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Duke, Clem or any one else who may have input; I have just had a 396 rebuilt, 060 over, oval port 110cc iron heads w/2.08 I and 184 E(i think)and pocket ported, cam is 238/242 duration with .550 lift, 110 degree lobe separation installed straight up. The pistons are JE with a small dome, 10.4 advertised compression (167/170 cranking pressure). Timing is 16 intial/ 36 total, all in by 2800. Intake is a single plane with a 750 Holley (vac secondaries). The motor runs fine, tunes fine, the problem is the horsepower is decidedly unimpressive, torque is above average from 2200 on up. I am thinking the motor should run much better than is does. Looking for advice here, not enough cam/ too much cam, heads won't work for this setup etc. Thanks in advance, I apreciate any advice. JOhn
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i would guess the single plane intake
would cause the problem on a street engine with closed exhaust. -
Re: i would guess the single plane intake
Are you sure the vacuum secondaries are opening? Also, what single plane intake are you using?Bill Clupper #618Comment
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Re: engine performance
Cam has way too much overlap, and a single plane manifold is lousy for a road engine.
How can it have above average torque from 2200 up and umimpressive horsepower? That statement makes no sense.
HP = TxN/5252
If you have chassis dyno pulls, scan and post the chart and tell us the 80 and 90 percent torque bandwidth ranges.
DukeComment
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What exhaust are you running?
If it's the stock Corvette dual exhaust, the cam is not ideal - too much duration, too much overlap. Should consider something up to about 220 duration with 114 degree lobe separation when running stock exhaust system. however, that cam will significantly increase your dynamic compression ratio (more pep), but will likely require that you drop back on your timing to an initial of 4 - 6 degrees with a total of about 34 degrees. Given the vacuum secondaries, if set correctly (not opening too soon) The single plane intake won't be nearly as much a problem as the overly aggressive cam.
MarkComment
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a 143 cam has 242/242 duration with a 114
lobe seperation and it works good in a 396 with big port heads,the 375 HP 396, and with oval port heads this engine should have good throttle responce unless you have a tall rear gear.i am used to running 3.70/4.11 with this combo. i would try a dual plane intake before changing out the cam. you can check the opening of the carb seconary by putting a small dab of grease on the baseplate where the secondary linkage hits for a stop,go for a full throttle run and see if there is a mark in the grease from the linkage.Comment
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after you account for valve lash
After you account for valve lash, doesn't that cam end up as about the equivalent of a hydraulic cam with about 230 duration at .050" (about the same as a Comp Cams 280 Magnum except the Comp has 110 lobe separation)? It sounds like his cam is about the equivalent of a Comp Cams 292 Magnum or an Isky 296 hydraulic.
MarkComment
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Re: engine performance
Duke; this engine replaced an over cammed 350 small block that was street driven and Friday night/Saturday bracket raced. The small block was far superior while accelerating and run quicker et's with about the same mph. While driving on the street this motor is impressive when "jumping" on the throttle at 2200/3000 but just does not have the "brute acceleration" it should. I think the small block would out run it and that should not be the case. It has a 750 Holley, 1 7/8" headers and 3" exhaust, 4.11 rear gears. Transmission is a turbo 400. Now the small block had a 700R4 behind it so maybe the gearing helped the small block. Unfortunatley no chassis dyno in the local area yet. JohnComment
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Re: a 143 cam has 242/242 duration with a 114
If you measure the .050" lifter rise duration ABOVE the tops of the clearance ramps, which is the number to use when comparing to hydraulic lifter cams, the duration is 231/231. Both sides use the same lobe, and this lobe, with a slightly different base circle dimension is also used on the inlet side of the LT-1 cam.
This big block SHP cam is an excellent high performance street cam, but compared to its 231/231 duration and 114 LSA a hydraulic 238/242/110 is way to big, especially the amount of overlap, for good road engine performance, but the configuration should make good top end power albeit at a big expense to low end torque.
The only way to know what it's doing is to get some chassis dyno pulls and then analyse the results.
DukeComment
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Re: a 143 cam has 242/242 duration with a 114
the orignal post did not say it was a hyd cam or did i miss that?Comment
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Re: a 143 cam has 242/242 duration with a 114
I made that assumption, and I will admonish everyone to state whether a camshaft is mechanical or hydraulic lifter when you attempt to describe it.
Duration at .050" lifter rise cannot be directly compared. A good portion of that lifter rise above the base circle is clearance ramp on a mechanical lifter cam, but on a hydraulic is virtually all turns into lift at the valve.
A rule of thumb is to knock 10-15 degrees off mechanical lifter cam .050" lifter rise direction to compare with a hydraulic lifter cam, but it depends on the clearance ramp height. For the L-72 lobe with a cleance ramp height of .012" the difference is 11 degreees. The 30-30 lobes' .017" clearance ramp height yields 15 degrees difference - 239 versus 254.
The Duntov's .012" exhaust clearance ramp height yields an 11 degree difference (as is the case with the L-72 cam) 231 versus 220, but only 8 degrees on the inlet side 228/220 because the clearance ramp height is only .008".
DukeComment
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THM 400 have very heavy internal parts
which slow the rate of accelleration,that is why a THM 350 is prefered at the drags. if you did not use light weight parts in the rotating assy that could also be the answer. those cheaper off shore rods and cranks are very heavy.Comment
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Re: THM 400 have very heavy internal parts
Clem, the crank is factory cast steel, the cam is Hyd, the rotating assembly was balanced, running the engine for cam break in was very impressive for throttle response, but when hooked to the trans not so impressive, maybe that's it. JohnComment
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