comp part number 12-443-8

2500-6300 rpm range on my 406

242/248 @.050
294/300 advertised duration
.540/.562 lift with 1.5 rocker
110 lsa

this is what comp recommended me for my engine

my engine is as follows
.030 400 (406)
standard dished piston 4 valve reliefs
Dart pro 1 aluminum 64cc heads
918 beehive springs (good to .600 lift and for roller cam)
ported and polished, intake flows 265cfm at .600 lift and 207cfm at .600 lift.
weiand stealth intake, idle-7000 rpm dual plane, want some low end
barry grant 750cfm mighty demon carb.
hooker long tubes with cutouts and a mufflex y pipe
car has no ac, does have power brakes,
stick car, will stay that way (t56 in the works) with at least a 3.73 gear

i know thats a pretty big cam, just want to know if it sounds feasble for my setup.

im assuming that i wont spin it over 6k on the track....

whaddaya think?

 

it's spot on for a dual purpose street/ strip car.

 

I agree,even though your duration may look a touch long to you,you can get away with it in a 406 w/out killing the off idle performance.

Should pull like a freight train through the midrange.

 

what do you guys think about off idle and low end, yeah its gonna get the crap run out of it, but think itll make good low end also with the dual plane intake?

 

due to the cubes, you'll have some low end. It'll be a little sluggish below 1500RPM. Below 2000RPM it'll behave like a stock 350 probably, nothing special. Not like a V6 car or anything, just nothing amazing. Once you hit 2000RPM, and then say 3000RPM, it'll pull like crazy.
Comp made a good choice on that cam I think. That's the XR294?
2,800-6,100 RPM they say on summit... I'm guessing you meant 2500-6300 due to the fact it's on your 406? I think it'd be more like 2300-5700 or something...

I didn't do the math, but what's your static compression ratio look like with that?

 

yeah thats what comp said for the extra cubes,

i know this engine has a dished piston with 4 valve reliefs. im trying to get close with a 64cc head... comps book says its recommended for 9.5:1 or better, i was thinkin id be closer to 10, but im told no.

any way of telling the dish size of a piston? i really dont have any markings that tell me what other than the overbore size.

i know its got alot of overlap, but im too trying to find out the best way to get my cr.

 

here's what I got with guess numbers.
Find out what volume the pistons have. They should have some kind of markings on top, or you can ask whoever you got the shortblock from. They say you want 10.5:1 or better CR for that cam, or else you lose more low end power. Since it does bleed off cylinder pressure, and you do have aluminum heads, I think 10.5-11:1 is realistic.

You can use a straight edge, and a feeler gauge to check your piston depth.

 

I agree, excellent cam choice.

You won't be able to keep it under 6k. Believe it. When it's still sucking your head off your shoulders at that point, you'll find it REAL TOUGH to shift.

I have a T-56 with the stock 3.73 gears still in my car. It's not enough gear, and all I have is a 305 with a XR264HR. You'll want 4.10s.

If these are some random "rebuilder" cast pistons and stock rods with used stock rod bolts, then that short block is not long for the world. Start saving now, you'll need another one soon.

To find your CR, you need the dish volume, and you need the deck clearance (how far below the deck the top of the piston is). You can measure the first with an old CD (how about a "free AOL trial" one) and some Vaseline or the like and some anti-freeze and a Nyquil cup or something; and the second with a straightedge and a feeler gauge. Bring back those numbers, and you'll then have something that can be plugged into a compression ratio calculator. Without ACCURATE those numbers, all anybody can do is just guess.

 

To help with the low end torque you will need to keep your dynamic compression ratio around 8.5:1 for 91 octane gasoline.

 

i thought it was 9.5:1 or better that i needed?

i did the piston volume and its right around 15ccs of water the dish held.

 

That's a pretty big cam, and you're using AL heads, so you can get away with higher. 9.5:1 would be too low. Above 10:1 for sure i'd say.
Shoot for 10-11:1 range I think. Do your math with that 15cc dish, and go from there

 

i keep getting over 10 with that cc on this calculator

Engine Compression Ratio (CR) Calculator

using a 4.155 bore, 3.75 stroke, 4.166 headgasket .039 compressed, -15cc pistons and stock deck height which should be 0 i would think

am i doing it right?

