[oldman]

Originally Posted by Tim M

I agree the AFM lifters are not ideal. May consider swapping them next year as another big project, but not over the driving months. No interest in a cam change.

But check my math...

LT1 intake valve diameter: 2.126 inches
Area of intake valve: 3.55
Boost impact to valve: 3.55 * 10.3 = ~36 psi decrement to valve spring pressure due to boost. (40 degree day today resulted in higher than 9.5 psi > 10.3)

Seems like one should offset this in either higher spring pressure and/or lighter valve weight (Chevy did both).

Admittedly, my interest is two fold - ensuring reliability and there is a bit of interest to see 200 mph with this car on the autobahn - again math says 6800 rpm in 5th should equal that number.

Besides the rpms sure sound nice!

Also note the LT4 cam has a lot less lift and duration on the intake side, I would also assume the ramp, especially the declaration ramp is very gentle.

When GM built the LT5, the basically took the off the shelf LT1 manual cam (no DoD), problem solved. Not saying or implying that any LT1 manual left the factory with the no DoD cam. I am saying the LT5 uses a much larger intake cam with no DoD and no other changes were done to the valvetrain. I like GM solutions, I figure they have to warranty their stuff against abuse. GM would not have changed out the cam design if there were no issues with the basic LT1 DoD cam. They did this for longevity, performance, and probably valvetrain stability / reliability.

To people saying to shift at 6400 RPM, I've only road race a few times, I do autoX, I do drive on the street. Many times in a road race you are HARD into the rev limiter. Add to that the higher shift point is not done because of power in the current gear, it is done to put the engine into the power range of the NEXT gear. For me personally, I think at some point boost + stock hollow intake valve + AFM lifter + bigger LT1 cam lobe => failure at some point in time. Valve bounce is a problem and IMO most failure is caused by valve bounce. I did not think so before as most of my boost stuff over the last 25 years has been 4 valve engines and there is NO issue with boost holding the tinney valve head open. But this is a HUGE intake valve, so IMO this should be carefully looked at.

Also to say xyZ is running zyz boost on stock springs / stock valvetrain. My thinking goes to how long? I want to undercut head solid pro flows on my last build just because my aim is to have a 100,000 mile and / or 10 year engine. Usually the first and only sign of valve bounce is the head dropping into the combustion chamber.


Back to boost and pressure on the back of the valve. It has been a non-issue to me due to my 4 v experience. Now on this board, um yeah, I learned something, it is an issue. Also think that there is an air column that is hitting the valve (that is why tuned length intake ports are there for, how much kinetic energy is being impacted on the back of the valved head?

[Tim M]

oldman: lots of good detail/comments.

I called BTR today and although the tech wasn't LT1 savvy necessarily he recommended the SK013 package. He couldn't tell me the spring pressure numbers, but Summit lists it as:

Installed Height (in): 1.875 in.
Seat Pressure at Installed Height (lbs): 165 lbs.
Open Height (in) (app): 1.225 in.
Open Pressure (lbs): 400 lbs.

I believe the installed seat height of the LT1 is 1.900 so drop the pressure to ~156. Along with your comments, I think it seems high for an OEM lifter...assume the AFM lifters are no more durable than LS7, etc.

I like your suggestion of BTR's PSI 1511ML .625" Lift Beehive Springs. 130 lbs @ 1.800", 370 lbs @ 1.175", Coil Bind 1.100", 1.290" OD, .630" ID.

Even without shimming, this spring seems to be the ticket for something slightly stronger than the OEM LT1 springs.

I will take you up on the offer to check the OEM LT1 spring for pressures at 1.900 and at full lift (1.349), if not too much trouble. I may just buy a single from Chevy and check it myself - seems like that info would be out there.

Thanks for the assistance.

[KingLT1]

1511's are great beehive springs...probably some of the best and have quite a bit more spring pressure then stock but they are more spring then you need for your application.

The stock LT1/4 spring #12678635 are a .570 lift spring. I don't know the open/closed seat pressures.

Another option that would work is the PAC 1215 .600 105/313.

[Tim M]

KingLT1: Thanks for posting. Reference the 12678635 spring - oldman posted that spring is used for both LT1 and LT4 applications now- ie same spring. I'd like something more.

BTR gave me three options - the LT4 spring (stock), the the PSI 1511ML, and the SK013 (which seems too very severe). He stated the rates are in that order.

Comp Cams was more helpful and provided a few details to piece together a 26918 kit.

But, I pressed the easy button with the PSI 1511ML kit. Believe the LT1 installed height is 1.900, thus expect the seat pressure to be reduced a bit.