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Discussion Starter #1
So i building a 351 to into my 1984 F150 and I'm not sure what to do about my compression ratio and how quench relates to it.

So question for the engine gurus. With the pistons I bought and the heads I plan on buying, either the AFR Enforcers (59cc chamber) or the Promaxx 9175s (60cc chamber), my calculations show I need around a .070" thick head gasket to get my SCR to around 9.5:1 and my DCR to around 8.4:1 which seems to be the recommended max with 93 octane to prevent detonation. Now, obviously this causes my quench height to be quite tall since it would have the .070" head gasket plus 0.18" deck clearance. Where does quench play a role in this with allowing me to run a thinner gasket and increase compression without increasing my risk of detonation? If I were to go down to a .040" head gasket with a 4.100" bore, my SCR is 10.2:1 and my DCR is 9.06:1 which the numbers say would have detonation issues. I don't plan on revving this over about 5000-5500. Just want a good torquey engine for a light-for-a-truck pickup (3800 lbs-ish).

Below is a list of the parts I'm using or plan on using
F4TE roller block, stock bore (decked .006")
Comp 35-320-8
Stock crankshaft
KB151-STD pistons
Scat forged I beam rods 5.955 length
Promaxx 9175 heads (2.02/1.6 valves, 180cc runners, flowing 255 @0.500” lift “) (Heads have not been ordered yet
Weiand Stealth intake
Holley 750 cfm carb
Longtube headers
static comp ratio 9.44:1
Dynamic comp ratio 8.35:1
Cometic MLS Head gasket .070" thickness, 4.100" bore.

If more info is needed, please let me know.
 

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I recently went through this. Quench is somewhat important if you want detonation resistance and .070 coupled with a small chamber is a bit too much IMO - you'll just be holding back engine performance if you worry about absolute DCR numbers. Factor in most DCR calculators don't take valve overlap into account so your numbers are a bit arbitrary. Lack of quench can make an engjne prone to pinging if you get aggressive with igniition timing or in your case loading the engine down low in a heavy vehicle - kind of important if you plan on using your truck for its intended purpose.

You can compensate for a bit higher static CR with appropriate valve events ie. a custom cam to get cranking compression within reasonable limits. It's a far better way to fix the issue than playing with thicker head gaskets. The cam you picked will hold you back IMO. The intake valve closing point isn't optimal for your engine, plus it doesn't have enough lift to take advantage of decent heads. 10:1 static CR isn't all that much for premium pump gas and aluminum heads. You'll be better off with a custom cam grind anyways, given you have decent static CR, plus I recommend a billet core instead of the -8 sadi core. I would talk to Ed Curtis or Bullet about this - there are ways to bleed a bit of cylinder pressure with the correct valve timing if they feel your cylinder pressure is too high for pump gas (which I doubt).

Also factor in decking the block may probably put you out of the hole by a few thou. Unless you're getting a good deal on the Promax heads, maybe consider AFR renegade 185s or TFS 11R 190s. Dont skimp on the carb either. I assume this doesn't have to pass any emissions testing?
 

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Discussion Starter #3
Correct, no emissions testing. I guess I’ll probably end up returning my cam to Summit and firing off and email to FTI. The reason I was planning on the Promaxx’s or AFR Enforcers was performance coupled with price.
 

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Got it. The heads are really something you dont want skimp on if you can afford it. You won't regret spending the extra money. What trans and rear gear are you running?
 

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Discussion Starter #5 (Edited)
AOD and 3.55s

So after running the numbers with the Trick Flows with 66cc chambers, the static compression ratio with a .040 head gasket was around 9.5:1. As far as heads, would the altered configuration of the Twisted Wedge valves allow me to run the 2.055" valves with no clearance issues? I'd still check PTV either way.
 

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Why are you trying to use premium fuel for a daily driver or work truck? If you are okay with spending an extra $.60 per gallon I understand, that's a lot of money to me for every day use.

You can use regular gas with compression in the mid 9:1 range, with the right custom cam and valve springs. I'm aiming at a 306 build for 87 octane, with something near 9.5:1 compression.

You might want to rethink the pistons when you know the bore needed, the compression, and the quench desired.
 

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Discussion Starter #7
Why are you trying to use premium fuel for a daily driver or work truck? If you are okay with spending an extra $.60 per gallon I understand, that's a lot of money to me for every day use.

You can use regular gas with compression in the mid 9:1 range, with the right custom cam and valve springs. I'm aiming at a 306 build for 87 octane, with something near 9.5:1 compression.

You might want to rethink the pistons when you know the bore needed, the compression, and the quench desired.
So it's not a daily driver, got a '03 Corolla for that. This is a weekend truck for fun. Won't be used for towing as I have a much newer truck for anything I might tow. So I know the bore needed, it's still stock size. I was somewhat limited on piston choice since it seems most aftermarket pistons are meant for overbores. And I was trying to raise the compression and get away from the 19(?) cc dishes on the stock 351 pistons.
 

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Very good. Then as 408foureye pointed out, you can use a higher compression in the low 10's, a custom cam made for that can accomplish it. I'd contact FTI about that and the quench, plus the head choices.
 

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AOD and 3.55s

So after running the numbers with the Trick Flows with 66cc chambers, the static compression ratio with a .040 head gasket was around 9.5:1. As far as heads, would the altered configuration of the Twisted Wedge valves allow me to run the 2.055" valves with no clearance issues? I'd still check PTV either way.
I'd consider the AFRs since they're inline heads and will give you easier piston compatability if you're already stuck with what you have. Not that there's anything wrong with the TFSs. Both heads are comparable and good pieces IMO. Don't shy away from the extra compression. It will only get you a better performing engine.
 

