hi - i have a strange problem i have been fighting for probably over a year now, with two more ideas of my own to try and one (bad?) idea from someone else -

somewhere just past 6375 (data logging limit on the '127/$6e) rpms, i start losing power like the ignition is misfiring, and if i shift after that the problem remains for several seconds before clearing up.

the setup is a forged/balanced 383, heavily ported superram, afr 190 heads, comp xfi 292 cam, 52mm tb, auto, 12" 3000rpm stall converter.
i have been burning my own chips but i went back to a formato chip i used to run with a different cam, when the car would pull to the rev limter, and had the same problem. i removed the msd 6al, replaced the accel coil with a gm coil, changed plugs. wires are fairly new taylor spiro pro.

i still want to try a new computer and a new ignition module in the distributor. oh - also have a new accel cap/rotor to try. and maybe a 58mm tb? then i am out of ideas. i dynod the car and power dropped off at 5500, supposedly the chart runs to 6550 but the dyno guy felt the problem and said it was the torque converter! says he has seen it with drag cars, and i need a 9" converter. i am having trouble understanding this.

changes to the car since it ran clean to 6500 rev limit are cam, rockers, intake and tb. i think the converter was changed before the engine changes. i have had a rash of problems, and now the timing is fuzzy... i was freshening the motor and went with a tpis cam, but they put the afr guideplates on wrong, i didnt realize it, had a hard time tuning and i broke pushrods so out the engine came again, and in went the bigger cam. but the problem at high rpms has stayed the whole time.

any ideas?
thanks,
sam

 

original setup:
58mm tb, ported plenum, asm runners, tpis big mouth
afr heads, 1.5 rockers
isky cam 211265/272 217/225 485/505 112 2000-6000

blown head gaskets, made these changes:
52mm tb, ported superram, ported big mouth
1.6 rockers
zz-409 226 /226 554 /554 112

never really ran right, then broke pushrods (exhaust slots on guideplates are longer, put them on backwards and the pushrods bind!)
pulled engine again, and switched cam:
comp 292xfi 242/248 584/579 113
afr hydra-rev kit (mostly for protection)
put the new pushrods, guideplates and rockers on myself this time..

 

Could it be a bad lifter??

 

hopefully not a lifter- the engine shop had them out and inspected them last time they had the engine. they are comp roller lifters. i called comp cams and verified spring pressure and that it does not appear to be valve float. tpis says they run the stock computer and ignition to 7k with the mini ram. LPE says set the rev limiter to 6200 and forget about it - the super ram is no good up any higher anyway. i am not buying it with the porting and cam i have. comp thinks it should rev to 7k easy.

 

How old are those valve springs?
I noticed that after a few years and 10000 miles that my zz9 cammed 350 just couldn't pull the 6900 rpm's it could pull when it was fresh. I mean it really pulled quickly back when she was new. The more years and the more miles I had on it the less Clean Rpm it could pull. It eventually liked to be shifted at 6000 rpm after 5 years and 20,000 miles because it just wouldn't make any good power above that. Thats just my observation but I believe the valve springs were getting weak after a while. You could be experiencing a bit of valve float due to the lifters pumping up. It seem to occur earlier in the prm range in the higher gears (more obvious). It just about seem like hitting a brick wall and would nose over really hard at 6950 even when that motor was fresh. (playback tach verified).

 

Check your alternator at high RPM, I had a miss last year above 6000 RPM and the problem was my alternator would sometimes not charge above 4000 RPM. When the alternator quite charging (I run both eletric fans at the track) the battery voltage would quickly fall to low 12's or high 11's and the car would develop a high RPM miss. I rebuilt the altermator and the high RPM miss went away. My theory is that the ignition system on my 1986 needs a solid 13 volts at WOT, high RPM to fire the plugs cleanly.

 

Well, based on experience with these setups your most likely pushing the limits of several things, hence the breaking up.

(1) with a SR and 383 CI motor, 6k+ RPMs are suspect in itself. Your likely several hundred RPMs past peak power.

(2) high rpm breaking up is usually caused by irrative valve train motion. In other words you are floating out the valves. Valve float is alsways controlled by the valve springs. At high rpms, if the spring is not strong enough, the valves will bounce MORE than typical. Yes, all valves even with proper spring bounce off the seats Typically about 3 times to be exact. Compliments of Reher-Morrison case study of valve trains.

Having said the above, I would highly recommend that you get a spring rate checker on your springs. Moroso makes one that can check the springs on the head without disassembly. Floating out the valves DESTROYES springs in a hurry - so make sure they are still to spec. You may even find a broken spring in doing the check. All springs should be nearly exactly the same.

