Flattop pistons and valve reliefs

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Worship79

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Alrighty, currently finetuning my engine after an upgrade (heads, cam, etc.). This will result in somewhere between 200 and 240, maybe 250 rwhp (first dynosession: 205 rwhp). Posted about this before I think, but not really relevant for my actual question (I think).

For my next upgrade I need to address compression. I calculated it by the official manufacturers numbers and that resulted in a puny CR of 8.48:1 (see below).

To really have some fun, but still be able to drive on pump gas (95 RON) I'd like to boost CR to around 9.5:1. For this I think I need to change my lousy dished, 4 valve relief, 13cc pistons to flattops.

Could someone educate me on the usefulness of valve reliefs? How does having 4, 2 or none relate to the CR?

calculated_compression.jpg
 
Something is wrong, the stock L48 had around 8.5:1 CR and that with stock 76cc chamber heads. You have 64cc heads so your CR should be closer to 9:1 with stock pistons and stock gasket thickness.

This calculator shows 8.9:1 CR for your engine using a .040" thick gasket and having the piston .025" in the hole (stock deck height 9.025")

http://www.race-cars.net/calculators/compression_calculator.html

here's a screenshot with your numbers:
21486e06072c91e.jpg
 
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Stock L48 yes, new Goodwrench no :rain: ChevyHighPerformance magazine did the 'Goodwrench quest' and they found out compression on the base 220hp (flywheel) engine was a wimpy 7.8:1.

If compression is at 8.9:1 I should get around 250 rwhp on the dyno (after I replaced all my plugwires) or more?
 
Usually the deck height is 9.025 also on those hecho en mexico LM1s/Target Masters/ Goodwrench blocks. From there the piston usually sits around 8.995 so total down the hole is about .030.

Do you have the correct gasket diameter? To lower CR, use a THIN gasket, as thin as you can get your composite one (stock uses steel but your deck is most likely not properly flat and certainly not rough enough, probably wire bristled or roloc bristled right???)

4 valve reliefs are much cheaper to produce, 1 piston type fits all holes and no install care has to be taken for orientation apart from noting the notch for forward. Same w/ symmetrical 2 relief or 1 big relied (l82 TRW 464664 piston & similar forged 327 model) but relief must be orientated on top of cyl.

4 valve reliefs = install for dummies :)

Actual CR will be even lwoer than calculated, the area above the top rings (side of piston to bore gap) needs to be added too :)

That CR is perfect for a huffer :)
 
Thanks for pithcing in Marck :hi:

We used a corteco engine kit, nr. 32222-1CS. Only info on thickness a could find was that the head gasket uses a .015" shim, whatever that may mean. Gasket diameter might not be correct as I took the most commonly used diameter: also unfindable information! Fit right around the cilinder, though I know thatofcourse is not accurate enough. My engine doesn't leak anymore, so in that respect I'm very happy with these gaskets :)

Indeed we first scraped the old gaskets with a gasket scraper and then wire bristled the deck for further cleaning. For the build see here (Dutch, but pictures speak for themselves). Below I listed my complete current setup, for further information. A huffer is something like a tow-truck, right? :skeptic:

How does a valve relief affect the CR? In which does one choose 2 or 1 or none? :search:

Goodwrench 350 SBC
- 4 bolt
- cast alu dished pistons 4 valve reliefs
EDL-60909 64cc performer heads
EDL-2701 performer EPS manifold
EDL-1411 performer 750cfm carb
CompCam XE268H cam:
- Duration at 050 inch Lift: 224 int./230 exh.
- Advertised Duration: 268 int./280 exh.
- Valve Lift with Factory Rocker Arm Ratio: 0.477 int./0.480 exh. lift
- Lobe Separation (degrees): 110
- Grind Number: CS XE268H-10
CompCam Magnum 1.52 roller tip rockers.
CompCam Double roller timing chain and gear
EDL-9629 .100 over pushrods (Edelbrock specs)
Corteco engine gasket (spacer shim 0.015”) kit (32222-1CS)
NGK-R BCPR5ES spark plugs, 3/4" reach (Edelbrock specs)
15W40 mineral oil
ACCEL HEI Super coil (plus new rotor and cap)
Hooker side-exhaust headers 1/5” primaries, 4” collector
Reverse flow baffles
TH350
Stock stall-convertor
 
As far as I know you can not use a .015" steel shim gasket with aluminum heads. Is that really a .015" thick gasket or what ? I googled the number 32222-1CS and it comes up as a .015" steel shim gasket.....
I use Mr Gasket .028" composite gaskets and I was told that these and the Victor Reintz .026" composites are the thinnest you can use with alu heads.
 
