we've had some problems with the car breaking up at higher rpm. If I test seat pressure and it's where it should be, are the springs OK, or is there more to it than that?

We use the PSI 1248's FWIW, installed at 2.100"

the cam is 308/318 advertised and 276/286 @ .050 with .782/765 lift.

Valves are 2.30/1.88 stainless on conventional heads

The springs probably have nearly 100 passes on them. Everyone says "change them, it's time" but how do I know

 

check seat and check them open. I never checked mine and ended up with 40# of seat pressure

 

Check the pressures and report back.

.200 long valves?

 

I don't have the means to check open pressure. I can check seat pressure though. I don't have any idea on valve length. I'm sure it's whatever comes in a set of BB2Xtra w/CNC.

 

Brady.
Did this just start?
Ya may need more spring than recommended,
260-270 seat and 8-900 open would be my guess maybe more with the blower.
Take one spring off and get it checked at your lift.

 

1-E 260# I 320#
2-E 280# I 300#
3-E 250# I 270#
4-E 280# I 300#
5-E 250# I 270#
6-E 250# I 280#
7-E 300# I 260#
8-E 280# I 270#

It just started, but we're have fuel delivery issues too- waterman pump was dying... I'll have the heads off in another hour or so to see if we melted anything...

Isky's webpage says the springs should be 375# on the seat and 1045# open... FWIW.

(waits for captain obvious to suggest changing the springs..)

 

I would check them before replacing a high dollar spring set.

May be the pump as lean/rich will cause that too. If its lean it will usually just lay down,rich will cause the miss/studder. Now this depends on the degree of rich/lean also.

 

Wow, those pressures are all over the place!

And DAMN that car sounds wicked idling!

 

I would be checking installed heights to make sure they're the same - 50lbs is a huge amount of variance.

What's the Isky Toolroom spring #?

I would be looking at ignition issues once you get the springs set up.

 

The seat pressures were all close to the same and over 300# when I checked them in April.

New springs are on the way, and when I took cap/rotor off- it's apparent there are some "issues" there as well...

The springs are actually PSI-1247 all installed right around 2.060

thanks!

 

Keep digging,no telling what you will find on that high class thing.

 

Ok

Rule #1 installed height is MORE important than installed pressure. Lot of old hats are going to disagree with me on that from a lack of understanding spring dynamics... that's ok, controlling the spring is all a product of how they are used.

Who is the cam from?

Int lobe is 308 @ .020"? 276 @ .050" ???? @ .200" and .460" lobe lift? (1.7:1 Rockers?)
Exh lobes is 318 @ .020"? 286 @ .050" ??? @ .200" and .450" lobe lift?

I'm hoping this is no more than 196 @ .200" int and 201 @ .200" exh

I'm guessing you are using 145g+ and 135g+ valves here...

The PSI 1247 Tripple 1.660" Diameter spring is NOT light by any means you are easily talking a 175-200g spring!

The problems I can see here....

1. You need roughly .100" shim under each spring from where they are now
2. You need gods pushrod in there with rouchly 1700lbs at the pushrod/lifter. At minimum I would run a .125" wall 3/8ths and even then with a 8"+ BBC PR I would rather run a 7/16"+
3. I think you have the wrong springs here, I'd look more at PSI 1242 in @ 2.000", 1571 @ 2.000" or a 1573 @ 1.980", that's 250-275lbs seated.... 675-750open. I'd also throw .015-.020" more shim at the exhaust springs as well. I'd go with the 1500 series stuff myself so I didn't have to run into this problem again anytime soon.

Bret

 

Bret,
I will be running that much spring pressure N/A with a 2.08 SS valve.
Closing against the BIG boost he has, is that enough? Just curious.

You loose more by not enough spring than too much.