383 timing question
 

I've got a Gen1 383 in a kit car (Ferrari Testarossa replica). It's not a Corvette (although some parts of it are from C4s), but I love Corvetteforum and respect the knowledge here and I know you C3 guys really know these gen 1 engines well.

The engine has a forged bottom end, ported and flowbenched AFR 190R aluminum heads and an unknown cam, port matched Edelbrock Torker II intake and a holley 700 cfm double pumper mechanical secondary carb with #72 jets in the primary. The secondary jets are unknown at this time. Headers are 1 3/4" long tubes. The ignition system is a MSD 6AL box, blaster 2 coil and a MSD pro billet distributor with NO vacuum advance. The distributor has the black advance stop bushing installed for a total of 18 degrees of mechanical advance and has the small blue springs that bring in full advance by 2500 rpm with the black bushing.

I know the engine builder dyno'd it with his "house carb and dist" and made 465hp.

I currently have the base timing set for 18 degrees...so combined with the mechanical advance should be seeing 36 degrees total advance. I need to verify this with my timing light yet.

The car starts, idles runs and drive fine. At idle it makes 12.5" of vacuum. The car is mid engine and the transmission is a Porsche G50 5-speed manual and is geared very low.

This car should go like a scalded ape but it just feels weak to me. The exhaust smells rich at idle to me. When driving It feels to me like it wants more advance. When I got the car the base timing was set to 16 degrees BTDC and it idled so rich it would sting your throat and make your eyes water. I increased the base timing to 18 degrees and retuned the idle jets and it made a big improvement in exhaust smell and power. But it still smells rich and doesn't meet my power expectations. The car is very loud and has 315 width tires in back. Maybe it's faster than it feels to me because of all the noise? I'm just puzzled.

Does this thing need more timing?
I've ordered a MSD recurve kit. I'm planning to try 20 degree base timing with 18 degrees mechanical advance for a total of 38 degrees. Does this sound reasonable? Should I do more? Should I put the blue advance stop bushing in the MSD dist for 20 degrees mechanical advance and go for 20 base plus 20 mechanical for a total of 40 degrees advance?

Also, the MSD box has a 6k RPM limit module in it. I've ordered other modules and will swap in a 6900RPM module when it arrives. The way it is now, when the rev limiter hits at 6k it's still clearly wanting more.

Sorry for the long post, and thanks for reading.

What are the cam specs, fuel rating and compression ratio?

10.2:1 - flat top Arias pistons with (I'm guessing) 75cc?? combustion chambers if that makes sense.
I'm also trying to find out if it has 5.7" or 6" rods as I understand that could make a difference. The crank and rods are forged Crower parts.

what do you mean fuel rating? It's got a tank of 93 octane pump gas in it now.

I've got a call in with the engine builder for cam specs. Richard Kippley built this engine back in the early 90s and it has sat since. I talked with him today and he is going to look back in his records for the engine data hopefully including the cam specifics and rod length.

I would not increase your total timing but probably retard it to ~32* for aluminum heads and increase your initial timing.

I doubt your problems are anything to do with timing and throwing more aftermarkets widgets at it will not make much difference.

An engine with 10.25 compression and 12.5" vacuum doesn't sound like a candidate for making 465HP. Maybe 365 (gross) HP or about 275 net.

Your heads are good heads, and don't need 36 total timing: They will make best power at 32 - 34 degrees, so you need to back off your total just a tad. Make sure the full total comes in at about 2500 rpm.

If you chose to run without vacuum advance, you need more initial: I would shorten the curve so you can run about 22-24 initial with the 32-34 total. This will require a very short advance curve.

If I were you, I'd consider dropping in a distributor with vacuum advance - you can run one successfully with the amount of vacuum you're pulling: Run a B28 vacuum advance limited to 12 degrees; run 18 degrees initial with 32-34 total. The 18 initial with 12 degrees vacuum will allow the engine to idle at 30 degrees of advance, and it will get you very nice off-idle throttle response (it won't increase peak power, but backing it off to 32-34 total will improve the top end a little with the AFR heads).

Lars

I agree with you that he probably don't to buy more parts, but I'd disagree that the op is only making 365 horse the zz383 is making 425 horse with 9.6 to 1 with lesser heads.

18 degrees mechanical advance is the lowest the distributor offers with the black advance stop bushing.

black = 18
blue = 21
silver = 25
red = 28

OK, I think the first thing you need to do is get a timing light on there and find out where the all-in timing is actually coming in. Just because the black bushing says 18 degrees, does not necessarily make it so. There can be a wide discrepancy between manufacturer's claims and the real world.

That being said, let's assume we do have 18 degrees of mechanical advance, and it's coming all-in at 2500 rpm. Setting your initial timing at 14 will result in an all-in timing figure of 32 degrees. Is your distributor an HEI type? If so, use the AR 31 vacuum advance canister. It's about equivalent to the B28, (for non-HEI distributor) and should give you around 16 degrees of advance. This will let the engine idle with around 30 degrees of advance at idle, and at higher rpm cruising, about 48 degrees, which all sounds just about right. With the more advanced timing at idle, (using a manifold vacuum source, of course!) you may actually see an increase in idle vacuum and idle quality, and the need for some carb fine tuning. Shoot Lars an e-mail and ask him for his papers on vacuum advance units, how to set timing, and just for the heck of it, how to tune a Holley. You'll be glad you did.

