Greg, i'm still not 100% sure why most of these 496 stroker kits use a (.250") longer rod but i'm going to find out for sure. Like i said before though, i'm pretty sure it has to do with easier machining assembly with less clearance problems.
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.
Mmmm....781 Heads w/Big Valves - Greg72's 496BBC Build Thread
- Thread starter Greg72
- Start date
4X4HIGH said:
I just got done building a 496 and can shed some light on the longer rod issue. The main reason for putting the .250 longer rod is to bring the crank/rod ratio back closer to stock after increasing the stroke from 4.0 inches to 4.250 inches.
When using the longer rod with the 4.250 crank you will also need to use a custom piston that has the wrist pin farther up on the piston. On these pistons, the wrist pin actually gets into the the bottom of the oil ring.
You can use the stock length rod with the stroker crank, but most engine builders do not suggest it because it puts a lot of stress of the skirts of the pistons/cylinders.
A good place to find a lot of info on BBC build ups is www.chevelles.com
Sandrat said:
Do you want to share any other specs on your 496?
I'd be curious to know if you bought the stroker parts as a "kit" from a vendor like Speed-O-Motive, or if you pieced it together yourself?
Did you do a roller cam? Any model #'s or cam specs?
Have you dyno'ed your engine (chassis or engine) to see what she puts down for HP/TQ?
It sounds like Scott is shooting for about a 9.0:1 CR on mine based on using a flat top piston and based on the CC's of the heads after final surfacing. Ideally it would be nice to be up in the 9.3 or 9.5:1 range, but he told me we'd be into the realm of domed pistons and would probably have to machine them for valve clearance.......sounded like a real pain in the wallet!!!
Greg, it's not a matter of cutting the pistons for valve clearance. It would be the only way to achieve 9.3-9.5:1. It would require a domed piston then remove some of the dome to add CC's back into the combustion figure to keep the compression down. A domed piston also doesn't allow for proper flame propagation.
Greg72 said:Do you want to share any other specs on your 496?
I'd be curious to know if you bought the stroker parts as a "kit" from a vendor like Speed-O-Motive, or if you pieced it together yourself?
Did you do a roller cam? Any model #'s or cam specs?
Have you dyno'ed your engine (chassis or engine) to see what she puts down for HP/TQ?
It sounds like Scott is shooting for about a 9.0:1 CR on mine based on using a flat top piston and based on the CC's of the heads after final surfacing. Ideally it would be nice to be up in the 9.3 or 9.5:1 range, but he told me we'd be into the realm of domed pistons and would probably have to machine them for valve clearance.......sounded like a real pain in the wallet!!!
Yes, the stroker kit came from Scat as a kit. It consisted of a steel 4.250 crank, 6.385 h beam rods and small domed SRP pistons. I got the kit from Gromm Racing in San Jose, he did all my machine work for my engine.
I just got done changing my 496 over to a more radical build, 12.5 compression, Brodix 2 plus heads, big solid roller, but the original build is very similar to what you are doing. Here is a run down of the original combo.
.060 454 block
4.250 scat forged crank
6.385 h beam scat rods
small dome SRP pistons 9.6:1 with 119 cc heads.
merlin oval ports with 2.25 intake and 1.88 ex. pocket ported.
Comp hydraulic roller custom ground. 271 advertised duration intake 276 exhaust. 215 @.050 intake 220 exhaust, 110 lobe sep, installed at 106centerline, .567 intake .581 exhaust.
Performer RPM manifold, 750 performer carb
1 7/8 hooker super comp headers
Crane roller rockers
Melling oil pump, Milodon pan and windage tray.
Desk top dyno said 500+ horse and 600 ft lbs. All I know is it hauled my 84 crew cab on 38's with no problems. This motor made power from idle to 6000 rpm. I could not have been happier with the combo. It idled with a noticealble lope but not objectionable, I even had a towing style Art Carr converter (low stall speed). It ran on 91 octane and never pinged.
Let me know if you have any other questions.
Todd
BTW I'm in San Jose also.
bowtiepower00
1/2 ton status
Greg, on your application, the difference between 9.0 and 9.3 CR is going to be practically nil. Factory engines were as low as 8:1 or so. 9:1 will work well with what I assume to be a smallish torque based cam, and will allow you to run 87 octane gas to save money. You can always play with the LSA, etc., to keep the cylinder pressure where you want it. Are you still thinking roller? There is a guy on some of the Chevy forums- Camaro or Chevelle tech, I think, or possibly Thirdgen, who goes by the name UDHarold or something similar. He is a fantastic cam designer who has worked for Comp and others, and I believe he designed the Voodoo cams for Lunati. I would get in contact with him if you have any cam questions. I think he recently started his own cam company.
