I need some help wit hsome suggestions of what my problem might be. I have a Fast Burn 385 with the hot cam and 1.6 roller rockers installed. I have hedman "D" port headers and Dual exhaust, Flowmaster Delta 40's. This is 2.5" outlets. I have a Holley 750cfm mechanical secondaries and doble pumer carburator(model #4150). I am running a low profile K&N air cleaner, which i believe is 12" diameter. Okie, here's the problem...In August, I dynoed my 79' vette and the best pull I came out with was 277.15 hp and 305.80 tq. This engine is suppose to be 425 at the fly whell so with a 15-20% loss it should have been around 340-360hp. Where am I losing almost 100 hp? Are my jets too small( they are 75/75)? I also just purchased an MSD distributor and Blaster 2 coil to fix my timing. What else will cause this 100 hp difference?

 

Isn't the 385 fast burn advertized at 350HP ? With the cam and the headers you should see around 400HP. My only guess is that the 1.6 ratio rockers are not a good idea (don't know what should be in there - 1.5 or 1.6)... I heard that a hot cam and the rockers may not work well... I never tried different ratio rockers so I can't tell.... My only advise is to put stock rockers in and see how it improves (if any) the output...

 

try a K&N xtreme

 

GM says that the hot cam and the 1.6 rockers produces 425hp

 

Well, they do say approximately 425 hp, so you have to take that for what it's worth. The Hot Cam was designed to work with 1.6 rockers.

What's your timing set at? Manual or auto, and with what gear? I'll guess you have an auto, I think the driveline losses are greater with an automatic transmission. I'd figure about 25-30%, but maybe someone else has a more accurate number. Does it run good? Rich or lean? I'm sure the dyno shop could have hooked up an oxygen sensor in the exhaust to determine if you're running the right air-fuel mixture, which would be determined by the jet sizes in the carb.

 

Why are you running 75/75 jets? Your supposed to have a 8-point spread. LARS the god of all carbs tore apart my brand new Speed Demon 750 and installed 68/75 jets replacing the 75/75 jets. Suggest you e-mail LARS at v8Fastcars@msn.com for more info.

 

Another member (I do not remember who it was ) dynoed the same setup - Edelbrock heads, cam and carb that I have installed at 320HP. I was very disappointed but still I am happy with my car's performance.... numbers are not everything...

Based on the other replies I guess the 1.6 ratio rockers are correct - try a bigger air filter, even if the hood does not close you'll see if this solves the problem.....

 

My timing was way low and the distributor was not advancing when i did the dyno test. I don't remember the exact number but that is part of the reason i'm putting in the MSD one.

The car is an auto with the 3.55 gear.

My dad and I believe the car does run good but we are not happy with the dyno results...100 hp difference would be noticable.

I don't believe it is running rich, unsure if its running lean thats why i ask about maybe the jets being uncorrect.

The dyno was a chassis dyno and they didnt do air/fuel unfortunatly.

 

Well then that's it. If the distributor was not advancing the timing as the RPM rose then you will not get good hp numbers. The faster the engine revs, the earlier the spark must fire or you won't get the full pressure rise out of the air/fuel mixture. You said you installed an MSD, have you dyno'ed it since then? Does the car feel faster since then?

With the old engines I shoot for 38 degrees total timing somewhere around 2500-3000 RPM. The new engines are certainly better, I don't know what it should be but I imagine probably around 36 or so. Set your total timing and test it again.

 

I have not installed the MSD yet but plan to very soon...unfortunatly, winter is coming very quick.

 

seems that we found the problem... let us know if that was not it...

 

I hope so, my dad still believe that it is not only the distributor that will make a difference but he still believe something is not right with the Carb.

 

You need to make sure the carb and timing is right. You will probably find that with the fast burn heads total timing is between 32-34 degrees. Are you using the intake that came with it, part # 12366573? I would change it to a Edelbrock Vortec air gap. I have a ZZ430 with the air gap and it runs superb. One of these days I'm gonna have mine dynoed.

