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Old 05-07-2022, 12:30 PM   #1 (permalink)
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'98 Trans Am 40mpg

Hey all,

I bought a 1998 Trans Am with 4 speed auto for cylinder head development, and I'm doing an mpg project in the interim. The key points are decreased pumping losses, increased thermal efficiency, and lean burn.

I'm spring boarding off of Legit Street Cars' 40mpg Corvette. While his engine was basically stock, I'm taking it much further.


The Corvette has lower CdA and 6 speed manual (12% lower final drive), which is certainly an advantage over the TA. fueleconomy.gov actually has the auto TA getting 15/23 vs the 6 sp Corvette at 16/25 (his stock car got 30 mpg @ 65mph). I predict the mods will be worth 30+ mpg highway running stoich, 40+ with lean burn.

To reduce pumping losses I'm looking at a 212* cam on a 118* LSA. This will reduce pumping greatly, while keeping vacuum high for good fuel vapourisation. Overlap leads to low vacuum, which causes misfire, which just dumps fuel out of the tail pipe and kills fuel economy. It should also work fine with 87 octane, which is 20% lower fuel costs vs 91.

I have 1-3/4" headers and will get a free-flowing cat-back exhaust.

I'm using 706 truck heads to bump compression to around 11.25:1, which is a 2.7% increased Brake Thermal Efficiency over 10.1:1. The heads have been ported to increase air speed. This improves mixing and atomisation for a more complete and faster burn. I should be able to remove several degrees of timing for less negative work. The preliminary engine sim is showing around 470 bhp (with no knock) and low .400 BSFC.

Alex talks about lean burn and how it raises NOx emissions. Sort of. If you run 1.1 lambda, like he does, this is true. Most charts end there, but if you keep going leaner NOx (and temps) plummet. I would like to go much leaner (24:1). A key enabler is a strong ignition system capable of 140+ mJ. Stock coils only provide around 80 mJ and ICE Ignition LS1 coils with their 10 Amp booster will generate 142. This will come later.

I should have the car up and running mid-June.

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Old 05-07-2022, 05:14 PM   #2 (permalink)
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I wish you the best of luck.

Pumping losses are directly attributed to high vacuum. Which is why diesels have low pumping losses.

Most high efficiency engines/hybrids use a cam profile or cam phasing to achieve late intake valve closing. This gives lower dynamic compression ratio but increases the expansion ratio.
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Old 05-16-2022, 10:52 PM   #3 (permalink)
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Quote:
Originally Posted by ksa8907 View Post
I wish you the best of luck.

Pumping losses are directly attributed to high vacuum. Which is why diesels have low pumping losses.

Most high efficiency engines/hybrids use a cam profile or cam phasing to achieve late intake valve closing. This gives lower dynamic compression ratio but increases the expansion ratio.
Diesels have "low vacuum" because they use turbos to fill the vacuum. By their nature they must open the valves late so that they don't hit the pistons. If they were naturally aspirated they would create lots of vacuum. The turbo is there to help fill the cylinders because of their limited lift duration.

if what you're saying were correct, then I should use a lopey cam and expect better mpg. the truth of the matter is engines need vacuum to run properly. If the intake valve opens too early you get lots of misfire. Vacuum creates the air speed that entrains, atomizes and emulsifies the air/fuel mixture, which creates a faster burn. A Toyota engineer gave me the Prius cam specs (which was at 40% BTE at the time). They phase the cam so that it closes at 100* ABDC, which means it opens the valve much later than typical...which means lots of vacuum. They give up a little pumping loss for better combustion and a much lower compression ratio (negative work).

I'm replacing broken sensors and fixing air leaks atm. My lean burn baseline should be around 40mpg...it's after that that I may need a little luck
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Old 05-17-2022, 12:11 AM   #4 (permalink)
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Quote:
The heads have been ported to increase air speed. This improves mixing and atomisation for a more complete and faster burn.
What do you think about the work described in the X-Prize Sonata thread? ecomodder.com/forum/showthread.php/x-prize-sonata-40234.html#post668238
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Old 05-17-2022, 01:26 AM   #5 (permalink)
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There are naturally aspirated diesels and they too have very low manifold vacuum. With no turbo they just don't get boost under load.

