Cylinder head question

Hey Steve,

I know that you mentioned you installed 10:1 pistons in the 302, but the actual compression isn't 10:1, is it? With the stock heads?

I have flat-top pistons in my '84 302, and GT40 Explorer heads, and my compression is only around 9:1. Maybe the pin height in your pistons is different from mine. Mine are just a stock replacement, the same as what was in the truck factory, but for 0.020" bored over.

No Cory, I am going to say with the stock heads it is around 9: 75 to 1 compression ratio. I put in .010 over Wisco pistions in a .010 over sized block.

Rembrant wrote

Hey Steve,

I know that you mentioned you installed 10:1 pistons in the 302, but the actual compression isn't 10:1, is it? With the stock heads?

I have flat-top pistons in my '84 302, and GT40 Explorer heads, and my compression is only around 9:1. Maybe the pin height in your pistons is different from mine. Mine are just a stock replacement, the same as what was in the truck factory, but for 0.020" bored over.

Bout like mine, 302 with a 0.030" over bore I dont know the deck clearance as I havent gotten the short block yet but the builder claims that short block with a 60cc head would produce 9.0:1 compression and with my 58cc heads should be around 9.2:1 compression.  But I dont know what thickness head gasket he is using for his figuring of 9.0:1 compression with 60cc heads.  But his short blocks are flat top pistons with four valve reliefs.

I put off any decisions until Monday, and in the meantime came up with this compression estimate using the calculator on the Summit website and based on stock 351w specs and the following assumptions:
- E7TE combustion chamber size: 62CC (table says 62-65CC)
- Sealed Power 336P pistons (.040 over): +6CC valve relief, 1.719 compression height
https://www.summitracing.com/parts/slp-336p40/make/ford
- Fel-pro head gasket compressed thickness: .040 (just a guess based on a review of different parts sites)



At 8.9:1 compression comes out lower than I imagined. Anyone see any glaring errors here?

Lucas

I've gone back through your calculations and I don't see anything wrong.  But I'm no pro at this.

Gary Lewis wrote

I've gone back through your calculations and I don't see anything wrong.  But I'm no pro at this.

Thanks Gary, much appreciated.

Lima Delta wrote

I put off any decisions until Monday, and in the meantime came up with this compression estimate using the calculator on the Summit website and based on stock 351w specs and the following assumptions:
- E7TE combustion chamber size: 62CC (table says 62-65CC)
- Sealed Power 336P pistons (.040 over): +6CC valve relief, 1.719 compression height
https://www.summitracing.com/parts/slp-336p40/make/ford
- Fel-pro head gasket compressed thickness: .040 (just a guess based on a review of different parts sites)



At 8.9:1 compression comes out lower than I imagined. Anyone see any glaring errors here?

Lucas

Looks right, after all the guy I am leaning towards doing my short block offers them in 0.030" and 0.040" overbore and states 9:1 compression with 60cc heads using flat top pistons with I believe the valve reliefs are typically are 6cc.  With 62cc 8.92:1 sounds pretty accurate in my book, I estimated mine should be 9.2:1 with 58cc heads but without knowing what compressed thickness headgaskets he use for his figuring of 9:1 @ 60cc chambered heads I have no way of knowing if mine really will be 9.2:1 or if it would be 9:1.

I did look up his pistons he uses in the photos he provided on the ebay listing and got things such as compression height for the pistons and I know he uses reconditioned stock rods and stock crank which gives me a good idea of the build, down side is he makes no mention of if he zero decks the block or if he brings the block down to a specified quench figure.  All his ebay listing states is

then they all  get squared up by our BHJ fixtures which gives you a truly squared up deck surface after we mill them (a plus for holding a head gasket tight under high stress applications)

His add also states the one I am looking at for $1,884 states the pistons are coated sealed power hyper pistons and the photo reference shows the short block with a H273CP sealed power Speed Pro hyper pistons which has a 1.605" compression height and a +8.00cc piston head volume.

This coupled with OE rods and a OE stroke comes out the following for me.

