Advice on Compression Ratio needed

[lewtot184]

I was just out in the garage monkeying around and took a quick cylinder pressure check on the engine in my 65 coronet. engine was dead cold, only took one plug out (lol), small mech cam with lash about .005" tight for the alum heads, 9.8:1 static compression, pistons a little loose (another lol); anyhow, 155-160psi. it would probably do close to 170 hot. very pump gas friendly. I think my dynamic is about 7:1.

 

[BSB67]

For what it's worth, here's my real world example. My 500" motor runs just fine on 92 pump no ethanol. With head off, no evidence of detonation on piston or head.
Just re-ran the numbers on my motor:
4.385 bore
4.15 stroke
6.6 cc valve relief
88 cc EDE Performer RPM's
.010 deck
.051 thick gasket
4.41 gasket bore
10.35 static CR
6.768 rod
63* ABDC intake @ .015 lift
1000 ft
8.37 dynamic CR

Nice. What are the cam's actual specs, lash setting, and cranking cylinder pressure?

 

[66Satellite47]

Nice. What are the cam's actual specs, lash setting, and cranking cylinder pressure?

It's a Comp XS274S 236* I/242* E @.050 (274/280 @ .015), 502/511 lift w/1.5 rockers, 110* LS, IC 106*, .016/.018 lash. Don't recall what the cranking pressure was. really good street combo, kills the tires, not big high rpm (above 5200) power though. One of my antique 300/310 -565/585 Cam Dynamics cam made a bunch more high end power in this motor. It ran fine on the street too.

 

[Hemirunner]

So I’ve calculated my BCR to be 23.23:1 what fuel should I use?

 

[Glenwood]

So I’ve calculated my BCR to be 23.23:1 what fuel should I use?

 

[lewtot184]

So I’ve calculated my BCR to be 23.23:1 what fuel should I use?

95% nitro, 2% hydrazine, 3% water. just kiddin.

 

[66Satellite47]

View attachment 576214

By God, I think you've got it!

 

[wsutard]

Circling this convo back a little... My plan is to tweak the compression with different head gasket thicknesses. I plan to have the engine dynoed after I put it together so during that I we will figure out where it wants to live its life.

Any comments on this idea?

 

[wyrmrider]

Tighter quench is worth more than less
Lower compression by lowering quench is a death spiral

 

[BSB67]

It's a Comp XS274S 236* I/242* E @.050 (274/280 @ .015), 502/511 lift w/1.5 rockers, 110* LS, IC 106*, .016/.018 lash. Don't recall what the cranking pressure was. really good street combo, kills the tires, not big high rpm (above 5200) power though. One of my antique 300/310 -565/585 Cam Dynamics cam made a bunch more high end power in this motor. It ran fine on the street too.

I figured it was a 274° in at 106° based on your numbers. Probably blows a pretty good number too.

 

[BSB67]

So I’ve calculated my BCR to be 23.23:1 what fuel should I use?

Better than average

 

[Hemirunner]

Better than average

But my DCR is only 7.94...

 

[wyrmrider]

you could get 1.6 rockers for your intakes if your springs and keeper to stem seal has the room

 

[lewtot184]

don't try to take things to the limit and then later find out you went a little beyond the limit. you'll play hell trying to walk it back.

 

[IQ52]

I want to pull the most power from the engine as possible while keeping it safe to drive.

don't try to take things to the limit and then later find out you went a little beyond the limit. you'll play hell trying to walk it back.

Sage advice. How much power do you really need? We've built many street 440's that push 140-145 psi cylinder pressure at 4,500' elevation. Depending upon the camshaft they make around 500 lb-ft at 3,000 rpm, 560-580 peak torque and 550-600 hp at 5,700-6,000 rpm.

 

[wsutard]

Sage advice. How much power do you really need? We've built many street 440's that push 140-145 psi cylinder pressure at 4,500' elevation. Depending upon the camshaft they make around 500 lb-ft at 3,000 rpm, 560-580 peak torque and 550-600 hp at 5,700-6,000 rpm.

It isn't really that I "need" the power, its just that I have the ability to tweak the setup now. Why not take it to the top of the safe zone if all we are taking about is either thicker head gaskets or finding the correct cc piston?

Either way, I have identified what I need to tweak and where I want the engine to be DCR wise, (8.2-8.3). I want to use the Icon 822 pistons which will require me to run a thicker head gasket to get the DCR from 8.5 to 8.2-3.

This entire exercise has been very valuable for me. This is the kind of stuff I wanted to learn about when building this 440. I plan to have the engine dynoed so I will report back what we find out.

 

[Garys1969RR]

After thinking about it for a couple days, I've decided that BCR must mean Boosted Compression Ratio, and of course DCR is Dynamic C Ratio, and RCR is, I suppose, Regular C Ratio. Is that right??? I had never heard of BCR before, but at 23 to 1, that would mean 23 Cubic inches of fuel/air is squeezed into 1 Cubic inch of space. Kinda like in a black hole. Right? I'll bet that would make a real powerful explosion when it goes off. LOL

 

[IQ52]

It isn't really that I "need" the power, its just that I have the ability to tweak the setup now. Why not take it to the top of the safe zone if all we are taking about is either thicker head gaskets or finding the correct cc piston?

Either way, I have identified what I need to tweak and where I want the engine to be DCR wise, (8.2-8.3). I want to use the Icon 822 pistons which will require me to run a thicker head gasket to get the DCR from 8.5 to 8.2-3.

This entire exercise has been very valuable for me. This is the kind of stuff I wanted to learn about when building this 440. I plan to have the engine dynoed so I will report back what we find out.

The thing that really bothers me is knowing for sure where the "top of the safe zone" is. As each engine is an experiment of one, calculations on paper are not going to tell you for sure where the 'top' is until you physically experiment. I've never had a customer who was willing to pay $2,500 dyno time, plus parts, while I changed cams, pistons, heads, gaskets, etc, until we found where it wouldn't work and then backed it down to where it was just into the 'safe zone'. So we just made a good safe pump gas engine for the street that they couldn't control until they had a tuned chassis.

 

[Hemirunner]

After thinking about it for a couple days, I've decided that BCR must mean Boosted Compression Ratio, and of course DCR is Dynamic C Ratio, and RCR is, I suppose, Regular C Ratio. Is that right??? I had never heard of BCR before, but at 23 to 1, that would mean 23 Cubic inches of fuel/air is squeezed into 1 Cubic inch of space. Kinda like in a black hole. Right? I'll bet that would make a real powerful explosion when it goes off. LOL

Correct, with the exception that the regular CR as you called it is usually refered to as SCR or static CR. This one is a twin turbo combo.

 

[wsutard]

And now there is a TWIST! Just watched the machine shop CC the heads. Eddy advertises them at 84cc, they measured out at 79cc. If I had just bolted this stuff together I would have detonated like crazy. That 5 CC drop moves the SCR from 10.3 to 10.8 and the DCR from 8.5 to 8.9. WOW.

Now we are looking at dished pistons to account for the CC difference.