69L71

OK, I'm at a crossroads. After a long hiatus, I'm back to building my car. Thought this would be a good time to do a little bench racing and ask opinions.

When I last went down this road, my intent was to build the engine 100% original.That meant SpeedPro/TRW L2268F30 pistons and the Crane 969961 copy of the 143 cam. With the factory 840 heads that should put me at or near the factory 11:1 CR. Running through the Wallace DCR calculator, that puts me at a DRC or 9.72, with cranking pressure of 210 psi. Everywhere I read I see folks saying that 200 psi is about as far as you can go with pump gas, so I figured I'm gonna be looking at some sort of octane booster or race gas blend. Since this is never gonna be a daily driver, but rather a weekend cruiser/fun-car, I found that to be unfortunate but not a showstopper. But before I button her up for good I thought I'd gather some opinions. Do I continue down this route or swap out the pistons for something closer to the L2300F30s and drop the compression to closer to 9.3 or 9.5? That would free me up to shorten the cam a bit and drop the LSA, and still stay well below 200psi. Lastly, if i go the lower compression route, should I stay solid flat-tappet or go full modern with a hydraulic roller? I understand these Mark VI blocks have shorter lifter bores and the tall HR lifters can be kinda unstable. And then there's the question, will the HR lifters fit under the factory low-rise tripower intake? Anybody done this?

For reference, the motor in the car is original, numbers matching. The car has 4:11s, 4-speed, no PS or PB, but does need to draw enough vacuum to run the headlights and wiper door.

Thoughts?

v2racing

The compression on these engines was never as high as advertised. The chambers tend to run oversize, sometimes by a lot, and the pistons are usually down in the hole about .025". They were on average about a point lower than advertised. The only way to find out what your actual compression is going to be is to cc out out the chambers and get your actual piston to head clearance. Then it's time to decide what cam to use.

Mike

Little Mouse

To me would want it to run on pump gas, a good hydraulic roller can easly make up the difference in the tq loss of less compression.

69L71

Thanks Mike. I haven't cc'd the heads, but they were listed to be around 107, but most people who have cc's them say they typically come in close to 109cc. Using 109cc with a Cylinder Bore Size or 4.28, Stroke Length of 3.76, Gasket Bore Diameter of 4.37, Compressed Head Gasket Thickness of .039, Piston Dome Volume of 35.5cc, and Piston Deck Clearance of .025 -- yields a CR of 10.96:1. Even if the heads were off a bit i don't see the CR coming down much. I need to find AT LEAST 10cc somewhere to get down close to 10:1

derekderek

I take it you are married to these 840 heads? Cause these are the true bath tub combustion chamber shroud the hell out of both valves. I would get to 10cc's by unshrouding the valves quite a bit and opening them up to more like 115CC chambers. On the other hand you might not wanna butcher original unported unmodified 840 heads. But you'll pick up quite a bit of power if you do open those chambers up a bit.

domenic tallarita

Funny, when we talk about fake, it's OK to have a engine stamped 427/435 engine with lower compression and the HP unknown. But what you can see on the outside all has to look original. If it's a 435 HP then I say keep it that way. Nothing like hearing the roar of the correct CR and HP. Change the color from the trim tag is not accepted, but change the pistons & cam seem to be OK.

Dom

stingr69

Play with retarding the camshaft timing. You can take some DCR out of it by retarding the cam a few degrees. The later intake closing point helps lower the value. Hurts the low end torque a little but adds back at the upper RPMs. Worth a look.

cv67

Seen a few guys reuse original pistons in sloppy bores really believing doing things right would drop the value. Insanity lol

3X2

I'd build it as a true 435. Since you said it is not going to be a daily driver, like my 435, I run $9/gallon unleaded with an addition of $12/gallon 112 octane leaded, About 6:1 unleaded to leaded. For the few hundred miles a year I drive, it really isn't a lot of money. But boy, when you put your foot into a 435! Especially yours with a 4:11.

