I am not trying to start a "what cam is best" argument (theres enough on here) but i am trying to figure out which way to go with my setup because it seems my DCR is too high, i was told 9.1 was too high for 93 but maybe i'm wrong). also my quench is .049 so i really dont want to drop my SCR any more.

anything i've found so far thats smaller then a 280 xfi (230/236 .576/.570 w/ 1.6 rr) has a 9.1 DCR or higher.

From what i got the 280 xfi will have SCR 11.34:1 & DCR 8.72:1

The next smaller cam (268 xfi) RPM range seems to be better for my setup (6400 max) but the DCR is to high. would i be better off dropping my SCR a little to compensate or would it be better to get the bigger cam and let it bleed off some of the compression?

i would get the 280 xfi but i dont thing my heads will support it, they are stock lt1's PP,

flow numbers
Intake
.100- 65
.200- 128
.300- 187
.400- 231
.500- 252
.600- 240

Exhaust
.100- 54
.200- 109
.300- 154
.400- 180
.500- 186
.600- 191


as you can see they max out at .550 and the 280 xfi goes past that. would that be wasted lift? i know a custom cam would be the best but its pointless to spend the money on one until i get my new heads.

if it helps, i have all the bolt ons with a fresh .030 stock bottom end with forged pistons, M6 and stock gears (for now). the car needs to run on pump gas as a its weekend car

So....what do you guys think? should i get the bigger cam and hope its works well with this set up or get the smaller and drop the SCR? Thanks in advance!

 

Lol 9:1 is not too high. Trust me...

 

how high do you think i can "safetly" go? safetly= not pushing my internals to the limits

i might have to rethink my whole cam choice.

 

You have a forged bottom end, what are you worried about? As long as whoever built the bottom end was competent, don't be afraid to throw some compression at it. My DCR is well above 9:1 and static is close to 13:1. Runs on 93 just fine.

Also, .049" quench is SUPER wide. You can safely run a much tighter quench as long as you have an adequate cooling system and a decent tuner. I have a quench of .032" on my 383 and can daily drive it if I want.

 

yep ........


and as for DCR its about as important as a screen door on a submarine.......

if your static is too hight regardless of what cam you toss in there at WOT under a load its gonna detonate........end of story......

dynamic compression is not a factor of volumetric efficency........it would work great if our motors never turned........ but they do.......actual compression changes dramatically due to other factors such as cylinder head, headers, intake, and the scavenging affect that takes place.......at 11.34 you should be able to run on pump gas just fine.......a friend of mine just built a 12.1:1 motor and its on pump gas.........i used to run 11.3:1 with no issues at all on pump gas..........

another factor to show u how much dynamic compression matters is...........
my dynamic is in the 9.3 range.......with a static CR of 14.3:1

now a car with a lower IVO and a lower static may create the same dynamic compression as me........but will he make the same power??? i think not......


i know this post will probably generate alot of contreversy with people praising DCR but it seems as though DCR is just a number that the internet crowd has overly inflated to be pertenient to proper engine design........

hope that helps,
Mike

 

if it was all forged then i wouldnt be worried but only the pistons are. the rest is stock other then the crank turned .010.

I know i have a lot of room on the quench and the static thats why i didnt want to drop the static to lower the dynamic

 

i wouldn't be concerned about it.......my old 355 i ran for two years ran great i put over 10k miles on it........it was 11.3:1 SCR with a good size AI cam in it and i revved it 6800 on the regular with stock crank/ stock rods with ARP bolts and forged Speed Pro Slugs..........when i took it apart it looked perfectly fine.........11.3:1 is not THAT high in the realm of what these engines are capable of.....i say keep the compression and run the cam......with a good tune and a properly operating ignition and fuel system you shouldn't have any issues....

 

evey little bit helps Mike. the only thing better than getting is building it, breaking it, learning from it then rebuilding it again. not enough cash for that!!!

 

"and as for DCR its about as important as a screen door on a submarine......."

So are you saying not to worry so much about my DCR when i'm picking out a cam?

 

lol i didnt break my 355 thou im keeping that shortblock for my truck lol

 

DCR is ONE factor you need to be concerned with, but its not going to make or break how the car runs.

Ignoring it completely is not wise either

What Mike is talking about is cylinder pressure. You can have high DCR and high SCR and the porting on the heads in particular and the overall efficiency of the engine will hav a huge impact on the octane limits. Cylinder pressure is what is truly "dynamic" as it changes with RPMs, load, etc. CP is what makes power, and also what breaks things if you go too far.

Ever wonder "why" too much ignition advance will destroy an engine, bend rods, crack pistons, pound rod bearings to death? Its not simply detonation, its the corresponding rise in cylinder pressure when timing is advanced. That's why you pick up power when the timing is advanced to an ideal point. Beyond that it's your enemy.

