Leaking Exhaust Valves

I did a leak down test on my 565 BBC with Pro-1 heads after ~35 passes. I"m running a 14-71 Littlefield on alky at 20 psi boast using a 44 mag set at around 34*. To my surprise I found 5 leaking exhaust valves. My egt's ranged from 850* to 1113* through the season.

This is my first season with a blown motor; I've always been a nitrous guy. My buddies say I have gone to the dark side. My question is, is this common with blown motors or did I just screw up?

Thanks for the input, Joe

 

depends on how bad they are leaking if it was the intakes it would fix before the next pass but on the ex. side we have run the car with some leakage out the ex side..Dave

 

The first things that come to mind:
1-springs got too weak and valves kissed pistons
2-springs are too weak and aren't on the seat long enough (because they're bouncing) to get rid of their heat
3-margins are too thin so they won't get rid of enough heat

Is there enough valve/rockerarm clearance? Maybe its almost being held open because the clearance is being set at TDC and not on the base circle of the cam?

 

Mike, the valves are Manley stainless and I haven't pull the heads yet. I'm using the Comp. Cam triple drag race springs with an installed force of 332# closed and 949# open. The minimum exhaust piston to valve clearance is 0.150" and the intakes are 0.175"; however, I have turned the motor to ~8300 rpm. You might be right. I guess I'll have to pull the heads and see?

SoDak, I think you got the answer. I now remember that earlier I did have to machine the rocker studs to allow for more rocker clearance. When I was setting the valve clearances one time I discovered some of the rockers hit the top of the studs were they transition to the larger section to support the stud girdle.

I'm now convinced that the valves must of kissed the pistons before I machined the studs.

Thanks guys, Joe

 

Back when we first went from Nitrous to the "Dark Side" (when IHRA made blowers legal in TD) we had this problem once we hit the 6.80 range w/SS valves. Bit the bullet and put Ti in and problem solved! Just depends on how fast ya want to go!

 

years ago i had a blown chevy ss valves ran it a few times leaked it down a bunch of leakage i thought WTF call the guy that built the motor he said they do that at times that open valves tweak as there cooling and i had good springs he the blower boost will seat them up and good springs will help so when it heated up they should be ok i ran that motor for 3 years no problems ran low 4.0s in an altered i thought if the guy could make 7 of those motors run together on a tractor and be 10 time world champion what the hell i ran the dog shit out of it YES everyone call it a tractor motor (lol)

ps make sure you not hitting any pistons then run it

 

I discovered what you are going thru with exhaust valves leaking during leakdown also .mine was caused by changing brands of cylinder heads and pushrod length was incorrect and valves kissed pistons as rockers put pressure on valve retainers opening valves at the wrong time during trying to start motor with no compression is hard.
the motor ran fine after putting in correct pushrods as there must have been enought pressure to close will running it was not till i took heads off again and noticed valve marks on piston

 

Is that enough spring pressure to control those heavy stainless valves? I would think that you need more.

 

Hmmmm, you wouldn't be talking about Tim Engler???

 

Everyone has given good advice, we have run alot of BBC combos and I agree you need more spring pressure ,we were never under 360 and would put them in about 380+ with Ti , you can keep shimming up close to coil bind , as long as you have good push rods and rocker system more is better

 

i wasnt going to mention any names but yes it was tim that was in 92 he done me a great job the price was right it made a ton of power for a chevy back then i always tried to get him to go drag racing but he like the sprint cars