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Discussion Starter #1
I hurt my sbf block about a month ago . Get new block and have it prepped , same machinist .... align honed , bored and honed (w/plate's) with an extra .001 clearance (over old block) , decked to match old block .

New bearing's (main , rod , cam) with same clearance's as old block . Also replaced oil pump which raised my oil pressure 10 lbs over old pump . Same rings , looked good and I had installed them new at the beginning of this season . Rotational torque required to turn over new short block - 15 ft pds ..... old set up was 12 ft pds .

Don't know my horsepower .... 2750 lbs , went from high 5.90'[email protected] to low 6.10'[email protected] .
I've been racing for the past month and I am (without a doubt) down a tenth and 2-3 mph !

I'm running a vacuum pump with a low tension ring package (which leads me to the point of this book:) ...... Since the build I've had a small oil leak around rear main area when the engine was not running . Pull engine yesterday to fix leak and find my lost power ...... I can't find ANYTHING to explain my power loss ... other than a leaky (new) rear main seal .... So , my question is would the loss of effectiveness of my vac pump and the added 10 lbs of oil pressure hurt me that much ?

Thanks in advance and sorry for the book :oops:
 

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Do you know how much vac you where pulling last year compare to know ?
a vac pump can give you 15-20 hp
 

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Discussion Starter #3
Do you know how much vac you where pulling last year compare to know ?
a vac pump can give you 15-20 hp
To be honest , going down the track I don't know for sure . I have checked the pump vacuum at the pump (it's the same as always and within spec) . I have a small (cheap) gauge in car that I can't see going down the track :) . Plus I haven't tampered with the relief valve so it will be pulling the same vacuum as before ..... except for the leak's that I've created ...... Thanks !
 

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Discussion Starter #5
Disregard this thread !!!!!! I found my hp ....... apparently the machinist DID NOT deck the new block to match the old one . My piston's WERE .020 out of the hole ..... now they are flush ! I should have checked that but I didn't . New head gasket's for an easy fix :supz:


Thanks fastlx !!
 

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SBF TTFMF
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Disregard this thread !!!!!! I found my hp ....... apparently the machinist DID NOT deck the new block to match the old one . My piston's WERE .020 out of the hole ..... now they are flush ! I should have checked that but I didn't . New head gasket's for an easy fix :supz:


Thanks fastlx !!
Wow...20 out of the hole before? What'd you run, a .066 head gasket? Glad you found your missing ponies 8)
 

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TwistedTerror
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Glad you figured out the problem...a tenth from having the piston out an extra .020...That's crazy! I think I'll have my heads milled now! ;)
 

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Discussion Starter #8
Wow...20 out of the hole before? What'd you run, a .066 head gasket? Glad you found your missing ponies 8)
Yeah , .061's to be exact . I bought my old block fresh out of a machine shop and just figured whoever had it had it decked to 9.480 causing my piston's to be out so much . But everything worked ok with that so I asked my machinist to just match the new block to the old one ....... we're gonna talk :mad: !


But , I'm doing the happy dance for the moment ! :)
 

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You think an .020 thinner gasket is really going to pick it up a tenth and 3mph in the 1/8?
seems to be down quite a bit, it is more probable that it is a ring seal issue.
I just don't see that kind of gain coming from the small increase in compression ratio.
You might see a couple hp but... Not on the order of 30-40hp.

The oil leaks probably aren't helping you pull vacuum either.
What stands out to me is that you used old rings in the new block...
 

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Discussion Starter #11
You think an .020 thinner gasket is really going to pick it up a tenth and 3mph in the 1/8?
seems to be down quite a bit, it is more probable that it is a ring seal issue.
I just don't see that kind of gain coming from the small increase in compression ratio.
You might see a couple hp but... Not on the order of 30-40hp.

The oil leaks probably aren't helping you pull vacuum either.
What stands out to me is that you used old rings in the new block...
I didn't realize it until I just went through my log book and the ring's have 116 passes on them now ..... it's a fairly mild bracket engine and my ring's usually last a lot longer than that ...... But , I think I'll go bolt my heads back on and do a leak down .....
 

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I didn't realize it until I just went through my log book and the ring's have 116 passes on them now ..... it's a fairly mild bracket engine and my ring's usually last a lot longer than that ...... But , I think I'll go bolt my heads back on and do a leak down .....
ring seal and compression...that's the most important thing in the N/A section:p

Ask up in the prostock thread how long there rings last...the short answer is....not long
 

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Milling the heads may not have the same results as raising the piston outta the hole though!
exactly.....the larger gasket and raised piston ... just like a big old gas port...top ring was sealing good before.....its not now
 

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Yeah , .061's to be exact . I bought my old block fresh out of a machine shop and just figured whoever had it had it decked to 9.480 causing my piston's to be out so much . But everything worked ok with that so I asked my machinist to just match the new block to the old one ....... we're gonna talk :mad: !


But , I'm doing the happy dance for the moment ! :)
The first design 351W blocks were 9.480" deck height. Your old block was probably one of those. The newer ones were 9.503". I bet your new block is one of those. I can't remember what year they changed but I think it was '70 or '71?

I used to have a street/bracket '67 Cougar with a '69 block in it. When I stepped up the build once, I got a deal on a set of forged flat tops that were designed for the newer block, & subsequently they were .020" out. I made a fixture out of a 250 Chevy six cylinder aluminum cam gear & chucked them up in my friend's lathe & took .015" off the top (yes I measured the crown thickness first lol). Had them balanced & ran .043" gaskets.
 

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Discussion Starter #16
Thank's for the response's guy's !!

leak down test @ 100 psi
#1 - 7% #2 - 8%
#3 - 6% #4 - 7%
#5 - 8% #6 - 7%
#7 - 9% #8 - 6%

cyl's 2 , 5 and 7 , I could feel a little air moving out of ex. port's , brass hammer tapped the valve's with no change . All the other's I could hear some leakage past the rings .

That seem's pretty good to me ! .....thought's on ring seal ?

BTW , my piston's are latterly gas ported .
 

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Discussion Starter #18
.020" deck difference is a bunch. you will see it in ET.

did you account for weather differences?
All 4 week's that I ran the air was lousy .... very hot and humid . I don't know the exact condition's but my car "settle's in" after the first timed run so 1 week I'd run 6.11-12 and next week it might be a 6.13-.14 . The last passes I made with my old block was similar weather and it was 6.01-02 range .....in good air it's ran 5.92-93 .

If I can get this pig back together I'll find out this weekend if I helped it or not ......................... probably be 6.20's :)
 

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Discussion Starter #19
Welp , when I put it on the trailer a week ago it had just ran a [email protected] . When I put it on the trailer Sat. it just ran a 5.99 @113.8 . The air was quite a bit better but not THAT much better . So , getting the compression back and sealing the engine tight got it right back to where it should be ..... :)
 

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Welp , when I put it on the trailer a week ago it had just ran a [email protected] . When I put it on the trailer Sat. it just ran a 5.99 @113.8 . The air was quite a bit better but not THAT much better . So , getting the compression back and sealing the engine tight got it right back to where it should be ..... :)
n/a engines don't like loose piston to deck clearances, that's for sure!
 
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