AustrianVette

Hi,

I had my 67 Vette on the dyno today.

It is a matchin #´s 435hp car.
Engine was rebuilt approx. 1000 mls ago.

Since the orig. side exhaust is really restrictiv I built a nice free flowing system with mandrel bend tubing and straight through mufflers (2 1/2inch). Further I put a K&N filter in.

Since I did this the car starts to stutter at 4000 rpm.
I expected that the hight air flow will need re-jetting.

I put QuickFuel metering plates in the front and rear carb.

Idle is fine and AFR also up to 3800 - 4000 rpm.

Approx. at 3500 rpm hits 13.5 AFR the first time.
After that it goes up until I close throttle at least at 4500 rpm.

Spark plugs are really light (not brown) more white.
No big mods done:

- port match, ports cleaned out
- full roller rockers
- stock solid cam
- exhaust manifolds ceramic coated
- 2.5 in mandrel bend exhaust, straight through mufflers
- scoop on hood functional


I´m running 44degrees ignition total.

What do you think. Go on with richer jetting???

So I´m definitly still to lean.
I´m running jet size 80 at front and rear now.
Front and rear carb open completly.

So I´m wondering if that car can REALLY use more than 80´s.

MiguelsC2

Sure sounds like it. The exhaust flow increase is dramatic dropping the sidepipes.

Been almost 30 yrs year since I built and tuned a Tri-power set up or messed with BBs. But the lean condition is obvious.

JohnZ

44 degrees is a LOT of timing - are you sure it isn't detonating when you get over 4000 rpm? At what rpm is it all-in at 44 degrees?

67*427

44 degrees is a LOT of timing. On my big blocks, I run 36-38 total (exclusive of vacuum advance) and have it all in by 2800-3000. If you're really running that much advance, I'd pull it back a bit and try it again.

Steve

OOps, John got there before I did....

bluzrocker

I understand from suprem Delux..15* initial timing and end up w/ 32-34total advance max..wont over heat and rattle through the early part of the torque curve...all in @ 3000-3500..how ever his was oval port and 93 octane fuel...also some vacume line re routing...the springs on some dist need to be fine tuned w/ the springs...in some cases by cutting one coil at a time to get the advance started... and in @ the desired rpm...There is quite alot to this and I'm only a student of this practise but feel its the path to good milage ,plugs that stay light brown,easy to start and no pinginning..no overheating...I'm sure there are some FORUM members that can be more help than I am...what compression Ratio are you running??

Bluzrocker

AustrianVette

I have 44 deg. timing with vacuum attached.
36 deg. vacuum disconected.
All in at approx. 3000 rpm

I´m running it on 94 Octane

Doesn´t sound like it´s pinging to me.

I have no idea what this is supposed to do, but I had 379 RWHP at 5000 rpm.

I have my own dynojet here in my shop. So I spend my time at the weekend here. Playing around with things like that

I took about 5 hours today and played arround with jetting and timing.

But I didn´t expect to run lean with #80 jets in both end carbs.

I will go on with richer jetting.

65tripleblack

Does the 1967 L71 run ported vacuum advance? If so, then you'll be getting too much total advance at WOT. Change to manifold vacuum.

67*427

At WOT, ported vacuum should be the same as manifold vacuum. The throttle plates (all three!) should be wide open and the vacuum advance should be out of the equation.

JohnZ

That's more like it - timing readings above idle with the vacuum advance connected are meaningless; total timing (initial + centrifugal) is always measured with the vacuum advance disconnected and plugged.

63Corvette

I concur, but I have an additional question: Have you opened up (the front of) your hood scoop? If so, it will lead to problems like you are describing (engine falls on its face at full throttle and 5000rpm.)
Do NOT open the front of your scoop, or air flow issues will arise.

AustrianVette

The open hood scoop may be a problem on the road.
But it doesn´t have any effect on the AFR with the hood opened on the dyno.

AustrianVette

timing is 44 at all.
I had a friend of mine checking the timing with vacuum attached while one "power pull" up to 4800 rpm.

It stops at approx. 4000 rpm.
Timing NEVER goes more than 44 deg.

AustrianVette

Did anyone put a free flowing exhaust on a 427/435 ???

What jets are you running??

Mike Ward

You're in Austria, which rates gasoline by RON only. Euro 94 is equivalent to our 89 AKI. I'm not sure that 89 is enough for your engine to stay away from detonation.

Westlotorn

I believe all these early engines should connect to Manifold vacuum as suggested already.
I don't think ported vacuum timing was introduced till the smog engines were created after 1971. I know my 1976 Buick 455 used ported vacuum timing.
If you are connected to ported vacuum and pulling 44 degee's the engine would certainly lay down with detonation, if this is the case fix it before you break a piston. Detonation is very hard on parts. Lean burns and excess timing contribute to detonation by increasing the chamber temps, when chamber temps go up sharp edges in the head and cylinder will glow red and work like a spark plug to kick off your fuel charge, unfortunately this happens before your spark plug fires creating an explosion before your piston hits top dead center. Cylinder pressures go through the roof.
Normal wide open throttle cylinder pressure will be 600 to 900 PSI. With Detonation going on Cyl pressure increases to 3,500 to 5,000 PSI and things break.
Sometimes the Detonation ends up with two flame fronts colliding, one starts with detonation hot spots and the other starts with the normal spark plug fire. When the two collide they make a swirling Torch that can burn nice smooth holes right through a piston head.
When a engine lays down at high RPM find the fix soon before you damage things.
High octane slows the fuel burn, lower octane speeds up the burn so a High Octane fuel allows a little more timing to be used but even with good fuel your total timing at 36 is probably good.
Good luck.

AustrianVette

That was "converted" for US-members here.
Austrian rating in RON on the fuel I run is 100 Octane.

Mike Ward

Alles klar!

Steve439

Sounds like the QuickFuel metering plate uses regular jets?
If so it would seem like it's the same as a regular 4150 secondary side with no power valve which would use a pretty big jet...

65tripleblack

If you are connected to manifold vacuum, then at WOT your spark advance will not exceed 36 BTDC (based on what you said in post #6). Please verify with a vac gauge that you are indeed connected to manifold vacuum. Maximum spark advance (if you are connected to manifold vacuum) will be 44 degrees BTDC @ idle and at steady highway cruising on a level grade (again, based on post #6). If this is the case, then your WOT (initial plus centrifugal) is near optimal, but your cruise/idle advance (initial + centrifugal + vacuum) is short by about 6-8 degrees!

All gasoline burns at the same rate, regardless of octane rating. It is the ignition temperature that rises with the octane rating, and that is why it is more detonation resistant.

If you have no power enrichment valves in your QF metering blocks on the outboard carbs, then power tuning will be made by means of main jets. Looks like larger mains are in order based on your spark plug description and 13.5 AFR is on the lean side. Increase by 2 numbers for starters and look for AFR closer to 12.5:1.

Free flowing mufflers by themselves should not change jet requirements by much. What are the components of your sidepipe muffler system?

midyearvette

simply remove the air cleaner while you have the car on the dyno and slowly start to close the choke on the center carb when you are getting a stumble. if you get to a point that it runs better, then you have your answer as to if you are too lean......good luck