Stock 305D Dyno Results

[Clarke]

After pulling the engine out a couple of years ago, performing tune up (included valve adjustment), replacing gaskets/seals and putting color on it, I decided to experiment with a couple different size jets on the dyno.

Configuration: Stock engine, stock ignition and rebuilt stock carb except one size up (larger) jets. According to the dyno guy, he said I could go up another size or two on the jets.

Results attached... Topped out at 91.4 HP

52 HP @ 1685 RPM (43.5 MPH)
72 HP @ 1925 RPM (49.7 MPH)
80 HP @ 2165 RPM (55.9 MPH)
88 HP @ 2406 RPM (62.1 MPH)
90 HP @ 2650 RPM (68.4 MPH)
82 HP @ 2890 RPM (74.6 MPH)

Not sure if these results are expected? I know there is some drop between the crank and the wheels. The tag in my truck has 165 GHP @ 3800 RPM and 142 NHP @ 3800 RPM, but I told the dyno guy I didn't want to beat the engine up.

Anyway, I thought I'd share with the group.

[turbobill]

Seems a little lower to me than what would be expected. I'd think 110 RWHP at 3600-3800 RPM would be more in the ballpark.

The factory Gross HP figures are on an unencumbered engine taking horsepower directly from the crankshaft. Net HP figures in those days accounted for engine accessories installed and operating. Transmission/rear axle losses account for roughly15 to 25% (automatics absorbing more) and a factory single exhaust may kill another 10 percent. (that becomes more noticable as airflow through the engine increases)

Tuning is an important part of the equation too, and over the years, I've found a few extra degrees of spark advance always produced increased power you could feel. After a full power run, read your sparkplugs for determining any mixture changes needed. (a wideband oxygen sensor is the best way)

The thing I found interesting is that your HP peaked at 2650 RPM. I would have expected HP to continue increasing up to near the factory rated peak HP RPM. This tells me that there may be some intake/exhaust restriction lurking and/or the centrifugal advance mechanism in the distributor may not be working properly. A dyno run upto 3600 RPM or so might further confirm this.

One last thing. Over the years, I found many vehicles in which the throttle didn't open fully. (that falls in the intake restriction category) With any sort of power complaint, I always check that first.

[bozzhogg]

I take it those figures are at the rear wheel's???

[turbobill]

The dyno sheet says "Chassis dyno".

[Clarke]

Thanks for the feedback turbobill.

I'll follow up on the items you mentioned... I too am curious why the curve dropped where it did.

Speaking of tune up, I rembember (after driving the truck for several years) I adjusted the timing and points per the specs in the manual, because I don't know enough to deviate (as you mention). I remember being surprised how far the timing/dwell was off (advanced?, had higher number but don't remember what it was). In fact, I stopped with the tune up process after the adjustments and took it for a test drive to ensure I didn't screw something up. The truck seem to run the same, except I noticed less power on the highway, almost like I was hauling a load. After returning home, I double checked my timing and points adjustment and they matched what I had set per the manual. Again, not knowing enough to deviate from the manual and not wanting to cause possible engine damage, I left the settings per the manual.

I set the dwell angle at 31 and the timing at 7.5 (V6-305D). What do you recommend setting these at, and what is the associated long-term risk (if any) of advancing the spark?

Thanks again,

Clarke

[turbobill]

On an engine with a factory setting of 7.5 degrees before top dead center, I might venture up to 15 BTDC or so in 2 or 3 degree increments. While you have the timing light on the engine, make sure the centrifugal advance advances the tinming as the RPM's increase.

Set initial timing with the vacuum advance disconnected and the vacuum source pluged. Also check the centrifugal advance with the vacuum advance disconnected and the vacuum source plugged.

One last item. Set the dwell angle to the factory setting and don't forget to lightly lubricate the breaker cam and rubbing block. As the rubbing block wears, the points will open later, thus delaying the spark and retarding the timing from what it was set at when the points were installed and adjusted. Depending on how much you drive, you may want to check the dwell and relube a couple times a year. As the rubbing block wears, the dwell angle will increase.