A valve relief affects the CR because it's a dish and it means you have more cylinder volume because of it.

unless we know what head gasket you have (compressed thickness, bore diameter) it's all a guessing game.

Agree with Karsten, the reinz one is about the thinnest out there.

Running a steel shim on a non trued deck with the incorrect roughness is asking for trouble.
 
Okay, but other than that no advantages to a relief? Quench perhaps?

As for the gasketsl, correct or not: those are the ones on it. I'll e-mail corteco for more exact specs.

What kind of trouble could there be? Blown gasket?
 
quench is a factor of deck height (or more precisely piston down bore) and compressed gasket thickness.
 
Thanks for clearing that up, so putting in 'flat' flat-tops would be best, unless they would increase compression too much to allow for the use of normal pump gas.

Once I know my compression and I am sure everything is in order, I was thinking of putting in hypereutectics like these: http://store.summitracing.com/partdetail.asp?part=UEM-KB106030-8&autoview=sku

What would be good rings to go with this type of piston?
 
There's no way you can raise the CR too much with flat tops and your combo. Also, CR is just a number, little significance. A rule of thumb more of sorts. DCR will give a better idea of what to expect, at least it's sort of related to (full throttle) cranking pressure and actual operating conditions (cyl pressures). It too is a number and a guide, but more significant than CR

If you opt for hypers, make sure you run a wider top gap as hypers retain a lot more heat in the crown because of silicone content and this heat has to be dissipated into the walls by the rings, therefore top ring gets hotter and needs a wider gap. use piston manuf. specs for this gap. As for KBs/Silvolites. I would use a different brand than those. Federal Mogul or Speed Pro would certainly be favored over KB/Silvolite IMO.

Why not measure ACTUAL valve lift first? Make sure you didn't wipe any loves. You can simply put a dial indicator on the retainers and measure valve lift, then compare them (and recalcualte using your actual rocker ratio to get lobe lift) With the actual ratio possibly being a little off, comparing the lobes is key.

How will you "know" your compression? The wording on a compression tester is a little confusing. it's a cylinder pressure meter. It does not measure the compression (ratio) and the 2 are not directly linked. In other words, if you say get a pressure reading of 180psi, it in NO WAY says anything concrete about your CR.
 
Worship79 said:
Thanks for clearing that up, so putting in 'flat' flat-tops would be best, unless they would increase compression too much to allow for the use of normal pump gas.

Once I know my compression and I am sure everything is in order, I was thinking of putting in hypereutectics like these: http://store.summitracing.com/partdetail.asp?part=UEM-KB106030-8&autoview=sku

What would be good rings to go with this type of piston?
Click to expand...

These pistons (+7cc dome) will raise your CR to over 11:1, too high. I base the calculation on 4.030 bore, stroke 3.48, piston .025" in the hole and a standard .040" gasket (with a thinner gasket the CR is even higher).

With the -13cc dished pistons that you have and a .028" gasket you're in the 9.2:1 CR ballpark. Perfect for pump gas and a mild cam like the stock L82 - the 224-230 that you have is a little hot for the CR. The carb is too big IMO also, it'll work but a 600cfm would be big enough.
 
Those are similar in crown design to the stock L82 piston, also with a 7cc valve relief. No way you will raise the compression that high. More realistically it'll be right around 9.5:1 with that kind of piston.

I see where you goofed Karsten :) 7cc dome, it has a 7c dish, so you are 14cc's off on your calculation :D
 
BTW, the Hooker side exhaust headers have 1 3/4" primaries, 1 5/8" are better suited for a 350ci SB and the reverse flow baffles are also robbing HP.
 
