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Unread 02-28-2012, 12:24 PM   #1
danturner266
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SBC 350 L31 Cam Selection

I'm wanting to replace my aging SBC 400 with a 350 L31 engine from a 2000 Chevy Express E3500 van. It's a 4-bolt main, had the block cleaned and honed already, crankshaft and rods are in excellent condition, nothing had to be turned or undersized. I'd like to reuse my pistons, but chosing to may have an affect on deciding on what camshaft to use. I don't want to reuse the stock cam because it's so small. I'm going to be running an Edelbrock Performer intake and Edelbrock 600cfm carb. I have Headman headers for the SBC/CJ7 conversion to install and will be running true duals with standard turbo mufflers. For the 906 heads, I won't be doing any porting or de-shrouding, just upgrading the springs to Z06 beehives and Comp Cam retainers and replacing the stock rockers with steel roller tips. I want to run my current (stock) converter in the TH350/Dana 20 and I've got 4:11 gears front and rear.

I'm looking at two cams but wonder if my stock pistons will be okay, since compression ratio may affect whether the cam I select does a good job or creates tuning nightmares. It's my understanding that this 880 block has a 9.1:1 CR with the stock pistons, and standard bore. I don't want it to sound like a dragster, so "lumpy" isn't nearly as important as function, but a nice healthy sounding idle would be nice.

Stock Cam: Duration @ .050in (intake/exhaust) is 191/196; Lift w/stock 1.5 rocker: (intake/exhaust) is 414/428; Lobe separation angle is 111 degrees.

1st Cam: GMPP 14097395 - this is the cam installed in the HT383 Truck and RamJet 350 crate engines.
The duration at lash point in degrees (intake/exhaust) is 288/308; duration at .050" tappet lift (intake/exhaust) is 196/206 and maximum lift with 1.5:1 rocker ratio (intake/exhaust) is 431/451. Valve lash is zero/zero and lobe centerline is 109 degrees.

2nd Cam: Comp Cam 08-411-8 - The duration at lash point in degrees (intake/exhaust) is 260/264; duration at .050" tappet lift (intake/exhaust) is 210/214 and maximum lift with 1.5:1 rocker ratio (intake/exhaust) is 474/474. Valve lash is zero/zero and lobe centerline is 111 degrees. This cam also came recommended by Ryan at Comp (Tech Dept).

I've heard that the 383 cam is a good choice, but I may need to get my CR around 9.7 or even closer to 10:1 to get it tuned. If so, would standard flat top pistons get me there? Should I worry about the 109 LSA, putting overlap near 80?! which seems high for a daily driver/weekend trail rig? (I will have power brakes before this engine goes in)

If I go with the Comp Cam, will 9.1:1 CR be okay?

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Unread 02-29-2012, 08:39 AM   #2
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I'd go with a custom-ground Xtreme Hydraulic Roller from Comp.

Lobe #3301 on both the intake and exhaust.

Adv. Duration 266
Duration @ .050 214
.463 valve lift with Comp's 1.52 rocker arms
107 lobe separation angle on a 103 intake centerline. 107 +4

It should have a mildly noticable idle and make gobs of usable torque. Just off idle to roughly 5,200 rpm.

9:1 compression is about perfect so you won't have to fiddle around with pistons.

The cam Ryan recommended will put you right at the ragged edge of the maximum valve lift those heads will take and you just may have clearance problems. The wide LSA will kill your torque curve as well. I've never heard of running LS springs on a Vortec head so I comment on that.

This sounds like a fun project. Enjoy!


Shawn
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Unread 02-29-2012, 10:06 AM   #3
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If you wanted to offset the cost of a custom cam and most-likely be a little further ahead on power, you could go with an Xtreme Hydraulic/flat cam.