If the spark advance is over advanced, the main risk is detonation and the resulting engine damage. Detonation is also called pinging, and sounds like a crackling sound, much like diesel clatter. Lesser risk is engine roughness and loss of power. (I tend to think the GMC V6 would fall into the later category as the compression ratio is low)

While it contributes nothing to wide open throttle power, the vacuum advance increases spark timing under light loads. This aids driveability and fuel economy. Make sure it works too.

[phelan]

I'm having trouble trying to view the dyno from my windows phone but did you rev up to 4000 RPM? We have a dynojet dyno for motorcycles here at my work and depending on tune and exhaust, it is quite common to see a drop in power at certain RPM, then see the power come back up. This is commonly caused by reversion, most affected by the design of the exhaust. On Harleys and Buells, reversion is almost always present around 2800 to 3300 RPM when a 2-1 exhaust is used. My Buell in particular has a very noticeable drop at those RPM.

I'm curious what kind of power one of these motors would make if someone welded up the combustion chambers into a bathtub shape and used flat top pistons to match, to achieve a 10:1 CR and utilize a hefty squish band. Though it would be hard to design and set squish considering there is only one size of head gasket that I know, if you can find it at that.

[phelan]

I think the numbers sound normal considering the RPM you revved to. Such a small sample, but notice the 40 HP difference between 1600 and 2600 RPM? If peak power is listed at 3800 RPM, you should be looking at 130-140 RWHP at 3800, which is 1200 RPM higher than what you revved to. Without revving to those RPMs, we don't really have a full picture of what the motor is doing, so at the moment it doesn't sound out of the ordinary IMO.

[Clarke]

Originally Posted by phelan

I'm having trouble trying to view the dyno from my windows phone but did you rev up to 4000 RPM? We have a dynojet dyno for motorcycles here at my work and depending on tune and exhaust, it is quite common to see a drop in power at certain RPM, then see the power come back up. This is commonly caused by reversion, most affected by the design of the exhaust. On Harleys and Buells, reversion is almost always present around 2800 to 3300 RPM when a 2-1 exhaust is used. My Buell in particular has a very noticeable drop at those RPM.

No, only reved up to about 3000 RPM. In ref to my exhaust, I have a single pipe (inlet) to a Flowmaster Super 10, then dual exit (outlet) for the tailpipes.

In the spring I'll double check a few things and experiment with the timing as described by turbobill in a prior post, then run it on the dyno again and run the RPM up a little higher. I'll let you guys know what the results are.

Thanks for the feedback.

[Clarke]

Originally Posted by turbobill

On an engine with a factory setting of 7.5 degrees before top dead center, I might venture up to 15 BTDC or so in 2 or 3 degree increments. While you have the timing light on the engine, make sure the centrifugal advance advances the tinming as the RPM's increase.

Several weeks ago I did some inspections and adjustments as you suggested and I didn't find anything unusal. I must admit, I was nervous about running the timing up twice per the manual recommendation; however, it was a good experience. I learned a lot and I believe there were positive results. Thank you.

I double checked my dwell setting, which was 33 degress and my timing was 7.5 degrees at 500 RPM (very close to what I set two years ago when I pulled the motor, resealed and painted it).

I completed two dozen timed runs on the highway with my factory settings. I adjusted the timing to 10 degrees and completed the same two dozen timed runs and the results were very, very close when compared to the runs with the factory settings. I then adjusted the timing until the engine started running rough, which was approx at 20 degrees. The RPM change from 7.5 degrees to approx 20 degrees was an increase of 60. I barely backed the timing off and the engine was smooth again, so I continued to back it off until approx 16 degrees, then I locked it down. My RPM was now 550, so I readjusted to 500. Once again, I completed the same two dozen timed runs and they were consistently faster. I also noticed the gas pedal seems more sensitive/responsive than before.

I've driven it a couple hundred miles and haven't noticed anyother changes like running rough, back firing, or anything like that. I still have that slight hesitation or sputtering when holding around 30 mph, but when I punch the gas pedal, it goes. According to the Motors Manual, it states it's normal for that carb to have a little hesitation at steady cruise around 20-30 mph (if I remember correctly).

Note: I'm not trying to make a racer out of this, just want to take advantage of whatever power I can get out of that big monster.