So why are people running 9.5:1 compression? I know that if I run like 12:1 I need race fuel, but is there a problem running 10:1, 10.5:1?

Secondly, I'm looking at cnc-motorsports short blocks to start my build, they can get an assembled forged crank/rods and choice of pistons for under $3000 to my door. I don't think there would be a better way to get a stout bottom end and get it professionally put together than this. And I have yet to find any negative feedback accross other forums. Is there a better bang for my buck?

You all have convinced me to spend the extra cash and go with 195cc ARP heads and I figure a custom cam grind. Like I've said in the past I'm going to use my high flow TPI manifold/runners/and plenum. However, if I ever decide to go to a miniram, nos, etc I think this setup would be the best for later expansion.

Thanks again, more questions to come.

 

you do not need race fuel.. I had 11:2 on my previous 358 build
my current 383 I have 12:2
depending on your cam the bare minimum would be 10:1

 

You dont need a custom cam save your money.

With that head you can run more compression without worry.

9.5 youre leaving power on the table. Need to know which piston your using, how far in the hole it will be approx..then select your cc size for head. 10.5 or so is reasonable....depending on the cam you could run more but better to leave a touch of room there.

Cam will decide it if likes it or not.

Im runing 11.5:1 on 91 pump but 99% of people out there wouldnt want the cam in it that makes it work.


All about the package

 

I've been down this road and wear the same shoes.

9.5 is too conservative...Go at least 10.25:1....More if using more duration than me. I suggest familiarizing yourself with dynamic compression by reading this website. Try downloading and running the calculator when you've gotten a better idea of cam choices.

In my case, I went from a 350 to 383, then debated a longtube setup vs something with better topend. I also wanted to give some consideration to emissions, motor longevity, looks, usage, etc... In the end, I picked something in the .550ish lift range and a single pattern.

Why? Because a longrunner intake can benefit from more duration on the intake side while the AFR (not ARP) heads don't need a split-pattern cam to flow well out the exhaust. It also turns out that extra duration on the intake will make a faster motor IF you swap to a shorter runnered intake. So, the question comes down to exact lift and duration -- and how to decide the best/correct compression for that combination. (This is where dynamic compression knowledge can help a LOT.)

More duration favors a higher spinning motor. More lift favors more power within the given power range. Since a higher spinning motor can use more duration, you have to pick a cam decent for both long-runner and short-runner use. (Note neither will be perfect for an individual intake, but you're only compromising about 10rwhp with this compromise.)

I like the superram cam for your situation. If you know about the superram, you'll know it's the intake that's halfway between longrunner and shortrunner setups. As such, it's cam package provides the compromised best fit for your situation....lots of lift, not too much duration, and single-pattern.

(The only consideration from my perspective is that a split-pattern cam has a more traditional rhythm. (IOW, a slightly better gallop sound.) But, any cam will sound "healthier" than stock.

If you pick a cam in the SR range/config, you'll definitely be safe with at least 10:1 compression....probably closer to 10.5:1. OTOH, if you're not sure you can ALWAYS buy the top octane, less compression might be reasonable. So, stick with 10:1.

There are other issues you'll learn about. Quench is a biggy. When a combustion chamber is designed correctly, you can run more compression w/o causing ping. The motor's coolant range also plays a factor. The more things you don't do right, the more you should stay in that safer range -- as you see many people do. Building motors can be a complicated project.

BTW...You'll find cnc-motorsports will recommend staying at/under 10;1 compression. I bought some of my parts from them. While I do think they have well-priced stuff, I ended up pissed at them because they don't know chit about C4 balancers. They ended up selling me one with a very bad reputation -- saying it was my only choice. Turns out there were several choices. I returned the "crappy" balancer as soon as it showed up at my door. They charged a re-stocking fee even though I considered the sale mis-information. When I spoke to them personally, I found out they have don't-give-a-crap attitudes. As such, I won't buy from them anymore.

 

More compression needs a bigger cam with more overlap to bleed off cylinder pressure, the bigger cam (duration) also works better at higher RPM. What is the max RPM you want to spin the motor to? Remember if you make peak HP at 6500rpm and the motor doesn't just choke off entirely, you're generally going to want to spin it to at least 6800rpm to maximize torque under the curve when shifting.

