Power loss over 6,000 rpm

-
midnight340

midnight340

Well-Known Member
Joined
Mar 19, 2014
Messages
462
Reaction score
276
Location
Lawrence, KS
Recently semi-rebuilt the 340 and added the Lunati solid cam 235/243 @ .50 and .526/.546 lift (Howards EDM lifters) with a stated rpm range of 2400-6600.

So this is a 9:1 motor, iron ported X heads, LD340 intake, 650 AVS2, headers, all MSD ignition, etc. Heads set up with dual springs recommended from Lunati. Carb all dialed in by A/F meter (good numbers from idle to WOT), runs really sweet throughout lower rpms, nice power from 2000 up, feels strong, etc. All my testing was up to 5500 to maybe 6000.

Got a chance to put it on the strip and to my surprise it seemed to fall flat past 5800-6000 though still running fine. Really just leveled off like there was nothing beyond. With the old .458" lift cam it would run past 7,000 easily. This was with my old Edelbrock 750 carb (much worked over.)

So does it just want more carb, or is there anything else that could cause this???
 
I'm not surprised. I run 20 degrees more duration just to shift at 7000. I'm betting 6600 is peak HP. Maybe 6500.

Also, if you went to a faster lobe, you start getting to the point where you can't get enough spring load on the seat and keep from getting too much over the nose and you get seat bounce. I'd love it if you could get a spring that was 340 on the seat and 360 over the nose. Life would be good. But no such spring exists.
 
I had the same thing happen when I put my .572 cam in it just fell on it's face at 6000, changed the springs and it was still pulling at 7500.
 
What is the spec on the springs, and did you test them before installing them.
 
Is it still making power over 6000 even if you could get past it? 650 sounds a little small. What intake?
 
Last edited:
I put the info you provided into Desktop Dyno and guessed on the cam centerline, intake, and open headers. Yes it does start falling off at 6000 r's.
 
toolmanmike said:
Is it still making power over 6000 even if you could get past it?
Click to expand...

That’s the thing Mike. 1; The springs may not be all that. But there’s more to it than that of course.
2; He reported it feels like it noses over before hand. So he can’t tell exactly could be beyond that rpm range. If all else was good if the spring is the issue all by itself.


midnight340 said:
Recently semi-rebuilt the 340 and added the Lunati solid cam 235/243 @ .50 and .526/.546 lift (Howards EDM lifters) with a stated rpm range of 2400-6600.

So this is a 9:1 motor, iron ported X heads, LD340 intake, 650 AVS2, headers, all MSD ignition, etc. Heads set up with dual springs recommended from Lunati. Carb all dialed in by A/F meter (good numbers from idle to WOT), runs really sweet throughout lower rpms, nice power from 2000 up, feels strong, etc. All my testing was up to 5500 to maybe 6000.

Got a chance to put it on the strip and to my surprise it seemed to fall flat past 5800-6000 though still running fine. Really just leveled off like there was nothing beyond. With the old .458" lift cam it would run past 7,000 easily. This was with my old Edelbrock 750 carb (much worked over.)

So does it just want more carb, or is there anything else that could cause this???
Click to expand...

I don’t know what your ported heads flow. That could be one of several factors of performance leveling out, AKA, out of breath/head limited.
Going smaller on the carb is also a breathing issue. IMO, go back to the 750. You have enough engine for it.
Then there is cam vs cam. Knowing exactly what was there in order to compare would be good. There is more to the eye than the simple listed specs of the cam going on with cams.

In all honesty, the way you describe the engine/car performance, it all sounds about right. These things (engines as per your equipment) all seem to nose over around that point given your parts and listed specs.

When people build engines like yours and report what seems like way higher peaks in power, (RPM based only, not actual hp/tq numbers, another story in itself) there breathing track is normally consistent with better intakes, well ported heads etc... and sometimes secrect trickery that never gets listed.

Sometimes they never say/list all the little tweaks that make the difference! Degreeing the cam in at a different spec, head coatings, piston top coatings, polishing those parts, trick crank finishes, modded intakes, the list gets long!

I remember a local dyno test being done that I walked in on and it was a test of cylinder heads for the local roundy track. The trick aluminum head made less power than the pro prepped valve job only head and a Slaughter occurred in front of my eyes. It was over 50 horse difference. Another fine example is a simple addition to your valve train geometry. Mike at B3 has kits that fix this issue. (Which isn’t a huge thing but, as you read on... maybe it is and worth your time, your call.) In my opinion, it’s a good to do. A member here local to me, got a kit installed it ran his car at track and picked up a 10th+ at the low 11 second ET mark.
That’s easier than ripping weight out of your car.

(A shout out to Kevin! Ya man!)
 
toolmanmike said:
I put the info you provided into Desktop Dyno and guessed on the cam centerline, intake, and open headers. Yes it does start falling off at 6000 r's.
Click to expand...

Cam is a 110 lobe separation, installed at 107* centerline.

The springs are rated:
Seat Load: [email protected]" ;Open Load: [email protected]" ; Rate (lbs/in): 402. They are installed at 1.66" height.