Rod Length : 5.090"
Stroke : 3.00"
Deck Height : 8.206" (OE not accounting for squaring or decking)
Deck Clearance : 0.011"

This combined with my 1011-1 recommended headgasket from AFR for my Renegade 165 heads with 4.100" x 0.041" head gaskets come up on Summits Compression Calculator as 9.15:1 and with 60cc heads 8.95:1 which tells me his 9:1 compression has to be with a thinner head gasket than the one I am running which is the thinnest recommended one for my AFR heads.  That also is assuming he doesnt deck the block that much.  If he decks the block down to 0.008" - 0.009" deck clearance he could get 9:1 compression with 0.041" thick head gaskets.  If he did that then I would still be right around 9.2:1 compression.

I will see if I can find this other compression calculator I found years ago, you actually punched in all the measurements and it did all the work for you, no math having to be done to figure things such as deck clearance.

Where did the deck clearance number come from? Is that typical for a 351w? My ‘84 302 was 0.014” stock, and was decked down to 0.005” when the shop built it for me.

Rembrant wrote

Where did the deck clearance number come from? Is that typical for a 351w? My ‘84 302 was 0.014” stock, and was decked down to 0.005” when the shop built it for me.

Deck Height : 9.503" (71-96 351W)
Stroke : 3.50"
Rod Length : 5.956"

3.50" / 2 = 1.75"

1.75" + 5.956" + 1.719" = 9.425"

9.503" - 9.425" = 0.078" Deck Clearance.

Thats assuming the block height is 9.503.

I ran into this problem years ago when trying to piece together a 351W build as I wanted to hotrod my 351W some, just about all piston combos with rods gave a huge deck clearance like as seen here.  Only way to really correct it would be to throw in a longer than OE rod or deck the block quite a bit.

Ahh, OK Rusty. That math works out for my engine as well then. I ran my specs through the Summit compression calculator, and it's pretty much bang on with my previous calculations which my engine builder confirmed. My '84 302 now has 65cc GT40 heads, 3cc flat top stock replacement +.020" pistons, and they decked the block down to 0.005", which puts it right at 9:1 compression.

Hey Lima,

Can you find a set of pistons with smaller valve reliefs, or different pin height? With a deck clearance of 0.078" you have some room there. I'm no expert though...lol, all my experience is with the 302;).

Rembrant wrote

Ahh, OK Rusty. That math works out for my engine as well then. I ran my specs through the Summit compression calculator, and it's pretty much bang on with my previous calculations which my engine builder confirmed. My '84 302 now has 65cc GT40 heads, 3cc flat top stock replacement +.020" pistons, and they decked the block down to 0.005", which puts it right at 9:1 compression.

Hey Lima,

Can you find a set of pistons with smaller valve reliefs, or different pin height? With a deck clearance of 0.078" you have some room there. I'm no expert though...lol, all my experience is with the 302;).

Yep, I can only guestimate my compression cause I dont know how much deck height would be on my short block.  Guy isnt really saying he is decking the block just squaring it up but who knows how much material would have to be taken off to square the deck surface up.  Like wise is the deck height on the low or high end of the range as there is a variance in deck heights which could change your clearance slightly and slightly can result in some fairly decent changes.

Its why I am fairly comfortable with my build being at least 9.0:1 ratio as I figure up 9.1:1 ratio with OE deck height so mine shouldnt be below 9.0:1 which is good as my cam wants 8.75:1 to 10.00:1 and I am shooting for in the middle between 9.0:1 to 9.5:1.

He could find some smaller valve relief pistons but I dont know what is out there.  Most common size I see is -6cc valve reliefs and the ones the builder I will probably use for my short block uses pistons with -8cc valve reliefs which as sealed power states can handle up to a 1.98"/1.64" valves which should be perfect for my AFR Renegade heads which sport 1.90"/1.60" valves.  But he does need to do something about that deck clearance if it is that much, I am going from memory but ideal quench is around 0.012" clearance I want to say and that has to calculate the headgasket thickness as well

Guys, thanks for pointing out the detail wrt quench. I overlooked that!
I reran the calculator with these Speed Pro pistons which get me closer to a better quench number with more compression height, but now compression is up to 9.4:1 even with the bigger relief cuts (+12CC).
https://www.summitracing.com/parts/slp-h336cp20/make/ford
I'm ok with <=9:1, but 9+ makes me nervous that I'll have to run premium with my iron heads...



