Compression ratio
Thread Starter
Racer
Joined: Apr 2005
Posts: 402
Likes: 4
From: Nassau
Compression ratio
I just worked out the compression ratio of my 496 and found out its frigging 11.85:1. There is no way I can see being able to run pump gas with this thing. I have aluminum heads with a 4.310" bore 4.25 stroke trw L2268 pistons with a -34.20 dome (I think) I got the dome volume after doing a bunch of internet searching but its always listed between -34.20 and -35.50cc. The heads are 118cc chambers and the block has been 0 decked. I calculated the compression ratio with a .041 thickness gasket.
My question is what would you guys do? Can I get a .080 thick head gasket and drop it down to 10.87 ish and even if I do would that be low enough to get by with maybe just adding an octane booster? I could also maybe get av gas and just run it at 11.8. I will have a problem getting race gas here. I have to import it myself and its expensive to ship because no one wants to deal with it.
I'm not interested in changing pistons so the only option is a thick head gasket or just to deal with using av gas. Can anyone recommend a particular gasket thats thick enough?
Thanks for any help
My question is what would you guys do? Can I get a .080 thick head gasket and drop it down to 10.87 ish and even if I do would that be low enough to get by with maybe just adding an octane booster? I could also maybe get av gas and just run it at 11.8. I will have a problem getting race gas here. I have to import it myself and its expensive to ship because no one wants to deal with it.
I'm not interested in changing pistons so the only option is a thick head gasket or just to deal with using av gas. Can anyone recommend a particular gasket thats thick enough?
Thanks for any help
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Maybe not one you would consider but with that compression ratio you can run on E85. I don't know what that would mean for carb and fuel system but it might be easier to buy there than AV gas and it's got a high octane content.
I think a thicker head gasket might make for a less than optimum quench which is usually around .041 if I recall correctly.
I think a thicker head gasket might make for a less than optimum quench which is usually around .041 if I recall correctly.
Drifting
Joined: Oct 2008
Posts: 1,496
Likes: 6
From: springfield ohio
It's been a while since I've seen the information about octane v/s
compression level but I believe you can as much as run 12 to 1 on 92/93
octane with out any problems. I have a sport quad (four wheeler)
that has 11.5 to 1 and it does fine on 92 octane.
Having the av gas as an option if the 92/93 octane isn't enough
you have a back up.
You may want more replies before you make your decision.
Might also do a search on the web about high compression and
octane levels needed.
Riggs.
compression level but I believe you can as much as run 12 to 1 on 92/93
octane with out any problems. I have a sport quad (four wheeler)
that has 11.5 to 1 and it does fine on 92 octane.
Having the av gas as an option if the 92/93 octane isn't enough
you have a back up.
You may want more replies before you make your decision.
Might also do a search on the web about high compression and
octane levels needed.
Riggs.
Thread Starter
Racer
Joined: Apr 2005
Posts: 402
Likes: 4
From: Nassau
The cam I have now:
Opens Closes Max Lift Duration
Intake 13.0 BTDC 51.0 ABDC 109 244 °
Exhaust 67.0 BBDC 9.0 ATDC 119 256 °
Here is a link to the cam card http://www.cranecams.com/?show=brows...tType=camshaft
Opens Closes Max Lift Duration
Intake 13.0 BTDC 51.0 ABDC 109 244 °
Exhaust 67.0 BBDC 9.0 ATDC 119 256 °
Here is a link to the cam card http://www.cranecams.com/?show=brows...tType=camshaft
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Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Burning Brakes
Joined: Feb 2005
Posts: 894
Likes: 91
Rod length!
I just worked out the compression ratio of my 496 and found out its frigging 11.85:1. There is no way I can see being able to run pump gas with this thing. I have aluminum heads with a 4.310" bore 4.25 stroke trw L2268 pistons with a -34.20 dome (I think) I got the dome volume after doing a bunch of internet searching but its always listed between -34.20 and -35.50cc. The heads are 118cc chambers and the block has been 0 decked. I calculated the compression ratio with a .041 thickness gasket.
