Yeah, I like to set the Ls1's at 8.5 DCR to be a little conserative. LT1's run very well at 8.5-8.8. Don't forget head flow (VE) can also make a combo more prone to detonation even at say 8.8 DCR (I'm talking big numbers over 300CFM). And most that have those numbers have big compression to run their large mechanical roller cams.

C Rules...
The whole combo is whats important.....If your gearing and heads can only support a cam in that range then shoot for the correct DCR and Quench with that cam. They are both important. I almost bet if my quench was over .050 I would run into problems at my DCR level and have to run less timing to compensate (not good). In fact, I do have a little experience with 10.8 to 1 and no quench with trying several cams when this was a 396. Max power was at 34 degrees and adding more did not make a difference. I changed cams out quite a bit and any big cam on that combo did not make any more power over a 305 comp cam....Never could figure out the problem until you figure out DCR and no quench. My old stroker combo made 405 SAE rear wheel with stock 10 bolt and a manual tranny. My new combo makes 460 SAE thru a 12 bolt and a auto tranny. Huge change in horse power....

If your thinking of building a 383 cube "ABA 383" (i think thats right) has had really good luck with a cam in that duration range. I would try something similar but my bigger heads need high lift and I would need to open my chambers up a little to bring down the DCR with a smaller cam....To much work for me... I might try a 230/236 comp magnum lobes on say a 110ICL/lsa. My DCR would jump to 9.4 so this would only be a trial to see what happends, "ET wise." I probably could not run to 7k on juice anymore either so, it might push my car into the 10's (full weight...NA) but I like the low 10's high 9's on juice idea better.

Engineer.....
I keep an eye out on similar combos....
I got 1.52's sixtys (with skinnies up front) the first time out with my new combo on a PI3200 (flash rpms was like 4300-4400 rpms). Unfortunatley, that converter bit the dust taking my tranny and a bunch of hard parts with it. I have a SS3600 Yank now (to tight for me) best sixty of 1.61 with full weight 17's up front though so, not a direct comparison...Looks like I can either rebuild the PI or restall the Yank to SS4000 (should flash to 4300-4400). The yank would have a higher extension (great for motor) but not so great on juice. My old PI 3200 had an extension of 5100-5200. The Yank is 5k right now. Decisions. decisions..... I would imagine that PI3600 of yours flashes to 4600-4700 and has an extension of like 5300 ?

 

yeah ABA has a very similar cam we've spoken before. hopefully he tunes in. the best bet for me is the 10.8:1 i think because my dcr is around 9.3 with a 54cc heads on 11.8:1 compression.

 

I you sure your numbers are right? I would think you would need 11.5 SCR to get to 8.7-9 DCR with that cam size...... 10.8 is no higher than an LT4 and they had small cams with low abdc's (I've heard). That can't be right I used the little LPE 74211 (I believe that the right number) cam with 211/219 at .050 duration with dinky base duration and fast ramps with no problem on my old 10.8 to 1 compression combo. And your cam's abdc would be much higher allowing more SCR...I would double or triple check your numbers...or better yet post on here your ideal combo and i can check on my calc..

Just ran some numbers.... with a basic 383 combo with 6 inch rods 10.8 SCR comes up with 8.4 DCR with your 68 ABDC. At 11.5 SCR I get 8.93 DCR.

 

Cam advertised duration: 274/288
@.50 224/236 574/572 lift 111 lsa 111 centerline
Bore: 4.030
Planned stroke: 3.75
Combustion chamber volume: 54 not the 58 I originally thought
Planned rod length: 6
Planed head gasket Thickness: .039 (Felpro 1074)
Head Gasket bore: 4.030
Piston to Deck Clearance: Not sure what it is. I was using different numbers from .006 and larger
Pistons: Flat or dished 16cc gives me 11.0:1 if I go with the 5cc I get those high DCR #s and a really high compression ratio

 

you can run 15:1 with pumpgas and 5 degrees of timing its just not practical...to keep timing up there and stuff is say 12.5:1 or so...with 93 pumpgas of course

 

I run 12.6 to 1 on SHell 93 with 32 degrees total timing.

David

 

13.5-1 but our cam is quite large and brings the dynamic down...

 

Zero deck that block if using the felpro...

If it were my combo, I would find pistons or have some made that gave me a SCR of 11.3-11.5 so, your DCR stays up. Your would need to have the combustion chambers cc'd also to be right on. If you plan this right, and don't skip steps due to time, you can come out with a fast running 383 on that small cam. If you rush this process, you'll have one running High 11's low 12's, instead of mid to low 11's (full weight)..

Also...that head gasket bore is 4.125 for that felpro (I believe). That won't change much on the calc, but you want everything right on...

 

cool thanks jim. i definitely don't want to rush. that's why I'm planning everything before the motor is even out the car. i want to have a plan for any scenario that might pop up after everything is measured. should I be shooting for a minimum combustion chamber size or does that not matter a great deal as long as I achieve the proper DCR and SCR. I'll make sure I use a gasket with the proper bore. I'll run some numbers with different pistons and see if JE makes a set from with that relief.

I'm glad this whole post turned out to be a pretty good one.

