The Stovebolt Site Forums The Shop Area Engine Shop Timing Question... Among other things...

I that a 350 installed in my 72 Stepside. The engine was overhauled not to long ago and I have just replaced the cam and heads for performance. What I have is one of the comp cams extreme 4x4 cams and a set of World Sportsman II heads. Now here are the problems.. I ran the engine to break in the cam like you are supposed to. At idle I was at 8 degrees advanced. After break in, I drove the truck down the road and it felt like I had 100 hp instead of the 350 - 375 hp that I was hoping for. What advance should I need for this setup? What am I missing??? Did I do something troubly wrong? Please help me!! What else could the problem be if not the timing??? The motor was in good shape I hope I didn't ruin it!!! Thanks!

How long did you have the motor setting before rebuild? A buddy of mine put a carb that had been idle for some time on a new crate motor and spent a lot time and effort finding out the carb was gummed up. he couldn't get it back up his drive it ran so poorly. What other items did you replace on the motor. Is there a chance you put a 302 balancer on it?

Once you get the RPM up above 2500 or so, what happens? You might have over-cammed the engine, and the torque converter won't let it wind up into its power range on acceleration. The initial timing needs to be advanced, but you're going to need to do some modification to the advance curve in the distributor to avoid over-advancing at high speed. When I set up a roundtrack engine distributor, I restrict the centrifugal advance about 7 to 10 degrees, and set the initial timing ahead by the same amount. You will get the best acceleration with about 15 to 18 degrees initial advance, and a total advance, initial and centrifugal, of 36 to 37 degrees. You can add another 5 degrees of vacuum advance for fuel economy, which will drop out almost immediately when the manifold vacuum goes down during full throttle operation.

What is the duration and lift of the cam, and what compression ratio are you running? If the heads are 64CC and you're using flattop pistons, the C/R should be about 10.5:1 or so. That's going to need 93 octane gas, a torque converter with a 2,500 RPM stall speed or above, and a 3.90:1 or lower rear axle ratio.
Jerry

Beltfed: The engine was in good running order before. I drove it in the garage, tore it down to the short block and put it back together. The intake is an edelbrock RPM Performer and it has an edelbrock 650 carb. I replaced the cam, lifters, pushrods, heads and 1.6 rockers. I re-installed the stock balancer.

Hot Rod: I told the tech line at summit what I had, which included a stock converter and 3.08 gears. They recommended the comp cam made for torque in this application. The specs are Duration 262/270 and lift .462/.480. The heads are 64CC with flat-top pistons. Would bumping the timing up to 15 to 18 degrees at idle help a lot? I'm also thinking that I need to re-set the valve lash. How do you modify the advance curve? This is a stock HEI distributor. Thanks for the help!

If the guys at Summit were real mechanics, they wouldn't be manning a tech line, they'd be out there bending wrenches. They're mostly there to convert the contents of your back pocket into profit for the company. The duration on that cam sounds OK for street use with a stock converter, but using 1.6:1 rockers turns your lift into .493"/.512", which is a bit extreme. The 1.6's also need about $500.00 worth of head modification by a very good machinist to work right, relocating the rocker studs 1/16" closer to the pushrod side of the head, and adding a set of hardened steel pushrods, guide plates, and screw-in studs. The advantage in lift isn't worth the hassle, plus you're putting the valves dangerously close to the piston tops unless you've milled the pistons for clearance. Take those 1.6:1 rockers off and throw them as far as you can!

The advance curve modification takes a distributor tester, and involves installing a couple of 1/8" roll pins in the slots where the advance weights move to restrict their travel. Whatever gain in initial advance is made must be subtracted from total advance to prevent over-advance at high speed, and detonation/piston damage. When I do that mod, I also modify the reluctor on the distributor shaft to allow speeds of up to 7,000 RPM without spark scatter. A stock HEI distributor starts to crap out at about 5,500 RPM without some special attention.

Did you install real flattop pistons, or do you still have the standard dish-tops the stock engine came with? The original pistons have a .200" dish, plus a chamfer around the outer edge, and the 64CC heads make about 9.5:1 with that setup, just barely streetable with 87 octane gas. Mid-grade gas will perform better, especially with the advanced timing. I still believe your poor performance is more of a gear ratio/torque converter problem than a major error in engine buildup. Email me about the distributor mods if you like.
Jerry

Looks like the cam is about right but I don,t think the 1.6 rockers should go with it unless they were part of the package. Did you disconnect the vacuum advance tube when you set the timing? Try it with 10-15 degrees BTDC. You can get an advance kit for the HEI that has different weights & springs but to really program it you need a timing light with advance feature or put a timing tape around the damper. Hot rod Lincoln is an expert on engines. Hopefully he will be back with some more help.

One more thought - Is the cam installed "straight up" or is it advanced or retarded? This makes a big difference in how the engine runs. I'm assuming you also used a new timing chain (?)

The heads came with screwed in studs and guide plates.. World said that the Sportsman IIs will work with 1.6 rockers without machine work. I installed new comp cams pushrods. The pistons are stock "flat tops" and have the double dishs in them. I'm not too knowledgable in the distributor department.. Maybe I should just buy a new MSD distributor. I probably should've left the dang engine alone.. It ran fine, just didn't have any power.

I'm hearing some slight ticking noise from the right side, so I'm probably going to re-set the lash on all the lifters. Then I'll try advancing to 12-15 degrees (after I get some octane booster). Anyone know a knowledgeable guy in the Louisville area? I'd like to bring it to someone.. I've never done this before and I'm probably second guessing all that I've done. It'd be nice to have an experienced set of eyes to look at it and diagnose the problems.. You guys are a great help.. I'll let you know what happens.

Waldo - The cam was install "straight up" and I used a new chain.

I do plan on gears.. I was thinking 3.55s due to my frequent interstate driving.. Would that be enough? The Tranny is a 350T.

Looks like Hot Rod got back before I did. I would leave the 3.08 gears in there. Anything geared lower would be uncomfortable on the freeway & have horrible gas mileage.

Let's hope the "slight ticking" isn't the valves and pistons getting together. Over .500" lift at the valve without special machining is playing Russian Roulette, but I've given you my opinion, based on building several hundred round track engines, up to 600+ HP on the dyno, and they stayed together. What you do with it is up to you. If you advanced the timing without altering the advance curve in the distributor, get ready for some melted pistons.
Jerry

The purpose of changing the cam and putting better heads is to make the motor burn more gas which equals more horsepower. If you don't increase the amount of fuel going to the cylinders, how is it going to run better.Re-jet the carb,put the distribitor on a machine and get the timing up to a total 37-40 dgrees. Then get a hotter spark( better Coil). With the Sportsman heads you should be good to .550 lift with Flat-top or dish pistons because they relocate the valve angle which allow the motor the work with more valve lift than the stock heads.

I also forgot to add that you need to find and mark the crank for true top-dead center.