Irish69427

I have been chasing a performance problem where the engine just seems to lay down above 5000 RPM. Have been through the entire ignition system and verified the WOT mixture is solid with my LM1. After eliminating everything else I came to the conclusion I may be experiencing some valve float or bounce. Engine runs great otherwise but simply noses over about 5k.

I don't have the history of this engine except that it is supposed to be a 427/400 tri-power. I took a closed valve spring reading on several valves and they averaged 60-70 lbs. at installed hight. This seems very weak to me and could cerainly be the problem but I don't know the specs for this type of head/cam combo.

Heads are the correct 290 oval port casting for 69 400 tripower. Cam may be a slightly hotter aftermarket grind as the measured lift at the valve appears to be about .560 not the .460 for a stocker. Don't know the duration.

Given the "blueprint" cam from crane calls for 121 lbs. on the seat, I think I am way off here.

Experts, please chime in.

gkull

I would not use sub 120 pound springs even on a H-flat cam. Valve float can be very destructive. I've changed springs with just the valve covers off

TheSkunkWorks

Yep, time for some valve springs. If you're not using oil, particularly during warm up, just change them with the heads on the engine.

FYI, the L88 cam has .560" lift.

VetteLS6

hmmm well you know you can just spend a couple of hundred bucks and get a cam in there that you know the real specs on....the L88 cam mentioned above would be to much for the heads you have on the short block.....it would need to have upper RPM flow to work not to mention it likes compression. Comp Cams or any other manufacturer with a cam in the 280* duration range should give good Idle with some nice stomp....Rev to 6200rpm, with the easy care of a hyd valve train. A Solid Cam like the square pattern LS6 cam would be nice also....10:1 compression works good here and it has a nice sound with decent idle quality.

Irish69427

Thanks for the responses. I had forgotten I had a spare set of oval port heads from my old 427 sitting in my storage unit. Went by there and tested the springs and sure enough they were about 100 lbs. on the seat. Those heads were on a bone stock 390 HP 427 so I definitely have a problem.

Also, I retested the valve lift at the spring very closely and the intake is about .550 and exhaust about .520, well above stock so I suspect an upgraded cam was installed whenever this engine was done last. It does have a nice lope at about 900 rpm and makes about 11 inches of vaccum so maybe 220-230 degrees duration? Stock is 214/218. Don't really want to change the cam as this engine is on borrowed time anyway. Will be building a 505 or 540 soon.

I suspect with the spring change this thing may really surprise me. What are recommendations for new springs? Around 120 lbs on the seat, maybe 280/300 open? I have the tools to change them on the car so that is no problem.

TheSkunkWorks

That 120# minimum seat pressure gkull mentioned is probably about right, likely with a rate of somewhere between 300-350# (the more aggressive the lobe profiles the higher). As an example; 120# seat with 300# rate and .550" lift yields ~285# at max lift. IMHO, you're on the right track here.

CRITICAL: Don't forget to match retainers to the springs and make certain your installed height doesn't put you into coil bind at max lift (you'll want about .060" minimum clearance).

I think you should be quite pleased with the results.

Irish69427

Well, finally got this spring change done. What a pain in the rear. Obviously these heads have been on a long time as I had a terrible time getting the keepers to release. Used every trick I knew. Found one intake valve stem seal was bad and replaced it.

Went with a single comp cams spring with damper so I could be sure I would clear the guides and umbrella seals. The originals were dual but very close to the guides. Specs were about 114 on the seat and 330 open.

Test drove it today and WOW, what a difference! Pulls hard to 6000 rpm now without a hint of the previous problem. Definite power gain. Went ahead and put on some roller rockers at the same time. The better rocker ratio and spring control have changed the idle a bit (more rumpity rump) and tone as well. Sounds great. Now if I can get the final tripower problems worked out I will be a happy guy!

TheSkunkWorks

Glad to hear...

What problems are you having with your 3x2?

(You'll probably get more help if you start a new thread for that, if you haven't already.)