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Poor performance after cam swap


Jeff G 78

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I am at a loss and need some help. Here's the background on my car:

I rebuilt the engine a few years ago and it has about 6500 miles on it since the rebuild. It has an N47 head and flat top pistons. The head was brand new when I built the motor. It was a takeoff head that had been sitting on a shelf since 1977. It didn't even have a wear pattern on the cam when I got it. I had a new Web Camshaft that I was planning on using, but chose to rebuild the engine with the stock cam that came with the head until I knew it ran right to take a variable out of the equation. The engine ran great for two years. I had 180psi across all cylinders, <10% leakdown, 18" vacuum at idle and it got between 23 and 26 mpg on 93 octane. I drove it to the Cleveland convention last October and then parked it for the winter.

Over the last few weeks, I began upgrading to the mild performance cam, springs, and rockers. First, I pulled the rockers off and swapped the springs. Once that was done, I set the engine to #1 TDC and painted match dots on the chain and cam gear. I wedged the chain, pulled the gear and slid the old cam out. I slid the new cam back in and turned it so that the dowel was at 12 o'clock. I put the gear on the chain with the dots matching and the cam's dowel lined up with the #1 hole in the gear. I reinstalled the cam gear, lash pads and rockers. I set the lash to the cold specs and buttoned up the engine.

Once the radiator was installed and all the fluids were filled, I fired up the engine. It popped right off, but wouldn't idle at all. I had to adjust the idle screw from almost all the way in to almost all the way out to maintain a ~800rpm idle. The engine revved smoothly and sounded fine, but idle was unstable and "lopey". I drove the car around a bit and it ran OK, but felt weak and continued to idle poorly. Once warmed up, I checked the vacuum and did a compression test. Both were bad. My vacuum was stable at only 10" and the compression was between 115 and 120psi on all cylinders. :mad:

The engine is obviously down on power and testing confirms that something is wrong, but I don't know what it is yet. I still need to recheck valve lash now that it has run a bit, but they were all set properly when cold. I know I set the cam timing correctly by lining up my paint dots, but that's the only thing I can come up with that would cause these conditions. I will also check leakdown on at least a few cylinders to see if it's sealing properly.

Any ideas as to what is off? I have rebuilt many engines and I was very careful to make sure timing was right, so I really doubt I messed it up, but SOMETHING is obviously wrong. If the timing IS off, I hope I didn't get any contact :tapemouth. The only other thing I can come up with other than an error by me is if the cam was made wrong. If I can't figure anything else out, I will get a degree wheel and see if it matches the specs.

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You don't time an aftermarket cam with the factory marks. Here is a writeup on timing a cam: http://forums.hybridz.org/showthread.php?t=111523

Even if you have the cam timed correctly, you can expect crappy results with factory EFI, because factory EFI is crappy and can't self-adjust to a large cam, so everything gets thrown out of whack. You might check with the ITS racers who are running 280Z's (if you can find one). I think a couple years ago they were given the ability to chip the computers, and so there might be somewhere that you could send the computer to have it chipped if you were really keen on keeping the EFI. If that doesn't work, then to properly utilize a larger cam you will really need aftermarket EFI of some kind or carbs.

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John is right. The stock EFI needs more vacuum to run correctly than the aftermarket cams generate. Consider going back to the stock cam, switching to carbs or, aftermarket programable EFI. I use Simple Digital Systems (SDS) on my 914.

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Jon and Rob are correct Scheider Cams make different cams for carbs and EFI. The lobe centers are different to develop vacumm for the EFI. Which cam did you buy?

Here are the specs.

Web Racing grind #91

It is a new cam with stock base circle

.450/.450 lift

260°/260° duration

238°/238° duration @0.050"

Their website doesn't list it, but I'm pretty sure I remember the lobe centers are the same as stock at 108°/111°

Thanks Jon and Rob. I will do a sanity check to make sure nothing is way off and then pick up a degree wheel. I do have a stopper and dial indicator. I will likely swap back to the stock cam until I figure out what to do. Carbs or a new FI system are not in the budget right now.

Is there any harm in running the stock cam with Schneider springs?

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I have a follow-up question for Jon or Rob. I will degree the cam as soon as I can get a degree wheel, but in the meantime, do you think that low compression and low vacuum are more likely caused by a retarded cam or an advanced cam? Also, how much would the cam have to be off to cause a 33% drop in compression test readings and a 45% drop in vacuum?

I'm just trying to get a feel for whether or not I could get it to run any better with the stock gear until I can get a Nissan Comp gear. If the cam is currently retarded, I can advance it 4 or 8 degrees with the stock gear's adjustment holes. If it's currently advanced, I'd have to back it off a tooth which would be (if my math is correct) 18 degrees in hole 1, 14 degrees in hole 2 or 10 degrees in hole 3.

Normally, I would simply order the gear and wait for it to arrive, but we have a Z meet this weekend and I don't want to miss it. It's unlikely that I'd get the gear in time to get it andjusted and back together by Saturday morning. If I can't get it close, then I'll hold off until I get the gear, but if I can get it in the ballpark I can at least drive it to the meet.

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A bigger cam will get you a lower compression reading and that is totally normal. That's why you hear people saying that cams "bleed" compression and you lose low end power. It's true that they lower static compression, but they do so to increase efficiency higher in the rev range. I don't know enough to say that your new compression readings are "normal" with a big cam. I think the common consensus is that the reading you get on a compression tester shouldn't be used for anything other than to compare the readings to the other cylinders.

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Well, I went ahead and removed the Web Racing camshaft and associated parts and returned everything to stock. My Z once again purrs along at an 800 rpm idle and I have 18" of vacuum.

Thanks to all who helped me out. I guess I will store the cam, rockers, pads and springs for now. Someday, I might switch to a MegaSquirt, TEC2, or even triple carbs, but until then I'll live with the stock cam and a properly running engine.

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