Need engine advise
Thread Starter
3rd Gear
Joined: Jan 2008
Posts: 3
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Need engine advise
I am hoping someone can point me in a direction. Are my expectations too high or is there something wrong with this combo. I just built a 10-1 .040 over 4 bolt 350 with a balanced scat rotating assembly, kb flattops, new Brodix IK180 heads, compcams x4254H cam hyd. cam, gear drive, roller rockers, hei with 65k coil, Weiand xcelerator dual plane intake, edelbrock 600 cfm carb, and block hugger headers connected to new 2.5 inch exhaust with flowmaster 50 series mufflers, a new 350 built tranny a new converter with 2000 rpm stahl, 3.42 gears. Everything on the motor is brand new. Timing is set to 14 initial and about 36 total. I have adjusted the timing to where it seems to be best. My issue is I was expecting about 350 horse from this motor and it appears to only be making 280-300. Desktop dyno put it at closer to 375hp. The motor runs good and is responsive so I am confused as to why my power is so low. This is in a 78 corvette. I ran the car against my 91 that is stock except for KN filter and aftermarket mufflers and the cars are dead even across the rpm range.
Instructor
Joined: Jun 2004
Posts: 182
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From: Sun Peaks British Columbia
I built a similar setup, and initially used a Holley 600 cfm w/ vac. secondaries. I changed up to the Holley Avenger 750cfm and found a big difference.
I personally think you're under carbed.
YV
I personally think you're under carbed.
YV
Le Mans Master
Joined: Apr 2005
Posts: 6,339
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From: Seattle Area WA
IMHO, the cam seems pretty small for what you are expecting based on that part number. Comp refers to it as a 4X4 low end cam, it has very short duration of only 210/218, and with those heads you could use some more lift. I would also look at that Edelbrock carb, it could be better.
With that compression ratio and aluminum heads, I think you would do well with the L-82 cam advanced 4 degrees, a CC 270 magnum, XE268 to name a few.
With that compression ratio and aluminum heads, I think you would do well with the L-82 cam advanced 4 degrees, a CC 270 magnum, XE268 to name a few.
Thread Starter
3rd Gear
Joined: Jan 2008
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Ok, I understand the thinking behind bigger is better but I put all those cams into desktop dyno and they actually lost horsepower and torque with my combo. The lift with the cam I have and the 1.6 roller rockers is 470/486. I would expect this motor to put out right at 350 hp with the parts selected. The quadrajet carb is a great design for street manners. Smaller primary bores to speed up air velocity in lower rpm. I know a holley will give me a little more horsepower but the edelbrock should not cost me 50 horse either. Yes it will lose some in the higher rpm's but I am not looking at anything above 5500 rpm.
Le Mans Master
Joined: Apr 2005
Posts: 6,339
Likes: 416
From: Seattle Area WA
That combo with a L-82 or "151" cam should make 300+ crank horse and those cams are a bit larger than this RV cam. This RV cam should have very nice manners and good low end but I would expect it to start giving up relatively early in RPMs. Also, as you know, if you use a stock Q-jet you are up to 750 cfm.
Is the 36 degrees with the vacuum disconnected and between 2700 and 3,000 RPMs??
Just as a comparison, using a well tuned Engine Analyzer 3.0 program against actual dyno results from various motor builds, and using my kids motor for example. It is a 350, with 10.25:1 compression running a "151" cam along with a 8004 dual plane intake and Q-jet. That program puts this combo at 300+ crank HP at 5,700 RPM.
DD could just be very optimistic with the data, also what do you mean by it appears to be making 280 HP, you need real dyno data, not SOP data otherwise its all meaningless assumptions.
It would be interesting to hear the specs on the pistons, the head gasket, assuming 64 cc Brodix heads and if the deck was blocked etc.
I'm also curious if you have had any detonation issues with this aluminum head engine when all your timing is in because your computed DCR is over 8.7:1??
Is the 36 degrees with the vacuum disconnected and between 2700 and 3,000 RPMs??
Just as a comparison, using a well tuned Engine Analyzer 3.0 program against actual dyno results from various motor builds, and using my kids motor for example. It is a 350, with 10.25:1 compression running a "151" cam along with a 8004 dual plane intake and Q-jet. That program puts this combo at 300+ crank HP at 5,700 RPM.
DD could just be very optimistic with the data, also what do you mean by it appears to be making 280 HP, you need real dyno data, not SOP data otherwise its all meaningless assumptions.
It would be interesting to hear the specs on the pistons, the head gasket, assuming 64 cc Brodix heads and if the deck was blocked etc.
I'm also curious if you have had any detonation issues with this aluminum head engine when all your timing is in because your computed DCR is over 8.7:1??
Last edited by Scott Marzahl; Jan 24, 2008 at 01:44 PM.
Thread Starter
3rd Gear
Joined: Jan 2008
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The pistons are KB Hyper with coated skirts and 4 valve reliefs netting -5 cc's. The pistons are -.016 in the hole. The head gasket is a felpro 1003 (4.166 bore with .041 compressed thickness). The rings are filed to .026 gap. I am using a -2 degree bushing in the gear drive. With all of these factors, I came up with a DCR of 8.3 and total compression at 9.97. I expected DD to be high at 395 hp. But I would expect 350 without a problem. The car has not pinged at all at any rpm range yet. Of course I have only taken it up to 5200 rpm so far. I am not just going by the seat of my pants. I ran the car against my 91 vette. The cars were side by side from stop until we let off which was at about 90 mph. The 91 vette is stock except for a KN filter and aftermarket mufflers. I am guessing here at maybe 260 horse. Taking in account the 78 vette weighs 200 lbs more and less aerodynamic is why I am guessing about 280-300 hp. The car will barely spin the tires but pulls hard from a launch all the way to 5200. With this combo it should be making tons of torque at 2000 rpm.
Instructor
Joined: Jun 2006
Posts: 179
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From: Melrose NY
I might guess that you are under carb'd as well. I just finished building a 327 with stock heads and a mild cam-9.5-1 compression, I called edelbrock and a tech guy said I would be losing Hp with a 600cfm, and he recommended that I step up to at least the 650cfm.
