bigkern76>http://www.geocities.com/bigkern76</a>
 
Get a set of headers and true dual 2 1/4" exhaust (bolt on Dynomax kit) and I'd bet that'd help immensely. Ditch the cat. Cheaper without it.

As long as you don't have a visual inspection you can pass a sniffer test anyway. I helped my buddy's car pass over the weekend. No cats, no EGR, no smog pump, none of that nonsense, just an Edelbrock carb and intake and 1 5/8" headers and 2 1/4" duals with Flowmasters. It CAN be done.

Tim
'84 Chevy K10, lifted, loud, fast, and 3/4 ton axles
 
8:1 with iron heads will run on 87 octane
9:1 with iron heads will run on 89 octane
10:1 with iron heads will bearly run on 92 octane
these are no problemo figures that are MHO.
you can run more compression with less octane but then you really got to screw with timing. if you run aluminum heads you can add a half point to the above figures.
just my 2 sense and humble opinion

-Zakk
'77 K5 SB 400 TH350 NP203 p/t 6" Lift D60 14BFF 4.10's 36"Wrangler II's

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CK5's MOAB 2002 OR BUST!!
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I gotta keep my "headerfolds", they're cool $hit. Good flow with out all the header BS. 4-wheel and Off-road had them 101 small block upgrades. I live in town of about 85,000 so I don't have a wide abundance of exhaust shops. I'm trying to get my hands on a pipe bender so I can rig up a set of 2 1/2" pipes with flowmasters. It would be easy to run that setup because I had never heard of emissions testing until I started on this board, haha.

<a target="_blank" href=http://www.geocities.com/bigkern76>http://www.geocities.com/bigkern76</a>
 
i run a 10:1 350 with flat top pistons and iron heads and i MUST run 93 and set the timing around 4 deg after tdc or it pings like hell.
 
With Vortecs and flat tops, your compression is going to be well over 10:1. You should use dished pistons to keep it down to 9.5 or below.

Tim

70 Blazer CST 4X4 350 SM465 NP205
87 Burb 4X4 350
01 GMC 2500HD 4X4 Duramax/Allison
 
I honestly don't know how that post ended up in this thread. I am sure it was supposed to be for someone else. Oh well.

And as for keeping your manifolds, I thought so too when I first got my headers. Now that I have solved all my header related problems, when I look back on it, I'm very glad I've got my headers. My truck has more power now--and it sounds a whole lot better than a truck with manifolds.

Be careful when you try to put 2.5" pipe on your manifolds, the output usualy isn't big enough. When you make the flange, you'll probably have to grind it.

If I were you, and I wanted to keep my manifolds, I'd just buy the Dynomax manifold dual kit. I put one on a friend's truck and it worked out very well.

Tim
'84 Chevy K10, lifted, loud, fast, and 3/4 ton axles
 
My motor is 10:1. I run 93 octane always. I also have the flat top pistons and Dart heads. My timing is set at 8 after TDC. with total timing at 33. I use the MSD box and Pro bilit dist. The headers are Ceramic coated Hooker Competition, with 2 1/2 exhaust and 2 50 series flowmasters. I will be adding roller rockers soon and a different intake.

79 Blazer 6"lift 38 SX's locked/4:56 <a target="_blank" href=http://www.keith.rockcrawler.com>www.keith.rockcrawler.com</a>
 
I really just depends on the motor honestly. In hot rod about a year ago a guy was running 14.5 to 1 compression with 92 octane and aluminum heads on his small block in his nova.. He did it by using a big cam that blead really down quickly at low speeds, having a 6,000 rpm stall in the car, and running 4.56s with 26" tall tires. The setup had enough slip in the converter and blead down in the cam he got away with it. Now when he wanted to race he needed at least 110. Anyways if you stay with 9.5 to 1 you should have no probs.

86Jimmy 4"&35"MTRs,87 burb 4"&35"MT,69Firebird w/525 hp,and 70GTO live in PHX,AZ<a target="_blank" href=http://www.geocities.com/modifierperformance>My vehicles</a>
 
Mine is 9.2:1, it's a 305, and I run 87 octane. :)

Tim
'84 Chevy K10, lifted, loud, fast, and 3/4 ton axles
 
I think those other 20 guys in my class at the mud bog was 9:1 Cmpression also. Thats why I left with a fat wallet and the satisfaction of whooping them with" MY BUILT NOT BOUGHT" Chevy Blazer. :)

79 Blazer 6"lift 38 SX's locked/4:56 <a target="_blank" href=http://www.keith.rockcrawler.com>www.keith.rockcrawler.com</a>
 
I'll have to get a picture of them, 'cause they're really screwy. I hadn't heard of them till I got a set. My uncle sent them up from Colorado for my graduation. I'm gonna have to whip out the tape measure one of these days and take some measurements to see if I can get a 2.5" pipe to work.

<a target="_blank" href=http://www.geocities.com/bigkern76>http://www.geocities.com/bigkern76</a>
 
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