496 990 heads limited rpm? - Chevelle Tech
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post #1 of 22 (permalink) Old Jul 2nd, 20, 8:23 PM Thread Starter
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496 990 heads limited rpm?

combo ported 990 heads 2.25/ 1.88 valve, ill post the flow numbers down below. Rpm air gap 850 mighty demon. Heads cc are around 114 flat top piston zero decked.

first cam was a solid roller cb xr280 r-10 .646 .653 on lsa 110 242 248. With springs from a 700 lift solid roller. It would happy rev till 6500+ pulling all the way. This setup had a .040 tho head gasket at about 9 to 1 ( roughly)

Cam that's in the car now. Hyd roller howards 618/618 235/241 on a 108. This setup had the fat head gasket and now a .020. Calcs roughly 9.5 to 1 comp. It has what they recommended for springs ( triple spring ) this setup has has lots a power from 2500 to 5000 but hits a wall and won't rev past it. Now re reading this when I first installed the cam I think it went closer to 6k. Sorry its been 2 years.

Now my heads have a bad I/e ratio. But that didn't seem to matter on the SR before. Any guesses on whats holding it back maybe the distributor getting old? Smaller cam is more affected by EI?


Head flow number's

Lift intake. Out 300. 234. 128 400. 279. 158 500. 312. 180 550. 327. 189 600. 338. 197 650. 337. 205 700. 340. 211 750. 345. 216
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post #2 of 22 (permalink) Old Jul 2nd, 20, 8:41 PM
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Re: 496 990 heads limited rpm?

Those heads do require some more compression to make good power but down low the 990's are a lazy port. Put some mild ported 049 or 781's on that engine and it will come alive. Right now with flat top pistons you are you are close with that 9 to 1. Put some more duration in that 496 and those heads will pull a lot harder. Something in the 250 ish intake and 260 duration exhaust on a 108 lca. One thing you can try is to advance the cam a little, supposed to help the end with the big port heads.
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post #3 of 22 (permalink) Old Jul 2nd, 20, 8:47 PM
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Hi Scott my heads are very very similar to yours. 026. My solid roller pull effortlessly to 6800/7000. Not running a crazy cam or ridiculous spring pressures. I believe on a 107 icl. Maybe needs a little more exhaust duration, or a little more spring? I’m not sure on the hydraulic rollers but a dyno pull with the proper instrumentation will likely tell you a lot about what’s happening.
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post #4 of 22 (permalink) Old Jul 2nd, 20, 10:01 PM
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Re: 496 990 heads limited rpm?

fuel delivery is always suspect when an engine doesn't pull on top. got an AFR meter?
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post #5 of 22 (permalink) Old Jul 2nd, 20, 11:26 PM
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Re: 496 990 heads limited rpm?

Hate to throw a wrench into anyones party, but cylinder head flow rarely limits engine rpm's. Valve springs, ignition and fuel flow are all likely contributors. Cylinder head flow would be near the bottom of the list.
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post #6 of 22 (permalink) Old Jul 2nd, 20, 11:39 PM
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What is the spring pressure? I’ve never seen a hydraulic roller need a triple spring. I bet your collapsing the lifters.
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post #7 of 22 (permalink) Old Jul 2nd, 20, 11:54 PM Thread Starter
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I've had a few people tell me that 990 heads would be bad down low. However first and second are blow the tires away anyththing over 2500 rpm. Not saying the other heads would not be better.


Maybe spark related ill try a new cap and coil. I did notice some wear on the cap. Also I figure out what springs are in it and post back.

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post #8 of 22 (permalink) Old Jul 3rd, 20, 1:06 AM
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Re: 496 990 heads limited rpm?

You answered your own question....the solid roller cam version of the engine would turn the rpm no problem....the hydraulic roller cam version now will not....what does that tell you??!!!!.
It for sure says the heads have no problem going up the rpm scale.
After you account for the valve lash with the solid, both cams are nearly identical on lift and on effective duration, the minor 2° difference in lobe separation is a wash.

BBC valvetrain is heavy....hydraulic mechanisms do not like that under the best of circumstances. They require a very narrow window in valvetrain weight and spring selection.