In a couple of days I will throw the old 750 carb on it and see if that makes a difference.
 
toolmanmike said:
I put the info you provided into Desktop Dyno and guessed on the cam centerline, intake, and open headers. Yes it does start falling off at 6000 r's.
Click to expand...
Ha ha ha ha, good work Mike!
While those programs are good for what they are, I just don’t take it as set in stone. There freakin GREAT to learn from and actually they are reasonably accurate if your honest & accurate about the info.

Have a little fun of that window is still open, change the head flow values up 20%!
 
midnight340 said:
Cam is a 110 lobe separation, installed at 107* centerline.

The springs are rated:
Seat Load: [email protected]" ;Open Load: [email protected]" ; Rate (lbs/in): 402. They are installed at 1.66" height.

In a couple of days I will throw the old 750 carb on it and see if that makes a difference.
Click to expand...

I was pretty close. I used 106° and 110°. No choices for valve springs.
 
rumblefish360 said:
That’s the thing Mike. 1; The springs may not be all that. But there’s more to it than that of course.
2; He reported it feels like it noses over before hand. So he can’t tell exactly could be beyond that rpm range. If all else was good if the spring is the issue all by itself.




I don’t know what your ported heads flow. That could be one of several factors of performance leveling out, AKA, out of breath/head limited.
Going smaller on the carb is also a breathing issue. IMO, go back to the 750. You have enough engine for it.
Then there is cam vs cam. Knowing exactly what was there in order to compare would be good. There is more to the eye than the simple listed specs of the cam going on with cams.

In all honesty, the way you describe the engine/car performance, it all sounds about right. These things (engines as per your equipment) all seem to nose over around that point given your parts and listed specs.

When people build engines like yours and report what seems like way higher peaks in power, (RPM based only, not actual hp/tq numbers, another story in itself) there breathing track is normally consistent with better intakes, well ported heads etc... and sometimes secrect trickery that never gets listed.

Sometimes they never say/list all the little tweaks that make the difference! Degreeing the cam in at a different spec, head coatings, piston top coatings, polishing those parts, trick crank finishes, modded intakes, the list gets long!

I remember a local dyno test being done that I walked in on and it was a test of cylinder heads for the local roundy track. The trick aluminum head made less power than the pro prepped valve job only head and a Slaughter occurred in front of my eyes. It was over 50 horse difference. Another fine example is a simple addition to your valve train geometry. Mike at B3 has kits that fix this issue. (Which isn’t a huge thing but, as you read on... maybe it is and worth your time, your call.) In my opinion, it’s a good to do. A member here local to me, got a kit installed it ran his car at track and picked up a 10th+ at the low 11 second ET mark.
That’s easier than ripping weight out of your car.

(A shout out to Kevin! Ya man!)
Click to expand...

The old cam was a single pattern 284 adv. with 218 @ .050 and a .458 lift.

I have been talking with Mike at B3 as I'm running the old 273 factory rockers (rebuilt.)
Do you think the B3 kit with rollers would make a big difference??
 
rumblefish360 said:
Ha ha ha ha, good work Mike!
While those programs are good for what they are, I just don’t take it as set in stone. There freakin GREAT to learn from and actually they are reasonably accurate if your honest & accurate about the info.

Have a little fun of that window is still open, change the head flow values up 20%!
Click to expand...
I just chose HP pocket ported heads with a 2.02/1.60 valve. If I change to large valve fully ported heads it's a lot stronger at the 6-7000 rpm range.
 
Heads were actually ported in the old days by Bob Mullen (ex-Chrysler engineer who I'm told designed the W-2 head.) But I've never had them on a flow bench.

My instinct tells me that it needs more carb mainly. The motor likes a lot of fuel WOT, seat of the pants says it runs best with A/F at 10.9/11.2 or so. (and I've done a lot of back road testing on that, plug readings etc.)
 
These are 2.02/1.60 heads and with the old cam (and carb) would rev happily past 7,000. So it should do that at least.

And the springs are also recommended for bigger cams than this, so should be sufficent.

Maybe I do need to do the bigger carb plus Mike's kit.
 
Put the 750 on, when i had a 360 .480 cam it would struggle to pull to 5500 with a 600 cfm then it happily pulled to 6200 with a 750.
 