Twin_Turbo said:
Those are similar in crown design to the stock L82 piston, also with a 7cc valve relief. No way you will raise the compression that high. More realistically it'll be right around 9.5:1 with that kind of piston.
Click to expand...

Yes, -7cc dish gets you to 9.5:1, the pistons in that link are +7cc dome and raise CR to 11.3:1

Edit: the description on Summit's website says +7cc (does not say 'dished -7cc')

http://www.race-cars.net/calculators/compression_calculator.html
 
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The pistons in the pic are 7cc dished, I think Summit has the description screwed up. See, it says flat top, and a dome is no flat top. It also says 2 valve reliefs, but it has an asymmetrical shaped single large relief (2 together).

I'm 99.999999% certain that the actual piston is a 7cc dish :)
 
I checked their KB 4 valve relief pistons and they're also listed as +7cc. The distance from wrist pin centerline to the top is listed as 1.561 (4 valve relief). Therefore half stroke (1.740") plus rod length (5.700") plus 1.561" is 9.001". It is a flat top piston with 7cc valve relief volume, the description is wrong, it should be -7cc.

Hyper piston and 11:1CR would be a bad choice :cry:
 
That's fast, great :cool:

BTW, the Hooker side exhaust headers have 1 3/4" primaries, 1 5/8" are better suited for a 350ci SB and the reverse flow baffles are also robbing HP.
Click to expand...

Changing the Hookers isn't really an option right now, but the baffles are going to be replaced. Not going to put in expensive STS baffles (yet) right away; I was thinking about starting with something cheap(er) like Max Flows or similar. Just to see the difference versus the reverse flows and then decide. After all else checks out first ofcourse.

Why not measure ACTUAL valve lift first? Make sure you didn't wipe any loves. You can simply put a dial indicator on the retainers and measure valve lift, then compare them (and recalcualte using your actual rocker ratio to get lobe lift) With the actual ratio possibly being a little off, comparing the lobes is key.
Click to expand...

I will: a buddy of mine just contacted me I could borrow his (digital) dial indicator (Mitutoyo) and magnet bracket.

How will you "know" your compression? The wording on a compression tester is a little confusing. it's a cylinder pressure meter. It does not measure the compression (ratio) and the 2 are not directly linked. In other words, if you say get a pressure reading of 180psi, it in NO WAY says anything concrete about your CR.
Click to expand...

Oh, I was referring to me mailing corteco to inform about compressed thickness and bore diametre of the gaskets I used. With those numbers the compression ratio should be pretty accurate, right? I'm aware knowing DCR is more useful, I'll probably get to that too somewhere along the line. :)

Those are similar in crown design to the stock L82 piston, also with a 7cc valve relief. No way you will raise the compression that high. More realistically it'll be right around 9.5:1 with that kind of piston.
Click to expand...

9.5:1 would be what I'm aiming at, given that higher compression does provide more power. It does, doesn't it? Or am I seeing things wrong?

Let's say everything checks out and me replacing and rerouting the plugwires, new airfilter and other baffles still only get me 205 RWHP. Putting in new pistons (I'll leave type and brand in the middle for now) should increase horsepower and torque? :stirpot:
 
Worship79 said:
Let's say everything checks out and me replacing and rerouting the plugwires, new airfilter and other baffles still only get me 205 RWHP. Putting in new pistons (I'll leave type and brand in the middle for now) should increase horsepower and torque? :stirpot:
Click to expand...

I agree, leave the Hookers alone and add the Maxflows later - I have Maxflows too and like them, I posted a video in off topic if you want to hear them

My old setup was a L48 with flat tops and 64cc Edelbrocks. I ran the mild Performer cam and a performer intake and 600cfm carb. It felt a lot stronger than 205HP. With that mild cam (194-202) the engine had a ton of low end power but wouldn't go past 5500 (estimate, I never had a tach with that setup).
So yes, if you install flat tops you'll have a nice powerful engine, with your cam it should easily rev 6000 and higher, be careful with that cast crank :ill:
 
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