I need to wake up my '68 427/390hp [Archive]

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68427

Sep 13th, 06, 02:20 PM

Hi,

I have a '68 Yenko clone with a realitivly fresh 427/390 engine & a M21 4-speed. Here are the engine specs:

3916321 1968 427 2/4 385, 390, 400, 425, 430, 435, corvette
3931063 1969-71 427 2.06 1.72 Closed 100.9 heads

Unknown cam, hydrolic lifters, unknown pistons. Stock oil pump, etc. Smooth idle, probbably a stock cam. Period Holley steel intake & 780 Double pumper w/mech secondaries, stock fuel pump. MSD 6200 ignition - hooker headers & full 3" dual exhaust.

The car runs great, I'm just being a greedy pig & want more power! I would like to leave the engine in the car, & the bottom end alone if possible.

I was thinking aluminum closed port heads, (expensive, but the loss of weight would be welcome), aluminum intake & deamon carb (1000cfm?), roller cam & lifters.

I want it to run on pump gas, & be to finicky once it's tuned. A camy idle is O.K.

Any specific suggestions for parts? And HP/tourque estimates?

Thanks!

Pete

Larger Dave

Sep 13th, 06, 06:32 PM

First Yenko never shipped a 427/390 engine (that option describes a two bolt block with a cast iron Holley manifold and closed port iron Pass heads and flat topped pistons, motivated by a hydraulic flat tappet cam), only 425 horse 427's ,the cast iron headed L-88, or the all aluminum ZL-1 being the other catalog fillers.

Since the 390 horse had 10.25:1 pistons in it from the factory; I will assume someone lowered your compression since then to be able to run great on today's pump gas. I would stick with your stock cast iron heads as the aluminum closed port heads, are not only expensive, (but aside from the loss of 63 pounds each) would not contribute to adding any horse power to your engine.

An Edelbrock RPM aluminum intake, and a 750 cfm Demon carb, with a new hydraulic roller cam would contribute a bunch to your horse power equation. Unless you are going to punch out your engine on this rebuild to 582 cubic inches (I don't recommend it) you will not need a 1,000 cfm carburetor. I have one on mine to feed a few extra cubes you do not have, not for looks.

The cam and heads will be the deciding factor on how many ponies you can safely corral. You should be able to generate about 550 horse with the cast iron heads you have now with larger valves (2.19" intake and 1.88" exhaust valves) and some pocket porting. The cam you choose will also require matching valve springs, and the more aggressive the grind the bigger the springs are to control the heavy roller tappets.

Torque with a 427 isn't exactly going to happen if you get aggressive with the cam. Unless you want to live on the edge with 4.56 rear gears and a roller cam you can forget about pulling hard off the line with what has now become a production small block engine (2006 'vettes ship with a 7 liter). Big blocks are synonymous with torque because with very deep pockets you can grow one to displace 705 inches or more (a 705 cid mountain motor is an off the shelf engine if you have enough cash from Pat Musi or Sonny Leonard). On the more pratical front here is a link to a more reasonable build.

http://www.chevyhiperformance.com/tech/engines_drivetrain/complete_builds/0608ch_big_block_chevy_engine/

Larger Dave

68427

Sep 13th, 06, 09:03 PM

Yes, I know the Yenko's never came with the 2 bolt 390 engine. Like I said it's a clone. (and like most clones this ones not 100% faithfull.....)

I also assume the compression was lowered on the rebuild. What is the highest compression ratio that would be acceptable with this engine and today's 93 octane fuel? I know the compression ratio is controlled by the piston dome & heads. Will I be able to determine the current compression ratio by removing the heads only? Or do I have to know exactly what pistons are in it?

Thanks for the advice on keeping the heads. I think I should get what I can out of the current heads, and swap the motor down the road if I want big gains.

I have 3.73 rear & don't want to have to rev it to high. I would think 5500 would be the max not knowing if it's balanced, etc.

Straight-line-69

Sep 13th, 06, 10:49 PM

If you can afford GM Performance or Edelbrock's O-Port alum heads (same thing, approx $1700 pr) the rest is easy. 1000 cfm would be too much for the street,..I'd look in the 750-800 range but in a double pumper with your 4-speed.

Don't know what gears you're planning out back, but if you're in the 3.73 neighborhood, Comp's 280H cam (230 @ .50 & .520 lift, single pattern,..with your exhaust you don't need dual pattern) would give you idle that says you have more cam than a stocker, but won't shake and quake the car. With the alum heads mentioned earlier, 10.25 compression would work with this cam with decent octane (90+).

With the RPM Performer or a Junior and 1.75" headers, this combo would be good for 450 HP in a 427 (I'm guessng), but would be very easy to drive at lower rpm levels,..not to mention enough torque to rip holes in the asphault.

If you don't need power below 2500 go with the square port heads, but I think you'd be making a mistake if this a street car.

