Hot Rod Forum banner

Another Cam Dilema... I'm Torn!

2K views 15 replies 8 participants last post by  BuzzLOL 
#1 ·
Okay so heres a little info to catch you all up.
Had a fresh rebuilt chevy 383. Long story short machine shop didnt catch a crack in the lifter valley. So new block should be ready today. Upon tearing the "old" one apart (700miles on build) where the crack was leaking onto the lifter in cylinder 3, the cam lobe is wiped out.


As comparison, caliper was set to size of other good lobes.

ANYWAYS! That being behind us lol
So this cam is the Comp Thumpr cam. I chose this because i have not machined my vortec heads for higher lift and this cam sounded awesome.

What I have:
Block .030 over, decked, line honed.
Probe -10cc pistons sitting flush at 9.015"
Scat crank
Vortec heads cleaned up w/ valve job, new springs, retainers etc. and PRW sportsman roller tip rockers
Weiand speed warrior intake
Edelbrock 750 carb

With .041 compressed felpro gasket the CR is 10.38:1
From what I understand I have stock 2.29 or 2.73 gears in the rear. Will be changing rear end and 3.31 or 3.73 this winter.

As for the trans, rebuilt th350, stage 2 shift kit and 2500 stall hughes converter.
Curb weight of car is about 3200lbs

So what do you all think? I know about the whole max lift of the vortec heads. Im torn between the XE268H and XE274H (pushing lift close)
But i was also intrigued by the duel energy line, havent heard much about them but the 275DEH has a safer lift with a nice choppy idle. :confused:

Opinions! :)
 
See less See more
1
#2 ·
The 268 is the biggest I would go on the vortec heads but your compression is too high for a cam that small. This is a situation where the mismatch of your heads and static compression are going to force you to compromise on a cam that is too big or insufficient ignition timing to get this to run on 93 octane. That's an unfortunate place to be in.
 
#4 ·
Not sure how a thmpr ran even reasonably well with 12* initial, that's about 10* short from where it should be. That's always the unfortunate issue with vortec heads, to much scr and not enough flow to justify the cam needed. That being said the efficient chamber will allow you to get away with the 268 and premium fuel. Top notch cooling system, tune spot on and it will work well. Its been done many times.
 
#5 ·
I see at least 3 problems here.
1. You haven't learned your lesson about flat tappet cams yet.
2. 10.38:1 SCR is too high for iron heads.
3. 275DEH closes the intake at 35.5 degrees ABDC and will generate at 9.14:1 DCR, too high for pump gas with iron heads.

You should have built this as a torque motor if you were going to use L31 heads on a 383. 9.0:1 to 9.5:1 would have worked if mated with a short cam that would produce a 8.0:1 to 8.5:1 DCR and would sign off when the heads stall at just over 5000 rpm's due to the larger displacement that they were not designed to feed. 170cc intake runners can only do so much.

Sell the iron heads and move up to a set of 195cc intake runner aluminum heads. You're not looking for a torque motor, although that's what you built. You're looking for power and the L31 heads are not going to generate the power you want, not on a 383.
 
#6 ·
So im still learning about dynamic compression. But the only way this could run decent is if i get the 268 because it closes the intake 72* ABDC thus lowering the DCR?? Thanks for all of the information everyone, very informative! :) What DCR would i be looking at with 268 and would that be safe?
 
#7 ·
Using the specs above,I have your SCR @ 10.48:1.The 268 cam is gonna still put your DCR in the 9:1 range.The 274 isn't gonna be a lot better in the 8.8:1 range.You need to either swap heads or pistons.Either a larger volume piston/head,or,go aluminum.The 268 closes the intake @ 61* @ .006/38*@ .050.The 274 closes @ 64*@.006/41*@.050.
 
#8 ·
I dont really have that option right now :( lol i spent too much on everything and im going to be broke for a while. This whole cracked block thing didnt help the budget much... all i can really afford is a cam and gaskets. With how much money ive thrown into this build i could have bought a crate 572 lol Dont bash on me :spank: This was my first block up build with all new parts haha I dont expect for this thing to pull the wheels up or anything but whatever will make it run best with what i have.
Also i know its close but will .490 lift on the 274 cam be cutting it too close without the valve guides being machined? Again thanks for all the replies and information!
 
#9 ·
There are other options to create more lift than machining the heads.There are several drop in spring options such as the beehive springs.You can also use shorter retainers & last but not least,you can use the "Ghetto Grind" on your current retainers.You can safely grind up to .030" to get clearance for approx .500 lift on Vortecs.The next problem you may face tho with that much lift/duration is the studs pulling from the heads.Vortecs are not as bad as earlier SBC head,but,the high lift/duration & fast ramps of the XE cams can make it more likely than not.I would suggest @ the very least,pinning the studs.
 
#13 ·
I ran a Lunati 00010 cam in a similar compression 355 with 416 steel heads 58cc chamber with pressed in studs and didn't have any issues with pulling the studs or detonation. It 230/230@.050 with .480/.480 lift with a 109 lsa 107icl. The timing was set at 16* base and 26* full
 
#14 ·
Why not just bite the bullet and get the guides machined for more lift, rather than hamstring the whole set-up just because you can't fit a decent cam into it because of a lift restriction.

Seems pretty stupid to me, and the cost of machining the guides down should be pretty minimal.

You've got screw-in studs, so put a real cam in it. A solid, but not that ancient "30-30"

You can get an Elgin solid cam/lifter package from Competition Products that will fit the bill, and the money you save versus giving it to Comp Cams just for their name and decals will darn near pay for the guide work also.
 
#16 ·
.
. I'm seeing a lot of suspicious clues in what you've said: heads not clearanced for cam lift, wiped out cam lobe, cracked lifter valley... I'm really wondering if parts got forced together and caused all that?!?!? My experience is properly installed flat cams outlast roller cams in the long run because flat lifters ride on a constantly replaced film of oil instead of depending on the overspeeded needle rollers rattling around in a roller lifter...

. You can get up to .525" lift on Vortecs without machining with simply an Alex's No Mods Kit:

VALVE SPRING KIT, SB CHEVY VORTEC, HI-PERF HYD ROLLER - Alex's Parts Sales (get the milder springs for a flat tappet cam!!!) May need to switch from self-guided rockers to guideplate system. Or build your own setup with Crane or Alex raised retainers and Z28 valve springs...

Z28 VALVE SPRINGS

http://www.summitracing.com/parts/hrs-98214/applications

. You can run about any cam you want if willing to add an ounce or two ( 1/4 coffee cup) of kerosene per tank of gas (15-20 gallons) to bump the octane up, that's what I always use... a budget Summit K1106 234/234 cam or K1107 234/244 cam kit may meet all your needs...

http://www.summitracing.com/parts/sum-k1107

. You can use this 383" Vortec head build as kind of a guide:

GM HT383 Crate Small-Block - Engine Build - Hot Rod Magazine All Pages
.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top