I have been picking up some bits and pieces off the forums about calculating dynamic compression ratio. In the past, I recognized that a longer duration cam would bleed off pressure, but I had no way of quantifying how much. When I built my current 406 back in 2003, I found a "good deal" on a solid roller Isky on ebay, and just went with it. This spring I am replacing rings and bearings, so having the engine on a stand makes a convenient time to re-think my valve events. Using one of the online DCR calculators, I found that my combination only has 7.6:1 DCR. I was running some cam iterations on Desktop Dyno last night, and it looks like I can pick up ~25 hp by adding some exhaust duration/lift, and changing my IVO. I sent a cam selection request to Comp this afternoon to see what they recommend.
The current cam is an Isky P/N:201602 solid roller with 260/260° @ .050" and 0.602"/.602" lift. Valve events @ .020" are: intake 42° BTDC & 78° ABDC exhaust 78° BBDC & 42° ATDC. Lobe center 108° with 0° advance.
The engine is a SBC 406: 4.155" bore; 2.75" stroke; .051" gasket; 4.2" gasket bore; 14cc dish piston; 58cc chamber; I think pistons are .010" in the bore. Heads are some old Brodix -8 with around 200cc inlet runners and 2.05"/1.60" valves. I estimate the intake flow around 260 cfm. The computer simulation really picked up power (~ 100 hp) with a set of 300 cfm AFR heads and a split duration camshaft.
The car is a '69 Nova; 3500# with driver; 3.73 gears; 3800 stall; full 3" exhaust; 1-3/4" headers; 250 hp nitrous; full MSD ignition. Previous 1/8 mile performance was 7.3 @ 93 mph NA and 6.6 @ 104 mph nitrous.
So do you all agree that my DCR is too low? Anyone with a camshaft recommendation? Do you think i would see the estimated 100 hp increase with the higher flow heads and cam change? I want a little more performance when it all goes back together.
I have been picking up some bits and pieces off the forums about calculating dynamic compression ratio. In the past, I recognized that a longer duration cam would bleed off pressure, but I had no way of quantifying how much. When I built my current 406 back in 2003, I found a "good deal" on a solid roller Isky on ebay, and just went with it. This spring I am replacing rings and bearings, so having the engine on a stand makes a convenient time to re-think my valve events. Using one of the online DCR calculators, I found that my combination only has 7.6:1 DCR. I was running some cam iterations on Desktop Dyno last night, and it looks like I can pick up ~25 hp by adding some exhaust duration/lift, and changing my IVO. I sent a cam selection request to Comp this afternoon to see what they recommend.
The current cam is an Isky P/N:201602 solid roller with 260/260° @ .050" and 0.602"/.602" lift. Valve events @ .020" are: intake 42° BTDC & 78° ABDC exhaust 78° BBDC & 42° ATDC. Lobe center 108° with 0° advance.
The engine is a SBC 406: 4.155" bore; 2.75" stroke; .051" gasket; 4.2" gasket bore; 14cc dish piston; 58cc chamber; I think pistons are .010" in the bore. Heads are some old Brodix -8 with around 200cc inlet runners and 2.05"/1.60" valves. I estimate the intake flow around 260 cfm. The computer simulation really picked up power (~ 100 hp) with a set of 300 cfm AFR heads and a split duration camshaft.
The car is a '69 Nova; 3500# with driver; 3.73 gears; 3800 stall; full 3" exhaust; 1-3/4" headers; 250 hp nitrous; full MSD ignition. Previous 1/8 mile performance was 7.3 @ 93 mph NA and 6.6 @ 104 mph nitrous.
So do you all agree that my DCR is too low? Anyone with a camshaft recommendation? Do you think i would see the estimated 100 hp increase with the higher flow heads and cam change? I want a little more performance when it all goes back together.
On my computer here it looks as though the pistons are "upside-down".
Sometimes it's tough making internet-diagnosis' from photos up here!!
Thanks, Gary in N.Y.
P.S. The valve pockets on my monitor appear to be towards the bottom of the block, should be "up" towards the intake?? Maybe it's one of those "optical illusions"!
