383 sbc first dyno experience

[steve cowan]

Pumping losses, open that exhaust valve as high as you can.

From what I have seen in these types of combinations are less exhaust lift more duration, smoother exhaust valve motion nets more power over the curve, again its application specific.

[HQM383]

There is one other change on the exhaust side nobody has picked up yet,
Hint - it's at 92%.????

I was going to say intake to exhaust flow ratio but without reading thrifty the thread again I don't see flow numbers for these heads.

What was the difference in peaks between engine dynoed in first post v this dyno run with induction changes?

The 180cc SHP heads come with 2.02" - 1.6" valves
The 92% is the throat size on the exhaust after I put in a 1.5" exhaust valve, no other changes.
Theory says exhaust throat percentage goes up as rpm increases but in this scenario rpm is not going up it will be max 7500rpm if that.
If I have time after a meeting or 2 I will swap out to the 1.6" exhaust valves and test,I suspect it will run exactly the same.
The reason I do these sort of things is because I believe you can get away with alot more that's out of whack in a basic combo like this one.

Stupid auto correct. Should have said "without reading through the thread again".

Anyway, I/E ratio was a stab in the dark as I recall the exhaust on your other heads flow very well. Not 92% but I was fishing for the answer. I didn't see the change from 1.6 to 1.5 exh valve noted.

So what was the peak hp and tq rpm on the first engine dynoed with smaller heads and cam? Same tq, about 15 more hp this time and pump v MS109 this time.

[HQM383]

Pumping losses, open that exhaust valve as high as you can.

From what I have seen in these types of combinations are less exhaust lift more duration, smoother exhaust valve motion nets more power over the curve, again its application specific.

Simulation showed just that. With the 401 sbc engine I'm building. Exhaust lobe from the slow Comp High Energy family and intake faster CSZ family. Intake 1.6 rocker and exh 1.5. Close to .080" less lift on exh but 6° more @ .050" lift duration and 24° different @ .006" lift duration. This found most torque average in the window 5000rpm to 6500rpm.
As you showed me Steve, randy331 and CGT done a lot of testing that backed this up.

[Tom68]

Well we are putting less exh ratio on dual pattern cams, last motor I did got a single pattern. Work hack got a single pattern cam 30 odd years ago replacing a bigger single pattern cam. No scientific reasoning, they were all big enough cams that I didn't need over rev so there was no point adding to the overlap.

[skinny z]

Pumping losses, open that exhaust valve as high as you can.

Whether you're in jest or not Tom, that's been my understanding. For whatever that might be worth.
But still, that ratio split is a popular trend.

[Tom68]

Whether you're in jest or not Tom, that's been my understanding. For whatever that might be worth.
But still, that ratio split is a popular trend.

The faster and higher you can open and close a valve the less time it has to spend doing the wrong thing.

[HQM383]

Whether you're in jest or not Tom, that's been my understanding. For whatever that might be worth.
But still, that ratio split is a popular trend.

The faster and higher you can open and close a valve the less time it has to spend doing the wrong thing.

You don’t think there is a possible advantage to a more controlled release of the combustion pressure and gas volume through the exhaust throat?

Maybe there is a thread you could start.

[Tom68]

Whether you're in jest or not Tom, that's been my understanding. For whatever that might be worth.
But still, that ratio split is a popular trend.

The faster and higher you can open and close a valve the less time it has to spend doing the wrong thing.

You don’t think there is a possible advantage to a more controlled release of the combustion pressure through the exhaust throat?

Maybe there is a thread you could start.

Out of my league, but my thoughts are no, yes there's a balance to be had on overlap. But on Exhaust opening, late as possible, fast as possible.
Overlap, the quicker you can operate the valves, the less overlap you have to have and that helps on the street.

I see Billy Godbold does differentiate between race and street (we tend to be talking generally without specifying), loves a 383 for street, just because it does all the bottom end stuff so well yet still makes good power and frees up cam choices.

[HQM383]

The faster and higher you can open and close a valve the less time it has to spend doing the wrong thing.

You don’t think there is a possible advantage to a more controlled release of the combustion pressure through the exhaust throat?

Maybe there is a thread you could start.

Out of my league, but my thoughts are no, yes there's a balance to be had on overlap. But on Exhaust opening, late as possible, fast as possible.
Overlap, the quicker you can operate the valves, the less overlap you have to have and that helps on the street.

I see Billy Godbold does differentiate between race and street (we tend to be talking generally without specifying), loves a 383 for street, just because it does all the bottom end stuff so well yet still makes good power and frees up cam choices.