Hits a wall and won't rev sounds like valve float/spring out of control.

if you don't find anything in the ignition or fuel situation, put the solid rollers on the hydro cam, lash it up tight like .004-.006", and give it a test run....if the rpm capability is now back you've located the source of the problem.
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post #9 of 22 (permalink) Old Jul 3rd, 20, 8:07 AM Thread Starter
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Here's the spings that we're used

https://www.summitracing.com/parts/hrs-98636-k12

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post #10 of 22 (permalink) Old Jul 3rd, 20, 8:53 AM
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Re: 496 990 heads limited rpm?

They aint heavy enough. You need to go heavier imho, but I dont know if you have billet cam or cast core. It isnt the heads. heck they only work at rpm, they wicked soft down low. Please check with a pro on springs, but assume you need 150+/ 400+ to hold that rpm, OR more, so PLUS 100 on the nose.

PS my pro built 467 has 145/400 w/ Morels and has a self-imposed limit of 6200 rpm.

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post #11 of 22 (permalink) Old Jul 3rd, 20, 9:37 AM
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Re: 496 990 heads limited rpm?

Quote:
Originally Posted by scotty's chev View Post
Here's the spings that we're used

https://www.summitracing.com/parts/hrs-98636-k12

Those are not tripple, they are double with damper.

Like Gene said not enough spring pressure.
But before you change them check what lifters and core the cam is on. Howard has three different BBC hyd roller lifters. 91161, 91167 and 91162.

91162 is Morel 4603 and they will handle extra pressure. Think some run near 200 seat and 450+ open.
91161 are gaterman out of memory 165/low 400 is what they will take.
91167 are Morel street series they are limited to 180 closed/380 open.

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post #12 of 22 (permalink) Old Jul 3rd, 20, 11:10 AM
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Re: 496 990 heads limited rpm?

Yep, in light of that spring info, I'm going to agree with 427L88 and Malibu ss 64, not enough spring.
That spring is 348 lb/in rate, you need to get in the range of 410-440 lb/in or maybe a little more.

That current spring is barely enough spring for a SBC hydro roller to turn over 6000 rpm with that cam profile.

Howards #98632 installed at 1.900" height looks like a potential fit, 160 lbs seat @ 1.900" and 420 lbs open at your .618" lift., 413 lbs/in rate. It is 1.500" diameter, should still fit the current retainers for your 1.514" spring too.
That's if your lifter can handle over 400 lbs open.
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post #13 of 22 (permalink) Old Jul 3rd, 20, 12:01 PM Thread Starter
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Thanks for all the posts and info. Seems we you all have found the issue without even turning a wrench. I ordered the parts from a local guy and jist asked for matching springs. I didn't know they weren't tripple springs I just saw the 3 springs and looked like tripple to me.


Now for now springs... i don't have a part number for the lifters however by looks alone it's not the 61 style lifters. Here's a few pics of them I've looked at other and not sure what i have.
where the bars attach i have all the same steel color. Pics of both the others seems to vary. Probably not accurate.
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post #14 of 22 (permalink) Old Jul 3rd, 20, 12:04 PM Thread Starter
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This is a street car only sees 1000 miles a year tho. I assume the reccomend spring pressure will be somewhat fine for longevity.

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post #15 of 22 (permalink) Old Jul 3rd, 20, 12:55 PM
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Re: 496 990 heads limited rpm?

Sure looks like the 91167 Street Series lifter, with that hooded wheel, all the better rated lifters have an exposed wheel.
The 91167 is the lifter indicated in the Cam/lifter package for that cam if bought together as a kit.

Right now, if the current spring is set up at the correct seat height it is listed for at 1.875" for 130lbs seat load, it would make 345 lbs open load at your .618" lift.....so if those are the 91167 lifters you've got you could put another 35 lbs to them.

Spring you've got now has room to shim it .060" tighter and still have adequate clearance to coil bind but that would only get you 19 lbs more.

Got anyplace near you that could test check one of your springs to see what pressure they show now?? I'm wondering if they are just shot, and a new set put in shimmed .060" tighter might do the trick and not overload the lifters.
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