[mrmark1957]

The machine shop that did the valve job and assembled my #13 heads set the installed height at 1.675". When I asked if the springs should be shimmed to 1.6" the guy said the Comp 988 springs had enough seat pressure (110 lbs) at 1.675 and Pontiacs needed help being tight on real estate. Cam is a Summit 2801 with .442/.466 lift. Any opinions, should I shim the springs ?

[chiphead]

That's a pretty mild spring. Did they measure the seat pressure at the 1.675 IH? That's the real question.

How did they get the extra .075? Did they use offset locks, grind the seats, use offset retainers, or did the .075 come naturally due to stacked tolerances? Did they have to use offset parts to clear a Viton seal or something? From my recent install of a 2802, I found that it's tough to get a Viton to fit without machinging the guide down or using offset retainers or locks.

It matters, because if they used offset locks or retainers, you've got to check that the rocker arm you use doesn't hit the retainer. The valvestem stick-out is reduced, so the rocker arm gets closer to the retainer.

But on to the springs. The comp site says those should give 117lbs at 1.6. I did some math and figured about 23lbs of pressure for every .100" inch of travel. So that extra .075" reduces seat pressure by about 17lbs. So you're at about 100lbs on the seat, assuming the printed specs. Now if the guy measured it at 110lbs at 1.675", that could be accurate and due to the retainer used, which will move the numbers around.

Now you could use this to your advantage. With 100-110lbs on the seat you could likely break the cam in without removing the inners. Then once the cam is broken in and has a few miles, you could shim the springs back to 1.6" to get some pressure back.

[Jay S]

Leave the exhaust set up how they are. I would put a .050” or .060” shim under the intakes if they are 30* valves. 45* you could run it as is.

[steve25]

You have a weird combination there!

On my stock 13 casting with a stock valve and retainer I have a installed height of 1.570”

With a RA4 length valve installed ( .160” over stock ) I then have a installed height of 1.720”.

How you have 1.675” must be due to the spring seats being cut some, or a different retainer being used.

The problem I see with those 988 springs is that they only produce 235 psi of open at .550” lift .
That is not enough, and to make things even worse your not even running .500” lift.

This is going to leave you with very early valve float when you try to buzz the motor up, and you can’t even shim those spring up to give more open pressures because when you do your seat pressure will be too unless your running a hydro roller cam.

What I would do if your running a flat tappet hydro cam is cut your spring seats another .025” or so and run the 995 springs.

From a test I just did my new 995 springs set up to simulate a stock retainer and your 1.675” height I get 120 psi on the seat, and the open to .450” lift I get 280 psi.

These numbers are more like what you want.

[Jay S]

988 springs are a close equivalent to the HO performance VS-11 springs. I know on paper they look like they are on the light side, but they (VS-11 and the 988) work with a quite a bit bigger cams that the 2801 Summit, even RA 4 type grinds. Can do a search if you want to research it further on here.. I am guessing if you look up VS-11 and Ken Crocie (HO performance) you will find more info. VS11 show a bit more spring rate, but IRC Ken has said they are close to the same spring.

The 988 spring is the recommended spring in Comp’s master catalog on a XE262, which is also bigger than a 2801. I have done about 20 of those summit type grinds, with the problems flat tappets have lately I would not want to over shoot the spring pressures.

[steve25]

The factory springs for example used with the 744 cam had 129 psi on the seat and close to 300 open.
Even the springs used for all the 1967 and up 068 cam installs had 253 psi of open pressure.

[Jay S]

With it’s square looking lobes the original 744 was know for being hard on springs and needed stiffer springs. The 988 probably wouldn’t be a good choice if that is what a guys was trying to use.

I know the 068 routinely ran 120 to 130 seat pressure. I don’t recall seeing max lift pressure. The 988s generally run a bit higher pressure than they are rated for. I can’t think of a application they would not work on for stock installed height and a 068.


You can run stock springs on a 2801 with 1.5s if you want, it is a mild profile. With 80 to 90 seat pressure on the regular stock spring will make it to 5000 RPM with the 2801. With the 988’s it will make it to 6000 RPM, just make sure you have 120 lbs seat pressure. If you plan to turn the engine harder than 6K it would need more spring.

[mrmark1957]

The guides have been machined for Viton seals and the factory oil shields and o-rings have not been used. The oil shield accounts for .025'' of the additional installed height. The intake seats are 30*, the exhaust are 45*.

[mrmark1957]

Originally Posted by Jay S

With it’s square looking lobes the original 744 was know for being hard on springs and needed stiffer springs. The 988 probably wouldn’t be a good choice if that is what a guys was trying to use.

I know the 068 routinely ran 120 to 130 seat pressure. I don’t recall seeing max lift pressure. The 988s generally run a bit higher pressure than they are rated for. I can’t think of a application they would not work on for stock installed height and a 068.


You can run stock springs on a 2801 with 1.5s if you want, it is a mild profile. With 80 to 90 seat pressure on the regular stock spring will make it to 5000 RPM with the 2801. With the 988’s it will make it to 6000 RPM, just make sure you have 120 lbs seat pressure. If you plan to turn the engine harder than 6K it would need more spring.

Not planning on running the motor faster than 5000 rpm.

[Jay S]

If you stop at 5000 you don’t need to change anything.

[Cliff R]

+2

I'd say you are OK with that set-up as the cam doesn't have the RPM range to require more spring pressure. Plus less pressure is easier on the cam, lifters and other related parts.

Here I use the Crower 68405 springs when we get up near 1.700" installed height and the 68404's for 1.600". They are a nice upgrade from stock springs and allow more room for higher lift cams at those shorter installed heights.....

[steve25]

The question I have for the OP is what cid is this motor?

Since he's running high comp heads a am assuming it's 389 or 400 and will have about a 9.5 compression.
A
Either of these two motors with something like a factory 068 Cam wants to go to 5600, in fact these cid motors with street type performance Cams start to come on strong with there hp at 4800 on up, and do like to be shifted at 5600.

If it where me I would not set up those heads with anything less then 260 psi, at on the Intake valves once the Cams broken in.

If you ever add Aluminum body roller rockers which add more mass on the valve side your going to need more pressure just to over come that mass and hold on to the red line that you had before the rocker upgrade.

[Jay S]

Max power with a 2801 in that combo is going to be around 5000 to 5200, the 068 will out rev a 2801.

[steve25]

Any 400 I have ever had with the 068 Cam got shifted at close to 5600 to drop back to about 4900, and that calls for springs that can get you safly to 5800 to 6000.

Hey to each his own

[mrmark1957]

The motor is a .030 over 400 so it's a 406 cubic inch. Machine shop flowed the heads also at my request as I had cleaned up the exhaust bowls. Intake ports flowed about 200 cfm and exhaust about 175 cfm at .450 to .500" lift. Machine shops dyno program said 409 horsepower at 4800 rpm if everything else is perfect with that head flow and 2801 cam. I'm running log manifolds with a 2 1/4 exhaust system, so it's not open headers. Auto trans shift point will most likely end up around 4800 rpm. The motor came with heavy CompCams roller tip rockers, I'm thinking about going with a stock stamped steel rocker.

[steve25]

I have seen test with a freshly rebuilt RAIII 400s and the 068 cam, and even when fitted with factory ram air exh manifolds into head pipes with no mufflers the motor could not top 380 or 370 hp, I forget which I would have to go back and look.

These nothing wrong with the stamped steel rockers if there not worn, go for it!