I don't know Matt for sure but he says he has 5 vacuum a pretty high
18 initial to get it to idle at 1000 rpm, whatever cams in it must be
fairly healthy.

 

i expect it is the L88 cam.
w/12" vac advance it has 10" vac at 1000.
so it is 30 to get 10".
the reason i'd take it apart is to check it over; measure actual CR, etc. Once the real CR is verified a factual debate can begin. He doesn't want to lose HP if possible. Without a dyno test it is hard to know how much it has now.
I throw out "free" things first. A lash change is free. This only works for real street driven cars, however.
Bench racing, well a ZZ502 swap would get the power and the torque. Costs a bit more

 

IMHO, if you're intending to tear it down in the relatively near future, I wouldn't mess with changing the cam beforehand. As for living with the high comp in the mean time, as mentioned, a timing adjustment will help, as will going with a step or two stiffer advance springs should you have the lightest ones in there now. It may reduce performance some, but it will also lessen the chance you may hurt something before you re-work things. Another perf robbing tip that might help a small amount if you're hearing part throttle ping would be to hook up vacuum advance to carb port (if available) rather than manifold vacuum. Retarding ignition timing will raise exhaust temps, so that cold air induction will be even more important.

Also, the cooler stat will help a little, but I don't recommend going with no stat. Get a Robert Shaw and test it on the stove against a candy thermometer while suspending it in a pot of warming water. Yep, sounds pretty ****, but that one faulty stat in a million can cause some really ugly things to happen if you end up with it, and the test is just too easy (unless your wife catches you doing it). Better to find out in the kitchen whether it's right or not.

Some suggestions to consider when you go back to the drawing board...

One major item to account for is those rectangle port heads, which are tailored for heavy breathing at high RPMs. At lower revs, port velocities can drop off significantly resulting in less than optimal bottom end and mid range performance. Less cam will serve to bring those lower RPM velocities up along with bottom end / mid range torque, but it's easy to go too far that direction with rectangle port heads and end up quite disappointed by how much power is left on the table (BTDT). You don't want to make those big ports redundant when compared to oval ports, else why keep them? If you're only looking for grunt, you need the ovals with a 427.

Thus, staying with the rectangle port theme, I'd look more at re-designing to bolster bottom end / mid range while resolving the octane issue rather than attempting a wholesale shift of the power band. Besides, reducing compression is already going to result in a substantial reduction of overall power production. To do this, you want a cam with a relatively high lift, and (some of you know where I'm going) the best way to get that with a shorter duration cam is to go roller. For this "bolstering", I'd be looking at hydraulic or solid rollers with advertised durations in the 270'ish range, whichever one suits your needs better (hyd = less maintenance, solid = more perf). You've said idle quality isn't a big concern, so you could go with an LSA of 108*, but I'd stick with 110* with 4* advance. (For a flatter torque curve and improved manners, 112* LSA.) If you want to take the time to play with it, you might try +6*, but don't expect a night and day difference for any single 2* step. (Advance for bottom end, retard for top end.)

I don't mean to open the DCR can of worms, but it matters how much compression you have and when the intake valve closes. Since you're still going to have plenty of flow capacity for a strong top end, I'd hit a final CR that puts DCR between 8-8.2:1. With the high VE's you should have with those heads and a roller, going higher will only yield diminishing returns and will put you nearer that octane issue again. If your quench is lousy (>0.060"), address that or go with an even lower CR.

Again with the rectangle ports and managing velocities, you want to pay attention that your intake selection doesn't present runners with cross sections such that they yield a reverse tapper into the heads. You don't want the A/F mix to be decelerating as it approaches the valve. For a relative improvement over the 3886093 intake (and especially the later 3885069 "open plenum" version), suggest checking out the Edelbrock AirGap 2-R, or you might find an old Edelbrock C-427. I have the C-454 Dominator flanged version of that last one (dug it back out for my 496), which proved an outstanding piece on a moderate pump gas 427 I once had. And yes, I was running a 1050 on it.