... it looks as though the pistons are "upside-down"....
Wow, I am both ashamed and stunned. That pictures was taken when I first assembled the engine back in 2003. I put quite a few miles, many dragstrip passes, and many bottles of nitrous through an engine with the pistons upside down. I remember checking the piston-valve clearance, and since I have dished pistons I guess it had sufficient clearance. Here is a picture when I was disassembling it.
I wonder how that affected the flame front and quench area. What really bites is that I was almost finished re-assembly. I guess the only right thing to do is flip the pistons over on the rods. Got to love messing with those spiral locks.
Thanks for the good catch Gary. Your easy approach softened the blow too. Man I feel like such a rookie.
Wow, I am both ashamed and stunned. That pictures was taken when I first assembled the engine back in 2003. I put quite a few miles, many dragstrip passes, and many bottles of nitrous through an engine with the pistons upside down. I remember checking the piston-valve clearance, and since I have dished pistons I guess it had sufficient clearance. Here is a picture when I was disassembling it.
I wonder how that affected the flame front and quench area. What really bites is that I was almost finished re-assembly. I guess the only right thing to do is flip the pistons over on the rods. Got to love messing with those spiral locks.
Thanks for the good catch Gary. Your easy approach softened the blow too. Man I feel like such a rookie.
It's not the first time I've seen it. In my area here I delivered ALL the machined components for an original LT-1 (.125" domed slugs) and had the opportunity to catch that one as well.
I got a call from the customer to stop by and see the short-block assembled!
First glance I see the domes "up" and the valve pockets down.
I told him he did a great job and left. Really, I said nothing!!
The reason I said nothing was due to the fact I knew there was NO WAY he was going to install the heads, so I just left on that note!
I hadn't heard from him again until he called a few weeks later and told me how happy he was with the machine work AND that the unit was running great!
I could only assume then that he "corrected" the error!!
Thanks, Gary in N.Y.
P.S. Don't feel "stunned" or "ashamed", nothing "broke" and it DID run. I'll add also, IF you didn't place the photo there you would still be at "square-one". Lesson learned and at no cost!
I got 4 of the pistons flipped around on the rods this morning, then I went into Huntsville to hang out at a dyno day. There were only 2 Corvettes - an 01 Z06 @ 343 hp and a stock C6 with the Z cut-out mufflers put down 382 RWHP. When I left a Supra had the high number a 593. Well, back to spiral locks and piston rings....
They mostly cater to the import crowd, but Aurthur (owner) is a super nice guy. His 600+ hp Civic and 750+ hp Supra are impressive to say the least. I tested my Nova there a couple times, and he still has my 660 hp nitrous pull listed in his top 5 - although he hasn't updated that list since 11/05.
Now that you got some quench (pistons corrected) why don't you install some heads gaskets to attain @ .040 piston to head height?
DD2000 falsely awards way big split duration cams. If you are running an open exhaust you should run the exhaust duration within about 6-8 degrees.
If you are going to be racing I would look at the lobe profiles in the comp cams cat. I'm running a billet small base circle endurance road racing lobes with 1.7 roller rockers to get .685/714 lift 4-7 firing order swap and CC poly dist drive gear.
I talked to a Comp Cams rep yesterday evening, and he recommended a 266°/274° with .630"/.630" and 3° advance. I didn't write down what the LS was though. I think the lobe profiles were 4020/4016. I need to pull out my ISKY to see what base circle it is. I am running 3.75" stroke with 6" h-beam Eagle rods, so it may be a little close tight in there. I haven't ordered the cam yet, still crunching some numbers.
Since you already have a 260/260 SR 108 cam it should be easy to figure out what you need.
Does it need more or less Duration Intake?
I read an article about head flow CFM once that really went into dyno runs using relatively small flow heads on a big ci small block. The result showed that past a certain point the laws of diminishing returns set in on using more and more cam duration.
Then I read an article which proved what I thought all along. Used big CFM heads and less duration to get the same amount of peak power
Corvette Store
406 SBC cam selection & DCR discussion