10.5:1 is a good target and use Pat Kelley's DCR calculator linked above to at least help get a handle if not determine a good SCR to shoot for. Great tool there.

Any of these three would work well with both intakes and the first two you could find probably cheap enough if you had to but I'd go with one of the last two. Then you can do a custom grind when you switch intakes hunting for more horsepower. There will be different cams that work better for each intake.
GMLT4 HOTCAM- xxx/xxx 218/228 lift w/1.6s 525/525 lsa 112
LPE219 -270/270 219/219 lift w/1.6s 560/560 lsa 112
xfi268- 268/276 218/224 lift w/1.6s 570/565 lsa 113

If you think you are going to eventually want to spin it higher, then you might want one of these cams to come out a little high on DCR so when you have enough SCR to have a nice combination with more overlap.

PS. If you search you can find multiple threads on it, but if you do go with the 219 make sure you know whether it is ground straight up or with 4deg adv in it and either way you want it straight up (108icl I think?) and when you go to a short runner intake you can even throw 2deg of retard in it to pick up a bit more HP.

Sorry about the AFRs, spending money is a sickness around here!

EDIT: Should say, I've got the LPE219 (still need to retard it back as it has advance ground in and I original installed it for a SuperRam motor shooting for maximum torque with the combo) at 10.3:1 SCR and 0.039" quench. Motor runs well, almost put in a 280xfi (230/236) but I didn't want the RPM to go too high and IIRC it needed more SCR to get it into the optimal DCR range, so if you see a bigger cam in your future definitely figure out what SCR you need now or plan to mill your heads in the future.

 

Good follow-up post. FWIW, I'm not impressed with the XFI stuff -- though some like them for their bit less overlap. (slightly better idle). By contrast, I think some owners take their lift for granted, don't use enough spring, and end up with valve float in a short period of time. From what I see, I think the SR cam has a better rep with nearly the same lift. And, the extra overlap makes up for any difference.

I think the statement I quoted above might be misleading/confusing to anyone just starting out. But I have to assume you're referring to someone's desire to spin a motor significantly higher -- with a different intake AND more cam. Hopefully, the OP figures that out.

Even figuring for a single cam option (installed for potentially two intaked), there can be come consideration. As noted, installing a 108ICL cam with longtube runners would be best-advised in a retarded position. With a good timing chain set, you can mount it 2 or 4-deg retarded. Interestingly, if you went to a short-runner intake, that degreeing could be reversed (to straight-up or advance) if you want to retain as much torque as possible.

Nothing is really right or wrong. It's just where (rpms) you want to build power. I did the same as USAsOnlyWay and installed it for max torque. With a 383 and mega-ported longtube intake, I have some serious torque....probably a bit too much. I probably should have installed it retared to get a bit more top-end. (OTOH, I thought seriously about putting an HSR on it after a year -- and mounted the cam with that intake in mind.)

 

I think the xfi268 or custom 276/276 doesn't look bad but I'm still impressed by how far ahead of the curve the 219 was for its time, obviously you know the one I'm running, I'm biased.

But make no bones about it, it is possible to get gobs more HP with a different cam, I just know what I want out of the car isn't a peak dyno number but a strong RPM band where you will spend most of the time.

 

Ok, im going to read that site. Sorry about thr arp thing was looking at studs and typed the wrong thing.

What it seems like from what you said i would prob, instead of a compromise, go with a cam suited to my high flow tpi and change over to a different cam if i ever switched the intake over.

My max rpm goal would prob be around 6000. I doubt from what everyone says the long tube setup could handle anymore than that and really for autocross i have to stay in 2nd gear mostly so i like the lower torque band, and at carolina motorsports park i am in 3rd and 4th most of the time and i like that not having to hit 2nd and maxing 4th on the back straight.

I am looking to save money (id like to keep the final bill as close to 6grand as possible).