Sorry that's not the condensed version, but hope it helps. Anyway, that's my story, and I'm sticking with it.



edit - Can't believe I forgot to add that headers with smaller diameter and/or longer primary tubes will move the peak torque RPM downward noticeably. With a set of Hooker sidemounts, for instance, you're typically looking in the upper 4000's with a 7 litre.

 

Thanks for all of the ideas here. I think that I would like to wait a bit before I tear it down and reduce the compression. I am looking for some short term experimentation first and am willing to throw octane at it in the interim, as long as I won't cause damage.

I think that I will add the RPM airgap manifold (with cold air induction via an l-88 hood) and would like to experiment with a low cost cam to start with. I could really use your assistance in getting me pointed in the right direction with a cam. I found the following Crane cams that seem to fall in the range of recommendations and specify compression ratios similar to mine. Can you help with recommendations here.

The L-88 cam spec was .540 / .560. The later ZL-1 cam spec was 560/600. I was under the inmpression that not much else was different with these other than the block, but I don't know that. I do not know how muchvalve lift I could get away with here. I guess based upon your recommendations, I could measure the lift with a cylinder at TDC.

Crane Energizer mechanical
ADV: 294/304 @ 050: 238/248 .569/.595 114 3200 – 6200 10 – 11.5:1

Crane Powermax Hydrualic
ADV: 296/306 @ 050: 236/246 .553/.571 114 3000 – 6000 10 – 11.5:1

Crane Powermax mechanical
ADV: 304/314 @ 050: 238/248 .544/.567 114 3000 – 6500 10.5 – 11.5:1

Crane Powermax mechanical
ADV: 314/324 @ 050: 248/258 .567/.590 110 3800 – 7000 11 – 12:1

Crane Powermax mechanical Roller
ADV: 288/296 @ 050: 238/246 .595/.615 (???) 3000 – 6500 10 – 11.5:1
(This lists a valve lift greater than what I have seen for the L-88 cam)

Crane Powermax mechanical Roller
ADV: 296/304 @ 050: 246/254 .615/.636 110 3400 – 6800 10.5 – 12:1
(This lists a valve lift greater than what I have seen for the L-88 cam)

Thanks again for all of your assistance and support.

 

I'm generally a CompCams guy (their being just minutes away only one reason) but I must admit I like Crane's Powermax line, and you've posted a fine list of candidates to choose from. However, before narrowing the selection of cams to play with any further, verifying your true CR and which exact cam you have now would really help. Without that info, your guess is as good as mine.

Could you get away with that #134561 hydraulic on some VP 105? If so, that looks like one I'd put a star by, given it appears you don't really wish to spend roller money at this time. Or, maybe #134571 hydraulic with 306*/314*, .571"/.585", 112* LSA, 109* ILCA might be in order if your CR indeed proves to be 12.5:1??

The thing is, if you have an L88 stick now and are already having octane issues, that's certainly not going to improve with significantly less duration. Also, it might be noted that the L88 cam has slightly more duration, tho less lift, than the ZL1...

 

I would think you have way to much advance in there.

 

I use them on the street. Additives dont work period. Just a waste of money causing a net cost increase in every gallon you use. Aviation is 100 octane and a good reliable source of better fuel with which to run those beasts on the street. Naturally 110 or that turbo blue is preferred however the aviation works. Plug off the vacuum advance you dont need it, 36 +- is close. Dont expect very much vacuum with L88 or LS7 parts, headlite and wiper doors work but very slow. Thats the nature of those engines as they are as close to a over the counter race engine you could build and really only proper for off road use. I have put them in nearly everything. Todays fuel is the challange we face with those engines and that how I use them. ...

 

I guess I will pull the cam after I get my paint on in a month. I can't take the engine out of service, as I need to drive to the paint booth after I am primered. Do you have any suggestions for determining actual CR without tearing down the engine? I would like to avoid a tear down until I decide wether or not to re-do the internals and avoid doing it twice.

 

I agree retarding those would help by lowering DCR (those numbers were deduced from cam card specs), but then again it would also tend to edge the power band's RPM range back up. Seems to be no win/win situation here, what with the apparently high CR, so making that call will be up to which priority outweighs the other.