 

I can't offer any more technical advice here than already stated...I spoke with Joe Overton at length when he spec'd my cam and told him what I already had which was an 11:1 compression motor as built by Lingenfelter...Eric Bradby re-ported the heads and found a bit more air and then I relied on Joe's knowledge of what "fits in there" as far as cam lift, duration and lsa based on what I wanted to accomplish and knowing that I liked the way the smaller cams perform...We calculated with .039 felpros that we were at 11.2:1 now and my cam specs are 226/234 .568"/.568" on a 111 lsa...I run it at 12 degrees initial and 36 degrees advanced...I don't pretend to know everything about DCR and SCR, so I used those who were very trustworthy and knowledgable in this area...

Once the cam/head combo is selected the rest becomes what will work with this combo to provide maximum results...I think that this is the area where we tend to screw up, too...Converter choice and rear gear/tire height will make or break ANY cam selection...My last experiment in gear/tire size picked up mph from 119.75 to 121.14 without opening the hood! This is one of the only times where I suggest using the dyno to map hp and torque curves...

So, in a nutshell, I run 11.2:1 and am very satisfied with how the combo works...I hope that with some of the changes I made over the winter that I may get a 10.99 pass n/a...we'll see...

--Alan

 

I here really good things about Mahle (-sp) pistons. I used probe pistons (4 cc dish) that are actually set up for a cup engine. All that you need to know is your compression height correct (besides bore) . These wierd 35 something cubed pistons with a short stroke were perfect for my 5.85 rods and 3.875 stroke.

Also, for the shape of our combustion chambers you would need a real good porter to tell you what's, what. Me, I would not worry about the chambers only making sure they were hard rolled and or polished up nice, and you know exactly what the cc's are.

I think you may have the wrong idea about head gasket bore. You want them bigger than your bore...It allows good transition from your combustion chambers to the same size or smaller bore size (valve shrouding badddd). 4.125 bore is fine unless you have a head with huge intake valves or are on a 60/40 split. Then they may be close to not working. You would need a pretty big bore to go bigger than that size and lt1's get pretty thin after .030 bore. Mine was sonic checked and had no problem with a .060 over bore (not the norm) especially when you have to grind off so much at the bottom cylinder to fit the rods.


Listen to ABA....
Gears and stall are huge for a given combo and the correct parts assembly (including cam size, compression and really good bottom end) wil help you to that goal of running fast.

 

thanks guys. i'll need a lesson in hp and tq mappings but i'll save that for another post. so i guess i had that bore thing all wrong. you need a head gasket with a larger bore than your block. i had that all wrong, thought they needed to be the same size. i'll have to ask my builder what size bore gasket he recommends along with the thickness to crunch the numbers again. i see the larger bore gaskets bump the compression up. the larger bore actually gives me better #s.

i might end up having to change my stall. i currently have a 3K with my 3.91 gears i'll have to talk to my trans guy when the new setup is in place. the 3k is pretty tame even with my current setup so we'll see what happens with that.

one last thing about this whole DCR stuff. is there a chart that gives what octane you should run for a specific DCR range?

 

Generally speaking for n/a street/strip applications you want the converter to stall 500-900 rpm below the torque peak of the motor and the rear gear to get you fairly close to the max hp rpm of the motor by the end of the 1/4 in 3rd (1 to 1) gear...I was truely amazed at how well the car responded to the switch from a 26" DR and 3.73s to a 28" DR and 4.10s...I cross at 63-6400 as opposed to 6000 and picked up a goodly amount of mph...I have since had my Vig 3600 restalled to 4000 to more accurately follow my torque peak which is 5000....I erred on the side of caution based on PIs recommendation for an LT1 which sees lots of street duty...We'll see if I can drop into the high 1.4s to 60' next year also with the aide of a Spohn drag rear sway bar...

--Alan

 

I'm new to this auto thing so that's some useful information. I'm more worried with putting my car together before figuring out what I'm going to do with it, but many times I sit here and try to put stuff together.

 

12.4:1 SCR/9.1:1 DCR, 0.032" quench, 91 octane + octane boost for safety, 34* timing, no detonation.

 

It really comes down to the cam. One of the old 350s in my 3rd gen had a 11.8:1 wiht cast iron heads with a comp 280H. It ran it on 87 with no pinging or problems with 35* total timing. My engine builder runs his 14.5:1 gen 1 383 on 93 sunoco with no problems but his cam is a monster and bleeds alot off.

 

okay guys just an update as my engine is currently being built and close to completion. i got all my numbers from my engine builder and used the calculator. i went with forged je pistons with a 10cc dish. this puts me at 11.3 SCR and 8.79 DCR the block is getting zero decked so with the cometic mls gasket quench will be at .039. the engine will be a 385.

im really hoping for 11s all motor NA at full weight. its a street car so I didn't want to go too crazy on the stall and suspension. i did however upgrade to a pi3200 converter tubular umi k member and lower a arms along with adjustable agx shocks on all four corners. plus i have the adj ta, sfcs, lca with the relocation beackets, and phb. i'll keep my fingers crossed.

 

I'm running about 12.2:1 with a baby cam on 93 octane.

 

Wow old thread.

The quality of your gasoline is a big part of the compression you choose, as well as camshaft selection.

 

Power gain from raised compression (i know there is a gain, just wondering what it would be)??

Is there actually a measurable gain in power by raising the c/r one point (10.5:1 -> 11.5:1)?