My question is what would you guys do? Can I get a .080 thick head gasket and drop it down to 10.87 ish and even if I do would that be low enough to get by with maybe just adding an octane booster? I could also maybe get av gas and just run it at 11.8. I will have a problem getting race gas here. I have to import it myself and its expensive to ship because no one wants to deal with it.
I'm not interested in changing pistons so the only option is a thick head gasket or just to deal with using av gas. Can anyone recommend a particular gasket thats thick enough?
Thanks for any help
My question is what would you guys do? Can I get a .080 thick head gasket and drop it down to 10.87 ish and even if I do would that be low enough to get by with maybe just adding an octane booster? I could also maybe get av gas and just run it at 11.8. I will have a problem getting race gas here. I have to import it myself and its expensive to ship because no one wants to deal with it.
I'm not interested in changing pistons so the only option is a thick head gasket or just to deal with using av gas. Can anyone recommend a particular gasket thats thick enough?
Thanks for any help
I assume here you have a S/D (9.800") block? Even a T/D (10.200") block doesn't appear to "fit the bill" either.
Thanks, Gary in N.Y.
P.S. Don't believe this is a possible platform! The piston you mention is for a 427" C.H.!
Racer
Joined: Aug 2005
Posts: 347
Likes: 0
From: Pace FL
If those are your pistons... somehow?
you have a wide LSA @114*. The cam is not big for a 496 but because of the wide lobe sep and it being flat tappet you should be able to run without a problem on 92-93 oct. The flat tappet and 114 will blead off more pressure. I assume you are running aluminum heads. Your DNCR will only be 8.0:1.
good luck!
you have a wide LSA @114*. The cam is not big for a 496 but because of the wide lobe sep and it being flat tappet you should be able to run without a problem on 92-93 oct. The flat tappet and 114 will blead off more pressure. I assume you are running aluminum heads. Your DNCR will only be 8.0:1.
good luck!
Last edited by howarsc; Dec 2, 2008 at 10:40 PM.
Pro
Joined: Oct 2005
Posts: 541
Likes: 1
From: Freeport FL
I think you will be fine running pump gas at under 12:1, but I would dial my timing back to something reasonable, and then work my way forward. With my blower motor C3 I was running 36 degrees and was fine at about 8 PSI (13.11 effective CR) and didnt break a piston until I went to 10 PSI (14.26). 
I am interested in seeing the piston question answered though...
I am interested in seeing the piston question answered though...
Pro
Joined: Oct 2005
Posts: 541
Likes: 1
From: Freeport FL
If those are your pistons... somehow?
you have a wide LSA @114*. The cam is not big for a 496 but because of the wide lobe sep and it being flat tappet you should be able to run without a problem on 92-93 oct. The flat tappet and 114 will blead off more pressure. I assume you are running aluminum heads. Your DNCR will only be 8.0:1. good luck!
you have a wide LSA @114*. The cam is not big for a 496 but because of the wide lobe sep and it being flat tappet you should be able to run without a problem on 92-93 oct. The flat tappet and 114 will blead off more pressure. I assume you are running aluminum heads. Your DNCR will only be 8.0:1.
good luck!
Le Mans Master
Joined: Sep 2001
Posts: 7,739
Likes: 630
From: Unreconstructed, South Carolina
Just curious, how'd you get an L2268F piston with a 1.765" C.H. to work with a 4.250 stroke??
I assume here you have a S/D (9.800") block? Even a T/D (10.200") block doesn't appear to "fit the bill" either.
Thanks, Gary in N.Y.
P.S. Don't believe this is a possible platform! The piston you mention is for a 427" C.H.!
I assume here you have a S/D (9.800") block? Even a T/D (10.200") block doesn't appear to "fit the bill" either.
Thanks, Gary in N.Y.