Good luck!

sschevellefan

Sep 14th, 06, 12:32 AM

My buddy had a 427 in his old 55 chevy p/u. It was basically the same motor you have except he installed a comp 280 magnum cam. It had a nice idle and had plenty of low end power. He ran it with a rpm intake and a 830 at first but then switched to a tunnel ram and 2 650 dp carbs. It was too much for it but it did feel faster with the ram. it was mostly for the look but he had it tuned perfectly. I would just run the comp 280, rpm intake and 750 carb and you`ll be happy with that set up.

68427

Sep 16th, 06, 12:16 PM

Here is what I am thinking:

Performer RPM 2-0 Intake Manifold
BB-Chevy 396-502ci - Oval Port
4-bbl Square-Bore Carb Flange (non-EGR)
1500-6500
Part Number: 350-7161
$192.99

Edelbrock HYD ROLLER LIFTERS BBC
Part Number: 350-97443 $456.99

BB-Chevy Performer RPM Aluminum Cylinder Head
290cc Oval Intake Ports
110cc Combustion Chambers
2.190'' Int/1.880'' Ex
Part Number: 350-60459
$859.99 X 2

Hex-A-Just 1965-up BBC
Part Number: 350-7334 $153.99

BB-Chevy 1.7 Ratio, 7/16'', 16/pkg, (A)
Part Number: 249-1320-16 $271.99

BBChevy Retro HYD RLR High Energy Lifters
Part Number: 249-854-16 $419.99

7.725'' Int./8.684'' Exh. 1955-Up with Retrofit Hydraulic Roller
Part Number: 249-7815-16 $36.99

Performer RPM Hydraulic Roller Tappet Camshaft & Lifters
BB-Chevy 396-427ci 1967-72
Duration: 335°
Part Number: 350-2261
$202.99

Should be good for 500HP & 470lbft with my estimate of 9:1 comp. (I don't know what pistons I have yet...)

See any problems? Worth $3500? I plan to sell the curent iron heads to recoop a bit. Or should I hang onto them.....

sschevellefan

Sep 16th, 06, 02:06 PM

Busted Knuckles

Sep 16th, 06, 02:48 PM

A 1000cfm carb would make it a pig regardless of the bottom end or heads - way to big of venturis to get decent signal to the boosters until you got it really wound up. 850dp max, a well tuned 750dp might even be better.

Greg O

Sep 16th, 06, 05:02 PM

68427

Sep 16th, 06, 09:43 PM

Don't know if it would save you much money or not, but Comp does have complete kits for retrofit roller cams. Springs, retainers, lifters, cam, timing chain, cam button, etc. May want to check them out.

They don't appear to have a BBC Hyd roller kit.

Here's my latest dream build:

Jegs's Parts

1 350-7161 Edelbrock Performer RPM 2-0 Intake Manifold
BB-Chevy 396-502ci - Oval Port 4-bbl Square-Bore Carb
Flange (non-EGR) 1500-6500 rpm $192.99

1 350-97443 Comp Cams Hydraulic Roller Lifters 1990 & Earlier Mark IV BB-Chevy $456.99

2 Edelbrock 350-60459 BB-Chevy Performer RPM Aluminum Cylinder Head
290cc Oval Intake Ports
110cc Combustion Chambers
2.190'' Int/1.880'' Exh Valves
1.550'' Valve Springs, .700'' Max Lift
7/16'' Rocker Studs $859.99e $1719.98

350-7334 Edelbrock Hex-A-Just Roller Timing Chain Set 153.99

249-1320-16 Comp Cam Pro Magnum Roller Rocker Arms For Chevy 396-454
7/16'' rocker stud
1.7 ratio
16 per package $271.99

1 555-20120 Jegs Aluminum Roller Rockers $219.99

1 720-4405k Mr Gasket Camshaft install kit $61.99

Summit

1 60212 Lunti BBC Voodoo Retro-Fit Hydraulic Roller Tappet Camshaft; Lopey idle, High performance "street" cam. Likes upgraded intake, Carburetor and exhaust system. Needs 2800 RPM or better stall speed converter, 9.75:1 compression ratio and 3.55–3.73 rear gears.
• Advertised Duration IN/EX: 282/290
• Duration @ .050 IN/EX: 231/239
• Gross Valve Lift IN/EX: .600"/.600"
• LSA / ICL: 110/106
• Valve Lash IN/EX: Hyd/Hyd
• RPM Range: 2200-5800 $285

Total Cost $3363

Depending on the pistons (I don't know what's inside yet) looks like I could approach 570HP/530TQ assuming 10:1 compression.

I'll try to post the virtual dyno slip.. http://image59.webshots.com/759/2/96/22/2606296220097072264wfxVTI_ph.jpg

http://image59.webshots.com/759/2/96/22/2606296220097072264wfxVTI_ph.jpg

phel69

Sep 17th, 06, 06:57 AM

"Depending on the pistons (I don't know what's inside yet) looks like I could approach 570HP/530TQ assuming 10:1 compression"

I don't think so, 500 hp is probably closer. Why do you list Comp roller rockers and then Jegs roller rockers? You only need 1 set.