Short of actually cc'ing clearance volume at TDC, I think the next best bet is to warm it up, pull all the plugs and get that cranking compression PSI, as Matt had previously mentioned. Then look at what cam is actually installed. From that you/we can extrapolate within a much smaller window what your actual CR likely is. If you don't have any problem returning an installed cam or two, you could have a small selection on hand so you could promptly stick your choice right in.

 

I ran a compression test last summer (engine moderately warm, all plugs out, throttle plate closed because I didn't know any better at the time). I cranked through four cycles on each reading and got a high reading of 150-160. Where would I find the cam number? Maybe I can quickly pull the intake and get a number from it and possilby measure the height of the lobes, if this will help us.

 

If you're good with a dial indicator, you can get a pretty accurate read with the valve covers off. I'd set up on top of the spring retainers of the I & E valves over cylinder #1, making sure you're parallel with the valve and well clear of the rockers. Make sure to record the lash. That will take care of lift.

As for duration and timing, what you're essentially going to do is degree the cam in reverse. If you don't have a fully degreed balancer, install a degreed tape for the correct diameter. It would be a good idea at this point to verify TDC, so I'd pick up a spark plug type piston stop while you're at it.

Of course, if we're only trying to determine whether you have an L88 or ZL1 stick, then the lift numbers will tell that story in short order.

 

i would not paint the car until u pull off 1 head. u can call a mobile mechanic to help.
this mystery will be solved in 1 day.
u spent enough time typing to do it
.
as a side note GM ads say the cadillac CTS is 11.4:1 on regular, and 7000 redline. 304hp V6
.
Oh, and here was a chance for the DCR guys to prove their theory and they won't even try Like 427Hotrod says, DCR says u can run 12.5:1 Is it true? Who has tried it? Anyone? Or do u all wimp out?

 

You have to know the exact information to make a good cam choice.
.
Here we go again. You are comparing apples and oranges. The cadillac is fuel injected with a computer to run perfect A/F ratios, anti knock sensors, able to pull timing back via the computer, probably aluminum block and heads, more efficient cooling.. They are able to pull over 300 HP out of a V6 in a good part because of the 11.4 to 1 compression and the computerised FI and ignition timing. Some of the modern cars are actually using variable valve timing to use more compression, pull more power out of the motor and still run on pump gas. The engineers that design these motors use DCR programs and computer simulations to develop and refine these technologies. You can't compare a 50 year old, point distributer, carburated, pushrod engine design with the modern engines.
DCR is not a theory. It is the actual mechanical compression that a running engine sees with a given combination of parts. Even the rod length will change this number so you need all the correct information to use a DCR calculator accuratly. You can run 12.5 to 1 compression in a carburated car on pump fuel but the cam specs would make the car pretty much unstreetable which is the problem the L88 has. To use the existing configuration safely the only option is to run more octane as Ironcross does.. That is why most responders on here have told the guy with the L88 to reduce compression if he wants to run pump gas on the street. The L88 is pretty much a 40 year old all out race engine. The key for most of us is a streetable cam with compression that will allow that cam to be used on pump gas not a 12.5 to 1 motor that is cammed to run on regular fuel. How would you choose the cam on an 11 to 1 street motor with perfect quench, aluminum heads, carb and great cooling system that you want to run on 93 octane? Guess?? Call the cam tech line and ask them what they recommend??
I would use the intake closing point for a given cam to verify, via a DCR calculator that my combo would run on pump fuel. 8.25 DCR would be my target. Ther are a lot of other factors to figure in though, rear end gearing, trans gearing, intended use, How much sreetability you are willing to sacrifice to pull big power. This has been explained to you numerous times but you refuse to even consider there is any validity to using DCR calculation in designing an engine so everyone is wasting their breath trying to convince you. Keep guessing, or leave a lot of free power on the table when building an engine. I see where you posted you bought 4 cams for an engine and returned 3. Don't seem like the method I would use. With careful design and planning this is not neccesary. I won't waste any more time trying to convince you of something you obviously don't or won't try to understand. Merry Christmas!!

