[steve25]

Can some of you fine folks check out / comfirm what I have turned up here?

This info I amp posting up was durived out of Pete McCarthys great first Pontiac performance book.

First off let me post up the spring specs for a 1968 350 hp standard high comp D port motor with a manual trans.
This motor uses different springs for Intake and Exh, and these are dual spring psi numbers as in inner and outer.

Int closed 69/60 psi. total psi @ .421" lift = 129 psi

Int open 142/128 psi. Total psi @ .421" lift = 270 psi

Exh closed 69/41 psi. Total psi @ .421" lift = 110 psi

Exh open 143/128 psi. Total psi @ .421" lift = 269 psi.

The SD455 of 73 and 74 used the same springs on the Intake and Exh valves, as follows.

Int / Exh closed 117 total psi @ .470" lift.

Int / Exh open 292 total psi @ .470" lift

Now the issue here is that the production SD455 motors where made with a Cam of .421" lift, not the intented first 1971 preproduction Cam of .469" lift.

If you run out the spring rate numbers as closely as you can with the factory info it sure as heck seems like the SD455 motors where/ are running around with less open spring pressure then a standard 350 hp high comp D port motor!

I think these motors have only some 250 psi of open pressure to the D ports 270 open pressure!

[steve25]

Sorry, I left out info about the SD455 springs.

There installed height for the intakes for example is 1.820" and the open height is 1.350", so that would be .470" lift.

[shermanator2]

Using your numbers, I get 274 lbs open for the SD455. (292-117)/(.470) x (.421) + 117 = 273.76.

[Kenth]

1. 1968 350 hp standard high comp D port motor with a manual trans. uses the same springs for intake and exhaust #9779009 outer spring rate open pressure 168 lb and #9781476 inner spring rate open pressure 158 lb. Total for both springs 326 lb.
Open pressure is net valve lift times spring rate, plus seat pressure in pounds.
Also, the total lift is .407", not .421".

2. 1973-74 455 S.D. uses the same springs for intake and exhaust #493145 outer 70lb@1.812" installed height spring rate open pressure 261 lb, and #493144 inner 40lb@1.750" installed height spring rate open pressure 111 lb. Total for both springs 372 lb.
Open pressure is net valve lift times spring rate, plus seat pressure in pounds.
Also, the total lift is .0407", not .470", not .421" or anything else.

It is quite obvious the 455 S.D. uses more open spring pressure, not less, than the 1968 400 H.O. engine, and they should, due to the 455 S.D. #744 camshaft grind with its square lobes.

[steve25]

Ken, can you post up where you got this spring pressure info and also any spring pressure checks you may have made with stock springs removed from any heads you may have on hand because your pressure numbers are really really high, especially for a stock D port 400 cid 350 hp high motor!!

I know for a fact from talking with Mac K 25 years ago at a event that the factory had Cam and lifter lifter life concerns for the 320 pounds of open pressure that the RA4 motors would have with there spring package at .527" lift!

Also included in his comments was that this spring pressure is also what brought about the bigger diameter push Rods used in those RA IV motors since that factory engineers also had concerns about exceeding 280 to 300 pound of open pressure ( depending on the intended rpm usage) with the 5/16" OD push Rods used in all non RA motors!

Does anyone on the board here have stock springs removed from a SD455 to run thru some test for us?

[sdbob]

Yes. I have good used springs. New springs on new heads. When I first did my SD455 stocker build.i used new everything. Then I broke rear end car sat awhile as business was priority. Anyway when I got back to repairs I pulled heads and checked springs. Engine had maybe hundred runs. The springs checked out exactly as advertised in shop manual. I used local machine shop cking machine.

[PAUL K]

Originally Posted by steve25

Does anyone on the board here have stock springs removed from a SD455 to run thru some test for us?

What heights you want tested?

[PAUL K]

I thought I had some SD-455 springs here at home but I didn't find them. I did come across a couple of sets of RAIV springs and checked them. One set is off a well driven 70 RAIV and the other set are low mileage over the counter springs.

Using a factory retainer to compress the inner spring;


--------------- Old / Low mileage

1.812 seat 107 118

@ .470 lift 305 320

@ .517 lift 325 340



I remember testing a bunch of different springs when I was prepping a pair of performance SD heads using a cam around .510 lift and couldn't find anything that looked better than the factory springs for pressure. Unfortunately, I don't recall the numbers but want to say they were about 115 seat and 330 open.

[Kenth]

My info is from H.O. Racing Specialties.
Again, open pressure is net valve lift times spring rate, plus seat pressure in pounds.

Fact is that 455 S.D. and 400 RAIV uses practically the same valve spring rates at the same installed height, BUT 455 S.D. uses standard valve train 1.5:1 rockers and .407" total lift were of course RAIV uses a beefier valve train due to the total lift of .520", add special lifters to that.

Still, the 455 S.D. do NOT have less open spring pressure than 1968 400 M/T.

[sdbob]

My used springs came off an early Sept 74 car with less than 10k miles. I was there when owner put better springs in and he asked me if I wanted them,yep.

[steve25]

Ok fine, a good low mileage SD455 spring has 320 pounds of open pressure at .470" lift, but the Cam only produces .407" lift, so some .060" less lift!

From the springs both aftermarket and stock that I have tested / finagled with over the years those springs with only .407" lift would then drop down into the pressure range of new standard RAIII D port springs, that s all I am saying here folks.

And by the way that pressure is also why the SD motor does not come with the RAIV !rocker studs, if it ran a Cam of .470" and 320 pounds of open pressure I am sure the factory would have stuffed in the RAIV rocker studs!

[steve25]

While we have HO racing in the mix of our discussion here, here is some more spring pressure info from there still great to this day assy manual.

[242177P]

Originally Posted by steve25

Ok fine, a good low mileage SD455 spring has ...

PaulK tested some RA IV springs. Not SD455.