I have been learning about DCR from this site and have a question that doesn't seem to make any sense to me. The new Chevy 572 (street version) has a 9.6 compression ratio but yet the cam in that motor has 254/264 @ .050 duration. I don't know the actual seat timing on that same cam but I can't see it being much under 300 if not more. My understanding is that you want a DCR of around 8.0 if possible. I don't see any way this cam combo is even close to making that kind of DCR so what gives?

DCR has a lot to do with the intake valve closing point, as well as Lobe Separation Angle (LSA)/overlap. This engine is setup to run on 92 pump gas on the street and under load at the track. I couldn't find any info other than what you have on the cam, but the valve timing and LSA #'s would give us a better look at its DCR. If you notice the ZZ572/720R steps the SCR up to 12:1, which seems a little soft compared to Reher Morrison, and Shafiroff's 13-14:1 sportsman engines.

The zz572 cam specs are as follws:
254/264 @ .050
316/326 @ .004
.632/.632
112* LSA
97* overlap
Bob

Overlap will determine idle quality.
Intake closing is part of the DCR. Overlap will not influence DCR other than by the fact that it is controlled in part by the intake lobe. They are all related, but dcr is basically determined by rod length, CID and intake closing. Now you can play with the intake closing by changing the advertised duration checking point. If you use .006" lift, then that is a good comparison point. You can use about 4 degrees per .001 lift to tweak the numbers to .006. So based on the cam specs listed above, subtract 8 degrees from the intake duration and plug that number into Pat's calc.
This works for Hyd cams. It's different for solids. You have to take lash into account for them. Also, GM wants their engines to be reliable, so they use lower DCR's to minimize fuel problems. This is why some people won't buy them. You can build your own, with similar quality, but more power. If you know what to tweak.

And as the bore and stroke go up, the acceptable DCR for pump gas goes down.

I haven't ran the numbers but I would guess those kind of numbers are in the mid to high 6's, maybe even lower. That is quite a step from 8.0. I guess that is where my confusion comes in. I do understand the higher compression would be better, (Hot Rod Magizine showed that you could run the High Compression motor on pump gas) and I do understand that it is due to the seat timing on the cam lowering the cylinder pressure. BUT the street version has to have a very low DCR. So is that low of a number ok or should that motor have a smaller cam in it? HELP

why don't you run the numbers. If anything it will teach you how close or far your guesses are.

Low DCR's typically will cause poor fuel mileage, soggy throttle response and reduces torque throughout the band. But with 572 ci. All these symptoms will be muted. Except for the poor mileage.

OK had to guess on a couple of things, I set the center line @ 110 and used 6.8 rods in the calculation. I am coming up with a DCR of about 6.17. I ran the calculator with 9.6 SCR and installed the duration numbers at .004 provided. This just seems really low and it goes against what I am hearing. Not trying to pick anything apart but actaully trying to learn something. I am building a 496 and the roller that was suggested to me dropped my DCR pretty bad (6.78) but is this an acceptable cam for the SCR?

Ok I went and ran it. Had to figure out the bore/stroke for a 572. And find where I hid the calc on the work computer.

Got 6.7 dcr, used 9.6 cr, 308/316 112, 6.5" rod
I wouldn't go lower than 7.5 dcr.
Now you know why it makes only 620hp.
Needs about another point of compression for about 50 hp.
Put's new perspective on those magazine dorks installing a smaller cam too. Wonder who told them to try it?

So far this is great information! I didn't take into account the seat duration at .004, I know the SAE is .006. But if I pull 8 degrees off the intake like suggested and throw a couple more degrees advance in the cam (only 2 degrees advance ground in)I can get it up to 7.47 so I am see better numbers that make sense for my 496. As far as the 572 goes I know we are guessing on a few numbers but the DCR still seems really low.

The 620 horse is a very conservative rating.....
Many have reported in excess of 700 hp on the dyno.
GM doesn't want to be criticised for a low power figure.
BTW...the 720 engine makes in excess of 800hp.

Personally....I still think compression ratio is way over rated.

Ron

Looking back at a lot of factory engines, the DCR is almost always quite low. It's a surprise if they reach 7.5. That's a way to insure that they don't have detonation problems. Chevy engines make good power but usually much less than can be got with a less conservative build and tune. They have to warranty these things.