Intake lobe #5432 Exhaust lobe #5232

Adv. duration 262 268
Duration @.050 218 218
Valve lift with .451 .433
1.52 rocker arms

107 +4
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Unread 02-29-2012, 12:21 PM   #4
danturner266
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What should I expect a "Custom-ground" roller cam from Comp to cost? They're advertised and third party vendor prices for standard of the shelf grinds are $300 or just under. I'd like to stay with the roller lifters but I suppose I could be convinced otherwise. There are a bunch of write ups about guys using Z06 springs in the 062 and 906 vortec heads to achive more lift without having to machine the guide bosses. The article I've been refencing is from Car Craft here: http://www.carcraft.com/techarticles...ade/index.html
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Unread 02-29-2012, 03:42 PM   #5
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Quote:
Originally Posted by danturner266 View Post
What should I expect a "Custom-ground" roller cam from Comp to cost? They're advertised and third party vendor prices for standard of the shelf grinds are $300 or just under. I'd like to stay with the roller lifters but I suppose I could be convinced otherwise. There are a bunch of write ups about guys using Z06 springs in the 062 and 906 vortec heads to achive more lift without having to machine the guide bosses. The article I've been refencing is from Car Craft here: http://www.carcraft.com/techarticles/vortec_valve_spring_upgrade/index.html
How cool is that. I didn't know a simple spring swap to beehives would cure it. Very cool!

That added lift opens up the door a little bit. Here's what I would do:

Comp Xtreme Hydraulic Roller.

Lobe #3313 on the intake and exhaust

Adv. Duration 270
Duration @.050 218
Lobe lift .330
1.6 rockers = .528 valve lift
107 +4

Should be just about perfect!

Comp charges $75 or $100 bucks for 'custom' but it's money WELL spent! You just give up too much torque with shelf grinds.
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Unread 02-29-2012, 04:18 PM   #6
danturner266
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Question for you Shawn, in all the cams you've selected, you mention 107 +4 (LSA) correct? How does this number compare the the 111* of the 08-411-8?
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Unread 02-29-2012, 06:50 PM   #7
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I could write a post long enough for you not to want to read it so I'll do my best to stay short and sweet. At least I'll try

The lobe separation angle (LSA) or lobe centerline angle (LCA), same thing, is the distance measured in camshaft degrees between the intake centerline and the exhaust centerline. It's also the single most important number to get right if you want to get the best ouput from an engine. It's also the least understood.

I use a mathematical formula from David Vizard that factors in the intake valve diameter and the volume of the cylinder it needs to feed. Adjustments are made from there regarding rocker ratio, compression, discharge coefficient of the valve seat at low lift, how aggressive the cam is at getting the valve off the seat, etc.

As you know, our engines have four strokes or cycles, yes? When you start building performance engines, there is a fifth cycle that MUST be optimized. That fifth cycle is the exhaust-initiated induction cycle; which is basically the exhaust pulling on the intake at TDC.

Believe it or not, a well-specced cam and exhaust system will pull up to a -7 psi vacuum on the opening intake valve and build as much as 90 miles per hour through the intake port while the piston is still parked at TDC.

Getting the correct LSA for a given engine combination is your best chance at making that happen. Once you know the right LSA, you have to select the right duration to get an overlap area that best-suits your particular application.

A camshaft on a 111 LSA in a naturally-aspirated 350 with under 10.5:1 compression and a 1.94 intake valve diameter simply will not allow the optimal intake valve open and exhaust valve close events to make the most usable power. Sure, you could always add duration until the same overlap was achieved but then you'd have way too much duration for the application and the wider centerlines will open the exhaust valve too early and close the intake valve too late, making the bad situation even worse and further reducing torque.

Ok, this is already getting lengthy. Does that help at all?

