406 Cam Choice
Looking for some help in choosing a cam to match my parts that are ready for my 406 rebuild.
Specs for the parts I am using are:
Stock GM22N 400 crank, 2 bolt mains
Stock 5.565” rods with ARP rod bolts
TRW Hypereutectic H601P, +030, 12.5 cc dished pistons, sitting .020" below deck
Bottom end was previously built, blueprinted and balanced, (low mileage 406) so I will just be adding new rings and bearings
1.6 ratio roller rockers
Patriot Performance 195cc ported heads, 65cc chamber
- setup for hydraulic lifters with 550 max lift
- Flow numbers are:
- Intake - 400L = 223, 500L = 252, 600L = 263
- Exhaust - 400L = 170, 500L = 183, 600L = 184
Edelbrock Performer RPM intake
Quick Fuel Q Series 750 carb
Hooker 2210 long tube headers, with dual 2 1/2" exhaust
Transmission is a manual WC T5
The cam I am thinking of going with is
Xtreme Energy XE274H Hydraulic Flat Tappet
RPM Range: 1800-6000
In using this cam, I have run into issues with DCR, thus I am wondering which route is the lesser of the evils. With using the 1.6 rocker ratios the DCR should improve slightly from the below numbers.
1) Deck @ 0.020" and .039" head gasket, gives me 10.28 static compression and 8.32 DCR. The .059" quench concerns me.
2) Deck @ 0.0" and .041" head gasket, gives me 10.72 static compression and 8.67 DCR. The DCR number is too high.
I have calculated bigger cams to help out, but this is for street / strip use so want high mid torque numbers and don't want to spin past 5600.
First, changing the rocker ratio will have no effect on the DCR. The valves will open and close at exactly the same time. With increased rocker ratio the valves will move faster and lift higher, but the actual seat timing remains exactly the same.
Use a Comp XE284H-10 cam #12-250-3.
Install at 104 intake C/L.
Your 400SBC will eat up this cam duration fast.
You will not be over cammed with the 284.
Being that you are almost 4000 ft. altitude above sea level, in Calgary, your motor will tolerate a little more cr than what most DCR calc proponents tote.
What rear gear ratio will you be using? What fuel will you be using?
persoanly I would install the XE274 and go with it.
Be aware that 400SB's as a rule, often do not need quite as much timing for max power. This, combined with the good combustion chamber design and aluminum heads means you do not need to get carried away with total timing at WOT and or total timing+ vacuum adavance at cruise.
With the right heat range spark plugs, carb jetting, fuel octane and the right centrifical and vacuum spark timing curve dialed in for your motor, I do not see a big problem with 10.2:1 cr and the XE274.
If you run into big problems, (which with proper tuning, I really doubt you will) just install the next bigger cam (XE284 on a 104 in c/L)
the XE 284 will have a chunkier idle but is very very torky in actual use.
The rpm range shown for these cams is for "average use" Like when installed in a 350ci motor. A 400SBC really eats up cam duration fast compared to a 350. Its hard to over cam a 400 and easy to under cam it cause its so torky.
You'll find the XE274 mild and very torky, will be all done by 5200-5500rrpm and the XE284 all done by 5800-6000 rpm in actual use in a 400SBC.
Both are very torky, the 284 has a slightly stronger top end and slightly less low end grunt. The actual torque curves or the two cams used in a 400SBC are very simular. Installing the 284 slightly further advanced (+2 deg) than cam card specs (net 104 intake C/L) will boost the low end grunt a tade.
Put away the calculators and theory and pick one and try it.
Your tuning will have much more effect than the camshaft will.
The larger cam of the two cams will quarter mile better. Its a good bit stronger in the critical 4000 to 6000rpm range used for full out WOT acceleration.
Last edited by F-BIRD'88; 08-24-2008 at 06:48 PM.
Yeah, with all the new calculators I have been discovering / using, I think I may have lost focus on reality a bit. This is my first complete SBC buildup so I am just trying to ensure I do everything right and have considered all angles. Thanks for clearing up my incorrect rocker ratio DCR as well....makes complete sense..now that I think about it and not follow internet gossip.
I have been looking at that bigger cam too but was worried about driveability, possible brake issues and the higher redline it would create. It would definately get the DCR number inline though. For octane, I am planning to run 91 octane minimum, as this is not going to be a daily driver. I do want some lope to the idle so as you say installing a 284 slightly advanced is sounding to be my best starting point. I am installing a 2 piece timing cover as well so if need be.... With advancing the cam I would suspect this to also lower the maximum RPM of that cam as well would it not? With the stock bottom end components, I am concerned about having to spin it to 5800 - 6000.
The 10 bolt rear currently has 3.42 gears, but I am planning to go to 3.73 for a better 1/4 mile RPM. I will also be fully welding the axle tubes, adding a brace kit and an aluminum support cover to beef it up.
Do you have any suggestions where to start with the timing? I have the HEI distributor that was running in the engine previously (dynoed at 385 HP using ported camel hump heads) so was going to use its existing centrifugal and vacuum advance settings as my starting point and tune from there. The ignition system will be upgraded with an MSD Digital 6 and MSD Blaster HVC Coil.
With the 284H in my 400 I run the mechanical timing locked out @36deg.
Vacuum advance is 15deg, controlled by ported vacuum. Crane makes a cool adjustable vacuum canister. My PB's work just fine. Idle in neutral is 800-850rpm, rough but rock steady. Throttle response is excellent. The overall rpm range is the same. The XE284 just has a stronger top end. The 284H does like to be cruising at 2000rpm and above. 91 octane is going to be on the edge. Do they not have 93 94 octane there.
Your motor will want something around 32 to 36deg timing.
The only way to know is to try it.
Again, the 284H does not make it into a big rever. I've had mine well over 6000rpm, but does not need this for best performance. Anywhere around 5500-6000rpm shift points are fine. 3.73's are just fine.
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