I'll have to call my guy Monday and talk to him again. My estimates are based on book numbers, and while he didn't take the deck down any the block has been through a machine shop at least once previously.

LD

Try the calculation with a 0.047" compressed thickness for head gasket. Does that drop it down a bit?

https://www.summitracing.com/parts/fel-8548pt2

Rembrant wrote

Try the calculation with a 0.047" compressed thickness for head gasket. Does that drop it down a bit?

https://www.summitracing.com/parts/fel-8548pt2

Yes, 9.2:1 using the same numbers. But then the thicker gasket moves my quench number in the wrong direction...

Lima Delta wrote

Yes, 9.2:1 using the same numbers. But then the thicker gasket moves my quench number in the wrong direction...

Right. I remember going through all this when I was building my 302. Two different engine shops told me to keep it around 9:1 compression, so that is where I ended up. I was building it for more power, but with daily driving ideals in mind (not that I daily drive it...I certainly don't, but I was trying to get as close to 300HP as possible, and still trying to keep it nice and smooth and as easy on fuel as possible). How much is gas where you live? It's back up to $1.10/L here, so I'd be doing everything I could to make sure I didn't have to run supreme...lol.

With proper quench you can run higher compression on lower octane fuel as the quench helps prevent hot spots from forming.

Its a reason why I myself am shooting for lower compression as I dont mind running 93 octane in my 306 build but I rather be able to run regular grade 87 since this is my daily driver.  Its why I invested in aluminum heads as well as fuel injection and will be having the DSII dist. I bought new recurved for my setup.  I want to do all I can to reduce my reliance on 93 octane premium fuel, if you have an engine builder doing the work you can work with you can get the block decked some to help improve quench but for me I feel my quench should be right considering using OE specs my deck clearance is already 0.011" and with my head gaskets puts me at 0.052" so it can only get closer with material removed from squaring the deck up.

I just double checked it and quench should never be below 0.035" and summit lists for street use under 6,000 rpm as being 0.035" to 0.045" and if you turn over 6,000 rpm 0.038" to 0.043".

Also states not to have more than 0.060" quench as this would slow combustion down and lead to detonation.

For me I would love my quench to be more around 0.040" but only way that could happen is to pretty much zero deck my block and that would get me at 0.041".  But still even at 0.052" it would have me below 0.060".  I could scrap my 1011-1 headgaskets for 1011-2 head gaskets they are basically the same except 0.039" compressed thickness instead of 0.041" and they are a pre flattened copper wire ring over the 1011-1 pre flattened steel wire ring.  The 0.039" compresed thickness would put me right at 0.050" quench using OE specs but I rather use what AFR recommends which is the 1011-1.

Using summits calculator if I were to zero deck my 302 and run the 0.041" gasket I would be right at 9.4:1 compression.  If I zero deck it with 0.039" gasket I would be at 9.45:1 and still my quench would be 0.039" which would put me being close to the max.  I just dont know if I could talk with the Creb before I order the engine and see if I can work with him to get the deck zeroed out and just run the 0.041" gaskets I currently have.  I just dont know if its worth the extra time and money to do that on what will amount to a daily driver.

Rembrant wrote

Lima Delta wrote

Yes, 9.2:1 using the same numbers. But then the thicker gasket moves my quench number in the wrong direction...

Right. I remember going through all this when I was building my 302. Two different engine shops told me to keep it around 9:1 compression, so that is where I ended up. I was building it for more power, but with daily driving ideals in mind (not that I daily drive it...I certainly don't, but I was trying to get as close to 300HP as possible, and still trying to keep it nice and smooth and as easy on fuel as possible). How much is gas where you live? It's back up to $1.10/L here, so I'd be doing everything I could to make sure I didn't have to run supreme...lol.

