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Low Compression Street Engine's & Tuning Techniques Idea's...How Too's on low compression Engines.

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Old 01-31-12, 11:20PM   #1
SPDMETL
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Default 87 octane-how much do You give up?

Possible cheap-ass 455/200-4r combo for 2nd gen T/A.

School Me, Dudes!
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Old 02-02-12, 11:19AM   #2
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A "cheap-ass 455"? I am going to venture to say "A LOT" since you said 87 octane and not X amount of compression. You've got the basics for a real nice combo there and it really wouldn't take much to make 475hp with cast iron and over 530hp with aluminum for a given driveability. Generally speaking, when $ enters the issue you find yourself not being able to address certain issues which enhance octane tolerance:
  • The combustion space must be configured to enhance combustion efficiency. That includes the chamber, piston top, compression ring, head gasket thickness, and deck clearance.
  • The drivetrain can go a long way to aiding octane tolerance: rearend gearing, 4 speed overdrive transmissions, high stall torque convertors..... anything that helps "unload" the engine at low rpm where lugging occurs.
  • Camshaft design: It's not just a longer duration but lobe placement for cylinder pressure "management" if you will. Longer and more aggressive lobes require an upgraded valvetrain as well.
  • Cold air induction is a benefit for detonation suppression.
  • Aluminum cylinder heads: This is the big one. You can/should run more compression to equal the thermal efficiency of an equal cast iron head. With the addition of modern combustion chamber technology, the aluminum heads have the advantage for sure. The larger ports offer more midrange and top end power for a given camshaft combination as well.
It may be argued that some of these measures are a crutch to enable the use of higher compression, but the funny thing is, these components and measures are done anyway in the search for speed but many guys only run 9.x:1 compression with an aluminum head. I've had a number of customers call and ask about the validity of aluminum heads because they were told they won't make any more power and aren't worth it. Go figure.
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Old 02-02-12, 12:17PM   #3
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Lets start with What do you have?

Heads?

Do you have iron heads right now?
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Old 02-03-12, 11:26AM   #4
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87 octane summer blend or winter blend? Big difference..

Depends on what your set up likes and the only way you'll find that out, is to road test it and make adjustments to the vacuum advance, initial advance and mechanical advance.

My 462 seems to get some additional off idle snap with lower octane. I know when I have it perfect by the lope at idle and the crackle of the exhaust.. With no ping anywhere of course..

Remember on the street, conditions are always changing, so you need to allow for that.. Now that my motor has over 50,000 miles, I just run 93 all the time... But I used to get that thing screaming on 89 with a little fine tuning. Trouble is, the next day the weather changed, so back to 93 to be safe...
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Old 02-03-12, 11:37AM   #5
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Here ya go Wife;s daily driver.

https://sites.google.com/site/ksmsin...-s-rookie-seat

The build, car and track results. On a real mild motor..........also with nitrous and also an ATI Procharger.

Happy reading.
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Old 02-03-12, 12:52PM   #6
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87 octane summer blend or winter blend? Big difference..
Depends on where you live
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Old 02-03-12, 03:39PM   #7
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thats a big x2 MB125, also depends were you get your fuel. we got all different types around here even thou they maybe same octane rating. There is a big difference in how they run.
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Old 02-03-12, 03:50PM   #8
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I know here in AZ there is alot of alcohol mixed in the gas. Any rubber in the fuel systems don't last long.
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Old 02-04-12, 12:21PM   #9
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Quote:
Originally Posted by 1967 Tempest View Post
Lets start with What do you have?

Heads?

Do you have iron heads right now?
Several sets of iron heads including 62's and 48's. Not sure about the others-haven't looked at them in awhile. I should probably just worry about a short block right now...
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Old 02-04-12, 12:32PM   #10
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Several sets of iron heads including 62's and 48's. Not sure about the others-haven't looked at them in awhile. I should probably just worry about a short block right now...
Hmm..

Id like to see a different head. Both of the heads above on a 455 will be very high compression. Over 10:1. Now you are out of the low SCR and depending on gas out of pump gas as well.

Plus if you have to bore out the 455, you are further increasing SCR. Not to mention that there is a big variance in Pontiac head casting. those heads might be 71CC right now and might need milling. Now you might be closer to 11:1 compression.

The 6X-8 with around 98-100CC will keep you close to the 9:1 SCR. And a 6H or 6S milled to 100-102CC plus head gasket will keep you around 8.5-8.75SCR.


8.5 is great for 87 octane, plus depending on what cam you go with you can run some serious initial timing. This makes for great pull right off of the line.


Is the short block together? What pistons are you planning on using?


I used Wallace Racing's chart for estimated SCR's.

Dave

http://www.wallaceracing.com/cratio0001.htm
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Old 02-04-12, 01:37PM   #11
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Just food for thought
I am using a set of 16 d ports with a 72cc chamber on a +.030 455 with a 14cc dished piston.
With a standard Felpro gasket i calculated 9.8 to 1
I already have it in my head i will have to mix in race gas, and i don't care i want power and longevity.
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Old 02-04-12, 03:45PM   #12
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I was thinking of as cheap a longblock as can be dependably(?) built from a 73 455 shortblock/iron heads I've had stored indoors for many years. Later going to a forged stroker built from a 400 block.
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Old 02-05-12, 04:04PM   #13
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Just food for thought
I am using a set of 16 d ports with a 72cc chamber on a +.030 455 with a 14cc dished piston.
With a standard Felpro gasket i calculated 9.8 to 1
I already have it in my head i will have to mix in race gas, and i don't care i want power and longevity.

That 14CC piston, is that adding to the area or taking away? I have the NP411 piston now in my 400 and it takes away. I think that it is around 15CC~ish. Effectively lowering SCR. Sounds like your 455 is gonna FLY!!!

Dave
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Old 02-05-12, 05:41PM   #14
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That 14CC piston, is that adding to the area or taking away? I have the NP411 piston now in my 400 and it takes away. I think that it is around 15CC~ish. Effectively lowering SCR. Sounds like your 455 is gonna FLY!!!

Dave
Its a - 14cc
If all is correct it should fly, but in all reality i wish i could have 15:1 compression, but i got a really good deal on these rods and pistons so i work with what i have.
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Old 02-07-12, 09:31AM   #15
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Depends on where you live
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He's getting the same gas we get...
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