BTP,
Here's what the current CamQuest assumptions look like:
You can take a look at the recommended cam and see if you agree with their assessment....interestingly the software only recommends ONE cam for this engine given the criteria I provided.
The only thing I'm not sure of is the flow numbers for the 781s with larger valves. Scott found a website that had accurate flow numbers for the big valve 781s, but I can't remember if this calc has those or not.
Here's the link to the flow numbers:
http://users.erols.com/srweiss/tablehdc.htm#BBChevy
Anyway, Scott has a friend with a flowbench and if I'm willing to pay $85 for his time he will measure my heads and give me 100% accurate numbers to plug into CamQuest. That could be very cool data to have.
Here's what the current CamQuest assumptions look like:
You can take a look at the recommended cam and see if you agree with their assessment....interestingly the software only recommends ONE cam for this engine given the criteria I provided.
The only thing I'm not sure of is the flow numbers for the 781s with larger valves. Scott found a website that had accurate flow numbers for the big valve 781s, but I can't remember if this calc has those or not.
Here's the link to the flow numbers:
http://users.erols.com/srweiss/tablehdc.htm#BBChevy
Anyway, Scott has a friend with a flowbench and if I'm willing to pay $85 for his time he will measure my heads and give me 100% accurate numbers to plug into CamQuest. That could be very cool data to have.
Last edited:
bowtiepower00
1/2 ton status
That thing is going to be a torque monster. Quite a few of the 454SS guys run that cam, the factory TBI is capable of controlling it. It's also reported that it passes emissions pretty well, too. It's been my experience that as soon as Comp hears the words Fuel Injection they automatically select very low duration grinds to cover their butt with regards to computer compadability. If you're going with an aftermarket EFI system, or a carb, you can easily add a little more cam without issue.
You've been playing with Desktop Dyno, correct? I would try running the numbers for something thats got 10-20 degrees more duration on both sides and perhaps a little more LSA, 113-115. The cam you're looking at now would work in a TBI 350, and you've got a lot more cubes. I guess it depends on what you prefer, torque is nice and is what really gets things moving, I just think that with that many cubes that cam is going to run out of breath pretty quick, though it's going to pull like a diesel.
Another thing to consider is that although Comp's Extreme Energy grinds are among the best around, they are a bit dated and there are some newer profiles on the market that pull another 10+ HP/ TQ out of thin air. I'm getting really picky now, you're going to have enough power no matter how you go. I understand Cams, but guys like UDHarold really take it to another level. I've seen some of the Car guys pick up some really good HP using one of his cams over your typical off the shelf Comp/ Crane stuff.
Like I said, I'm really being picky now, but I know how much a BBC Roller is, and I just want to make sure you know of all your options before you lay down a Grand. With that much torque, you could get away with a TH400 and 3.08's.
You've been playing with Desktop Dyno, correct? I would try running the numbers for something thats got 10-20 degrees more duration on both sides and perhaps a little more LSA, 113-115. The cam you're looking at now would work in a TBI 350, and you've got a lot more cubes. I guess it depends on what you prefer, torque is nice and is what really gets things moving, I just think that with that many cubes that cam is going to run out of breath pretty quick, though it's going to pull like a diesel.
Another thing to consider is that although Comp's Extreme Energy grinds are among the best around, they are a bit dated and there are some newer profiles on the market that pull another 10+ HP/ TQ out of thin air. I'm getting really picky now, you're going to have enough power no matter how you go. I understand Cams, but guys like UDHarold really take it to another level. I've seen some of the Car guys pick up some really good HP using one of his cams over your typical off the shelf Comp/ Crane stuff.
Like I said, I'm really being picky now, but I know how much a BBC Roller is, and I just want to make sure you know of all your options before you lay down a Grand. With that much torque, you could get away with a TH400 and 3.08's.
Greg72 said:BTP,
Here's what the current CamQuest assumptions look like:
You can take a look at the recommended cam and see if you agree with their assessment....interestingly the software only recommends ONE cam for this engine given the criteria I provided.
The only thing I'm not sure of is the flow numbers for the 781s with larger valves. Scott found a website that had accurate flow numbers for the big valve 781s, but I can't remember if this calc has those or not.