Is there a good place to get a timing chain set/cam/lifter/pushrod set or would piecing together be the best. I know i can get a set of 1.6 roller rockers used for around 250, and find a cam that is best suited for my goals for what maybe 300? And then say another 250 for pushrods and lifters? Then what about the timing chain and cam degree tool? Would that be better to let a machine shop handle or try to do myself?

Ive assembled and engine before and feel confident i could learn but if its difficult i would rather learn on my ****** old buick engine in my hearse, not my high hp corvette.

 

wow! That article blew my mind, it took me a few reads to grasp the most basic point. It would seem to max out my power i need flow numbers on my intake with the heads to figure out what the best valve seat time would be before i can figure out the best cam. It looks like i might just go with a cam that is close (like the three suggested) and then having some upgrades to do later down the road once i have a baseline.

If the afr are sold in 64cc and 74cc chambers is there a prefered choice to manipulate the pistons or afr chambers? Seems like 74cc would be best and could be milled later if needed...

 

I think you're going to find that with your pistons and deck height along with your head gaskets, you are going to want the 64 cc heads. I bet if you did a static CR, you will get low 10s:1.

 

well, what would be the down side to getting a 74cc? When I get my block through CNC they will put whatever pistons I need, so I'm thinking if there is no difference, I should go with 74cc to have more wiggle room...

 

Going with the 74s will drop your CR super low. Then you are going to have to pay a machine shop to mill them down. Figure out your deck height and piston volume and head gasket thickness. Then compare compression ratios. Shoot for 10.x for a decent runner that you can still run pump gas through.

 

Even with domed pistons?

USAsOnlyWay, the numbers I'm finding for the LT4 hot cam arn't matching up... are there multiple "Hot Cams"?

Summit has the cam listed as

279/287 218/228 lsa 112

not: GMLT4 HOTCAM- 268/280 218/228 lift w/1.6s 525/525 lsa 112

 

Like I said, you have to calculate. Don't get hellbent set on something before you know for sure. The easiest way isn't always the right way.

 

Go with the summit ones, I swore I googled that to check last night but those were just off the top of my head. It looks like the other ones were right. Good thing you checked that, my apologies. Looks like I just had adv duration off.

Also, here is the post detailing the difference in 219 grinds
http://forums.corvetteforum.com/1571878780-post6.html

 

Old skool way of gettting CR up when only large chamber heads were available.
Current thinking has flatops and a small chamber to get better flame propagationn

 

ok, well doing the calculations best I can using the DCR and some values i'm having trouble with.

Crank Stroke is 3.75
Rod Length is 3.7
Intake Close: 41* ??????? is this correct? comes to 41.5 in calc but
the cam spec pic you showed says 41...
dynamic stroke: 3.5005

Compression came to this....
bore diameter 4.030 (might not be .030 but just assuming to be safe)
dyn stroke: 3.5005
Combustion chamber: 64cc
Head gask thick: 0.041
Head Gas Bore: 4.166
Piston Deck Clearance: Have no idea, put .04 to see what would happen. ?????????????????????????

Just trying to get a baseline here to get my head on the right track.

Its saying I am getting a 355ci displacement with over 8.8:1 dyn compression. Something isn't adding up....

 

Rod length is likely 5.7 not 3.7.

What cam are you modeling? Trying to figure your compression before your build?

 

Domes are typically the least desireable option. Flats tops are next. And, inverted domes are thought to be the most efficient.

If you figure out the reason people build for good quench, you'll hopefully see it. Squeezing the charge into a smaller ball (with inverted domes), gets the fastest flame speed. Spreading it out (with domes) slows it down and can end up with the highest probability of detonation issues (when running lower octane fuels).

Of course, a better flame-front = more power too.

 

Using these numbers (kind of) with a 4.166" head gasket bore, 3.75" stroke, 4.030" bore, -7 cc piston head volume (this one is super important and you don't have it listed), 64 cc heads, 0.041" compressed gasket thickness, and your Piston deck clearance at 0.015" (again, VERY important) Stock is typically 0.025", my number would allow for 0.010" decking of the block:

Static CR is 10.4:1 and you would have a 0.055" quench. I hope I did the quench right.