P.S. Don't believe this is a possible platform! The piston you mention is for a 427" C.H.!
was wondering very same thing.If it ain't been put together yet, then get some different pistons ... sp book says sixty over popup L2268F has +34.2 dome volume. And, they've already been milled at factory ... that's how the volumes vary across the same p/n piston but different oversizes... different dome heights milled at factory ... the bigger the O/S, the more has been milled off top.
If you run 4.310 x 4.25 w/ 118cc on street pump gas ... suggest popup piston w/ about +10 to +20cc dv. If no spray, no blow & No/limited race ... hyper should work fine ... sealed power H112CP60 is 4.310 w/ 1.525"ch & +21cc popup dv & fits std 9.8 deck w/ OE rod length (6.135).
SP also has H552CP60 ... has shorter dome & less volume (+11.2cc) but same 4.310 x 4.25 w/ 1.525"ch.
Me thinks Mahle may still have forged that's about +18cc but requires 6.385 rods.
BTW, pretty sure that cam is hyd roller, ain't flat tappet.
Last edited by jackson; Dec 3, 2008 at 12:48 AM.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
DCR = 1 + (B2 x ((S + R) – ((√ (R2 – (sin(IVC) x (S/2))2)) + (cos(IVC) x (S/2)))) / [(CV + GV + DV) – PV]
…where B is bore, S is stroke, R is rod length, IVC is intake valve closing, CV is cylinder volume, GV is gasket volume, DV is deck height volume, and PV is piston dome volume. For a dish piston, you add the volume of the dish rather than subtracting. (The 2's in italics are exponents.)
Or, you can download a comprehensive calculator from the site below, which includes a good primer on DCR...
http://www.empirenet.com/pkelley2/DynamicCR.html
Actually DCR is a misnomer for this theoretical statistic. True dynamic compression ratio is SCR x VE at a given rpm and load, which is a good bit more difficult to surmise as you either have to build and test the engine in question or use elaborate simulation to arrive at fair estimates.
BTW, run 6.385" rods with a 4.25 stroke. (Need a set?)
…where B is bore, S is stroke, R is rod length, IVC is intake valve closing, CV is cylinder volume, GV is gasket volume, DV is deck height volume, and PV is piston dome volume. For a dish piston, you add the volume of the dish rather than subtracting. (The 2's in italics are exponents.)
Or, you can download a comprehensive calculator from the site below, which includes a good primer on DCR...
http://www.empirenet.com/pkelley2/DynamicCR.html
Actually DCR is a misnomer for this theoretical statistic. True dynamic compression ratio is SCR x VE at a given rpm and load, which is a good bit more difficult to surmise as you either have to build and test the engine in question or use elaborate simulation to arrive at fair estimates.
BTW, run 6.385" rods with a 4.25 stroke. (Need a set?)
Last edited by TheSkunkWorks; Dec 3, 2008 at 01:51 AM.
Thread Starter
Racer
Joined: Apr 2005
Posts: 402
Likes: 4
From: Nassau
GOSFAST I just checked the stroke again and you are right. I based it on what the mechanic I got the engine from told me and a friend supposedly measured it when I first got the engine and we were checking everything out so its just a 427 bored .060 over. Its my fault for never measuring it myself. I measured everything else and took for granted that my friend measured right and the mechanic knew what he had. I got it real cheap but it still sux thinking it was a 496 now its just a 427 lol. On the bright side the comp ratio is only 10.5 so I don't have a gas problem any more.
I hope the cam I got isn't too much for the motor now. I'm glad I checked it but am pretty bummed out at this point. I guess Ill throw everything back together and get another short block next year.
Thanks for all the help guys
I hope the cam I got isn't too much for the motor now. I'm glad I checked it but am pretty bummed out at this point. I guess Ill throw everything back together and get another short block next year.
