compression ratios and octane
Safety Car
Joined: Apr 2005
Posts: 3,624
Likes: 431
From: Dale City VA
It dosen't matter as his is a reverse cooled LTX. LTX to L98, is an Apples to Oranges comparison when it comes to comparing DCR and octane requirements.
I'm running 9.1 DCR and 11.66 SCR in my personal LTX on 91 or 92 with no problems here in the NOVA/DC area. Keep in mind I have .036 quench, Manley reverse dome pistons and AFR 210 Eliminator heads with very efficient combustion chambers.
8.5 DCR LTX's will run hard on 89 octane and never have a hint of pre ignition. I personally built one last year (thread stickied at the top of C4Tech) for a frined that has run on 87 octane on a road trip with no problems and pulled down over 420RWHP on 89.
An L98 with stock heads will have a hard time running on 93 octane with a DCR approaching 9.0, even with good quench (.040 or less). The problem lies in the inefficient combustion chamber desgin and where the quench areas are located in it. Generic D shaped "Dish" pistons make it worse IMO (than flat top's) becasue they have a big dead spot right under the spark plug and another one directly across on the backside of the valves.
Now you can get the DCR down below 9.0 by retarding the camshaft intake centerline, increasing the camshaft intake lobe seat timing (advertised duration) , reducing the SCR or a bit of all 3. The first 2 methods delay the intake valve closing event/point. How much you can move this event around depend on car weight, how many RPM you want to turn the engine, gear ratio and stall converter speed.
If it were mine: I'd ask the machine shop to take a seat cutter or fly cutter and "unshroud" the combustion chamber around the intake valve and the exhaust valve all the way back almost to the fire ring on the head gasket. That will get you a couple of CC's in the heads around +4 or +5cc over what you have now and then I would move on to the camshaft.
That camshaft is pretty mild. If you want to keep the motor tame you should try to find a lobe set with more seat timing (advertised duration) but the same/near same duration@.050. If you don't mind stepping up to a bit more radical you can just stay in the same lobe profiles you already are looking at and step up a set/couple of sets of lobes.
Try to keep the quench around .040 and for god sakes use the most accurate caclulators you can in order to determine you SCR and DCR. I use and recommend Pat Kelly's. You can read up on his site and down load his calculator here:
http://www.empirenet.com/pkelley2/DynamicCR.html
Will
I'm running 9.1 DCR and 11.66 SCR in my personal LTX on 91 or 92 with no problems here in the NOVA/DC area. Keep in mind I have .036 quench, Manley reverse dome pistons and AFR 210 Eliminator heads with very efficient combustion chambers.
8.5 DCR LTX's will run hard on 89 octane and never have a hint of pre ignition. I personally built one last year (thread stickied at the top of C4Tech) for a frined that has run on 87 octane on a road trip with no problems and pulled down over 420RWHP on 89.
An L98 with stock heads will have a hard time running on 93 octane with a DCR approaching 9.0, even with good quench (.040 or less). The problem lies in the inefficient combustion chamber desgin and where the quench areas are located in it. Generic D shaped "Dish" pistons make it worse IMO (than flat top's) becasue they have a big dead spot right under the spark plug and another one directly across on the backside of the valves.
Now you can get the DCR down below 9.0 by retarding the camshaft intake centerline, increasing the camshaft intake lobe seat timing (advertised duration) , reducing the SCR or a bit of all 3. The first 2 methods delay the intake valve closing event/point. How much you can move this event around depend on car weight, how many RPM you want to turn the engine, gear ratio and stall converter speed.
If it were mine: I'd ask the machine shop to take a seat cutter or fly cutter and "unshroud" the combustion chamber around the intake valve and the exhaust valve all the way back almost to the fire ring on the head gasket. That will get you a couple of CC's in the heads around +4 or +5cc over what you have now and then I would move on to the camshaft.
That camshaft is pretty mild. If you want to keep the motor tame you should try to find a lobe set with more seat timing (advertised duration) but the same/near same duration@.050. If you don't mind stepping up to a bit more radical you can just stay in the same lobe profiles you already are looking at and step up a set/couple of sets of lobes.
Try to keep the quench around .040 and for god sakes use the most accurate caclulators you can in order to determine you SCR and DCR. I use and recommend Pat Kelly's. You can read up on his site and down load his calculator here:
http://www.empirenet.com/pkelley2/DynamicCR.html
Will
Last edited by rklessdriver; Sep 16, 2009 at 06:57 PM.
Safety Car
Joined: Apr 2005
Posts: 3,624
Likes: 431
From: Dale City VA
Sorry but nope. That chamber is for lack of a better description a bath tub. Lots of "dead area". Typical of 1986 when it was desgined. GM basically made some very minimal improvements over the 041, 292, 461 and 462 chambers of old.
Now look at this SB 2.2 chamber. This is about as good as it gets for a wedge cyl head. See how the area between the valves has been sculped into a small well defined quench pad? See how the dead area has been shaped by that quench area to form a "figure 8"? This forces the mixture to burn near the center of the piston/chamber where it can provide the most efficient power stroke.
Even modern 23* heads are taking advantage of this kind of work. Look at an AFR Eliminator chamber.
Now you can't make a 113 into anything like these without a crap load of welding and grinding - but you don't have to... You need to drop about half a point of compression (SCR and DCR)
Take the head gasket and lay it on the head (line it with the head bolt holes properly). Mark the area around the sides of the intake valve and the exhaust valve. Remove that metal over hang, either with a die grinder or using a fly/seat cutter. Gain 4 to 5 cc's and drop some SCR. Your part of the way to where you need to be.
Be careful not to grind around that little square spark plug "quench pad" too much. The chamber is very thin in that area.
Will
Now look at this SB 2.2 chamber. This is about as good as it gets for a wedge cyl head. See how the area between the valves has been sculped into a small well defined quench pad? See how the dead area has been shaped by that quench area to form a "figure 8"? This forces the mixture to burn near the center of the piston/chamber where it can provide the most efficient power stroke.
Even modern 23* heads are taking advantage of this kind of work. Look at an AFR Eliminator chamber.
Now you can't make a 113 into anything like these without a crap load of welding and grinding - but you don't have to... You need to drop about half a point of compression (SCR and DCR)
Take the head gasket and lay it on the head (line it with the head bolt holes properly). Mark the area around the sides of the intake valve and the exhaust valve. Remove that metal over hang, either with a die grinder or using a fly/seat cutter. Gain 4 to 5 cc's and drop some SCR. Your part of the way to where you need to be.
Be careful not to grind around that little square spark plug "quench pad" too much. The chamber is very thin in that area.
Will