P.s. I calculated your LSA based on a 30-over 350 or 355 and it was on the verge of a 108. Since you aren't boring the engine, a 108 +4 is perfectly ok.
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Unread 02-29-2012, 08:13 PM   #8
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This is getting to the good stuff Shawn. Although I'm not all that great at math, I can find a few calulators out on the interweb that relatively agree with each other. So, with the rest of my drive train in mind, not wanting to break something everytime I take it out in the woods and get on it, is this last cam recommendation going to do it for me? Are you able to decern any estimated hp/torque numbers assuming a stock L31 pushing 255 hp @4600 rpm and 330 ft-lbs @ 2800 and instead of FI going to naturally aspirated? I'm pretty certain I'm going to take a hit for the carb instead of FI, but since the cam is the heart of the engine, I think I can get all of that back and then some with the right one. I really sure I don't want to exceed 350hp/420ft-lbs...just thinking about the power to the pavement thing, it has to go somewhere and everything else needs to handle it.
I like your brealdown of LSA, and I understand (like a newb) what overlap is, I guess I'm trying to wrap my head around the +4 (and it's probably jr of me) but are you referring to locating the crankshaft timing gear at the +4 position? If so then that makes sense....
So roughly 4 bones for a custom cam? Oh, I really didn't want to spend that much, but I really do realize how much the cam can mean, which is why I asked in the first place. I've never bought a cam before that didn't come in a box off the shelf, and if I should use a custom one that fits the bill then so be it.... just wanna make sure I put green down for the right one.
We haven't really talked about compression ratio. Is my 9.1:1 good enought no matter what? Should I resuse my pistions and get the rotating assembly balanced now or wait it out until the cam decision is made?
Appreciate your expert option on this stuff Shawn....
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Unread 02-29-2012, 08:33 PM   #9
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You're more than welcome!

Do me a favor. Spend tonight thinking long and hard about EXACTLY what you want this engine to do and what you want the engine for. If crawling around in the woods, up and over logs, etc. is what you see and has your priority, tell me. If all of that is a big part of it but "mall-crawlin'" with a good sound is important as well, tell me. Tell me everything that's important to you.

To be honest, it's much more difficult to spec an engine/cam combo for a street driver than a balls-out race car. Give me that stuff tomorrow and I'll run it through my engine analyzer for a tp/torque estimate.

What was the transmission, gear ratio and tire size again?
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Unread 02-29-2012, 10:10 PM   #10
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Ty Shawn! I've got a TH350 with a stock converter (I assume since I didn't build it), Dana 20 Txfr and D30 front AMC20 rear with one piece axle shafts and 4.11:1 ratio. Tire are 33x12.5x15 BFG MT KMs. When I'm running 60 on the hiway the tach's sitting at 2200-2300 or so, idle is 700-800 and rough right now, at a stop light with the brake down, I'm usually tempted to pop to neutral cause the engine surges and wants to pull. I have no idea what going on in the 400 since I didn't build it either, and IMO it runs warm, like 220* most of the time but I can cool it down to 190* if I let it idle and let the Flex-a-lite twins do their job.

I do drive it to work (about 4 miles away) often during the summer months, and on the weekends we take it out on some nearby trails, mostly firetrails, but there are times I wish I could run down the interstate at 60 for an hour or two to get to other locations to get back into the woods, but worry about the engine temps. At 60 it's sitting at 220* and won't actually cool down until I slow down. Hoping the 350 will be better at cooling! We don't "mall-crawl" - actually I avoid the physical place itself, but understand the term. We do drive it around town week nights when we want to go out to eat or stop by someones place to visit (crusin' I guess) so if I can keep mileage around 12-15mpg that would be good. Right now the 400 gets 10 taking it easy and probably 6 gettin' on it.

So, to answer your conjecture, I do want to be able to "crawl" over a log, etc. but I still want to be able to drive it there and not have to trailer it. If I have to get rpm up to the "crawl" capability so be it, but was thinking that the idle-to-5500 rpm specification might mean that power was available directly off idle. I don't know if idle (800 rpm) and the 1500 rpm where most intakes/cam packages start to give power means the same thing since I'm planning to run my stock converter.

You are educating me alot here on cams, there are so many different variables and I can use the direction of someone who has done this many times before. I know what you mean about it being difficult to spec it out for a street driver than a balls-out race car. I also know that guys have built these 880/906 engines with much more power than I really want, and pretty easily what I've seen. With very little, these are good to 500hp!!! So, I feel as if I should be conservative, but want power to come on at idle (by power I mean torque, since I think the only real place for horsepower in a Jeep is if you are muddin!!!).
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Unread 03-01-2012, 10:09 AM   #11
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I've been playing around with a few different lobes this morning and am of the opinion that the first cam I recommended (Xtreme Hydraulic Roller #3301) on a 108 +4 is the best choice when paired with 1.6 rocker arms.