Ignition still all good? If it rev'd to 7000 before, I'd think real hard about what changes were made that could affect it up there. I agree that it it probably wants more carb, but if it worked before..?..ignition components can go from good to bad as fast as my wife's mood and for no apparent reason so that might be somewhere to look.
And yes from me on Mike's stuff. It'll keep your geometry nice up there and keep the cam doing what it was meant to do..
 
just for comparison
I had a 223/230 Hughes cam in a zero-deck 360LA with OOTB eddies. With an AirGap,a 750DP, TTIs and their dual 3" exhaust; I could feel almost exactly where the power peak was, later proved by an accelerometer. It was just over 5200, and started dropping~400 later. But it revved sweetly to the rev-limiter at 7000. My Scr was 10.95 and the pressure ~180 or a bit more.
That cam died and I swapped in the next larger Hughes cam, a 230/237FTH with new springs, and took about .012 off the decks. The Scr dropped a bit and likewise the pressure. Thing is; in the first two gears, I can't tell where this cam peaks at. It should be about 5400, but the thing is so flat and it spins the tires forever so IDK where the peak is. So I just run it up to 7000, cuz it sounds sooo great.
With 3.55s, and a manual trans, 5000 is a tic over 80 mph in third gear and 7000 is 113 ; so the peak is in there somewhere; I just never go there, getting caught would be the end of my driving for a long time. And the one time I went to the track, I trapped 93 in the Eighth, not yet into third gear,lol; rather second-over with a GVod in splitter mode.
You have about the next bigger cam to mine, which might power peak around 5400,possibly a tad higher, depending .
So, I don't think my 750DP is too small yet. But the 750Vsec I tried was. And the 600 was pitiful.
So I guess IMO, I agree with those who point to the 650 as being too small.
Thing is, I have had a 340 2bbl that went well into the 6000s no problem, in the first two gears at least.
I also agree with those who point to springs. I have only experienced valve float three times since 1969, and each time the rpm just went flat, as in stopped reving any higher. This exact thing happened to me on a 750GPZ Kawasaki motorcycle. But in this case the Mainjets were too fat. I down-sized 4 numbers, and the thing went to 11,000/12000 no problem, which IIRC was on a 10,000 redline engine. No it didn't seem to be any faster,lol; it just revved higher,lol; kindof like a smogger-teen with 340 valve springs.
One time some friends of mine bricked the gas pedal on a smogger-teen, just to hear it blow up. After some 10 minutes of listening to the ruckus, one brave soul un-bricked it. As the Rs came down, it quit running and would not restart. Some time later they sold that car to me, and when I tore the engine down, you could see the witness marks where the valves had kissed the pistons. As I recall the exhaust valves were all bent. They said the rpm just went flat, as in, it revved no higher.
 
Last edited:
That was worth reading through just to hear about the 318 torture test, lol! The only thing that will hurt one is wanton and reckless abuse...
 
Is this new cam/engine combo stronger up to 6000 RPM than the old cam? With the much bigger .050" duration numbers, the higher ramp rate, and the much higher lift, I would certainly expect so. If that is the case, and the new cam is simply pulling more air in and you put on a smaller carb..... it seems pretty well expected that it will run out of carb at a lower RPM.
 
nm9stheham said:
Is this new cam/engine combo stronger up to 6000 RPM than the old cam? With the much bigger .050" duration numbers, the higher ramp rate, and the much higher lift, I would certainly expect so. If that is the case, and the new cam is simply pulling more air in and you put on a smaller carb..... it seems pretty well expected that it will run out of carb at a lower RPM.
Click to expand...

Yes, stronger up to 6000. And once you put it all in front of me like that, it makes a lot of sense. I went back and forth between ordering the 650 or the 800 AVS2 carb, should have gone the other way I guess.

I like the Edelbrock carbs for this Kansas climate. Along with the extreme temperature range the humidity can range from 35-40% to 98% at times. I can change mixture rods in a couple of minutes. (I actually had labeled sets of rods as Winter, Spring/Fall and Summer.)
 
nm9stheham said:
Is this new cam/engine combo stronger up to 6000 RPM than the old cam? With the much bigger .050" duration numbers, the higher ramp rate, and the much higher lift, I would certainly expect so. If that is the case, and the new cam is simply pulling more air in and you put on a smaller carb..... it seems pretty well expected that it will run out of carb at a lower RPM.
Click to expand...

An update here: You are so very spot on here!!! :)

I removed the new 650 AVS2 and reinstalled my old 750 Edelbrock that I had dialed in pretty good and the motor now zooms past 7,000 rpm happily!!!! The Lunati solid lifter cam with .526/.546 lift is now happy again.

The AVS2 is a very nice carb, with the annular boosters very responsive at low rpms and all. Had I gone with the 800 I would have been in good shape. May still do that when the budget allows. (First I''m getting info to Mike at B3 for some rockers with his kit.)

My ancient (2000?) Eddy 750 still had the "pinched" secondary emulsion tubes that SOME had, so I drilled those out larger (so it should suck more fuel.) While in there I filed away any casting marks, and polished things up. Anyway, with mild early fall air, I am jetted at 107/113 with 73x52 rods I have larger .043 squirters and the stronger accelerator pump from the 800. My A/F readings are 14-ish at idle (900-1000 rpm) and pulling 10-11 hg at idle, 15 hg at road speed. 14-ish A/F at steady 2500, 12.5 part throttle, and 11.2 WOT.

All is good. Thanks for the input from all of you!
 
Last edited:
midnight340
roller tips and B3 would make no difference just $$$
dbl check spring requirements
light retainers?
headers ok
did you post your head flows- I have not read the whole thread
any chance you are opening valves into the turbulence area?
 
You must log in or register to reply here.
-

Back
Top