Return to Billy’s book page 106 to 108. The effectiveness of overlap scavenging starts at EVO point and ramp shape.

I’m interested in hearing from Steve and his thoughts on choosing 1.5:1 exhaust ratio for this engine. I assume he wanted to keep exhaust lift down for a reason.

[skinny z]

You don’t think there is a possible advantage to a more controlled release of the combustion pressure through the exhaust throat?


Maybe there is a thread you could start.

Out of my league, but my thoughts are no, yes there's a balance to be had on overlap. But on Exhaust opening, late as possible, fast as possible.
Overlap, the quicker you can operate the valves, the less overlap you have to have and that helps on the street.

I see Billy Godbold does differentiate between race and street (we tend to be talking generally without specifying), loves a 383 for street, just because it does all the bottom end stuff so well yet still makes good power and frees up cam choices.

Return to Billy’s book page 106 to 108. The effectiveness of overlap scavenging starts at EVO point and ramp shape.

I’m interested in hearing from Steve and his thoughts on choosing 1.5:1 exhaust ratio for this engine. I assume he wanted to keep exhaust lift down for a reason.

Partly the reason I asked.

As for race vs street. It seems there's no real defining line. One person's race is another person's street. Unless we put the definition on race as something that never sees street duty.

[Tom68]

Return to Billy’s book page 106 to 108. The effectiveness of overlap scavenging starts at EVO point and ramp shape.

Billy is one of the few that could play with ramp rate but really you can only slow things down.
You can also do it with pushrod pivot height in the rocker arm (depending on where it was originally setup) but of course that also affects the closing. I've done it with Jesels, I did it to soften the closing, driver had a bad habit of over revving on downshifts.

[steve cowan]

You don’t think there is a possible advantage to a more controlled release of the combustion pressure through the exhaust throat?

Maybe there is a thread you could start.

Out of my league, but my thoughts are no, yes there's a balance to be had on overlap. But on Exhaust opening, late as possible, fast as possible.
Overlap, the quicker you can operate the valves, the less overlap you have to have and that helps on the street.

I see Billy Godbold does differentiate between race and street (we tend to be talking generally without specifying), loves a 383 for street, just because it does all the bottom end stuff so well yet still makes good power and frees up cam choices.

Return to Billy’s book page 106 to 108. The effectiveness of overlap scavenging starts at EVO point and ramp shape.

I’m interested in hearing from Steve and his thoughts on choosing 1.5:1 exhaust ratio for this engine. I assume he wanted to keep exhaust lift down for a reason.

Not from my dyno testing but for my application there seems to be not alot of benefits from lifting much over 600 thou on the exhaust.
I am talking 383 cubes that peaks 6500rpm- 7000rpm.
Blowing the cylinder down slower makes sense to me,I understand the thought of pumping loss but how much effort is required to push exhaust gas out ABDC when piston is on its way up.
Exhaust valve opened at BBDC.

[HQM383]

Return to Billy’s book page 106 to 108. The effectiveness of overlap scavenging starts at EVO point and ramp shape.

Billy is one of the few that could play with ramp rate but really you can only slow things down.
You can also do it with pushrod pivot height in the rocker arm (depending on where it was originally setup) but of course that also affects the closing. I've done it with Jesels, I did it to soften the closing, driver had a bad habit of over revving on downshifts.

We'll keep the discussion on track which was prompted by skinny's question:

That raises a question I've had since the first I read about someone using less ratio on the exhaust as opposed to the intake.
What's the logic there? Is it compensating for a cam spec or working with the specifics of a particular port?

It seems EVO point and rate coupled with exhaust design influences pressure waves that determine the effectiveness of overlap. It seems for engines like Steve's with typical exhaust used the 1.5 ratio creates a better valve movement that in turn sets up for more effective overlap period.

[HQM383]

Steve,
I'm curious with a few other aspects between your two 383 builds.

Both had different heads. Both sets of heads were ported. One set iron heads, 1.94" intake valve and in the 180cc vicinity. Other heads aluminum, 2.02 intake and about 20cc more intake volume. What was the flow difference? Do you have velocity data on the two heads?

I noted peak tq figure was about the same 520ft/lb between the first post original engine and this second engine but the first used MS109.

[steve cowan]

The port architecture between the two heads are very different even though they are both Dart heads.
Cast heads are a Wissota spec head that I originally bought to learn on,$300 AU each head bare delivered from Summit.
The cast iron head has a better top cut to chamber transition and taller SSR so the turn is not abrupt like the alloy 180cc SHP heads (early casting)