If all that checks out, lower your RPM. Again we have had much experience with these 383/SR combos and 6k is TOO high!

~5500 rpms is going to be max.

I went back and read the entire posting, have a habit of just reading the question and that is it..........having read the entire post....you will NEVER EVER see 7k RPMs with that motor. Oh it will make noise up to that rpm, until it smacks the piston with a valve, then it will make a big noise - but it will never EFFECTIVELY make good power to 7k. I BARELY reach 7k with HUGE 227cc heads and a cam over 700 lift. Trust me when I say ~5500rpm.

Also having seen that you busted up PRs and other things on the motor, its likely that you have bent valves and definately suspect springs now if all that was not changed out. Take the advice and have all those things THOUGHLY checked out. Last thing you want to have happen is a bent valve breaking off in the motor. You can at that point say goodbye to the bottom end and your heads.

If it were me, I would be pulling those heads off and having the entire head assembly checked over REAL good.

Oh and on the converter. Rule of thumb is the smaller the converter, i.e. 11", 10", 9", 8" etc....the higher you can stall the converter and keep efficiency. In other words, a 10" converter with 3000k stall will be less efficient than a 8" converter with 3k stall. Most higher end converters are 9" and the real good ones are 8" - IMHO 8" is the only way to go once you get over 4k stall.

 

6E (89) is still MAF, right?

How does the MAF data look at or near the breakup. Does it also drop, or is it maxed out?

Maybe your flow is exceeding the max airflow vs. rpm 247 @ 6000 or 255 @ 6400, causing a lean condition.

If the maf stays high, I would think it points to either ign or fuel related problems.

If the maf drops, I would think valve float or lifter pump up or a possible intake coupler collapsing. Are you using the accordian style or a smooth power coupler?

For a test, you could always set your lifters to zero lash to eliminate pump up as a possibility (an old 60's trick). This could cause lifter failure, so proceed with caution

 

Sams problem seems to take a few seconds to clear after the 1-2 shift and therefore does not exhibit typical valve float symptoms. I agree with the cam Mfgr. in that the bump stick grind should rev past 7k, perhaps not making any more power but allowing one to avoid that particularly long gear change at places on an autocross course where an immediate downshift would follow.
I am highly suspect of a lifter punping up and taking a moment to settle back down.
Where exactly is the restriction that prohibits this combo from making power to 6500 or reving more freely to 7k?
What would be difinitive in diagnosing this malady?

 

thanks for all the input! i have more ideas / things to check now..

after the pushrod fiasco, the engine was sent back to the engine shop and tore down, inspected, and put back together. supposedly new valve springs matching the new cam were installed. this shop builds circle track engines all day so they know how to build engines that rev over 7k.

here is some data from the last runs i made while dropping timing down - it was suggested that too much timing could be the culprit - i saw little difference.



i understand that power will drop off over 5500, but with the original mild cam and long tube runners the engine would at least spin to 6500 and cleanly hit the rev limiter. now it will not do that. not saying i will be expecting power there, but there are times it would be nice not to have to shift just to have to immediately downshift. plus that first gear is soo low it would be nice to start second a little higher into the powerband.

voltage looks good, alternator was recently rebuilt with a 140amp stator

valve springs should have < 3k miles on them.

coupler between the maf and tb is now a homemade PVC and rubber drain coupler deal. had the smooth gasket material coupler and it did collapse.

data logging shows ~900mv at max rpm, dyno wbo2 says 13.8 to 5500 where the graph ends.

hopefully tomorrow i can check valve lash, and maybe try the new computer and module. and see if i can borrow a 58mm tb to test.

trying to decide if it is worth investing in a smaller converter if i stay with 3000 stall - drove a car at the track with a 3600 stall converter and it was brutal - if you got on it too much coming out of a turn it would break the rear loose and come around like nobody's business! i guess all that stall is great in a straight line but in a turn let's just say it can make life interesting
thanks!
sam

 

The MAF flow is clearly dropping from 253 gm/sec to under 220. This is roughly proportional to the contribution of 1 cylinder. 253/8 = 30.

You're suddenly loosing approx. 30 gm/sec flow or 1 cylinders contribution to the total. This would not happen if ign or fuel was temporarily cut. Something is interrupting the air flow.

My guess is a pumped up exhaust lifter causing the intake stroke to be short circuited.

 

i'm convinced - first stop will be checking valve lash tomorrow. thanks!

 

You may have damaged your cam,broken push rods don't sound good.