 

Stop worring about DCR. It is a tool to be used in very fine tuning and first time calibration. Besides your DCR will change with the altitude. so it doesn't really matter very much in deciding what cam and SCR to run.

The reason you increase SCR is to keep the DCR from dropping too much in the lower rpms. well...besides the performance increase in the upper. lol

another thing is that 9.1 DCR, or even SCR, doesn't react the same on every motor. look at a few examples... LT1 motor was produced with reverse cooling so it was able to have a SCR of 10.4/1, also the LT4 with the same design is able to hold a 10.8/1 SCR. but when the ls1's came out they only had a 10/1 SCR. why? the LS1's were not reverse cooled. So, gm lowered the SCR (not saying a ls1 can't have more CR, just stating an example).

Don't worry about it. keep the cooling system up to par, and you won't have any issues.

 

This. It's just another stat that online cam experts throw around to make themselves sound more intelligent.

 

i guess i was basing too much on DCR because of what i have read lately. with this in mind what would you say the major factors are in picking a cam?

just to be clear i'm not asking "whats the best cam" because we all know there isnt one for every application. what would like to learn are the appropriate steps to picking out the correct cam for my application and performance needs.

 

yes

 

first thing u gotta ask urself is ........what is the intended purpose of the car? max effort track?
daily driver?
nice mix of both??
u want a lopey idle or a smooth one?
what is ur intended max rpm?
steepest gear ratio you'd be willing to run?
and vehicle weight?
also biggest converter u want to run if its an auto??

 

i hope i'm not coming off the wrong way but thats the easy part. i can pick at least 4 different cams out that all will fit my needs based on the manufactures power curves and advertised rpm ranges. all of them within a few degrees and rpms of each other. i would think its more mathematical then that, or am i wrong and i'm making this something more then it is (i seem to be good at that).

in everything i do thats new to me i try to learn as much as i can so if ask dumb questions or too many useless questions sorry. i'm just trying to learn as much as i can.

to answer all the stuff in the above quote...
track a few times a year (i have another motor i'm building for the track)
a DD if it has to be, more of a weekend car though. i have 2 other vehicles
i like choppy but its all about the HP at the finish line
i want it to stay together until i'm done with the other motor so id say shifting at 6500-6700 on track days is it
i have 3:73's but i could go to 4:10's
3400 lbs with me in it. i know i know. me and the car need a diet!
M6

and thanks everyone for taking the time to help. maybe when i'm done i'll know a little about a little

 

ok you probably already know this but as a general rule of thumb.......

lower duration = more grunt towards the low end of the rpm range and a peak power and torque number thats lower in rpm

higher duration = more grunt up high and a higher peak power point

lower lsa = gives you more of a plateau of power........you'll build power and torque quickly but also loose it quickly at the end of your power band........and have more of a flat spot of power at the top of your curve than a peak......also a lower lsa can help to keep a high duration cam down to a managable rpm range........the drawback to this however is increased overlap.....resulting in that choppy idle and also low vacuum, in addition to your drivibality decreasing...

higher lsa = quite the opposite.......you get a wider broader power band with more of a peak to it than a plateau.....however its a gradual gain up to that point and a gradual fall off from it..........a higher lsa will keep the overlap lower which intern will let the car drive better and exhibit better street manners and creat more vacuum.....

lift = lift is a good thing to a point.......too much lift can create very agressive lobes that can beat up valve train......also excessive lift on a cylidner head that doesn't flow well at that lift is also useless.......its a match of cylinder head vs. cam lobe design to make proper power and do it effiecently and with durability....


that just scratches the surface........theres alottttt more to it that i dont really have the time to type lol.......do some searches on the rest of it however just as a fair warning.......dont belevie everything u read here on tech lol.........books are the best way to learn


anyway my suggestion for ur car.....

Ai 230 / 238 - .575/.575 - 110 LSA Primary Operating Range: 2300-6400rpm
This is our largest stock long-block cam for M6 cars. Excellent for rebuilds with 11:1 compression. Lopey idle, good drivability with gears & tuning, and excellent mid-range with peak power at approximately 6300rpm. Properly tuned w/ supporting modifications, M6's typically produce 350-365rwhp SAE corrected.


its an AI cam..........

 

Thanks for the info. Its the stuff "below the surface" that i am very curious about . I have spent a lot of time reading on the web but who knows if its fact or opinion. i wish i had the cash to swap stuff in and to really find out for my self. hell next week i might not even get paid!!...stupid government!! Me---> <----Congressmen

that 230/238 AI cam was one i was looking at but i wonder how much of that .575 of lift would be useful to me since my heads drop off right at .550

another thing is do you know how much they go for? i didnt see a price on AI's website.

 

yeaa that cam will work good with ur motor...the lift is a bit but it will be fine espically when u get to massaging them head runners a bit.........i think they go for like 400$ id email em though to make sure....