Ive seen people with proper quench run 10:1 compression on cast iron heads with 93 octane pump gas.  They swear up and down with proper quench you can get away with 9.5:1 compression on regular grade 87 pump gas with cast iron heads.  That is what I went by for my aluminum heads as my quench may not be ideal or proper but the aluminum should help create a buffer to prevent detonation with lower grade gas.  Can also use the fuel injection to properly go rich under load to help cool the cylinders down to prevent detonation better than going with a simple carb.

Ok, so this time worked the calculator using these speed pro dished pistons (+19CC) then prescribed a deck clearance of .012" (.052" quench with a bit of wiggle room), and lo! 9:1 compression on the nose!
https://www.summitracing.com/parts/slp-h653cp40/make/ford


Working backwards I need a deck height of 9.483" to get here...
I would feel comfortable burning 87 octane at 9:1.

Regular is 121.9/l in town (Thunder Bay) right now, so yeah, looking to stay away from the premium here too lol.

LD

Guys, first, thanks for your comments. They've been helpful in motivating me to play around with the numbers.

New question: On the priority list of considerations, where does "quench" fit?
I mean, I understand the benefits of having good quench spacing, but really, is keeping it under 60 thou really that critical in an engine making around 9:1 compression? Or is it one of those things that becomes more critical the higher the compression?

Reason I ask, is that:
A) Going over the numbers with various combinations of pistons, including "stock replacement" pistons, there are none that I looked which, when combined with a "normal" head gasket and stock engine specs, actually meet the <0.060" quench limit.
B) If I want to have a properly quenched engine, I will almost certainly have to have the block milled, then also the intake side of the heads so that my intake will fit. The extra cost might be justified if I needed that extra quench effect to push some compression limit, but I don't feel like that's what I'm doing by shooting for 9:1 with my set-up.

Ideal quench is basically head gasket thickness (if not less), so why aren't any of these blocks zero-decked from the factory? Doesn't it seem strange that so much modification should be required to re-fit stock replacement parts?

Lucas

Lima Delta wrote

Guys, first, thanks for your comments. They've been helpful in motivating me to play around with the numbers.

New question: On the priority list of considerations, where does "quench" fit?
I mean, I understand the benefits of having good quench spacing, but really, is keeping it under 60 thou really that critical in an engine making around 9:1 compression? Or is it one of those things that becomes more critical the higher the compression?

Reason I ask, is that:
A) Going over the numbers with various combinations of pistons, including "stock replacement" pistons, there are none that I looked which, when combined with a "normal" head gasket and stock engine specs, actually meet the <0.060" quench limit.
B) If I want to have a properly quenched engine, I will almost certainly have to have the block milled, then also the intake side of the heads so that my intake will fit. The extra cost might be justified if I needed that extra quench effect to push some compression limit, but I don't feel like that's what I'm doing by shooting for 9:1 with my set-up.

Ideal quench is basically head gasket thickness (if not less), so why aren't any of these blocks zero-decked from the factory? Doesn't it seem strange that so much modification should be required to re-fit stock replacement parts?

Lucas

Honestly long as you have less than 0.060" quench you should be good.  The problem is when you are over 0.060" quench you drastically increase detonation chance as you are disturbing the combustion process.

Honestly its not super important to get it down to ideal, but like wise you dont want to have it too much then you harm combustion and introduce detonation chance.

Thats why me personally I am not splitting hairs over my quench as I am looking at 0.052" quench at the absolute most and 0.011" deck clearance.  From what I could find a stock 302 had a piston compression height from Ford of 1.61" which puts OE deck clearance as 0.006" and OE head gaskets as far as I know were 0.040" which makes a factory 302 in the neighborhood of 0.046" quench from the factory.  Mine at 0.052" inch is a decent 0.006" difference.  But still I dont know what my deck height will actually be.

Also most of these engines we are dealing with are being setup for emission compliance not efficiency.  For efficiency you would want to go to these lengths to try and get all you can out of your engine while not being wasteful.  Its why before I purchase my short block I am going to have a talk with Creb and see what deck clearance I am looking at to see what I am looking at quench wise, if I come up with in the neighborhood of 0.046" or less I will not do anything extra as it would be better than factory.