scottyp99

Hi, folks, I know this isn't a Corvette specific issue, but I know there are some pretty knowledgeable engine-builder types on this forum, and I'm just gonna throw it out there and see if anybody can help me out. OK, here goes.....

I have a Dynamic Compression Ratio calculator, and I have been playing around with different combustion chamber volumes, head gasket thicknesses, and cam timing in anticipation of a cam and head swap for my stock 1980 L48 engine equipped 'vette, with the goal of making a combo which will give me some more power and also run good on 87 octane gasoline.

Question #1: Can anybody tell me what sort of dynamic compression ratio to shoot for in order to run good on 87 octane gasoline?

Question # 2: I have run a bunch of different camshaft's timing thru the DCR calculator, and have noticed that the stock L48 advertised duration is really long (280 intake, 288 exhaust) compared to it's duration at .050 lift (195 intake, 202 exhaust) and so, the DCR calculator shows that with this cam, I can run a pretty high static CR and still have a pretty low DCR. All of the other aftermarket cams I have plugged into the DCR calculator have given me a much narrower range between advertised and .050 duration, and hence, between static and dynamic CR, for instance, the Comp Cams 252H has an advertised duration of 252 on both intake and exhaust, but a duration from .050 lift of 206. My question is, is this some kind of quirk in the way that chevy measures cam duration? Or does the stock cam just take a really long time to ramp up to .050 lift? I would sure consider running the stock cam if its accurate, less work for me, and I like the powerband charachteristics of the stock cam, also.

OK, I hope some of you gurus out there can help me out, thanks for your time, I'm out!

Scott

Belgian1979vette

I would suggest you stay around 8.0 DCR.

As for the cam numbers. You need to take into account that you keep loosing compression pressure untill your valve is completly closed. Only from that point on, you can calculate how much air is trapped in the cylinder.

63mako

What calculator are you using? Are you using aluminum or iron heads? You might like the powerband characteristics of the stock cam put it doesn't make any real power and won't no matter how much compression you run. Stock L48 cam able to run on 87 octane you would be lucky to get 250 HP. It has real lazy ramp rates compared to modern cam grinds.

scottyp99

OK, maybe I didn't phrase my question correctly. What I really want to know is this: Does the stock L48 camshaft REALLY have THAT much duration? It sounds like an awful lot!! Thanks'


Scott

howarsc

if it isn't an everday driver then design it for 91-93 octane. Prie is not that much different. It is more complicated than just DNCR but static CR and other factors play a role (even assuming optimal timing). I would not run 87 at sea level in the south on a static CR more than 8.5-9.0. Dynamic should be closer to 7-7.5 for 87. But Vehicle weight, gearing, temps, DA, driving intent (towing, racing, ect) all play a role...

gkull

The total flow is what throws all the calculators out the window.

Lets say motor "A" has a stock air cleaner/carb/intake/heads/wimpy lobe profile/ exhaust gas cross over and or EGR/ restricted exhaust system


Every motor is an air pump

Motor "B" has free flowing everything without EGR and heat and the cam is the same as motor "A". Motor "B" will have a higher dynamic compression ratio because it can pack more cold air in.

Going with something like open air cleaner/aftermarket aluminum intake/heads/cam with bigger lobes and the "DCR" climbs

Going from h-flat to solid lifter to roller cam profiles with big CFM flowing heads makes cam duration touchy.

scottyp99

Yes, that's called volumetric efficiency, and forced induction makes an even bigger difference, like blowers and turbos. That's why you sometimes see engines with great big blowers that have, like 6:1 static CR. Their DCR is probably even mor than SCR!!!


Scott

jetnoise

Until you know where your slugs are at in the hole, your exact chamber cc, and where your cam is phased...your guessing at best
DCR is only one part of the equation. Don't get hung up on just it.
Your stock cam probably looks like a broom handle. I'd go by the .050 number if your trying to compare.
A small step up in cam can yeild you even better results even in the same power band as your oem peice and then some...

here some reading material for you

http://www.empirenet.com/pkelley2/DynamicCR.html

MotorHead

Volumetric efficiency and that increases with good heads and intake.

so of you had two identical short blocks with different tops ends ( heads, intake and carb etc. ) but same static and dynamic compression ratio's ( same cam in both motors ) the one with the crappy heads and intake might run on pump gas while the one with good heads and intake might detonate because it is filling the cylinders better

Stick with 7 -7.5 DCR for low octane gas

Grumpy 427

I have played with BBC engines mostly. I had a 427 with 11.25-1 compression with Aluminum heads and a cam with .245/.255 @.050 ran great on 92 octaine with 34 degrees total timing.