The DCR thing works very well but there are a lot of folks who think it holds water as well as a sieve. The rendition used here was brought to my attention by David Vizard. He (along with many engineers over the last 70-80 years) spent a lot of dyno and track time determining the CR ratios that work. Chevy sells these engines to anyone with the requisite green and needs to insure they will run no matter who owns, drives, or maintains them. Consequently, they get a conservative build. These engine will make more power with higher compression.

Actually, SAE calls for .004 or .0045, not .006.

All this DCR theory is wonderful when planning an engine, but the real test is cranking compression. If the 572 makes at least 170PSI, it will make good power on pump gas. If it's down to 130PSI, then I'd call it mismatched.

Even though the DCR is 6.7:1, this takes NO account in volumetric efficiency! For all we know (and I don't think anybody here does), the hundreds-of-thousands of dollars that GM put into R&D work may be actually payed off in the way of VE. It's possible that the components on that motor are matched well enough to attain a high VE, and would thereby have to lower the compression to accomodate pump gas.

Alot of people seem to disregard the 572 because of it's "low" dyno numbers. Honestly, when did we start putting so much faith into the dyno? We all know that different dynos give different results. I seriously doubt the dyno that GM uses is the same as the local performance house. The real test is the drag strip, plain and simple.

Besides, it's a HUGE engine! Max power comes in below 6000. And the thing make massive torque. A 454 will pale in comparrison.
Shoot, it's more than 100 CI bigger than a .060 454.
The heads I believe are unique to this engine. Not the same as the over the counter Alum perf head. Still might be made by Edelbrock though.
Put a little more cam in it, bump the CR a bit, maybe either port the heads or replace them with something better and get out of the way!
I wish I had one in my car. Although the 12 bolt might not like it.....
Ron

The DCR calculators I have seen ask for closing point at .050. If you do this using these #'s you get 8.16 for 110 IC, and 8.35 for 106 IC. Don't know which one is correct, but maybe some of the recommendations you have seen are based on .050 timing #'s. You'd be hard pressed to get 8.0 #'s using seat timing, unless you 1) have tons of SCR, or 2) you are closing the intake incredibly early.

The VE of the engine plays a big role on things too. High VE will pack the cylinders full of more intake charge thus negating the low calc. DCR #'s. I put a new solid roller in this winter that had 2 degrees less LSA and a tad more duration. It was supposed to be installed on a 104 CL, but I backed it up to 106 like my old cam. So, the intake is now closing a bit later, but the tighter LSA should be helping VE in upper RPM ranges. One would think cranking compression would go down, but cranking compression went from 165-170 with the old solid roller to 175-180 with the new solid roller. This new cam is also approx. .05 quicker in the 1/8th, but approx. .02 slower in 60'. Wasn't much of a gain, but he old cam had an issue and had to be sent back. I just had them regrind the new one this way to see what happened.

Think about this, with 1.7 rockers the valve is open .085" at .050" tappet lift (hydraulic cam, less with a solid). How much compression will be made with a hole this size in the chamber. Figuring DCR with .050" intake closing time will give a false number. Seat times have to be used since the chamber must be sealed before compression can take place. The 7.5 to 8.5 DCR recommendations are based on seat times. I don't know the exact numbers but to use .050" lift, the DCR needed would be quite a bit higher. Just a guess but probably about 1.5 ratios higher (7.9 seat DCR is about 9.4 at .050") and it still wouldn't be as accurate as seat times.

DCR, as calculated by my calculator, does not have any inputs or output for VE or cylinder pressure. These vary with load and rpm. It simply substitutes the piston location at intake closing for the stroke length. For more information, I highly recommend reading David Vizard's book on cams and valvetrains (pg 25 in particular). He goes into quite a bit of detail (although there is an error in his calculations at one point).