Thanks for all the help guys
Pro
Joined: Oct 2005
Posts: 541
Likes: 1
From: Freeport FL
DCR = 1 + (B2 x ((S + R) – ((√ (R2 – (sin(IVC) x (S/2))2)) + (cos(IVC) x (S/2)))) / [(CV + GV + DV) – PV]
…where B is bore, S is stroke, R is rod length, IVC is intake valve closing, CV is cylinder volume, GV is gasket volume, DV is deck height volume, and PV is piston dome volume. For a dish piston, you add the volume of the dish rather than subtracting. (The 2's in italics are exponents.)
Or, you can download a comprehensive calculator from the site below, which includes a good primer on DCR...
http://www.empirenet.com/pkelley2/DynamicCR.html
Actually DCR is a misnomer for this theoretical statistic. True dynamic compression ratio is SCR x VE at a given rpm and load, which is a good bit more difficult to surmise as you either have to build and test the engine in question or use elaborate simulation to arrive at fair estimates.
BTW, run 6.385" rods with a 4.25 stroke. (Need a set?)
…where B is bore, S is stroke, R is rod length, IVC is intake valve closing, CV is cylinder volume, GV is gasket volume, DV is deck height volume, and PV is piston dome volume. For a dish piston, you add the volume of the dish rather than subtracting. (The 2's in italics are exponents.)
Or, you can download a comprehensive calculator from the site below, which includes a good primer on DCR...
http://www.empirenet.com/pkelley2/DynamicCR.html
Actually DCR is a misnomer for this theoretical statistic. True dynamic compression ratio is SCR x VE at a given rpm and load, which is a good bit more difficult to surmise as you either have to build and test the engine in question or use elaborate simulation to arrive at fair estimates.
BTW, run 6.385" rods with a 4.25 stroke. (Need a set?)
Seriously, thanks, that was good reading. I understood the concept, but thats a great little site. I'll have to plug in the numbers from my engine.
Le Mans Master
Joined: Sep 2001
Posts: 7,739
Likes: 630
From: Unreconstructed, South Carolina
so very very true common dcr program calcs are merely estimates ... fraught w/ inherent shortcomings ... and some have wide-open door for gross error built-in (recall KB's project-killer fault?). I also note the trouble one might reap if relying on a paid csr at a big name camhelp desk who knows how to operate his common dcr program but lacks practical experience & "feel" for cams & compression. Very few & rare are gearheads with both access to & expertise in CFD programs; not me nor any camhelp desk csr I'm aware of.
Anyway, OP's piston-compression issue resolved itself.
Me thinks 440" ain't no slouch ... you need only ensure cooling system clean, complete & in good shape ... hope you have 4speed or convertor ... or cam may be too much (that's merely an estimate too).
Racer
Joined: Aug 2005
Posts: 347
Likes: 0
From: Pace FL
so very very true common dcr program calcs are merely estimates ... fraught w/ inherent shortcomings ... and some have wide-open door for gross error built-in (recall KB's project-killer fault?). I also note the trouble one might reap if relying on a paid csr at a big name camhelp desk who knows how to operate his common dcr program but lacks practical experience & "feel" for cams & compression. Very few & rare are gearheads with both access to & expertise in CFD programs; not me nor any camhelp desk csr I'm aware of.
Anyway, OP's piston-compression issue resolved itself.
Me thinks 440" ain't no slouch ... you need only ensure cooling system clean, complete & in good shape ... hope you have 4speed or convertor ... or cam may be too much (that's merely an estimate too).
I have a masters degree in mech engineering and took the CFD courses. It all sucked and wish I had those years back now. lol. It makes pretty pictures but is just spliting hairs on 99% of the applications out there. It is useful if you only run on an engine dyno with controlled loads and temps. It is good for design of combustion chambers and port shapes. In addition to air temps, other variables such as fuel distribution, fouling, vehicle weight, traction, clutch vs TC, and variations in VE as you stated all influence detonation. Your intake manifold will heavily influence VE profiles as a function of engine speed.That said DNCR is just another useful tool that only as good as those who use it. It is fun to play with while you wait on the machine shop or UPS.
