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Where'd the horsepower go????


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Those are wierd cam specs. 275/280 with only 208/212 duration at 0.050". Normally a cam with that much advertised duration would have at least 230 degrees at 0.050". I guess that is a factory GM grind with very slow ramps. Is that the cam that came in the ZZ4 block?

 

The 112 degree lobe center is also hurting throttle response a little since you have a carb.

 

But, that cam is pretty small when considering the at 0.050" duration. So, I think your timing is about right. You could try 15 initial timing to see if that helps the low rpm torque. 20 maybe be too much from that cam. But on the other hand, the advertised duration (275/280) normally would run well with 20 degrees initial. The low duration at 0.050 is throwing me off. Where are you getting those timing recommendations from?

 

I limit the total advance in HEI's by placing some weld in one of the advance slots to shorten the travel. You will need to disassemble the distributor to get to the advance slots to weld them. I weld a little in one slot then grind it smooth and flat.

 

The vacuum advance can also be reduced with the kit that you have already found. That vaccum advance kit uses a stop plate and a controlled vacuum leak to make a custom vacuum advance profile but it doesn't limit the advance from the mechanical portion of the distributor.

 

Newer head designs (fast burn and vortecs) don't require as much total timing as the older style heads. So, in most cases there is too much total advance when older style hei's are used on newer style engines. That is why the length of the advance slots should be reduced.

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Yeah it's a factory GM cam. I didn't have a cam card or anything from the builder. The suggested timing I got from JEGS, basically a VERY similiar engine setup with one of their crate engines.

 

So, in summary.

 

Set the initial at about 15

And, limit the total on the vacuum advance to about say 32?

Then re-adjust carb, and I should be closer. Correct?

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15 degrees initial titming sounds about right. But maybe a little more total advance, 34 total.

 

Yes, adjust that carb. But one of these days plan on getting a regular old Holley. The Holley 3310 750cfm vac. sec. carb is the best bang for the buck carb ever made.

 

For better performance, consider a cam change. And get a cam with 108 to 110 lobe seperation (good for a carb) and look for a cam with steeper ramps. The CompCam hydraulic rollers XR282HR or XR276HR look like much better performers than what you have now (10 to 15 degrees more duration at 0.050" with the same advertised duration). However, your current cam should give decent power with a good idle, good mpg, and a long service life which is nice. Plus it is already paided for, right!

 

I tried a 700R in my Z and didn't like it. I ended up using a GM T5. It will hold the power as long as you shift a little slower. I ran 10's with a regular T5!

 

Good luck with the engine tune!

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  • 1 month later...

well crap, I basically decided that the torque converter was stealing the power.

 

Now, next question.............

 

Ok, off the line ...... running good

 

While cruising at about 50mph, then stepping on it ..... I'm getting a gasp up at the carb before the motor really grabs.

 

I've played with the timing, the carb, and the springs in the distributer and don't know what to do next............ where am I going wrong??

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I'll confess I didn't read all the replies, but just in case someone did not mention it....

 

Vortec heads like less timing than older chevy heads. From what I have seen you run around 36-38 degrees total timing with old heads and around 32-34 with Vortec heads. Because of the better mixing combustion chamber less timing is needed. This may not be the whole problem but something to think about.

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  • 2 months later...

If the fuel lines are too small it would only have an effect after a long full throttle run. It seems like you are having throttle response problems.

 

A pop out the carb is a lean backfire and/or retarded ignition timing.

 

Years ago, I ran the stock 240 gas lines on a V8 conversion and it ran mid 12's without any fuel starvation problems. And a 240 has smaller gas lines than a 280!

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Guest speedmon

I had the same problem once with an older vette.

 

High compression 350+big cam+high rise manifold=needs more gas

 

Check fuel line pressure....if OK

Try 750 Double pumper

 

If that doesn't work, take to engine builder for rework.

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Could it be the stock Datsun fuel line (hard)? I've been contemplating getting some 3/8" aluminum or stainless steel fuel line if it'll help. Whaddya think?

 

Well let me throw this out at ya. You say that your running a 600 Edlebrock Vacuum Secondary? My Engine and yours is very similiar except for heads and cam. I ran a 650DP Holley on mine with stock jets and the car had now power off the line or really anywhere else. Now this was a 650DP. I upped the jets like 4 sizes and she came alive. I then mounted the 750DP and it evolved to the beast of today. Your engine is putting out 385 horses bro....that requires more gas/air etc.

 

Im not BSin here, this is from experience. I think your carb is too small and she needs more fuel/air. If you have anyone that can let you put a 750DP on her, please try it. Im almost certain youll be glad you did.

 

Terry

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Thats a nice carb. I ran the 3310 series when I used a vac secondary. I forgot to ask what your intent was. If its daily driver then yes vac sec is better. If you want all you can be then DP is the way to go. Gas mileage will sufer however. The prob with vac secs on our cars is that its hard to get them to open up being the car is so light. Putting lighter springs etc help but a pain in the butt unless you buy the quick change kit. This is the carb I have:

 

750 cfm Classic Carb

510-O-4779C

 

I purchased the main body replacement from PROFORM without the choke tower for better airflow. Not saying you have to do that. The classic is awesome. I wish you could borrow a carb first to make sure thats what it is....it sure sounds like it to me though. Research and see what carb GM had on thier test engine that put out 385 horses... Im excited for ya...

 

Terry

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The 600 Edelbrock carb with electric choke is the least performance minded carb Edelbrock makes for the V8. The manual choke 600 is better but the 750 is the way to go if you must run Edelbrock.

 

I have been down this road, my Dad had a 600 Edelbrock on his 289 Cobra motor and couldn't understand why the motor was a dog. I let him use my Edelbrock 750 and he couldn't believe the difference.

 

That said I run a Holley 750 double pumper with a Proform main body. It is a gas hog but when you want to go giddy up it doesn't hesitate to. I have this carb on my 385 sbc and some say I need to step up to a 800-850 dp'er.

 

For performance it's Holley, for everything else it doesn't matter! J/K!

 

Mike

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Guest tony78_280z

Psst... Quadrajets are 750+, a rebuilt costs about as much as any other rebuilt, and it'll give better MPG than a holley. Or get one from a junk yard and rebuild it yourself for about $30.00! While your at it you can add some performance mods like those listed on my webpage. shh... You don't wanna piss off they holley guys or get into a this carb that carb flame war.

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Psst... Quadrajets are 750+, a rebuilt costs about as much as any other rebuilt, and it'll give better MPG than a holley. Or get one from a junk yard and rebuild it yourself for about $30.00! While your at it you can add some performance mods like those listed on my webpage. shh... You don't wanna piss off they holley guys or get into a this carb that carb flame war.

 

HA! caught you! J/K..True, I must admit a Quad is not a bad way to go and pretty inxspensive too. I have not run one in forever so I just forgot about it.

 

I have a Buddy who runs one on his big block nitrous fed Buick that hauls some serious butt for a big heavy car. 6.40's in the 1/8th!

 

Mike

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