Valve Springs and Boost............

[MS-75]

Yesterday I put the beastie back on the dyno with the new head studs and DC EWP, fully expecting to eliminate the problem I encountered last time.

See the power curve below-



That was from the previous session, but the power curve dent was IDENTICAL yesterday.

What you can see is the big drop in the powercurve. Last session i figured it was the head gasket leaking (23psi tends to do that to a stock set-up)

Anyway, yesterday we ran into the same problem again-even though the motor now has a full ARP stud kit in it. (I was F***EN SPEWING)

Up at 24.5 psi, she did the same again, and also was needing a whole lot of wastegate pressure to deliver the pressure we wanted at the manifold.

Until I got home, I thought that the head gasket was the culprit again (it still pushed out 100-200mls of coolant). However, when thinking about yesterdays session and the one before, I started smelling a rat.

Scenario-

Last time, the head gasket was leaking furiously (4 litres of coolant over a few runs-I had to top it up each run), whereas yesterday, it only spat out a tiny amount.

Compare to that the big dip in the power-curves from each session, and they were virtually identical. Same place, same size, and the more boost we shoved in, the worse it got. Also, the point at which it occurred was repeatable to a pound.

Looking at both those bits of info, it became rapidly clear that the head gasket leaks were totally different, but that the power drop was the same-telling me that they were not in fact related.

Finally it dawned on me.

At 21+ psi and anything over 5300 rpm the stock valve springs are not strong enough to pull the valves shut!! EUREKA!!

(there's a bit more to the deduction than that, but in a nut shell-that's the last remaining hurdle between me an 500rwkw (has made 446 rwkw on 23psi with the this problem so far)

Anyway, I'll run it as at Compak attak as I don't have time to change the springs before then, but it should still run a low ten to high nine, and then I'll fix the springs up for Jamboree.

Sean

[rob]

phew! for a second there i though you were going to say that you wont be visiting us for compak attack

glad to see u have nutted out the problem

cant wait to see this in the flesh!

[kingmick]

stock springs i take it?
have you had a chance to see what they are on the seat?
what revs are you going to take it to?
check to locks for stretching when you get it all out.

[chris davey]

Sounds like a feasible explanation to me

So what car will you get the valve springs from?

[oldcorollas]

nice work....

what size are the valves?
a 40mm valve will have 1.95 X Boost pressure removed from the spring rate...
24.5psi = 48lbs....

think of it this way... you can accurately calculate the required seat pressure now

[jeffro ra28]

what sort of engine(excuse my ignorance)?



Even Brand new springs will see a drop in pressure after just a few laps, it isnt un-usually to see drops of 40 pounds with some springs just after a few laps.

P.S. Higher spring pressure will not effect HP!

[kingmick]

jeff its an OHC engine so seat pressures are way down from ours!
tell the truth jeff your running 260's hahahahahahaah

higher spring pressure just slows spinup a fraction.

stew ive run 30psi with 55 pounders, was happy at that.smaller valves

[jeffro ra28]

ahah Mick! Thanx for outlining my nill experience with Jap engines although i should have realised the mechanical losses and valve train weight difference, duh jeff...lol

Actually 280, but aprox is the word there!

So can anyone specify what the Jap engines should see on the seat with NA and turbo?

[MS-75]

stock springs i take it?
have you had a chance to see what they are on the seat?
what revs are you going to take it to?
check to locks for stretching when you get it all out.

Yeah-totally stock dude. It seems they are a bit more prone to boost induced float than 1J or 2J's, but my guess is that's due to the fact the motor isn't boosted from factory and it has a much lower redline than the other two.

I haven't dicked around with valvegear much, so what do you mean by

check to locks for stretching when you get it all out.

?

As for RPM, the most I figure I'll ever rev it to is 6500, but at present, 6100 is max.

what sort of engine(excuse my ignorance)?.

It's a Landcruiser 4.5L, internally pretty standard with custom manifolding and a GT42. The spring pressure in a direct actuation system is extremely low compared to a pushrod system. When I disassemble and reassemble the head I don't need a spring conpressor-I can compress the springs by leaning on them.........

As far as what to use in place of the stock ones, thus far I haven't actually measured them up. Given that the 2JZ supra head studs fitted, my guess is that a set of upgraded 2J ones may fit-but I'll wait till after Compak Attak before I start pulling shit apart again.

I'm actually going to turn the boost down to race because on 23psi it made 446rwkw, and dropped to 422rwkw at 24.5psi.

Oldcorollas-yeah-that was the sort of working I was figuring on. What I'll try and do is compensate for the effect of 30psi boost against the area of the valve-plus another 20% or so to offset the 500rpm above factory redline. (anyone have any rules of thumb for %seat pressure increase per % increase in RPM?)

Anyway-it was a little frustrating to encounter the problem again-but now that I've worked it out, fixing it won't be a worry, and in terms of physical work it only means pulling the rocker cover and cams-which is less than a day. The key will be careful selection and checking of the replacement springs so I only have to do it once.

Sean

[oldcorollas]

as a super ultra rough guide of force vs rpm, forces increase with the square of velocity?

so... 6500/6100 = 1.066
squared = 1.135....

so if you uprate the springs 14% for 7% rpm increase rpm, should be ok?
so exactly what you suggested...

work out stock pressure at float, based on the stock seat pressure minus the 24.5boost, then uprate based on valve area and increased boost, and 15-20% for rpm (20% for a bit of leeway), and should be bananas (it's already nuts )

[kingmick]

sean when the valve train gets put under stress the locks tend to stretch a bit. as your going past the valve usefulness you prob find a few of them stretched. depending which ones they use you prob be able to use MOLY ones from something else. saves dropping a valve racing.

lol fixing problems is the joy of racing!

[BigWorm]

Once you work out your spring sizes & required rates check out http://www.performancesprings.com.au/index.html, they have more springs than you can poke a stick at, & are very helpful. They also do a good deal for Toymods members, I'd be suprised if they couldn't sort you out with what you need.
And good luck at Compak Attak, I only wish I could be there to see it! BTW, have you heard any news on progress at AIR?

[takai]

Joel.. off topic somewhat, but when did you move to SA? I always remember you being an NSW junkie.

There are more and more Toymodders in SA now, maybe we shall have to have a meet up sometime.

Good luck for CA though Sean.

[MS-75]

Once you work out your spring sizes & required rates check out http://www.performancesprings.com.au/index.html, they have more springs than you can poke a stick at, & are very helpful. They also do a good deal for Toymods members, I'd be suprised if they couldn't sort you out with what you need.
And good luck at Compak Attak, I only wish I could be there to see it! BTW, have you heard any news on progress at AIR?

That site is brilliant.

You root my boot.

[BigWorm]

Wait till you see their prices.