Well, we have all seen threads relating to dcr and how much is safe for pump gas of varying octanes (91-93 for the most part). Well I have always wondered how the cc500 which has specs of 258/264 (adv.) 206/212 (@.50)
lift of .480/.488 (1.5) and a 112 lsa with a 108 icl (112+4) is able to run safely in our engines without knocking especially on a factory tune and thermostat. I think the ivc would be 57* and with 54cc (or less in some cases) factory combustion chambers, .050 gasket, 4.5cc relief pistons, .020 deck height, and all the other necessary bore stroke etc. I get a dcr over 9:1

link to specs http://www.compcams.com/information/...ss/LT1_LT4.PDF

Now I don't know of anyone running this cam but it is a comp cams grind specifically for an lt1 motor. Just curious why comp would spec something with that high of a dcr if above 9:1 or even lower you run the risk of detonation especially if having to run 91 octane and factory fan settings. Any insight? maybe the lt1 can sustain more than the magical 9:1 with no problems but I don't know of anyone doing this and certinly not with stock cooling/tune.

 

It is ~8.8 DCR. The LT1 with a small cam is pretty much at the edge wrt detonation. Thew factory tune relys on the KS to protect the motor.

Rich

 

what specs did you use to get 8.8?

 

Basically the same as yours but 5cc valve reliefs and 56cc chambers. There is some vulme above the top ring that isn't accounted for anyway. But the explanation isn't based on the difference in calculation - it's based on the fact that a stock LT1 does detonate under many commonly encountered operating conditions, the KS saves it.

Rich

 

I know, but I was thinking that the detonation would be rather severe at this level so that the ks would pull a lot of timing which would lead to poor perf. etc. So are you saying that there is roughly 2.5cc of volume above top ring? or are you just using 56cc as your combustion chamber volume?

 

Wouldn't that cam have lower DCR than the stock cam, since it has more overlap?

 

1) more overlap means the whole chamber isn't full of available O2... some of it is spent gases that serve to reduce the available O2.

2) the later valve closing of larger than stock cams makes for a smaller DCR than the stock one. (i.e. the GM847 cam in my stroker is fine on pump gas, but a stock cam would detonate the gaskets to hell )

3) I have a feeling DCR "limits" can be somewhat higher in a LT1 due to reverse flow cooling... which is why you hear other enthusiasts talk about 8-8.5 DCR for targets, and LT1 guys look more at 8.3 to 8.8 (some as high as 9).

4) like rich said... KS saves the day. modern motors can afford to get teh DCR aggressive for better fuel economy at part throttle, and still have a decent top end with KS-timing control.

Any part-throttle condition severely compromises the VE an engine operates at and you have little to no problems with detonation (at least from high DCR). It's WOT that bumps the VE, and hense the DCR to its highest levels. Some people don't refer to this as technically DCR, more like the actual cylinder compression level or whatever, but its a way to look at DCR a step further and realize what conditions you need to watch for DCR.

Ever notice the stock GM tune runs the A/F ratio at almost 10:1 at WOT? Better WOT performance is found around 11:1 or 11.5:1, but I have a feeling this was one of the GM warranty "just in case" situations to avoid problems in high-milage LT1's detonating down the road. No proof really, but I have my suspicions....