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Discussion Starter #10
I'd consider the AFRs since they're inline heads and will give you easier piston compatability if you're already stuck with what you have. Not that there's anything wrong with the TFSs. Both heads are comparable and good pieces IMO. Don't shy away from the extra compression. It will only get you a better performing engine.
The only thing that I didn't necessarily like about the AFRs was they only have the 60cc and 72cc chambers whereas the Trick Flows have 66cc chambers which seems to be perfect. But other than that, I prefer the inline valve setup of the AFRs. And I'm definitely not opposed to compression. I just don't want to have to put race gas in a pleasure vehicle. As long as I get a strong engine with a broad power curve that runs on pump gas, I'm happy. I went ahead and reached out to FTI for a custom roller cam plus requesting recommendations regarding the rest of the top end.
 

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So i building a 351 to into my 1984 F150 and I'm not sure what to do about my compression ratio and how quench relates to it.

So question for the engine gurus. With the pistons I bought and the heads I plan on buying, either the AFR Enforcers (59cc chamber) or the Promaxx 9175s (60cc chamber), my calculations show I need around a .070" thick head gasket to get my SCR to around 9.5:1 and my DCR to around 8.4:1 which seems to be the recommended max with 93 octane to prevent detonation. Now, obviously this causes my quench height to be quite tall since it would have the .070" head gasket plus 0.18" deck clearance. Where does quench play a role in this with allowing me to run a thinner gasket and increase compression without increasing my risk of detonation? If I were to go down to a .040" head gasket with a 4.100" bore, my SCR is 10.2:1 and my DCR is 9.06:1 which the numbers say would have detonation issues. I don't plan on revving this over about 5000-5500. Just want a good torquey engine for a light-for-a-truck pickup (3800 lbs-ish).

Below is a list of the parts I'm using or plan on using
F4TE roller block, stock bore (decked .006")
Comp 35-320-8
Stock crankshaft
KB151-STD pistons
Scat forged I beam rods 5.955 length
Promaxx 9175 heads (2.02/1.6 valves, 180cc runners, flowing 255 @0.500” lift “) (Heads have not been ordered yet
Weiand Stealth intake
Holley 750 cfm carb
Longtube headers
static comp ratio 9.44:1
Dynamic comp ratio 8.35:1
Cometic MLS Head gasket .070" thickness, 4.100" bore.

If more info is needed, please let me know.
You’d be fine with the .040 head gasket. 10.2 is totally within the realm of being fine on 93 octane. If you’re pulling a load up a steep hill that might be a problem, but for a street rod kind of truck it’ll be fine.

Idk what you’re running for a trans, but you can for sure use more camshaft with that compression ratio though. It’ll more than likely make more power everywhere.
 

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Discussion Starter #12
You’d be fine with the .040 head gasket. 10.2 is totally within the realm of being fine on 93 octane. If you’re pulling a load up a steep hill that might be a problem, but for a street rod kind of truck it’ll be fine.

Idk what you’re running for a trans, but you can for sure use more camshaft with that compression ratio though. It’ll more than likely make more power everywhere.
Yeah, i think really it'll come down to the cam specs recommended to me by FTI. I know this will be an unpopluar statement, If I can get away with 60cc chambers, that would allow me to go back to the AFR Enforcers rather than spending $1800-$2000. Although the Renegades are sweet ass heads.
 

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I run 10.7:1 on my 427W using 93 octane. Haven't had any issues yet with ping/knock. Cam was specified by builder as a street friendly cam that will work with 93. Since you're getting a custom, this should not be a problem. What does 60cc bring compression to?
 

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Discussion Starter #14
So a 0.040" head gasket plus the 60cc chamber would give me 10.18:1 static comp ratio with 0.058" quench/squish. But based on the cam I currently have, it's my dynamic comp ratio that has me worried since it's over 9:1.
 

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Yeah, i think really it'll come down to the cam specs recommended to me by FTI. I know this will be an unpopluar statement, If I can get away with 60cc chambers, that would allow me to go back to the AFR Enforcers rather than spending $1800-$2000. Although the Renegades are sweet ass heads.
I can guarantee you won't have issues running straight pump gas with the higher SCR. Running a carb also cools the incoming air charge into the cylinder for even better detonation resistance than with efi, so you have that advantage too. The correct quench coupled with a small efficient chamber and aluminum heads will only need around 30 degrees total timing for peak hp/tq. That means your total timing won't make it detonation prone as opposed to big open chambers on iron heads with the piston down in the hole. I've seen lots of old 70s engines that had no quench at all (mopar 440 comes to mind) which made them chronic rattle boxes when you tried putting decent timing into them, even with premium gas. There are so many other factors that dictate octane needs.
 

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Discussion Starter #16
Yeah, I've definitely realized since starting this build that there's a lot more that goes into it than I initially thought.
 

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So a 0.040" head gasket plus the 60cc chamber would give me 10.18:1 static comp ratio with 0.058" quench/squish. But based on the cam I currently have, it's my dynamic comp ratio that has me worried since it's over 9:1.
Don't stress too much about quench was my conclusion after a lot of research I did for my setup. I'm at 0.051" or more. Don't tell Ed you're stressing about dynamic compression - he'll probably laugh at you. My DCR is 9.3:1 (edit, oops it's only 7.5:1) BTW. There's more that goes into the cam design than simply where the intake valve closes.
 
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Ditto, DCR is at the bottom of importance. Quench is up there, but much more important to the person selecting the cam, than to the builder or owner. It's one of several key items they use to build a proper cam.
 

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I have a 451 Mopar stroker motor that's 10:1 and runs fine on regular pump gas. It has Edelbrock heads and .040 quench. From my understanding anything more than .040 is not effective quench.
 
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