Just to put things into perspective, A friend was told that his new cylinder heads from a top tuner had 72cc chambers when in fact he was shipped the 62cc version. His 403 stroker was therefore running 12.4:1 static on 93 pump gas with a 236 cam on a 114+0 LSA, and we couldnt figure ot why the car was not running the usual 24 degrees timing but rather only 22 degrees. It was still stronger and ran without any knock retard. The net result of the added full point in compression was 2 degrees less timing yet it made 14rwhp and 20rwtq more at the peaks.

I state the above to illustrate how much a distorted view so many put on compression and how some people think there is some kind of off-on switch when you go from 11:1 to 11.5:1. Trust me, its not that close.
Just about every H/C car out there can run far more compression than they do.

 

Thanx for repeating your experience here, Guy. I had seen a similar post of yours before.

According to the data Joe_G sent me, and assuming the ramps on his LG cam are similar to COMP XE-R ramps, his theta is about 75 degrees, and his DCR is about 8.2 to 1.

If he can avoid having the valves get friendly with the pistons in his new heads, he can bump up his static compression ratio from around 11.3 to 12 to 1 and still have a DCR of 8.7. Interesting, no?

 

Lee you've got me focused on this now...I'm eager to get some mods done now that Moroso will re-opening in 2 weeks!

Sent a note to Roger at HPSFLORIDA to put me in touch with Livernois....maybe they can do something custom with the valves (raise them) or fill the chamber a little to give me some more compression without cutting! Worth asking. I'm not keen on cutting the pistions, though I know since I'll never use nitrous or FI I shouldn't be bothered...

 

Subscribed

 

I offered to host the spreadsheet for download, just to make it a little easier for those who want the Excel file. The Clevite Kid gave his approval, so you can now find a copy at the link below:

Clevite Kid DCR Chart

 

Thanx, Gannet.

 

All very true and all good points. My 8:1 boat motors now have 9.5:1 SCR and run 87 octane gas. People told me I was crazy but the numbers for the cams and heads I was running said otherwise...I could have run even more. As for timing advance, I've always said that sometimes less is more.

 

Thanx to Joe_G, I found out that the money I invested in sponsorship of this Forum does get me something - the privilege of posting a LIVE spreadsheet. So thanx to Gannet for hosting it on his website , but HERE IT IS for all to use and modify.

- Lee

 

The consensus (from SpinMonster, of course!) is that with a tight quench (0.030" or so) the heads for the LS2 (and LS3) can run up to 8.7 to 1 DCR on pump (93 octane) gasoline.

Any thoughts on what the DCR should be for 91 octane at sea level?

 

Does anyone have the closing specs for a stock LS7 cam to apply to the chart? Thanks for posting the chart.

 

I searched for this thread today to give a link to someone, and re-reading it, it's just too good to have buried down in the archives.

We have some real brain cells following this forum willing to give us the benefit of their knowledge for free. It's a great thing.

Lee how you doing buddy. Long time, did you ever buy HP Tuners?

TTT for those who haven't seen this.

 

Got HPTuners Pro - check your email, Joe.

 

I had seen this, and a year ago, it made my brain implode. I just woke up from the coma today, actually.

I failed math just about every year in school, so this thread is giving it to me with no KY right now, but it's definitely good reading. I'm doing my best to understand what's happening here by converting everything into verbiage that translates to my GED level education. And a GED of the California Public School System, no less.

Joe_G, interesting choice on the Livernois Stg 2's, by the way. I have a set sitting on my living room floor about 5 feet from me right now, with a ported/smoothed Fast 92 to match. If you haven't made your decision on heads yet, keep an eye out for my results. They'll go on shortly...once I sort this misfire issue, and procure a new clutch master cylinder. It never ends...

Clevite-

Thanks a hell of a lot for this thread. And more like it to come, hopefully.

 

Hey Slickshoes!
My trans troubles mean Im sticking with stock heads for a while.

 

Haha, exact situation here. I bought the heads months ago. They've been sitting since around August. Tranny took precedence.

 

CleviteKid's spreadsheet says theta for the LS2 cam is 68* ABDC (the point where the intake valve has closed to .004" off the seat), but this site claims that theta for the LS2 is 81*. That makes a fairly huge difference in stock LS2 DCR (7 1/2 vs 8 1/2). Corvette Action Center seems to have a direct line to factory engineers, so I'm supposing the cam specs they publish are accurate.

Anyone have different numbers for stock cams than those in the link? Or is this discrepancy caused by the difference between .004" lift and .006" lift values?

Jeff

 

Spreadsheet looks good, at first glance stroke should equal 3.622 and rod length should equal 6.098 (for stock LS2). To some trivial but when doing something such as this, carry at least 4 decimal places when possible.


Mike

 

The chart on the CAC site is incorrect. It has the correct .050" duration, lift, and centerline numbers but the valve events are way off. Take the intake centerline number and back calculate the valve events from the .050" and .004" duration numbers. For example, the LS2 intake centerline is 118° ATDC and we'll use 270° as the .004" duration. Half of 270° is 135° so you go 135° in both directions from the intake centerline to get the .004" open and close specs. Running the math gives you 17° BTDC/73° ABDC open/close while the chart has 9° BTDC/81° ABDC open/close. I'm not sure about the .004" duration numbers but they're not off by much if they're wrong. For instance, the chart has the LS2 intake at 270° (which I used) while Clevite Kid used 268°...I have no idea which one is correct. If we use 268°, that still gives us 16° BTDC/72° ABDC open/close which isn't 68° ABDC for the closing point that Lee calculated. A 72° ABDC closing would put the DCR at 8.16 which isn't much less than the 8.45 he calculated. The difference is most likely he used the .006" duration numbers while I used the .004" duration numbers. BTW, the .004", .006", and .050" lift refers to tappet lift, not valve lift.

The cams in my boat engine just happen to be on a 118° intake centerline which is the same as the LS2. I have 234° duration @ .050" tappet lift and 274° duration @ .015" tappet lift. They're solid rollers which is the reason for using the .015" tappet lift as solid lifter cams typically have the advertised duration measured at the recommended valve lash. Anyway, my .015" intake closing point I use for my DCR calculation is 75° and since I have more duration than the LS2 cam, there's no way the LS2 .004" intake closing point is 81° ABDC.

 

once again I searched for this to post a link....and again I'd like to bring it to the top for those that haven't seen it.

This kind of info makes the forum great. Thanks again Clevite Kid!

 

You guys are AWESOME !! -- -- --