motorsports302

alright so I'm trying to nail down my quench/compression ratio. Looks like a lot of folks are running 11.4:1 to 11.6:1 static compression ratios and 8.5:1 to 8.7:1 dynamic compression ratios on LS engines without issue from what I can tell. We travel in this car so I want to be able to safely drive this thing at sea level with 91 octane without detonation issues.

I have GM MLS head gaskets with a 0.051" compressed height and am planning for 0.010" out of block for a 0.041" quench. This should net me a static compression ratio of 11.16:1. The cam is ground with a 110 intake centerline and 277 advertised duration. Using a calculator program I got online intake closes 68.5 degrees ABDC, yielding a 2.711" dynamic stroke for a dynamic compression ratio of 8.61:1. Will this give me a safe combo? Anything special i should do or pay attention to when putting this together to ensure no problems? What would yall say a max "safe" compression ratio is for this combo at sea level with 91 octane?


Reference info
LS3 aluminum block (4.070" bore), 3.622" stroke, 243 64cc heads, flat tops with -3.2cc valve reliefs, 0.051" GM MLS head gaskets. combo will use a 228r cam, ported 243's, stepped long tubes, CAI, ported LS2 intake & tb, and ngk tr55ix plugs. shooting for 470ish rwhp daily driver '06 GTO.

Jimbo'sZO6

iTrader: (1)

You should be fine. I'm a bit over 11.8scr and right at 8.7 dcr in my 416 and all I can get around here is 91 octane. No issues whatsoever but I'm not running alot of total advance. (24* IIRC) and my cam has alot of overlap, (27*). My Wiseco pistons have a slight dome that was bigger but my engine builder shaved them down a bit to get the SCR and DCR down a bit,
Heads are big bore AFR 225's with 69cc chambers and I'm running GM MLS gaskets at .051 compressed thickness. As long as your tuner is good you won't have any problems. When I still had the 346 in my car I ran 11.5 scr and had the timing cranked up to 28* and it ran fine on 91 octane fuel.
Just curious, since you went with an LS3 block why didn't you go with a 4" crank and get the extra cubes for the hp and tq increases?

Jimbo

racer7088

Unless its a hardcore weekend warrior drag race deal we run low 11 to 1 range as it seems the higher you gp you do have issues with tuning and detonation etc. If you have a poor heads cam and intake combo that doesn't make as good a VE you can run more but all our good stuff will not run on pump gas at 12 to 1 really. We try and stay 11.25 or under FWIW on the long runner stuff oin general.

Jimbo'sZO6

iTrader: (1)

Eric,
I should have added that at this altitude where the car spends most of its time, (7500+') higher compression seems to work pretty well. Pete Incaudo did the heads and speced the cams for both my builds and recommended I go with these numbers. This car is by no means a daily driver, but I do take it on long trips and it's been to lower elevation plenty of times and seems to handle both environments equally well. The tune is by no stretch on the "ragged edge", (12.3-4 to 1 WOT on the track or the dyno) and it still gets decent mileage on the freeway. (25+ if I don't flog it too much)
Just want to let you know your expert opinion means alot and when I do my next motor you are going to build it!
Cam specs are: Comp XE-R 248/254 .615"/.647" 112+2

Jimbo

motorsports302

My stock LS2 bottom end let go, so I already had the heads and intake. I wanted to reuse them and didn't feel they would fully support a 416. Besides I'm a fan of higher bore to stroke ratios, guess that started with the 283 in my old truck I also wanted to maintain highway mileage but it looks like that isn't an issue with you getting 25mpg with your 416 haha. The 376ci will be plenty for me... especially coming from a stock LS2.

Looks like my SCR and DCR are on the higher side of what's recommended, but should make for a strong street setup. From what I read, the higher DCR should give some nice low end and throttle response, especially with my smallish cam (228 duration at 0.050" and 0.588" lift) and the flow of my heads peaking around 0.550" lift (~300cfm) to match the cam. This is a weekend and fair weather car but I like to break it out for daily driving sometimes and want it to be up for it all the time if I so choose! Hope the combo works good anyway!

racer7088

Sorry didn't see you were way up there in the thin air! Yes you can definitely run more compression up at that altitude for sure. Compression will also help your mileage too but down here in Texas we can't run as much since the air os more dense and sometimes the temp is also very high and the engines run hot.

hymey

I'm at 30* with L92s and 230s cam and compression is 11:1. It just loves timing like some tuners like Ed from VA mention with the L92s. I was considering going to 11.8 but just decided to stay at 11:1. We have a pump gas 355 Holden V8 running dash-9 yella terra alloy heads (same place that makes the rockers) It made 597 hp on 33 deg timing with a modern style chamber and 11.8:1, going to 12.3:1 picked up 30hp to 627hp 355 all motor still running 33* every motor is different IMO.

JonCR96Z

iTrader: (33)

Who builds a hotrod for highway mileage?

Your DCR is not bad, but why did you go with such a small duration cam? You could retard the cam a few degrees if you're worried about it, but it'll be fine either way. You could have gotten Gary to open your chambers up some while he had your heads.

Good luck.