Peak power timing anomoly on 383 sbc

[ian_uk1975]

My 383 made best power with 34* total timing on the engine dyno. Now the engine's in the car, I took it on a rolling road today and found peak power came with the timing advanced all the way to 43*!!! This is very odd and makes me think the excessive advance is a crutch for some other issue. Specifically, I'm wondering about exhaust restriction possibly? Can exhaust restriction affect fuelling and can fuelling that's off (maybe too rich) make the engine like more timing (thinking a rich mixture will burn slower)? Using quite a big cam with quite a bit of overlap. Interested in hearing sound theories on what would make an engine need more timing than it should.

[stevieturbo]

How are you measuring or controlling timing ?

Same in both cases ?

[ian_uk1975]

Measuring with dial-back timing light. Just using an HEI distributor... nothing fancy.

[stevieturbo]

The same timing light used in both cases ? IS there a TDC mark and timing marks you can measure off, rather than a dial back ?

Was dizzy position marked when dyno tuning was finished ?

just in case there is some human error in there ?

The latter timing of 43deg at WOT does sound quite high.

[ian_uk1975]

3 timing lights, all showed the same reading.

Balancer is degreed and checked against true TDC.

Dizzy position marked.

43* total is definitely too much timing... should be developing peak power at 34*, as it did on the engine dyno. Question is, what's making the engine 'like' more timing now it's in the car. I'm thinking it can only be exhaust restriction which is somehow throwing the fuelling off. Also, I did notice my fuel pressure was up at 8psi, which is a bit high for a Holley carb (think they generally like around 5.5-6psi). No regulator fitted (but I do have one spare somewhere), so couldn't dial the pressure down. Not sure if 2psi more fuel pressure could cause over-fuelling at WOT?

[stevieturbo]

carb or fuel injection ?

If carb, it should not make any difference at all unless it's blowing past the needle valve.

If injection, no chance 2psi would even make a measurable difference.

Are AFR's the same as on the dyno ? Intake temps etc ?

Although I havent a damn clue as to why there would be such a difference. Different fuel on the dyno ?

At 43deg, can any knock be heard ? or does that simply offer the best performance ?
Is timing rock solid at all rpms ?

[ian_uk1975]

Carb.

No way to measure AFR as no lambda bung in collectors and sidepipes have baffling which prevent a probe going up there. Lambda on engine dyno was 0.90, which is apparently pretty much perfect for max power. I've got a feeling that the fuelling with the engine now in the car is different, causing the engine to like more advance. I'm thinking it's too rich causing a slower burn, hence it liking more timing. The only things that could possibly affect the fuelling is #1 I'm using a mech. fuel pump whereas the engine dyno used its own elec fuel pump, but this shouldn't make a jot of difference as long as the fuel pressure is correct and #2 A full exhaust is now hooked-up to the engine.

Good to know re. fuel pressure... didn't think that would make any tangible difference.

Fuel on dyno was V-Power and I've got V-Power in the tank now.

No knock audible at 43*... tried knocking it back a couple of degrees and lost power. Tried advancing it a couple of degrees and also lost power, so 43* was the sweet spot.

Edited by ian_uk1975 on Saturday 1st October 20:21

[anonymous-user]

Was it dyno'd with the same intake/exhaust system ??

Typically you find engines run cooler in the car than on dyno (because you rarely actually hold at high load for very long on the road (and even the track, where corners tend to get in the way ;-)
The colder metal temps in car tend to allow more ignition angle when operating at detonation limited timing. However, you are saying that you can get to MBT without detonation. If you have to advance the ignition angle to get more power, it suggests that the charge is taking longer to burn (so you need to advance the timming to "light the fire" earlier to get Pmax in the most optimum angle)

This suggests that as you say, burn rate has fallen, this can be caused by changes in fuelling, exhaust residuals, or increased cylinder filling (more air/fuel takes longer to burn).

It certainly wouldn't suprise me to see a step change in AFR on a carb system when going from dyno to chassis install.

[ian_uk1975]

A step change in terms of jet sizes? Would you expect exhaust restriction to cause over-fuelling? Thinking the exhaust stream contains HC, so if it gets sucked in during overlap (and my cam is quite big for a street cam), the HC in the exhaust gas could lead to a richer mixture.

I might try leaning the jets out a couple of steps.