Here's the link to the flow numbers:
http://users.erols.com/srweiss/tablehdc.htm#BBChevy
Anyway, Scott has a friend with a flowbench and if I'm willing to pay $85 for his time he will measure my heads and give me 100% accurate numbers to plug into CamQuest. That could be very cool data to have.
I am wondering why the lift is so low on a this roller cam? This defeats the purpose of using a roller cam and a set of good flowing heads like the 781's.
I think you will be leaving a lot of power on the table if you go with this type of cam. With the power peaking around 4500 rpm peanut port heads would suffice. Big Blocks can take a lot of lift and duration and still remain very streetable while making great power and torque.
Another thing I noticed is you have the heads entered as wedge style chambers, big blocks chevys are cantered style heads.
Sandrat said:
I think the program shows lift at the lobe, not the valve?? I'm not sure what the "standard" rocker ratio is (1.6?) so maybe that is what's making those numbers look a little "weak" in the program....
The CamQuest program doesn't really let you screw around with cam specs the way that DD2000 does, but I can use the recommended ones from CQ and then run some "iterations" in DD2000 and see what happens with the HP/TQ.
I wouldn't mind trading off a bit of that TQ for a some more top end HP, but the whole purpose of this motor is to move a lot of weight (heavy truck) and TQ is what's going to get that done.
As for the question of economy.....who knows? I get 7MPG now with the 350SBC, how much worse could it get.
Greg72 said:I think the program shows lift at the lobe, not the valve?? I'm not sure what the "standard" rocker ratio is (1.6?) so maybe that is what's making those numbers look a little "weak" in the program....
The CamQuest program doesn't really let you screw around with cam specs the way that DD2000 does, but I can use the recommended ones from CQ and then run some "iterations" in DD2000 and see what happens with the HP/TQ.
I wouldn't mind trading off a bit of that TQ for a some more top end HP, but the whole purpose of this motor is to move a lot of weight (heavy truck) and TQ is what's going to get that done.
As for the question of economy.....who knows? I get 7MPG now with the 350SBC, how much worse could it get.
The lift specs you entered are at the valve, this includes the 1.7 rocker ratio.
I would like to see what the power looks like if you entered the cam specs listed in my 496. I think you would see a more fun/usable power band.
Sandrat said:
I will put your specs into DD2000 just for fun.....I'd be interested to see what the differences are.
What was the C.R. on that original motor?
Was the cam installed straight-up or advanced?
Any other "special" info I'd need to know to accurately build the model?
Greg72 said:I will put your specs into DD2000 just for fun.....I'd be interested to see what the differences are.
What was the C.R. on that original motor?
Was the cam installed straight-up or advanced?
Any other "special" info I'd need to know to accurately build the model?
Here are the specs right from the cam card.
Gross valve lift intake .567 exhaust .581
Duration at .006 tappet lift intake 271 exhaust 276
Valve timing at .050 intake (1) BTDC. (33) ABDC
exhaust (44) BBDC (4-)ATDC
Cam installed at 106 intake centerline
Lobe seperation 110
Duration at .050 lifter rise intake 215 exhaust 220
lobe lift .334 intake .342 exhaust (X 1.7 rocker ratio) gives you lift at valve
Also, if you can, try to change the cyl head info to cantered style instead of wedge.
Gook luck.
Ok, here is a more accurate screen shot.
Not trying to bash, just learn...
What's the point of going to the expense of a hydraulic roller when the specs aren't wildly different than what is available in a flat tappet set up? I know an advantage of a roller design is the ability to achieve more dramatic profiles so why not use this? I know there is a difference in reciprocating drag but surely that alone can't be enough to justify the cost.
Just wondering why y'all spent probably close to $1k instead of $200, especially when this is intended to be a "budget" type build for a towing application?
What's the point of going to the expense of a hydraulic roller when the specs aren't wildly different than what is available in a flat tappet set up? I know an advantage of a roller design is the ability to achieve more dramatic profiles so why not use this? I know there is a difference in reciprocating drag but surely that alone can't be enough to justify the cost.
Just wondering why y'all spent probably close to $1k instead of $200, especially when this is intended to be a "budget" type build for a towing application?
A roller cam has a much more aggressive opening and closing ramp that can't be duplicated with a flat tappet cam. Obviously there is way less rolling resistance as well. A flat tappet cam with the same specs as a roller cam will not perform anywhere near as good as the roller cam because of the fact i just stated.