Edit: Just took another look at Lars' post, and I see that it implies that your distributor does not have the capability of using a vacuum advance canister. Can you verify this? If so, throw my advice right out the window, or go get an HEI distributor that can use a vacuum advance can.:thumbs:

Scott

I'm running 36* of advance at idle with vacuum. 19* initial. If your running without vacuum advance I'm not sure how you would clean up the idle quality unless you locked the advance out at 34* permanently.
Running lots of advance at idle will clean up the exhaust a whole lot, improve your idle quality and your off idle response.
You may need to modify the mechanical advance timing slot by welding or filling in the slot with JB weld to limit it's travel then grind it back to get the exact advance you want.
If you don't have a digital dial back timing light to do all this, do yourself a favor and pick one up. It will save sooo much time vs using timing tape and a standard timing light. The digital ones come with an RPM read out too, very handy.:thumbs:

http://www.sears.com/craftsman-profe...1&blockType=G1

Lars is correct in his understanding that the MSD pro billet distrubutor in this car does not have a vacuum advance.

Thanks for the info. I've got a timing light with a dial back knob on the back I've been using.

It's beginning to sound like I should maybe change to a distributor with a vacuum advance module.

What is the purpose of this current distributor without any vacuum advance? It it supposed to be for racing applications only?

This is the one on it now:
http://www.msdignition.com/product.aspx?id=5679



Maybe it needs this one:
http://www.msdignition.com/Products/...m_Advance.aspx

timing
 

I agree with REELA completely - get a digital timing light with tach and timing readings. I bought one last year and it is the BEST tool I have ever purchased.

lets see of pics of this 383 installed in the Ferrari Testarossa replica :cheers:

More info to add:

I pulled the secondary bowl off the carb and checked the jets - they are 73s.

I re-set the float level on the secondaries - it was a little high but not too bad.

And I set the base timing to 20 degrees. with the 18 degrees mechanical advance that puts it at 38 degrees total.

Stupid me, I forgot to verify total timing with the timing light.

I re-tuned the idle screws for max vacuum, put a 6900 rpm module in the MSD box and went for a ride.

It feels like it's happy up until 6500 RPM, so I put a 6700 RPM module in it after I got back home from the test drive. It felt about the same as it did with 18 degrees initial timing.

It still smells way rich at idle. This is my biggest desire it to get the car to stop smelling so strong. My wife won't come near it or me after I've driven it the stench is so strong.

I'll keep my eyes open for one.

http://bahrnet.dyndns.org:2980/webshare/TR9.jpg
http://bahrnet.dyndns.org:2980/webshare/TR8.jpg
http://bahrnet.dyndns.org:2980/webshare/TR7.jpg
http://bahrnet.dyndns.org:2980/webshare/TR6.jpg
http://bahrnet.dyndns.org:2980/webshare/TR5.jpg
http://bahrnet.dyndns.org:2980/webshare/TR4.jpg
http://bahrnet.dyndns.org:2980/webshare/TR3.jpg
http://bahrnet.dyndns.org:2980/webshare/TR2.jpg
http://bahrnet.dyndns.org:2980/webshare/TR1.jpg

It's a tube chassis front bumper to rear with a Fiero cab integrated into it.

The front suspension crossmember is C4 Corvette but has been converted over to Carerra coil-overs. the steering is a manual rack and is fairly heavy feeling, but so were the real Testarossas.

The rear suspension is fabricated from Howe chassis components with custom machined uprights and two Carerra coil overs on each side.

The body is fiberglass. The trim items are Ferrari to complete the look.

Interior is pretty much stock Fiero except the engine cover behind the seats inside is removable for obvious reasons. It's also had extra sound insulation added in. It has a VDO programmable speedometer with a magnetic pickup and a complete set of Autometer gauges to monitor everything else.

Brakes are Wilwood 6-piston in front and 4-piston in the rear with two piece rotors all anodized. The 3 master cylinders / pedals for the clutch and brakes are Howe chassis components. there are two vacuum assist modules for the brakes, one in front and another in back with a supplemental vacuum pump running them. There is a proportioning valve in the back next to the rear vacuum booster as well.

The cooling system is two large radiators, one on each side of the engine feeding off the side vents in the body. They also have electric fans on them. Each radiator also has its own Howe chassis reservoir. It's all plumbed in braided stainless and AN fittings.

Under the front hood are two fuel cells, one large and the other reserve with a valve to switch between them, a large fuel filter and a fuel pump all done in AN fittings. I'm still looking for the fuel pressure regulator, it has to be there somewhere.

Exhaust is entirely hand built from mandrel bent stainless steel.

Wheels are 17" Dymag C4 Corvette with ZR1 11" wide wheels in the back.
Tires are 315 in back, 275 in front.

great looking car ! how much dose it weigh ? put your carb to stock factory jetting .and replace the power valve .what is the vacuum reader with the car in gear ?try different power valves to tailor your fuel curve .