My468 has 12.2-1 compression big dome slugs. the cam there is .262/.272 @ .050 on a 112 LSA. I was on the edge there. 92 octine with VP octaine booster. i would back the timing out too but could still drive it?

howarsc

Detonation is most likely to occur with high IAT, low-mid rpm, and high loads on the engine. That is ... lugging up a hill pulling a trailer in 95 degree heat and running low octane fuel.
The point at which VE is highest with respect to timing.

A few things to reduce detonation (naturally aspirated engines):
tight quench
Better cooling (ie. AL heads)
lower IAT
cooler plugs
lower SCR
More cam (lower DNCR)
higher octane fuel
Thermal coatings
more gear
lighter car
higher elevations
ect.

scottyp99

OK, it's obvious to me that I'm not explaining myself clearly enough. Let's put it this way:

Stock L48 camshaft:

advertised duration-------------------------280 int, 288 ex
duration at .050 lift-------------------------195 int, 202 ex
valve lift-----------------------------------.390 int, .410 ex



Comp Cams 240H

advertised duration-------------------------240 int, 248 ex
duration at .050 lift-------------------------192 int, 200 ex
valve lift-----------------------------------.390 int, .390 ex



OK, now let's compare these two cams;

Look at the valve lift......hmmmm.....pretty close to the same......

Look at the duration at .050 lift......hmmmmm.....almost identical......

Look at the advertised duration.........WOW!!!!!!!! The stock cam has WAY more advertised duration than the Comp Cams cam!!!

I wonder why that is? Is the advertised duration figure for the stock cam inaccurate? Or maybe it just has a really, really slow opening rate? Or maybe it's just some quirk in the way that chevy measures duration?

I don't know, but I'm curious because some calculations that I want to make regarding Dynamic Compression ratio rely on the actual seat-to-seat duration, and this stock cam seems to have a ridiculously long duration compared to it's .050 lift duration.

Just to make it perfectly clear, I need to know if the stock cam advertised duration figure is accurate or not.

howarsc

"advertised" duration can mean manythingsto different manufacturers...
Could be Seat to Seat, duration @ .006", or any other small lift up to as as much as .020".
Factory cams like yours probably were seat to seat.
Just used the same standard when estimating DNCR. I always use duration at .006".

63mako

Understood perfectly the first time. Difference in the two cams is the ramp rate. Factory cams were engineered for long life and no warrantee issues. If you want a fast ramp rate better off with a roller cam. Cam designers push the envelope on what is a safe rate to pull more power out of a given duration.
To try and clarify this there is a certain point as you make the ramp steeper that the flat tappet will have durability issues. A smoother transition is needed with flat tappet. A roller can have a fast rate due to the lifter design. It can snap the valve open and closed quicker.

TheSkunkWorks

The Comp has a more aggressive lobe profile, which opens and closes at a quicker rate than the stock cam, therefore doesn't need as much duration to reach .050" lift. When you compare a CompCams 280AH with the L48, you'd see equal advertised durations of 280*/288*, but the Comp's lift is .483"/.483" with durations at .050" of 232*/237*. With so much more area under the curve, tho having equal advertised duration the 280AH spends far more time at considerably more lift than does the L48. This all goes to show that comparing cams with such dissimilar profiles is akin to comparing road apples to a nice ripe Macintosh. More on how this relates to "DCR" in a bit...

As for theoretical "DCR" calculators which attempt to extrapolate seat timing from .050" lift, the above potential for variations of cam profile aggressiveness makes them hopelessly inaccurate. Better accuracy will be yielded by using one based on seat timing, such as the one which comes with this primer on theoretical "DCR", which uses advertised duration...

http://www.empirenet.com/pkelley2/DynamicCR.html

Now, the reason I use quotes around "DCR" when referring to theoretical calculations based solely on the intake valve closing event (IVC) is that true dynamic compression ratio is VE% x CR, which is rather more difficult to surmise. The former does not account for variations such as cam profile aggressiveness above illustrated. Further, as a result of this and many other factors, these two definitions of DCR only ever equate for the brief moment when VE% is equal to the percentage of stroke remaining at IVC. FWIW, it doesn't take a blower for VE's to reach 100% and more.

Theoretical "DCR" is still a worthwhile reference IMHO, but don't get caught up on it as being the holy grail of engine design. Oh, and remember that with any calculation it's "garbage in, garbage out". Guessing about any value makes the exercise moot. Hope that's worth $.02.


