I recently modified a 1973 400-4 motor with 60k original miles. After my modifications, the engine feels flat, especially off the line. It feels slighlty more powerfull around 4000 RPM and then falls flat again around 4500 RPM.

I checked cranking cylinder pressure and it is only measuring between 115-120 PSI on all cylinders. This seems way to low! Based on the article I found at http://www.thepontiactransampage.com...blueprint.html. I am afraid my engine combination may never run to my satisfaction. Here are the modifications that I made, any insight or suggestions are welcome!!!

I swapped out the 4x heads for 1971 #96 heads (new valve seats, H.O. Racing's VS11 springs and 3 angle valve job). The chambers measured out at 96.5cc. I used a 71 cast iron intake, new Ram Air Restorations D-port exhaust manifolds and 2.5" exhaust, installed an HEI distributor (12 volt power sourced), and a Carb Shop stage II prepared Q-Jet. I installed a new Comp Cams XE262 camshaft (218/224, 110, .462/.470), 1.52 roller tipped rockers, Comp Cams pushrods, and their timing chain/gears. Initial timing is set at 16 degrees advanced, with total timing set at 35-36 degrees (vacuum advanced not yet hooked up).

I thought that had been careful in putting together this mild combination. It was my understanding that by using a Comp Cams Xtreme Energy camshaft that I would be able to maintain some decent cylinder pressure. My stock motor would have been around 135 PSI. Losing cylinder pressure is not what I expected!

By the way the car is a 1973 Grand Am (4300 lbs), 3.23 gears, stock converter, and will be driven daily.

Thanks in advance for any help!

I recently modified a 1973 400-4 motor with 60k original miles. After my modifications, the engine feels flat, especially off the line. It feels slighlty more powerfull around 4000 RPM and then falls flat again around 4500 RPM.

I checked cranking cylinder pressure and it is only measuring between 115-120 PSI on all cylinders. This seems way to low! Based on the article I found at http://www.thepontiactransampage.com...blueprint.html. I am afraid my engine combination may never run to my satisfaction. Here are the modifications that I made, any insight or suggestions are welcome!!!

I swapped out the 4x heads for 1971 #96 heads (new valve seats, H.O. Racing's VS11 springs and 3 angle valve job). The chambers measured out at 96.5cc. I used a 71 cast iron intake, new Ram Air Restorations D-port exhaust manifolds and 2.5" exhaust, installed an HEI distributor (12 volt power sourced), and a Carb Shop stage II prepared Q-Jet. I installed a new Comp Cams XE262 camshaft (218/224, 110, .462/.470), 1.52 roller tipped rockers, Comp Cams pushrods, and their timing chain/gears. Initial timing is set at 16 degrees advanced, with total timing set at 35-36 degrees (vacuum advanced not yet hooked up).

I thought that had been careful in putting together this mild combination. It was my understanding that by using a Comp Cams Xtreme Energy camshaft that I would be able to maintain some decent cylinder pressure. My stock motor would have been around 135 PSI. Losing cylinder pressure is not what I expected!

By the way the car is a 1973 Grand Am (4300 lbs), 3.23 gears, stock converter, and will be driven daily.

Thanks in advance for any help!

Combo seems OK to me. Did you degree the cam? You could gain a little cyl pressure by moving the C/L a few degrees. First though, I would make sure the valves are lashed correctly. VS-11 springs, I thought I was the only one still running them. http://forums.performanceyears.com/g...on_biggrin.gif

That is a very low pressure for a mild cam. Did you floor gas pedal to fully open throttle plates while cranking?
Did you figure out what compression ratio you have? You mentioned chamber volume but the other big factor is how far the piston is down the hole at TDC. Did you measure that?

I did not degree the camshaft (this could have been my mistake). The compression is 8.0 to 1, which is why I went with the that particular camshaft. When I performed the compression test, the engine was cold and I did fully open up the carburator.

I spoke with Chris Mays at Comp Cams today. He felt that my particular engine combination with the XE262 cam should produce 150-160 PSI. Mays felt that I had purchaced an appropriate camshaft. He was of the opinion that I probably installed the camshaft improperly (retarted). If I did, then I will feel pretty retarded! I will have time to check this on Monday. Thanks for the imput gentleman.

Anybody else out there have a (modified) low compression 400? If so what combination did you put together and are you happy with it?

Your cam would have to be about 8* retarded to loose that much compression. Put #1 cyl at TDC and see where the timing marks line up.

Doug....are you talking about the timing marks on the balancer/timing cover or the dots on the timing gears?

I figure that if I installed the timing gears 1 tooth off, that would be an 8.57 degree difference in camshaft timing (There are 42 teeth on the cam sprocket and each tooth equals 8.57 degrees).

How did you come up with the 8 degree figure for my loss of cylinder pressure? Is there some type of formula?