 

what is the part number on that intake? will it fit under the 79' hood?

I do have the one that came with it.

 

If you know your dist is not advancing correctly, then it needs to be fixed before you even start messing with the carb.

 

yea... i know i guess i'm hoping to also find out what jet sizes to use and if that along with the ignition would give me the extra 100 hp.

 

I don't think the air gap will fit under a stock hood . It's not a problem for me , I have a high rise hood. A drop base air cleaner might work, not sure. The intake part # for a fast burn 385 is 12366573. When GM dynoed the ZZ383 they used a single plane intake to produce 425 hp. They also said you could use the same intake you have but it will decrease the hp by 15-20 hp.

 

I am following this thread because it raised the issue of jet sizes and using an O2 sensor to sniff the tailpipe to adjust the air/fuel ratio. I am going to do a quick search on these pages to see if the topic of where to buy such a tune-up tool has come up. If reasonably priced, we should all have one...

Understand that I learned and applied what follows on my 454 big-block car. Details may be different for a small block. Anyway...

Lars explains very well in his carb white paper the concept of total timing and adjusting each portion of timing that make it up. I would do it his way. If I can summarize from memory it goes something like this: plug vacuum advance canister temporarily and check mechanical advance, preferably with light springs temporarily installed, and car running by revving and noting the timing change. This is easiest to do if you have an adjustable timing light. It lets you move the timing mark on the motor with a dial on it. Starting with the dial set to zero you are reading an accurate timing value from the engine. At every 500 rpms or so, roll the dial to get the timing mark back to where you started, the "reference." You then read the amount of timing in degrees on the dial. Map it from 500 to 3000 rpms or so. It should all be in by then. If you used the light springs, you'll see sooner when no more timing comes in - quit after that. You then put the correct springs back in and set initial advance to whatever you need to add to the mech number to get 36-38 degrees or whatever you decide is right. Then and only then do you reconnect the vacuum can to a PORTED (not present at idle - comes in just above idle speed) vacuum source on the carb. Check for detonation and retard timing slightly if needed to avoid it... I think that in general our Chevy distributors give ~15 to 20 degrees of vacuum advance - Lars's paper gives stats on each GM stock vacuum advance canister - points and HEI. Note that on my big-block I could see upwards of 50 degrees of total timing at highway cruise - med engine speed, very little throttle (therefore high vacuum). Of course, when you floor it, the vacuum advance portion goes away as vacuum drops to near zero at wide-open throttle. You are then left with the remaining 36-38 degrees and you hopefully avoid detonation that way... By the way - use a vacuum meter to confirm that your can works - no leaks and full advertised advance coming in - with the timing light and engine running - similar to when you checked the mech advance... Map advance from zero suction to full vacuum from the hand pump. Compare to Lars's stats for your can. Mine was bad and I replaced it - what a difference!

After all this is done on your old ignition setup you will likely find the source of trouble that kept you from getting good advance. Fix that, try again, or just chuck it and redo with the new stuff.

I agree that the jets should have a spread. 4 equal sized jets implies tuning for wide open throttle use (like maybe in a boat...?) and not for our usual street driving.

 

Innovate Motorsports sells the LM-1 wideband O2 sensor for about $350 and you can add an RPM recorder for a little bit more $ if desired. We had a group purchase discount a while back on a different forum and the people that bought them were impressed. I will be buying one for my toolbox at some point. Very precise tool for dialing in a carb.

If you want to make your own narrow band O2 sensor using a Bosch sensor and a voltmeter (less precise), there is a realy good web site out there showing how to make a narrow band O2 sensor and how to step-by-step tune a Holley Carb. Best site on the net for dialing in a Holley.

http://www.bob2000.com/carb.htm

Check it out!

-Mark.

 

You did check to make sure that the accelerator pedal opens the throttle blades 100% didn't you?

Just a thought.

BigBlockk

Later.....