 

I have to again ask, why do you want to run this combo to 7k, when there is no way it will turn those kinda RPMs and make power? We are running very large solid rollers with extreme head setups, with SINGLE PLANE intake and do not see those RPMs! A SR setup will NEVER effectively move air up there unless its on a 2XX CI motor.

All your doing is beating the living daylights out of the motor, and things WILL start to fail, that is a for sure.

Not trying to be a pain, just trying to keep you from experiencing more grief.

Few other things....most ECMs will only record to 6375RPM its a glitch in the stock ECM configuration - therefore I ask, what sorta tach setup are you using to confirm your shift points/peak RPM?

If you do not have a good aftermarket tach, there is a very good chance your pushing that motor WAY WAY beyond what you think you are. The stock tacks are horrible at being accurate. Do you have an aftermarket rev limiter in the car? If so you might be hitting it, hence your breaking up.

Please understand that I am not trying to be a pain in the azz with telling you that your wasting your time and beating your motor trying to achieve 7k rpm with the SR/383 combo. I am simply telling you based on lots of experience that those rpms for that combo are not achievable, and trying to save you more damage.

I trust that the circle track guys can build a good motor, but that does not overcome the fact the SR intake is limited in upper RPM effectiveness. Its design was to improve low end power to mid 5k rpms. you can put the biggest cam you like in the belly of that motor and it will not overcome the physical limitations of the intake runner size/length. All that will do is narrow the rpms range more.

In summary, (1)verify your tach readings with an aftermarket tach; (2) Run the motor where she makes power and don't worry about achieving an RPM - it doesn't matter if a motor can only make power to 4.5k as long as it makes great power. Actually the longevity of such a motor would be tremendous over a high revver. Just remember one thing - RPMS KILL>

 

 

After all this, is the geometry correct from the pushrod, rocker etc.? Mine parts from them (tpis) had none of these issues. I checked plates, springs etc. did not not have binding pushrod issues etc.

I also agree w/ski down it on RPM. If you are going past max. for rpm purposes that would put you around 5500-5700. If at that point I had to push on it that hard to overcome the rpm drop to second, is it then possible to change trans.ratios?.

 

yes, the cam was trashed, something about it not liking the roller lifters turning sideways after the one broken pushrod let the lifter come up out of the bore and bend the spider... forgot about that. comp was able to regrind to original specs, but i put in the brand new comp xfi cam anyway. just to be clear, after this problem i pulled the engine and brought it back to the shop - they had fired the guy who put it together, and replaced the broken parts (other than the cam - the new cam was my idea) under warranty. they say it will turn 7k all day long.

i understand that the stock computer will not data log past 6375, but if the stock rev limiter is set to something higher that 6375 does it still work, or does the rev limiter also not know you are over 6375?

i do not have an aftermarket tach but now i think i will get one. and maybe swap out the 6a for a 6al... and get a shift light and set it to somewhere around 5700. maybe spend some quality time back at the drag strip on test night and see where i get the best time?

 

I always thought that the settings were only able to be set at even numbers 5000, 6000 etc. or deleted (stock chip/ ecm).

 

The high resolution rpm16 signal (16 bit) is used as an input for the rev limiter function. This signal should also be present in the aldl stream.

The rev limiter can be set in roughly 100 rpm increments anywhere up to 12000 rpm. Actual resolution is 65536 * 120/(value * no. of cylinders)

So a decimal value of 100 = 9830 rpm, 99 = 9929 rpm, 101 = 9733 rpm etc. Stock settings are 98, or 10031 rpm off, and 103 or 9544 rpm on.

The 6375 limit is the low res 8 bit signal with a resolution of 25 rpm (max: 255 x 25 = 6375)

Some more information (minor corrections):
The rpm16 signal that I mentioned previously is actually calculated from the distibutor reference pulse signal in ms, and is not an acutal signal in the data steam. My bad.

For example: drp = 2.50 ms, or .0025 sec/p when this is inverted = 400 pulse/sec * 60 sec/min = 24000 pulse/min / 8 cylinders * 2 (revs/pulse) = 6000 rpm

If you can see the drp signal in ms, you can calculate the rpm in this manner.

Note: I have not been able to verify that the drp based rpm16 is actually used for the rev limit function, but the resolution of the rev limiter suggests it. From my experience this also seems to be the case, but I have no data above 6375 rpm to confirm.

 

well, i adjusted the valves - some were slightly loose and one was slightly tight, but nothing more than 1/4 - 1/2 turn. i set them all to 0 lash, < 1/4 turn preload. made next to no difference.

a friend thinks maybe a lifter was damaged and the engine shop didn;t find it... we are thinking of trying to pull a plug wire at a time and see if it makes a difference...