69L71

Thanks 3x2. Good to hear from somebody that is driving one in the factory configuration. The sound of an old school High Compression Solid lifter motor is awesome! I've been looking on line to see what folks with L71s, L72, and LS6s are doing to drive their cars. Surprisingly there isn't much info about that. What's the $9/gal unleaded? I've seen I can get Sunoco 114 leaded. A 55gal drum is just over $600. If I used 5 gallons at a time that's 11 tank fulls at 2:1 93 + 114.

I did find an Icon piston with a 28.2cc dome. That would get me right at 10.2:1 with a DCR of 9.22 (cranking pressure of 197). That has me pretty tempted. Might still be on the ragged edge of pump gas but I'm pretty sure I could use a full tune and not have to compromise.

7T1vette

For reliability, performance, and 'freedom from flat tappets', I would recommend going with a full roller cam/lifters. Want stock heads? Go for it. Want modern aluminum heads (which can operate with a bit higher compression and not have pre-ignition)? Go for it. Most folks are just going to see the outside of the engine and gawk at the tri-power setup.

Bottom line: If you car is a museum piece and completely original....and you want to sell it as that in the future....go with all stock stuff. Otherwise, do what you think is best for YOU.

P.S. I have an iron motor and heads with 10.5:1 compression and it runs fine with pump premium. If it had aluminum heads, there would be no problem running it with 11:1 compression.

3X2

It's Sunoco 260 GT at 100 octane, not 98 like I mentioned earlier

JBrooke825

Wings field ,5,45 a gal . But you need a can.

427Hotrod

10.2'ish or so with a good HR cam would make a good package. Modern HR's will spin 6800 or so with good valvetrain setup. Or a nice solid "street roller"..which will spin all you want and be easy on parts.

As mentioned...lots of variation on parts to get compression. Need to be sure. I've run 11.2 in my 427 with iron heads on 93 octane with no issues (195 PSI cranking)...but tune needs to be right and cooling system needs to keep it happy.

A lot of what you can get by with is based on efficiency of the combo. Once RPM climbs and the cylinders get really full....that's when you have to make sure you have enough octane. The better the cylinders fill with good flowing heads and the right cam.....the touchier it gets.

But it's worth it!


JIM

3X2

av gas? I thought it is not legal to sell av gas for anything other than an airplane and that it is not really suited to automotive use.

kossuth

DITTO Unless you absolutely want to stay original on everything there is no reason not to run a hydraulic roller. Way better performance and you don't have to get super crazy about finding the correct oil etc. It's one thing if you intend to do the whole NCRS thing but you can actually run a milder cam as far as duration and get more power out of the car due to the how much more aggressive the roller ramps can be because of the roller lifters.

suprspooky

Don't worry about the lifter bores, I started with an L71, it's now a 460 with AFR heads and Hyd. Roller with the original Tri-power setup (rejetted) all Vac. items work and it pulls solidly to 6100. You gain a lot with the roller, I had mine custom ground. I ended up with 560ish hp at the crank with Tri-power (590ish with single Holley and Dart Intake), last fall I chassis Dyno'd it and they reported 481 at the wheels. Granted if you keep your CI heads I don't think the gain will be as big but it'll still be significant and no lash adj.

Little Mouse

Back in the olden days in a pump gas street engine you had really only two choices for real performance a solid flat tappet or solid roller and back then stuck with bronze dist. Gear so almost nobody ran around on the street with a roller cam. Something to think about today, you have 16 lifters and it only takes 1 to stop spinning and you are toast. Needless to say it costs worlds more money to fix an engine today. Even an old farm tractor harley davidson has had roller lifters in it forever.

69L71

Hey Spooky, did you run into any trouble with the roller rockers hitting the bottom of the tripower intake? I've heard that can be an issue....

JBrooke825

its 100 octaine. Lots of people go there and get it. I see you can get it in
Lansdale and Bridgeport also. Not sure about the price though. Maybe I will
try them this year and see if it makes a difference, I know it doesn’t help the
gas mileage . Yuch.