TSW

63mako

Found this quote by David Vizard that explains this better than I can:

There are safe profiles, and there are power orientated profiles, and just about everything between. The key to knowing which profile you may be selecting is determined by its "hydraulic intensity." This term, coined by Harvey Crane, describes the rate of lifter rise imparted by the profile. This number, for a hydraulic cam, is found by subtracting the 50 thousandths duration figure from the 6 thousandths duration figure.

Most factory cams have a hydraulic intensity in the region of 70°. Originally such large, and consequently low intensity, figures were the result of mild profiles brought about, in part, by cam cores having far less surface load bearing capabilities than current cam cores. Be aware that some cheap cams are ground on cores, which cannot support high surface stresses. If you feel the need to use such cams understand that you will need to apply extra diligence to the selection of other parts in the valve train and to the care of the valve train during subsequent assembly. My advice here is buy cams from a company that uses top grade cores.

If nothing else leads to its downfall, a profile ground on a good core can have a hydraulic intensity of 50° - 55° and be a pretty safe bet in terms of reliability. There are, however, many flat tappet cam profiles to be had, which offer substantial increases in performance by pushing the hydraulic intensity boundaries. All the big cam companies such as Crane, Crower, Comp, Lunati and Isky have them and, to stay competitive, they keep pushing the envelope. Lunati's new Voodoo range is a prime example here and recent tests in a big block Chevy showed they certainly delivered as promised.

The problem of wiping out flat tappet cams could be prevented by going to a roller design.

As you can see the HYDRAULIC INTENSITY of the factory cam is over 80 degrees. The HYDRAULIC INTENSITY of the comp cams cam is only 48 degrees. They have surpassed the hydraulic intensity boundary of what is considered safe from an engineering point of view. The fast ramp cams do deliver better performance. For how long before you wipe a lobe is the real question you need to ask.

scottyp99

OK, guys, I think I have what I need now, and thanks alot for your time and patience. Sometimes I may be a little slow on the uptake, but eventually the light bulb above my head goes "ding!", and I get it. Sorry to be such a PITA, but the duration numbers were making my DCR calculator kick out some really wierd numbers, and I needed to know if my data was correct. I was kinda frustrated because I just wanted to know this one little thing, and you guys were throwing all this extra stuff at me, but not really hitting on the exact thing I wanted to know, but looking back on all of your posts, I realise that I am now quite a bit smarter about the differences between factory and aftermarket camshafts, thanks to you guys.
The DCR calculator I am using is actually the one you suggested in your last post, Skunkworks, and I think it works really well, although I haven't used any others to compare it to. I also have read the primer, which was what got me interested in all this DCR stuff in the first place. The stuff on "hydraulic intensity" was news to me, tho, 63mako, and I think that is what explains the stock cam having such a large difference between advertised and .050 duration. I was thinking about leaving the stock cam in, because the lift and .050 figures were so close to the Comp Cams 240H, I figured it's power charachteristics would be the same, and it would save me the hassle of stabbing a new cam into my motor, but now I see that these two cams are not as close as they seem, so I will plan on installing new cam and lifters with the new heads I will be installing. Might as well throw a new timing chain in while I'm at it. I'm really going for low end grunt on this engine, as My vette has an automatic transmission, stock converter, and a 3.08 rear gear, and I don't wanna get involved in changing out any of that stuff. I figure I'll build it for low end grunt, shift it at 5 grand or so, and I will be happy. I am now thinking maybe a Comp Cams 252H. The CamQuest software shows a nice power curve for my intended setup, and the CR calculations show me a static CR of 8.9:1, with a dynamic CR of 7.6:1. A stock deck hieght of .025 along with a .015 head gasket gives me a quench of .040 in, which I am told is good for resistance to detonation. This will be my daily driver from april till october every year, so I want to get decent gas mileage and run cheapo gas. What do you guys think?


Scott

gkull

To get good MPG you have to make an efficient motor.

With a stock bottom end and your dished pistons I would throw on 58 cc heads like vortec or something with a Crane 268 or even 272 power max cam

DRIVESHAFT

Did Crane re-open?

gkull

summit racing still has a warehouse full of Crane items.

I would never buy a crappy pot metal cam from Comp Cams. CC is really a scummy company when it comes to how many flatten cams and destroyed motors they have caused. It is sad that they have good employees who work under corperate bean counters out to screw you.