I was talking about the balancer/timing cover. If I retard my cam 4* I loose 10 PSI, if I advance 4* I gain 10 PSI. It was just a guestamation. You're 20 PSI down from your estimated cranking pressure. Straight up my cranking PSI is 170 lbs.

Doug....thanks for the input. Using your figure of 10 PSI per 4 degrees, I could be losing between 21 and 43 PSI. Assuming that I retarded the cam 1 or 2 teeth.

Wood, i am running a 455 with a comp xe274 and about 8.5 compression and i only see about 135-140 on the compression, and i have done leak down tests and such, i think the xe cams bleed off cyl pressure that is why they work well in low compression engines. You need ot do a leak down test to check things out. I installed my cam straight up but did not degree it to check it.
Mike

I can't possibly see how bleeding off cylinder pressure would help an engine that has low cylinder pressure (compression) to begin with. The leak down test is a better way to see if you've got a "motor" there or not, but will not tell if you're camshaft is positioned for best performance like a cranking comp test or drag strip test would. The leak down will help tell if you've got the lash set too tight or a ring seal problem, but if the problem occurs with all cylinders, I'd look at cam timing. Put a wheel on it and see for yourself. While you've got the wheel on it, verify that your timing marks are accurate. I've seen aftermarket balancers off by as much as 30'!. Good luck. You're on to a good start already.

I would agree with the last post. Make sure your timing marks are correct. An easy check is to stick a screw driver in the cylinder and when it is not moving any more check the timing mark to see if it is at zero. Obviously turning it over by hand.

UPDATE: Well today, I took the front timing cover off and the timing marks sure looked liked they lined up properly. I put a degree wheel on and checked to see how the cam was positioned. The intake center line was right where it is supposed to be, 106 ATDC!

I am not sure where that leaves me now, except I know that the cam is installed correctly, my cylinder pressure is low (115-120 PSI) and the engine has less power than before! Perhaps I will try a leak down test next.

Gawd, does this sound like dejevu!

Wood, I have a '74 Grand Am that had a Comp Cams 280AH cam, 8.6:1 compression, blah, blah, blah. The 280AH was a complete mis-match for that compression, and never got more that 115 PSI cranking pressure. The car had a great mid range (3,000 to 5,500), but was a pig below 2,000 RPM. Once I changed over to a Summit 2801 cam, the car felt a whole lot better. The 2801 has 214/224 duration, pretty close to what you have, but has 112 LSA. If I'm doing my math right, you have about 6 degrees more overlap than the 2801, but I'm not sure that would cause your low cranking pressures.

Although your cam is milder than the Comp 280AH, you are also running about a half point less compression than I am. I would do what Doug said and try advancing your timing a bit to see what happens. I never did that with my cam because I had quit a bit more lift and had no idea what my piston-to-valve clearance was without tearing the engine down. Was easier just to have the cam changed.

One other thing to keep in mind is how heavy the GA's are. With 3:23 gears and 2 tons, I wouldn't expect eyeball flattening accelleration. I have 3:89 rear gears and 3:06 first gear in my 700R4 trans, feels pretty stout but not what I expected either.

Good luck! Got any pics of your car you could share?

Fake Goat.....what where your cylinder pressures after you installed the Summit 2801 cam? I don't want to jump the gun and yank the XE262 cam out just yet, but I was thinking about purchacing Summits 2800 cam.

Are you and Doug suggesting to advance the ignition timing or the camshaft timing? I was thinking about advancing the Cam 2-4 degrees. I don't have any digital pictures of my Grand Am just yet.

I haven't taken any cylinder pressure readings since I changed cams, but I'm curious what they are. Will check it out and post.

We are saying advance the camshaft timing. Correct me if I'm wrong, but this would move the overlap period more into the exhaust cycle and out of the intake cycle. Because less overlap time is spent in the intake cycle, less air is lost through the closing exhaust valve.

Are you absolutely sure the valves arent adjusted to tight?

You should have more than 115 psi with that cam. XE cams have a 110 deg. centerline and are ground with 4 deg. advance. That's why your at 106 which is correct. You should be seeing 150psi easily even with 8:1 compression. Double check your cam timing and valve adjustment and if you still have problems I'd perform a leakdown test and start looking elsewhere for compression loss.

I wouldn't be too concerned about the compression readings provided they are uniform and the engine isn't using any oil. Before making any drastic changes take a hard look at the distributor and carb. A LOT of power can be made or lost thru careful tuning. I have recalibrate a lot of Carb Shop (CA) q-jets in recent years. There is a lot of power to be made right in the carb and probably some more with the distributor......Cliff

Thanks for the input gentleman....I am going to try a different compression gauge today (Snap ON). By the way I live at 6000 feet, I am sure that has some effect on the end results. Does anybody know how much?