Pat, I am glad to see you are chimeing in on this. From what I read, you are the man to talk to on this subject. Matt/Ron, I was not saying the 572 is wrong but all I was saying is that the DCR seemed low. I agree a lot of R&D has gone into that motor. In fact I am using it as an example in a positve way. I have a 496 that I am assembling and the Hyd roller that I now have is a Lunati with the seats at 309/314 and I only have 10.1 SCR. the .050 numbers were fairly close to the 572 (250/255) but mine has a smaller seat time. I also have a little more compression (only 1/2 point). So I am trying to convice myself that this cam is not too large for the motor. I was just thinking that I have a better DCR than the 572 so I shouldn't be too far off.

As Ron suggested, this engine is huge so torque isn't a big problem. I suspect that the bigger the engine the less important having a high DCR becomes, at least for a street engine. What happens with low DCR is the bottom end torque suffers. With a 350, and I had an engine like this, putting a 300º+ cam in a 9:1 engine (DCR ~6.7 or so, depending on a number of factors) will cause the engine to be a total stone below about 3000 rpm. With an engine around 500ci or more, due to the shear size of the engine, it will make gobs of torque anyway so running a lower DCR is much less of a factor. If this was an all out race engine, you'd want to optimize the DCR, running as high of compression as possible. By running a lower DCR in a big engine, you take advantage of the size of the engine and eliminate any chance of detonation. That's my thoughts on it.

One possible advantage of doing what you are trying is, the engine may be easier to launch (since the lower DCR will kill a bit of low end torque), yet the big cam will produce power in the upper rpm range. Also, a 300º cam is almost tiny in a 500 inch engine compared to a 350 SB. Since a big bb draws a lot of air at lower rpm a big cam may be necessary just to feed the cylinders. I don't know, and this would be interesting to research, how much variation in rpm range there is between a 350SB and a 500 inch BB given the same duration cams. Will a 300º+ cam in a 500 (496) go to 7000+ rpm as it will in a 350.

Take an old 265, a 327, a 350 and a 400 and then switch over to the BB head and then a 402, 427, 454 and finally the 496 and run them all through D2K holding everything the same except motor size and it will be a good indication how cam size needs change due to motor size increases!!

Been there and done that little exercise and also changed cam sizes too later just to see how that changed too...

It is real interesting to watch the trends, or at least to me....

pdq67

Your right Paul. I've noticed for my 406, I can look at all the RPM ranges for camshafts but I have to subtract about 1000RPM because the catalogs are oriented for 350 motors...

Thanks Silver,

Between the stuff I learned at Pat's site and doing that, I picked up a lotta info on how to make "street-cammed motors"..

pdq67

PS., I also call them "grunt motors" b/c that's what it takes on the street, what with mild rearend gears and bigger tires and all..

I've seen the DCR calculators based on both seat (PK's) and .050 (Keith Black and others). Both have merit. On a fast ramp cam, the .050 will be close to the seat results as far as real world power. On a traditional HP cam, the ramps are more suited to higher rpm and literally outrun the air loss to make high RPM power. No matter how you cut it, a really low DCR based on seat numbers will be a slug in a street motor operating at midrange RPM.

Interesting comments on KB's site regarding their calculator:

"More about intake closing time. The calculator asks you to use intake valve closing time at .050” lift because this is the most popular printed valve checking point used by cam grinders. The real closing time of most modern cam profiles is about 15°'s later. By adding 15°'s to the .050” lift, expect to get a better, more real-world, effective compression ratio number. At most intake valve closing times the piston speed is slow enough that very low valve lift will still reduce the compression. Be sure to add 15°'s to your .050” lift closing time when using the "Effective Compression Ratio Calculator"."

Italics addedThe 15º is an estimate to correct their calculator to something closer to seat times. I ran my engine data through KB's calculator. Using seat times, they agree to 3 places. Using .050" + 15º gives a DCR, in my case, is .7 of a ratio higher than seat timings. I still think seat timings are best to use with faster ramp cams. With slow ramp cams, using .050 + 15º might be closer to reality.

I'm absolutely not an expert but I think the best way to calculate DCR is by actually measuring the cylinder volume at intake valve closure. An old street racer from years back told me when he ran his 302 Z-28 he dialed in his cam timing with a compression tester. He just kept advancing the cam untill he got the cranking compression he wanted, this was with the GM off road solid lifter cam. He smoked many big block cars with that little 302. It would be interesting to test some of these calculators by actually measuring the cylinder volume at different points with the different rod lengths, strokes, etc.