Old Mercedes diesels actually have a pump to create vacuum for things like the brake booster (and even door locks)

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Old 05-17-2022, 01:42 AM   #6 (permalink)
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Quote:
Originally Posted by ERTW View Post
Diesels have "low vacuum" because they use turbos to fill the vacuum. By their nature they must open the valves late so that they don't hit the pistons. If they were naturally aspirated they would create lots of vacuum. The turbo is there to help fill the cylinders because of their limited lift duration.

if what you're saying were correct, then I should use a lopey cam and expect better mpg. the truth of the matter is engines need vacuum to run properly. If the intake valve opens too early you get lots of misfire. Vacuum creates the air speed that entrains, atomizes and emulsifies the air/fuel mixture, which creates a faster burn. A Toyota engineer gave me the Prius cam specs (which was at 40% BTE at the time). They phase the cam so that it closes at 100* ABDC, which means it opens the valve much later than typical...which means lots of vacuum. They give up a little pumping loss for better combustion and a much lower compression ratio (negative work).

I'm replacing broken sensors and fixing air leaks atm. My lean burn baseline should be around 40mpg...it's after that that I may need a little luck
I had a natural aspirated diesel and it ran no vacuum. Then when I turbocharged it, I ran the same cam and same timing because that's how the factory made the turbo version.
I have a 5.7L 1989 firebird with a t-56. One time I actually drove the speed limit it got right about 30mpg, but ain't nobody got time for that. If I could run lean burn I could easily eeek out another 10%. I installed an eprom with the early 1990s tune on it that caused the engine to favor a lean burn "under highway driving conditions", said to be good for about 1mpg, so a pretty mild lean burn.
My next mod is I found another t-56 locally and bought it. But its not just any t-56, it's the t-56 with the 0.5:1 topgear. My current t-56 has the more common 0.62:1 topgear.
It has plenty of power has no problem going over east coast mountains on I95, I81, I40, I84 in 6th gear so I figured might as well over drive those rear wheels a bit more.
The only time it lacks power in 6th is towing and the solution to that is called 5th gear.
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Old 05-18-2022, 07:19 AM   #7 (permalink)
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Quote:
Originally Posted by ERTW View Post
Diesels have "low vacuum" because they use turbos to fill the vacuum.
Diesels have low vacuum because they don't have a throttle, they control engine speed with fuel.

Quote:
Originally Posted by ERTW View Post
if what you're saying were correct, then I should use a lopey cam and expect better mpg. the truth of the matter is engines need vacuum to run properly. If the intake valve opens too early you get lots of misfire. Vacuum creates the air speed that entrains, atomizes and emulsifies the air/fuel mixture, which creates a faster burn. A Toyota engineer gave me the Prius cam specs (which was at 40% BTE at the time). They phase the cam so that it closes at 100* ABDC, which means it opens the valve much later than typical...which means lots of vacuum. They give up a little pumping loss for better combustion and a much lower compression ratio (negative work).
I'm going to challenge you on that. The prius uses late intake valve closing which decreases vacuum because it pumps the intake gases back into the manifold, does not increase vacuum.

Hybrids do not need to have high vacuum for idle quality because... hybrids don't idle.
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Old 05-18-2022, 10:49 AM   #8 (permalink)
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Our hybrid car idles.
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Old 05-18-2022, 12:36 PM   #9 (permalink)
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Our hybrid car idles.
So it burns gas and the only reason is because the key is on and the engine has to stay running?
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Old 05-18-2022, 12:53 PM   #10 (permalink)
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The engine runs when the hybrid battery charge gets low from running the A/C while parked, driving at very low speed on electric power or when the heat is on.
With the heat and A/C off the engine doesn't idle when parked very much.

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