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Compression Ration SBC Mild Blown

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dusterdave173:
OK
I am putting together a short block for a mild SBC 671 blower engine for grins--not going to lean on it--have a decent 671 with some wear but ready--have a correct fuel pump, have a cam 260/270 at 050 on a 112, have a new set of aluminum rods 6 inch so getting ready to buy pistons--always heard 8:1 but in recent post DRE says he is tad over 10--now he is serious comp unit--mine is for fun --can I run that high CR? It seems lot more choices for 6 in rod pistons will put me in the 9-91/2 to one range unless I do expensive customs which for this build I do NOt want to do
I am going to be playing
Rinning mid 5 in 1/8th with a 350 stack --not aiming to go lots faster just have a blower unit for grins and fun
Thanks!

George:
Running 10.4:1 with 6:71 stripped 1/1 . We have seen 10psi on our tale tell gauge setup ( yea no computer) . Iron heads and no problems. The combo weight is #1700 +-. I am very impressed with the performance as it has run in the 4.50s at our local 1/8 mile track. I set it up with a .060 copper gasket so I could go thinner or thicker but so far no need. Its a 377 ,6" aluminum rods , Venolia pistons.

Timing was 30-33.

Frontenginedragsters:
Dave:
 The posting by George above mine is right on. 8)
10 to 1 compression will work fine. I'm sure your talking alcohol.
Start with 1 to 1 on the blower and about 20* timing. Sneak up on the timing.
Matt

dusterdave173:
THANKS! Like I said --just want to play around a little--The 20 degree timing is great advice--did not know to start so low  Yes alky for sure--
   I have a set of pulleys that are one tooth diff like a 35 and 36 so that should work


I really appreciate the forum and the help

dreracecar:
20* NO frickin way

  Alcohol burns as a liquid not a vapor like gas, thus it fires off when conditions are right AND changes fireing as conditions change---- thats why you run a magneto because of the long spark duration, so that the spart will be around during those changes.

10+ CR gives you that needed kick to get things going before the blower comes in. Alc is extreemly hard to pre ignite (detonation) Anything less ( in reletive numbers)and you are just making a pig out of it.

On a cold motor if you had 20* of timing, you would prob fire of the cyl at 14/15 deg btdc and warmed up only have 20* in it??. Minimum I would start with is 29/30* at anywhere below 2000' corrected and 1* per 1000' after that

 Its all about makeing the motor happy. The same engine I run, many years ago, because of class index, I ran 5 over on the blower, 29* of lead,and thru a lot of fuel at it (fat) to slow down to 7.60 and the motor was not happy about it. Now because of 7.0pro, I run 18 over, 34*lead, and trim the fuel back and use a high speed, now the motor runs clean and is happy (same spark plugs all year and 15 passes before I change oil)and still not even near a tune-up range that will start hurting parts.

 The amount of blower boost/overdrive is what makes the combo mild, but basic motor/fuel princibles must be adheared to first and foremost

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