[ridds]

What figures is it achieving, performance wise and how different was the exhaust system between runs?

Also, how good was the engine synod, any correction? Wad the inlet temp controlled?

[anonymous-user]

I'd want to get some kind of accurate AFR numbers from the engine in the car before i fiddled around with the jetting..........

(how hard is taking the exhausts off and getting a lambda boss(s) welded on??? )

[stevieturbo]

Or simply fitting an extension to the exhaust exit.

[Steve_D]

I would get the fuel pressure down before trying anything else. I believe 6psi is all a Holly wants and others are lower still.
Carbs are sensitive to fuel bowl level so those extra psi could fill them a little more before the float has sufficint load to close the valve.
Also the engine may be at a different angle in the car which again will alter float level. My SBC has a wedge plate under the carb to correct the angle and get it level.

Steve

[anonymous-user]

stevieturbo said:

Or simply fitting an extension to the exhaust exit.

You have to be a bit carefull with this, as sometimes you get pressure reversals at the tailpipe pulling fresh air back into the exhaust (although probably unlikely with a large cap/low speed V8)

[ian_uk1975]

I think I'm going to get another power run on the chassis dyno with the sidepipes off and some 1 metre lengths of flexible exhaust tubing connected directly to the headers in place of the existing sidepipes (thank God for ear defenders). This will, at least, take 95% of the exhaust restriction out of the equation.

[stevieturbo]

What sort of exhaust does it run that's so restrictive ? And shouldnt you be running it on the rollers in a fully installed, as you will use it manner ?

[ian_uk1975]

stevieturbo said:

What sort of exhaust does it run that's so restrictive ? And shouldnt you be running it on the rollers in a fully installed, as you will use it manner ?

Sidepipes with 2" baffle and restrictor plates.

I've already run it on the rollers in a fully installed, as I run it manner.

[Pumaracing]

There is no mechanism I can think of that would change ignition advance requirement at WOT as radically as you indicate short of WOT no longer actually being WOT which would obviously drastically reduce power if the throttles were only opening partially.

That suggests to me it isn't really 43 degrees of advance which would indicate the crank balancer has shifted and is no longer aligned with TDC.

If the bhp on the rollers was even vaguely comparable to that on the engine dyno then you can rule out exhaust, throttle and other calibration issues. Check the basics first. Compression test and double check ignition and cam timing back to actual piston TDC position with an indicator through a plug hole.

[ian_uk1975]

Pumaracing said:

There is no mechanism I can think of that would change ignition advance requirement at WOT as radically as you indicate short of WOT no longer actually being WOT which would obviously drastically reduce power if the throttles were only opening partially.

That suggests to me it isn't really 43 degrees of advance which would indicate the crank balancer has shifted and is no longer aligned with TDC.

If the bhp on the rollers was even vaguely comparable to that on the engine dyno then you can rule out exhaust, throttle and other calibration issues. Check the basics first. Compression test and double check ignition and cam timing back to actual piston TDC position with an indicator through a plug hole.

Carb secondaries are fully opening... one of the first things I checked.

Same balancer was used on the engine dyno and is virtually brand new, so doubt it has shifted. Will double-check though, just to put a tick in the box.

I'm still suspicious of the exhaust... this is the only real difference compared to the engine dyno that could change things. Cam is fairly radical (for a street cam - 248*/248* @ .050) and I'm wondering if the exhaust is drastically hindering cylinder scavenging, which might be throwing the fuelling off and drastically altering burn time, hence the need for more advance? Just an arm chair theory at this stage though, which will be proved or disproved when I get around to running it on the chassis dyno with effectively open headers.

Regarding the possibility of the engine being sick, I doubt it at this stage... all plugs look good, not using coolant, oil looks good, no excessive blowby evident, etc.

Only other thing, and I'm sure it's nothing, is when I first started the engine once installed, I had a fuel leak from the fuel log to the carb and fuel was dripping onto the intake/head interface, which shrivelled the excess RTV sealant visible and slightly wrippled/warped the visible part of the intake gasket. I'm sure the part of the gasket out of sight actually making the seal is unaffected though? No signs of any intake leak and, even if there was, it wouldn't account for these symptoms I wouldn't think.

[stevieturbo]

You should never use RT Silicone anywhere near petrol or anywhere that may come into contact with petrol etc. ie intake manifold gasket.

Although doubtful that's anything to do with it