Advice on Compression Ratio needed

[wsutard]

I'm working with my local machine shop on building my 440 and they are recommending that I back my compression on the build down. Based on my specs and components the dynamic and static compression ratios are calculating out to be 10.3 static and 9.284 dynamic, with about 182 cylinder pressure. The machine shop is worried that these, especially, the 9.284 dynamic are too high and thinks I should shoot for 8ish dynamic, 10 static. This machine shop is well known and respected in the area.

Everything I have read says that 10.3 and 9.284 should be fine with my setup. I called Hughes and spoke with them and they calculated out the same numbers as I did and commented on how the aluminum heads will make this work for premium pump gas.

The 440 will be street use with hopefully low end torque since I probably wont ever run it pas 6k.

Anyone have specific comments based on experiences that will help me decide what to do?

Here is the components that matter and their info for calculating the ratios.

Elevation: 100ft (Seattle)
Bore: 4.35 (30 over)
Stroke: 3.75 (stock)
Head Volume: 84 cc (Performer RPM)
Piston Volume: 4.5cc (Icon 822)
Rod Length: 6.760 (Molnar)
Gasket thickness: .039 (Felpro 1009)
Gasket opening: 4.410
Deck height: 0
Cam specs Intake @ .050: 44.5 abc (Hughes Solid flat tappet, STL3842BS3-9)

I used this calculator for my results. https://www.uempistons.com/index.php?main_page=calculators&type=comp

So, do I take the machine shop advice and back the compression down by either changing the pistons or the cam? Or do I run as is? I want to pull the most power from the engine as possible while keeping it safe to drive.

 

[wyrmrider]

Open chamber heads? 4 get it
.035 quench, closed chamber heads
whatever CC it takes to get compression goal in the piston on the spark plug side
you have to force the combustion chamber into a compact form near the spark plug not any on the far side of the chamber

 

[beanhead]

Just my opinion, but if it was me I would go with the shops recommendation and err on the safe side with the compression.If you like the cam that you've picked maybe have them look at piston height to achieve what you're after, rather than put in a lumpier cam to bleed pressure, which might disappoint you a little on the bottom end..... Backing off CR a little shouldn't cost that much power if done right and will save you headaches..

 

[69Bee]

If you give yourself a deck height of 0.010", your CR is 10.069:1 and with aluminum heads, you should not have any issues. You would probably be okay they way you are now. You won't notice that small of a change. My only concern is only allowing 0.039" piston/head clearance which is very close to the 0.035" min. Also, with those pistons, make sure the rods are PN#DH6760YTB8-A due to the pin diameter.

 

[wsutard]

If you give yourself a deck height of 0.010", your CR is 10.069:1 and with aluminum heads, you should not have any issues. You would probably be okay they way you are now. You won't notice that small of a change. My only concern is only allowing 0.039" piston/head clearance which is very close to the 0.035" min. Also, with those pistons, make sure the rods are PN#DH6760YTB8-A due to the pin diameter.

How do I go about changing the piston/head clearance? Thicker gasket or different pistons?

 

[Bicylindrico]

Did you add the 15 degrees on the Intake Closing Point figure? My figures worked out to 10.444 static and 8.58 dynamic with my camshaft

 

[wsutard]

Just my opinion, but if it was me I would go with the shops recommendation and err on the safe side with the compression.If you like the cam that you've picked maybe have them look at piston height to achieve what you're after, rather than put in a lumpier cam to bleed pressure, which might disappoint you a little on the bottom end..... Backing off CR a little shouldn't cost that much power if done right and will save you headaches..

I picked the cam based on Hughes recommendation for what I was looking for. Lower end power 10-10.5 CR. So I cant say that I "like it" vs it was what I was told to go with.

 

[wsutard]

Did you add the 15 degrees on the Intake Closing Point figure? My figures worked out to 10.444 static and 8.58 dynamic with my camshaft

Umm no. I literally add the degrees? 44.5 + 15 = 59.5?
I was wondering what that meant but figured it was some industry standard.

 

[Bicylindrico]

Umm no. I literally add the degrees? 44.5 + 15 = 59.5?
I was wondering what that meant but figured it was some industry standard.

The valve doesn't slam shut so they figure some bleed off as the valve closes

 

[wsutard]

Did you add the 15 degrees on the Intake Closing Point figure? My figures worked out to 10.444 static and 8.58 dynamic with my camshaft

That brings my dynamic down to 8.495 and the static stays at 10.3

 

[Bicylindrico]

That brings my dynamic down to 8.495 and the static stays at 10.3

My understanding is anything around 8.5 dynamic is the max you should run on the street with pump gas

 

[wsutard]

If you give yourself a deck height of 0.010", your CR is 10.069:1 and with aluminum heads, you should not have any issues. You would probably be okay they way you are now. You won't notice that small of a change. My only concern is only allowing 0.039" piston/head clearance which is very close to the 0.035" min. Also, with those pistons, make sure the rods are PN#DH6760YTB8-A due to the pin diameter.

My understanding is anything around 8.5 dynamic is the max you should run on the street with pump gas

Adding in the deck height change that 69bee suggested, plus your extra 15 degrees brings it down to 8.3 dynamic and 10 static. with 0 deck its 8.45 and 10.3.

 

[Bicylindrico]

Maybe get some other opinions here on the board but based on this information I am completing my build with this setup and plan to run the some 92 with a couple gallons of race fuel. I will only drive the car about 15 miles a month tops

 

[Bicylindrico]

Adding in the deck height change that 69bee suggested, plus your extra 15 degrees brings it down to 8.3 dynamic and 10 static. with 0 deck its 8.45 and 10.3.

Wish it were my 15 degrees but I am relying on the accuracy of these calculators and I have measured about every figure I used to calculate

 

[Cranky]

.035 minimum piston to head clearance? Only if you are running aluminum rods. I've done engines with much less using stock type or aftermarket steel rods. I will say that .019 is too close for using laughing gas Axe me how I know that lol

 

[1967coronet440]

@dvw @IQ52

 

[threewood]

I have an almost identical build...
Same 3842 cam
KB237 pistons .010" in the hole
Stealth aluminum heads 80cc
.030" Cometic gasket
.040" quench, 10.4:1 CR, dynamic is around 8.4
I run 91 pump gas without issue with the cam drgreed straight up, 24 degree initial / 36 total.
Edit---I'm running stock rods with ARP rod bolts, floated wrist pin.

 

[m79ded]

My understanding is anything around 8.5 dynamic is the max you should run on the street with pump gas

Yes you are right with those open chamber heads with loose quench. I was told can go to 8.75 aprox with tight close chamber heads like TFS240 and that the chamber is so much more efficient that you won't need as much timing. About 29 to 31 degrees will be enough as opossed to 36 for those lazy open chambers. I got .049 quench with TFS 240 and 10.85 comp and 8.78 dynamic. hope it works. I am running a 242-246 with 112LSA in it.

 

[Cranky]

Low compression lazy engines usually run pretty good with a lot more timing than 36.....just saying. No quench low compression engines can run good if you push the limits. The combo has to be somewhat right at least for a lot of lead timing.

 

[lewtot184]

I use the 84cc edelbrocks (these heads cc at 81-82 unmilled with a fel-pro 8519 gasket) with the Icon 836 piston. 9.8:1 static compression with a [email protected]" cam. I figure my static compression is probably over 10:1 with a carbon build-up. this combo is very pump gas friendly and makes more power than my old butt needs.