We can do a little better on power with two other similar lobes but hydraulic rollers can be tricky and it's easy to go the wrong way. Hydraulic rollers tend to distort from the sideload that occurs as the lobe sweeps around the roller and basically tries to force the lifter into the side of the lifter bore. This distortion will bleed oil between the inside of the lifter body and the plunger and can really effect the horsepower so I don't want to get too aggressive with the opening rates.

Based on that, I decided to stay away from the Xtreme 4x4 lobes that Ryan recommended and one of the 3100 series lobes that I was looking into for a personal build. For what you're doing here, I'd rather err on the slightly conservative side and keep your valvetrain stable and reliable.

You're also right on the verge of needing more exhaust lobe area. We could pick up another 10 horsepower by using a little more exhaust duration but it would be a direct trade-off in low-speed power and fuel mileage. I don't think it's worth it in this application.

With that cam, I'm showing 351 horsepower at 5,000 rpm and 390 ft/lbs of torque at 4,000 rpm. It should be making nearly 320 ft/lbs at 2,000 rpm so this should be a kick to drive!

I forgot to answer your question about the "+4" thing. That refers to where we position the camshaft in relation to the crank shaft. Taking your cam on a 108 LSA as an example, if the intake valve reaches max lift at 108 degrees after TDC, the cam is said to be "straight up." If we move the cam so that the intake valve reaches max lift at 104 degrees after TDC, the cam is considered advanced 4 degrees. 108 +4. If the same 108 cam was installed so the intake reached max lift at 112 degrees after TDC, we'd say the cam was installed with 4 degrees of retard, 108 -4. The 108 is the lobe separation angle and the +4 tells us where you want it to be installed.

Make sense?
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Unread 03-01-2012, 10:45 AM   #12
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Oh, I forgot about your piston question. The only concern I have with stock pistons at this power and rpm range is what style of dish they used.

If they used a true, D-shaped dish then you should be just fine, assuming you don't have excessive skirt/cylinder wall clearance. If they used a dish that goes around the entire circumference of the piston, however, I'm not so inclined to say that it's good. Without any quench pad, they're more prone to detonation. Got a pic of one?


Shawn
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Unread 03-01-2012, 11:31 AM   #13
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I like those numbers!!! The explanation of +4 is what I thought but wasn't sure, definitely thanks for clearing that up for me. The pistons aren't equipt with a d-shapped dish, the dish goes around the entire circumference. I've scrounged up that they are roughly a 16cc dish. Here are pictures of a piston and the cylinder head chamber.
0611phr_03_z-vortec_build-piston.jpg   0611phr_18_z-vortec_build-combustion_chamber.jpg  
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Unread 03-01-2012, 01:06 PM   #14
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Wow. GM really did a great job on those Vortecs. I wouldn't sweat it. Get that cam ordered and build that bad boy.


Shawn
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Unread 03-01-2012, 05:25 PM   #15
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I was just thinking about your cooling issues and I have another suggestion.

Since you're getting a custom cam anyways, have them grind that cam as a "4/7 Swap". That will change the firing order from 18436572 to 18736542.

With the normal firing order, 5 and 7 fire in sequence at the back of the block where the water is the hottest. With the 4/7 swap, 4 and 2 will fire in sequence at the front of the block where the water is the coolest. Ever hear about Chevy's dropping #7 in high-output applications? That's most of it right there.

Obviously I'd like to see you get the temps down to a stable 180* but this will act as some insurance and costs you nothing. Just don't forget when it comes time to install the plug wires... 18736542!

There are other things you can do for added insurance like tapping the back of the manifold into the water jackets and running AN line from the back of the manifold, through your heater core and on up to the front. That will also give the hot water another flow path instead of having to fight its way back through the head to the front.

If my description is clear as mud, check out a Performer RPM Air-Gap and you'll see the outlets that I'm talking about at the back of the manifold.

Ok, I think I'm done


Shawn

Edit: I just reviewed your manifold options and tapping the rear water jackets would be a PITA. The 4/7 swap is still a great idea though.
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