I really don't get it either.
It seems pretty odd to be getting 220psi (cam backed up to 0*) with a 10:1 or 10.26:1 (depending on stroke). The race fuel company stated that when 93 and 110 are mixed 50-50 it will yeild 99 octane. My current combination will run on 99 octane.
My question is this:
Would it be safe to asume that droping the CR 0.8 to 1 full point would let it run on 93 octane? I understand that camber design and quench play a role in this. Milling the pistons and openning up the cambers a pinch is less than ideal and some say it could even promote detonation.

_________________
67Cuda,FAST EZEFI,340cu,CR=10.25,RollerCam&Rocker (XR268HR,#20-810-9)(#1622-16)(EddyRPM#60779,#7576), (MSD6AL,#6425) A904, GearVendorsOD, 8 1/4,3.55:1, ClassicAir

A quench / squash area only works in a small zone between .030 and .060 of clearance. In theory, that range is .000 to .060 but with piston rocking, con rod streaching, crankshaft deflection, bearing and pin clearances... you need to leave some space so the piston does not hit the head surface.

An interesting finding is that you get more detination in the .060 to .100 clearance range then .100 and above. (a true open chamber)

You should get some clay and "mock-up" the chambers to see if you are getting into the clearance "high ping" zone inside thoses chambers.

The statement above still does not answer the key question:
Whay does this engine get 220 - 225 psi of cranking cylinder pressure?
DD

If the engine was designed around a 4.25 stroke and it's actually 4.35 that would go a long way toward answering some of these questions. I think that needs to be nailed down once and for all then proceed from there.

_________________
David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

All measurement have been verified except the stroke. Its either 4.25 or 4.35. I haven't had a chance to drive back down and measure it with a quality tool. But in any case the CR is either 10.05 or 10.26 respectivly. Is there any other thing that could cause the 220 psi cranking pressure? Is it possiable for a 10.26 motor the produce 220psi? I do have a late model starter that cranks it pretty fast. Maybe the compression tester is wrong? I didn't buy another one because the use of race fuel seemed to confirm the pressure. Maybe the pressure isn't that high and I am buying poor quality fuel at the Citco around the corner? Maybe I am using the tester wrong. It came with instructions. "Crank engine over 6 times and read pressure" (basicly stated) If I crank it 4 times over it reads 185 ish. 6 times reads 220. 7 and up is the same at 230psi. The motor is disesembled at this point so a different gauge isn't practicle.
I can read the measurements good enough to know that I don't have an above 11:1 motor here.
I am brain storming here and not coming up with any good reasons. I'll even take bad reasons at this point.

_________________
67Cuda,FAST EZEFI,340cu,CR=10.25,RollerCam&Rocker (XR268HR,#20-810-9)(#1622-16)(EddyRPM#60779,#7576), (MSD6AL,#6425) A904, GearVendorsOD, 8 1/4,3.55:1, ClassicAir

Now, I don't know how the knowledgeable guys here take their readings in cranking pressure, but I'll tell you how I do it: I beat the hell out of the engine, remove ALL plugs, remove carb and everything, connect the comp tester and crank it untill that cyl produces the reading. Never crank it more than 1 time per cyl per reading. Maybe I'm doing it all wrong? 185 would be a more realistic reading if your measurements works right. Now, seeing all the "findings" you've done, 180 ish sounds like the real deal... but then again, let's cool heads down and wait untill a real expert comes along. 220 PSI on your present confirmed numbers is still soounding high as heck to me.

_________________
Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

.....sez Doc
....sez Bren

OK OK the piston is 7 in the hole,,,and the head gasket will add a little but....

Adding 3.5cc to the chamber wont do it.

I think your chamber size indicates a need to open up around the valves. My head ended at a 77cc chamber size when all the chamber work was done. I milled the block for compression to end at 9.25 cr....the piston was still WAY in the hole.

I sez you built a 12 to 1 motor....by this math

If I back out of slants for a minute and look at a SBC with flat top pistons at ~zero deck with 64 cc chambers I get a 10.5/11.5 to 1 motor with a 3.48 stroke and 4" bore. (43.75 cid per cylinder vs 43 for Eileen).

I still think there is incorrect math here.

_________________
Yeah....Im the one who destroyed this rare, vintage automobile.....

I tend to agree.

With my 2.2/198 rod motor everyone tells me that it works out to 9.5-1. Everyone EXCEPT Mike J.

Mike J. says 10.8-1.

I tell everyone it's 11-1 anyway.

My eyes tell me that when you move the piston .160+ up the hole, you gain way more than 1.5 points of compression.

I hate math to start with. There's way too many numbers in there.

_________________
Official Cookie and Mater Tormentor.

I agree that simply adding 3.5cc to the camber won't do it. You have part of it, but missed the part where we will be milling the pistons .031 for a deck height of .038.

Can someone run these new numbers just to be sure.
Bore = 3.591
Stroke = 4.25 or 4.35 this has yet to be varifed but it is no less or no more, and it only moves the CR .25 max
valve reliefs = +3 (after milling pistons)
deck height = .038 (after milling pistons)
head CC's = 68cc (after grinding/cutting 3.5cc more in)
gasket compressed = .048
gasket bore = 3.66


I am coming up with 9.24 / 9.43 (depending on stroke)

_________________
67Cuda,FAST EZEFI,340cu,CR=10.25,RollerCam&Rocker (XR268HR,#20-810-9)(#1622-16)(EddyRPM#60779,#7576), (MSD6AL,#6425) A904, GearVendorsOD, 8 1/4,3.55:1, ClassicAir

At one time this may have made sence, but now it dosen't.

_________________
67Cuda,FAST EZEFI,340cu,CR=10.25,RollerCam&Rocker (XR268HR,#20-810-9)(#1622-16)(EddyRPM#60779,#7576), (MSD6AL,#6425) A904, GearVendorsOD, 8 1/4,3.55:1, ClassicAir

If your using flat top pistons.....And using the numbers you listed, I'm coming up with an 11:1 motor. (With the 4.25 stroke)

_________________
225 Cubic Inches of Iron-Head American Muscle

225 bored .040 /.100 off block, Schneider Cam 224@.050~ .480 lift - Stock valves, blended bowls, Offenhauser intake with 500 Edelbrock carb

I ran the numbers through one of the online calculators and got 9.48:1 using the 4.35 stroke. That makes me wonder about compression calculators.

_________________
David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

I got about 9.3 with 4.25 stroke and 9.5 with 4.35 stroke static compression ratio.
You may have posted it but I don't remember, what length connecting rods are you running?

_________________
Franklin
'61 Valiant

Yeah, Ive always wondered how accurate those are.
Here's the one I use: http://csgnetwork.com/compcalc.html

I did put piston volume at '0' last time, cause I didn't understand the valve-relief measurements. Assuming he means -3cc, it reads 10.7:1.
That's more believable.

_________________
225 Cubic Inches of Iron-Head American Muscle

225 bored .040 /.100 off block, Schneider Cam 224@.050~ .480 lift - Stock valves, blended bowls, Offenhauser intake with 500 Edelbrock carb

That calc also gives his displacement as 327.7 cid......

_________________
Yeah....Im the one who destroyed this rare, vintage automobile.....

Well, there's his problem!
Hehe!
Guess I better find another calculator.

_________________
225 Cubic Inches of Iron-Head American Muscle

225 bored .040 /.100 off block, Schneider Cam 224@.050~ .480 lift - Stock valves, blended bowls, Offenhauser intake with 500 Edelbrock carb