 

{insert Gong Show type buzzer noise here}

Stock deck height isn't 0. It's around .025". Then, most pistons, especially anything less than a pure racing one (which doesn't sound like what you've got), will add about .020" to that. Any cast piston, and most hypereutectics, even most of the TRW forged ones, are in that category. Sometimes it's in the specs, sometimes not. Sometimes they give a "spec", but all it really is, is the "design spec" for the motor; so they might say for example that the compression height is 1.56" (stock), but if you measure it you find it's really 1.54" - which is .020" farther down in the hole than stock.

Using those numbers, I come up with about 9½:1; not really enough for that cam.

On the other hand, if the deck clearance REALLY IS zero, it's around 10½:1; much better.

Only way to know for sure what you've got though, is to measure it. Guessing and "thinking" is a great way to trick yourself into making a bad decision.

I'd want to see mid-high 10s for that cam, and those heads, to make the COMBO work out right. Don't forget, you've got ALL THIS FLOW, but it's useless at 2000 RPM or whatever, because the cylinder has all the time in the world to fill. Those heads won't deliver their money's worth until around 5000 RPM. Same for the cam.

A low compression motor and a huge cam in a street car is a guaranteed recipe for a slug. All the cubic inches in the world can only go so far to help cover up a gross mismatch like that.

 

like we've mentioned, measure deck height with the straight edge, and feeler gauges.
Bring a piston to TDC, lay the ruler across the hole, from front of motor to back, across the center of the hole (this is above the piston pin centerline basically.) Then slide feelers under the straight edge, and find the clearance. do it at the edge of the bore. It'll be between 0-.050", so if you're in that range, you're doing it right.

Do a few pistons, usually 1&8 are good choices. Average the number. Say it's .035. Then re-do your math based on that.

Remember, quench is a good thing too

 

so... what do you guys recommend? i want a cam that will work with that, but i obviously have enough head to get a big cam, so what would fit me the best for what i got, id like to have a pretty erratic idle...
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would taking material off my heads be beneficial, although i dont want to do it, i want it to all work well together

 

I recommend that you measure your pistons FIRST, before you post another post. Then make that post, your measurement. Because it's WORTHLESS to try to optimize a motor unless you know the TRUE compression ratio. Which means, no guessing.

You only have enough head for a big cam, if you also have enough compression.

Remember: there's no such thing as TOO MUCH CAM; there's only NOT ENOUGH MOTOR.

 

pistons cc'd at 15cc...

 

so i dont do it in the middle of the piston, just the outside of the dish?

sounds fairly simple

 

That's right. The dish will give you ~.125" depth. Hence I said if you're in the 0-.050" spec you're doing it right. If you have a large gap, you're not at TDC, or you're measuring it wrong.

Use the lip. Measure on the lip, towards the front or back of the motor, hence, right over the piston pin. This will eliminate the rocking of the piston (if you did it on the side of the piston).

 

so parallel to the wrist pin?

 

yes

 

ok i put a feeler guage under a straight edge on 2 cyls, and came up with .025 that puts me a feather away from 10:1 right? anything i can do to put me over that mark or is 10:1 still too low?

 

You should really pull it apart,find out what your pistons are,if they are cheapos,replace them.Plastigauge your bearings while your there,inspect them,and make sure you have the guts to crank this thing like your planning to.If all else checks out,have .025 decked off the block,reassemble,and live happily ever after.Trust me,a corner cut now means 50 miles travelled later.

 

i would really like to, but i dont think the finances will handle that right now.... id just assume get new rings and and flat tops and just a small hone on the cyl. i wouldnt really need to deck the block if i was gonna scrap the pistons i could just flat top them and wholla! 10.5 or more depending on how i went

should i have them custom grind me a cam with 28* and 292 duration that will suit my compression ratio more? will stepping down to like 292 and 292 and trying to keep my lift at the numbers from the first cam? i know stepping down to the next lower one puts the lift at .520 and.542 which i think is not up to par with the flow numbers on the heads....

any ideas?