 

i understand what it is:
a useless calculation that is popular.
it is 100% real; just By ITSELF completely useless.
Once all the other MORE IMPORTANT factors are considered......
u just backpedal
bad old design(DUH! how about a MSD timing/knock retard? too simple?)
crummy pistons
yada yada.
u should jump at the chance to run this on 93; not cower in fear. if it doesn't work THEN make changes.
Personally, due to the LOW DCR i think i could get it to run good(if 12;1 OR UNDeR). u guys wont even try, that is a shame.
BTW, any of you thinkers try the 4hole plate/spacer? i see HOT ROD and other mags are using it. why not U guys?
Merry xmas!
THE 3 CAMS i got in '82 didn't have any specs, just A, B, C; all were WAY too big, but i'm glad i took a look.
i'll take a WAG that larry will find 10-11.5:1 and discover that the PO lied
larry never tried 93, afraid of a nuclear explosion? i at least will RUN what i've got 1st. no fear. How many of you found your vette runs great on 87? did u try/tune for it?
Vizard ran 14.7:1 and found only a 10-20hp loss on 91-93 well set up. is that so bad?

 

You can run 12.5 to 1 on 93 or even 87. Even 14.7 to 1. It is just not going to be streetable with the cam profile required to do it.
that to choose a proper cam he will really need to know deck height, bore, gasket thickness, gasket bore, chamber volume, piston volume, stroke, rod length. With this info the actual comp ratio can be figured.
That there are a lot of corvettes under 10 to 1 comp like yours that can be tuned to run on 87 octane. A real L88 spec motor ain't one of them.

But I disagree that DCR calculations are completely useless and will no longer try to convince you they have any use. They are totally useless to you because you won't consider the validity of the calculator and that is fine, your choice.
I don't need a 4 hole spacer. I have a 6 hole manifold.

I have an MSD Distributor, 6ALN box, RPM window switch and a Timing retard box on my build.

I still would like you to answer this question because I am really curious: How would you choose the best cam for an 11 to 1 street motor with perfect quench, aluminum heads, carb and great cooling system that you want to run safely on 93 octane without detuning? I would figure the DCR of the combination with the cam I am considering. What would your method be? You seem to have a secret for appropriate cam selection on a given build not related to DCR. I just want to know what it is!

 

I have since torn down the engine and found that it is not what I expected. I also found some shoddy craftsmanship and will need to replace my pistons. I also plan to replace the rods as a precaution. The new rods were about as much a having new bolts put on the old rods. I have posted all of the details that 63Mako outlined above. This is the thread:

http://forums.corvetteforum.com/show....php?t=1907339

Would you guys mind helping me figure out the DCR and ultimately select the best cam? I would appreciate the assistance.

Larry

 

the secret to not detuning:
optimize the tune.
ie, see AZDoug in c2.
dyno tune showed best power/tq
was 23 total timing.
not 23 plus base.
23 total.
his 427 made over 400whp
i use DCR a little, it can help a little,
but it does not make me an advocate of Dogmatic Cam Religion

 

Voodoo apparently still plays a more prominent role in conjuring up engine combos than science for those who refuse to accept that DCR is a factor worthy of at least some consideration.

VE (volumetric efficiency) and max cylinder pressure are ultimately what matter far more than simple DCR, but we can figure DCR and use that knowledge to help better project the likely VE's and pressures of a proposed engine before spending a dime. The only other way to get a more accurate picture is either thru SIMs (just very complex calculators that use mathematical formulae which happen to include IVC and DCR to make estimates) or by spending the money first to build an engine (and then hoping for the best and spending more money to make any corrections/improvements or just living with the results).

Some of us would rather shed as much light as possible on the target than stab into the dark, and however little you may believe DCR to be worth it does help illuminate things by at least some discernible amount, but only if we open our eyes. If one has the ability to view the situation with 20/20 insight, then all the better.

But, it should be kept in mind that not everyone has the facility from which to divine VE's or pressures with any measure of accuracy, and IMHO for them DCR is an even more important reference point, as long as it is used in the proper context. (A 13.5:1 build with a radical cam might well could have an 8.33:1 DCR, but I wouldn't advise attempting to run it on pump gas or driving it on the street.)



edit - FWIW, actual DCR = SCR x VE%, so I have since begun referring to simple DCR calculations based on IVC (as dscussed herein) as "theoretical DCR".