So is 9:1 DCR the actual generally accepted pump-gas limit? Some other posts have claimed that 9:1 DCR is nuts and that you have to shoot for closer to 8.5:1. Also, am I looking for the IVC at .006 or is it as soon as the lobe starts .00000000000001? I wouldnt imagine more than a degree or two discrepency between the two, so maybe it dosnt matter much.

As for the add 15 to the .050 number? I dont exactly agree with that. My duration at .006 is 281 and at .050 its 230. For my cam I needed to add ~25. I would imagine there are even less agressive profiles out there that would need even more added to the .050 numbers.

 

But you divide the difference your cam is duration wise at .006 by two cause duration account for that of the opening and closing side. So in other words if the difference of advertised verses at .050" is 40 degrees (which is pretty quick for a hydraulic lobe) you would divide that number in 2 to get 20 which is the effect it will have on each side of the lobe (opening side and closing side).

As far as the .ooooooooooooo1 you also have to consider there is valve train deflection and other events which may not actually make the valve move at all off the seat until .006 or even higher so the advertised number of .006" is probably pretty accurate.

 

Yeah, I got this part. I believe the f-13 has an intake duration of 281 @ .006 and 230 @ .050. Thats 51 degrees. 51/2 = 25.5. I rounded down to 25. if 20 is quick, 15 is even quicker for a hyd cam.

As for the second part, thats what i figured. Just go by advertised closing @.006.

 

Oops, sorry, missed that. Yeah, 15 is pretty much unheard of in hydraulics and fast for solids. The TK stuff from comp is their fastest standard rocker ratio lobe and it takes 28 degrees to get from .006 to .050 on both sides, 14 on one side. My street solid roller takes 38 degrees to get from .006 to .050 on both sides and 19 on one.

If you are only talking a difference of one or two degrees I don't think it will influence the DCR that much anyway. Good luck

 

keep in mind, the dynamic ion can be whatever you want it to be and the limit of pump gas is always determined by the actual pressure/temp in the cylinder (and well, timing).

so if you have a weak ultra low lift cam and a huge restrictor, no air is going to make it in the cylinder. pressure is low, no boom.

 

Well, heres the deal. I'm running the me-too-cam, the f13. It looks as though in order to keep my DCR limited to 8.5, my resulting SCR is a mere 11.0:1. I was willing to shoot for 11.5:1 SCR, but this whole DCR thing is starting to scare me. At 11.5 SCR the DCR rockets up to 8.86... Does it sound right that I should be scared to mill for 11.5 SCR with this cam?!? It just dosnt sound right that I should worry aout it.

 

I wouldn't be worried about it. Infact, I endorse it. Worst case? You run sunoko 94 or pull timing 1*.

Good tuning is an essential element to your scheme. Polish your combustion chamber, piston etc and use sodium exhuast valves. There are some good tricks to getting away with more ion. For some reason, good tuning is still a secret

 

Good tuning may not be available... Im the idiot behing the controls

I guess I woulnt mind dumping in a gallon of toluene if i really had to, but like you said, could always just pull timing.

 

I still come out high.

F14= (232+15/2 + 112) - 180 = 55.5 Plug that in with stock LS1 specs and 62cc chambers and even a .052" gasket nets me 9.337DECR

What am I missing???

 

Your adding 15 into your formula when you shouldn't be and your starting with your .050" duration, when you should be using the advertised.. Adding the 15 is if you already know your .050 IVC, then you add 15, or in most of our cases 25, to estimate your advertised .006 IVC. You use the formula if you know the advertised duration of the cam, but not the .050" IVC. In your case it would be 281/2 +112 - 180 = 72.5 . This is assuming the 112 is your ICL, not your LSA.

 

Yes, between 24.5 and 27 should be added if you are using a hydraulic roller. If you are using a solid it is likely somewhere between 19 to 15 that needs to be added.

 

Thanks, that puts me right at 8.1

 

Does DCR have a direct correlation to power? As in would a 9:1 DCR make more power, or is capable of more power as a 7.5:1 DCR? What effect does the gap between SCR and DCR have on engine performance. As in how does a motor with a 7.5:1 DCR and a 12:1 SCR compare with a motor with 8.5:1 DCR and a 9.5:1 SCR? I know these questions have MANY variables, but is there a general answer for either?

 

watered down explanation: more compression = more power
as for the second part- a smaller gap simply means your intake valve closed earlier. no more, no less. it just means the intake valve closes earlier in the compression stroke.

 

Theorhetically, you could run a high 16:1 SCR and a normal 8:1 DCR with a late closing intake valve. I know you wouldn't do this, but whats the point of knowing a motor's SCR when thats not the actual compression ratio during operation?

 

Once you get up to the RPM range for which the cam, exhaust and intake are tuned, you may have 100% or more V.E., thus DCR could exceed SCR. Normally, the reduced time factor prevents high RPM detonation, but when you're talking a 'lebnenteen to one' SCR...

PS: It would take an IVC of 'only' ~ 102 ABDC to get an 8:1 DCR from a 16:1 SCR..

PPS: Maximum safe DCR really boosts torque in the low RPM range where a big cam is weak.

 

I ran 16 to 1 SCR and 8.5 DCR on a 2 stroke Yamaha Blaster ATV (2 stroke 200cc air cooled single cylinder), along with re-shaping the chamber into a hemi.

Results : It took down a Banshee with headers and pipes (360cc twin cyl, water cooled 2 stroke)

Ihave also done 2 stroke RC airplane engines with good results.

Moral of the story? It can be done, but atv and RC engiones are much cheaper to rebuild than LS1's, so tread softly..

 

Well I put all my numbers in using both of the calculators and one came up 8.764 and the other was 8.64.