 

Here's what i'd do: (see attached pic)

Basically, use a .015-.020" thickness gasket. That brings up your CR from 10:1-10.3-10.4:1. That's in the useable range, AND gives you a .040-.045" quench distance. Ideal. Keep a low temp t-stat, and a quality waterpump. Run a bottle of watter wetter in the rad. You shouldn't have a smidge of problems.

You are naturally aspirated, so I don't think forged internals are needed. I'd make sure you have ARP main and rods bolts however. (I can't see someone building a 406 shortblock without it however.) Peace of mind would probably have me tear down, or at least find out every tiny piece of info I can, about the existing shortblock. Suprises later (bad ones), really suck. I don't think it's worth it for you however. To tear it down I mean. Just make sure you know it has some good parts in it...

Make sure the gasket, and heads, are ok to use on a 400CID block. Think steam holes.

 

where do i get gaskets that thin?

i have already had the heads drilled for the steam holes (well at least one side, as the other side on aluminum heads wont hit water no matter how far you drill?

these gaskets will be ok for a street car? and 10:1 will be ok for that cam, cause if thats not sufficient, i might just go to a custom grind and take a few degrees of duration off of it and try to keep the same lift numbers.

 

The holes WILL ALL hit water, if drilled properly.

The problem is the ones on the intake side. They need to be angled toward the exhaust side at about a 30° angle, instead of directly perpendicular to the deck.

Go back and re-do them that way, YOU'LL NEED THEM to be right and functional.

 

you are the first one who has said that... and everyone else ive talked to that run machine shops say as long as one side is drilled its ok...

confusing

 

No; both sides need to be drilled.

Look down in the block. You'll notice that there's no communication between the water jacket on the intake (lifter) side of the cyl assy, and the exhaust (exterior of the block) side. Therefore, a hole on only one side, won't let the steam out of the other side.

Drill the holes. They're free and easy. No reason to avoid the issue, just do it. If you want, epoxy up the ones that go however far up through the side of the head bolt boss, if they drilled them that way.

Now that you know how to do it, maybe you can teach your machine shops a thing or 2. Wouldn't be the first time that a "rebuilder" type machine shop got taught by a hot-rodder how to do racing work.

 

search summit, or phone them, and ask about a head gasket, for a 400CID block, that's below .023" thick or so.
Cometic can custom make you one, but it's pricey. Lets remember, a standard, plane jane, nothing fancy, stock GM gasket is .015" thick. They've lasted for many years, heck, they come stock.
And yes, head gaskets aren't poor quality, and go around failing all the time. That's usually due to USER ERROR. Ie, make sure you use RTV around coolant holes, and use good bolts (ARP), with washers, and use sealant on all bolt threads.

 

so should i get a stock replacement?

 

That should work

 

anybody know where i can get a thin head gasket, i hear they make a .015 copper shim head gasket, that would put me up to 10.4... where or how do i get one, and also would it be feasible since i dont have domes or flat tops and 64cc heads/stock deck?

 

im also gonna call cometic later this morning, they say on their website that they make their MLS gaskets down to .027 compressed, which would put me at 10.19XXXX (10.2)

would that be enough to solve this issue?

im struggling finding a gm head gasket for a 400, anybody know where i could get one?

 

.
.
um, GM? Did you call up a stealership?

Cometic would work great, but it'd be much more pricey.

 

i did look up on gmpartsdirect and found zero results.

if you were me which gasket would you go for a stocky gm .015 or spend the few bucks for the .027 cometic?

 

the guys at comp swear by this and ive called them a few times and told them my concern was the overlap, i told them that with the cometic gasket worst case scenario was 10.2:1 and he told me thats plenty enough, he also went on to say that even though with that duration you have some bleed off of compression, that the extreme energy cam has a faster profile that the valves open and close faster and that builds more cylinder pressure that makes up for the overlap. i also told him that if a step down on duration was necessary id even have a custom cam ground, and i also told him i diddnt want it to be a dog on the bottom (even though im not expecting it to be a torque monster either) he said since it has the stock length rods, he believes that the dual plane and the rod size will help on the bottom end.

these guys are the ones who build the cams, and i do know thta this cam will act differently because of the bore/stroke/cube difference. making what would sound like a huge cam in a 350 per say not so big when you add 56 cubes.

make any sense?


hate to ask so many questions, but its for the sake of information