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Pushrod V8 reliability and fuel consumption


karman1970
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Sold our old car the other day.  Life unfortunately dictates that our next build will probably be postponed a while, though no too long I hope.  Never stops one from bench-racing though.  So that said, I've been thinking about engine options.  Brought up two separate but related questions I haven't been able to figure out.  Hoping some of you more experienced guys might have some input.

 

1) I have read both here and the Lemons forum that American pushrod V8s (especially the SBC) don't do well in endurance racing unless you keep the revs low.  I've seen 5000 max mentioned quite a bit for both Fords and Chevys.  One Lemons team said they had to move down to 4000 max to keep their SBC alive.  Why is this?  From what I've read, the circle track guys don't have a problem keeping their stuff alive, and I've sure they rev way higher.  Other than a windage tray, good oil pan and pump, and making sure the oil drains back to the pan without restriction, I haven't seen any special stuff they do or run to keep them alive.  Definitely nothing that couldn't be done for little to no points in Champ or Lemons.   What is it about the pushrod V8s, and especially the Chevys, that cause them to be so fragile in endurance racing?  Do GM's 4.3, 3800, and 60-degree V6 suffer the same issues?

 

2) Been struggling to find a way to word this clearly and succinctly, and I'm pretty sure this fails both.  Here goes anyway... Platform in question is limited to around 190-200hp for swapping.  If you had a bunch of engines of different displacements and design but the same max horsepower, which one would theoretically have the lowest fuel consumption across the usable powerband?  Opposite ends of the spectrum are: multi-valve 4 or 6 cylinder making 200hp vs smog-era 400ci+ big-block from a truck also making 200hp.  All other things equal, which one theoretically moves a 2700lb tunaslapper around a track faster?  Is one option enough more fuel efficient to make a difference?  My gut says for a given max power, the larger engine will probably turn a faster lap, but it will also burn more fuel.  I'm also assuming the fuel burn is somewhat proportional to displacement and peak torque output?  Thus, it's kind of a toss-up in an endurance situation - go faster and pit more often, or slower and fewer stops?  Or is there a happy medium, and things go bad on the extremes (very small or very large displacements but all the same max power)?

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The "SBC" as it were, is old.  Not just the technology, but the physical age.

 

The roundy round guys turn 6K on "stock" built 305s all day long, sometimes for many seasons.

 

The issue with your typical lemons sbc is that its a wrecker engine that's 30+ years old, and has done 100K+ miles.  They're worn out, or sludged up, or more often both.

 

With a decent windage tray, pan, an uprated oil pump, and a properly spec'd out stock rebuild there's no reason the bottom end won't survive.  This only leaves valvetrain issues.  The cheap chinesium rockers are all ticking time bombs whether they're stamped steal or alloy rollers. 

 

Pretty much anything with an SBC should have room to take points for a set of high quality roller rockers.  Convert to bigger studs, and I'd bet you could spin 6k for as long as you can throw fuel at it.  It just won't make much of it unless you replace/port the heads and get a decent intake on it.

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1) Ditto to what was said above. Only other item to add, those vintage motors often have terrible oil control, lots of room to bunker oil away from the pan, and sometimes suffer from poor geometry stability (big heavy parts with poor stiffness allowing round things to end up less round). Good rebuilds, machining, bearings and oil control exist for these motors if you look. 

 

2) You are asking about BSFC for different motors. This is the fuel required to make a horsepower. For well tuned NA motors at wide open this doesn't vary much. Slow motors with good combustion (usually shown by needing little timing advance) and low friction shine here. 

 

What does stand out, if you end up 200 lbs heavier for a motor with equal power your fuel to weight will suffer at the same power and BSFC. The weight will require more power\throttle on time to reacellerate and your burn rate will go up. 

 

Bigger motors will have better mid-range power for the same peak. Will go faster. Will burn more fuel if you use it, even if BSFC is same the fact you have more power down low will burn more fuel. If you drive the bigger motor soft footed and make the same power vs rpm as the small motor (limit throttle down low) and it has same BSFC, you will burn the same amount (assuming car weight is the same). 

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5 hours ago, Black Magic said:

If you drive the bigger motor soft footed and make the same power vs rpm as the small motor (limit throttle down low) and it has same BSFC, you will burn the same amount (assuming car weight is the same). 

This part is incorrect. Typical car engines make peak (or nearly peak) BSFC when at full throttle. It has a lot to do with “pumping losses” and “friction losses. Basically, floor it always. A small engine pinned at 4000 rpm will have a lower (better) BSFC than a large engine soft footed at 4000 rpm making the same power. 

 

The bold part is the invalid assumption.

Edited by enginerd
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2 hours ago, Black Magic said:

 

2) You are asking about BSFC for different motors. This is the fuel required to make a horsepower. For well tuned NA motors at wide open this doesn't vary much. Slow motors with good combustion (usually shown by needing little timing advance) and low friction shine here. 

 

What does stand out, if you end up 200 lbs heavier for a motor with equal power your fuel to weight will suffer at the same power and BSFC. The weight will require more power\throttle on time to reacellerate and your burn rate will go up. 

 

Bigger motors will have better mid-range power for the same peak. Will go faster. Will burn more fuel if you use it, even if BSFC is same the fact you have more power down low will burn more fuel. If you drive the bigger motor soft footed and make the same power vs rpm as the small motor (limit throttle down low) and it has same BSFC, you will burn the same amount (assuming car weight is the same). 

 

Yes, obviously weight would be a factor that has to be considered, but I was trying to ignore that for the moment.  I know stuff can get wonky at part throttle.  So if we assume the engine is tuned well, then the WOT fuel consumption of pretty much any engine would roughly follow the shape of the power curve?  (I know BSFC is not a constant and changes with load and RPM)

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This was my power goal as well.

 

I went gm 60 degree v6.  There is a manual trans that bolts right up for fairly cheap (wc t5 for around 250$).  The engines themselves are cheap (200 to 500$) .  

 

Never heard of an issue with one that was maintained (replace intake gaskets on older ones).

 

You can get speed parts for them (cams, port and polish, etc).

 

Easy to service (pushrod engine).

 

Available from around 130 hp up to 260 hp.

 

We burn just about 8 gph with a 201 hp engine.

 

We used to run sbc back in the day.  Did all of the mods.

 

It seemed they would assplode in new and different ways almost every race.

 

Pushrod through the hood....  spun bearing....  massive oil leak...  part of the cam broke off...

 

We had fresh rebuilds, a gm crate engine, you name it.  We just couldn't keep them together for some reason.

 

Kept to 5500 rpm max.

 

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1 hour ago, enginerd said:

This part is incorrect. Typical car engines make peak (or nearly peak) BSFC when at full throttle. It has a lot to do with “pumping losses” and “friction losses. Basically, floor it always. A small engine pinned at 4000 rpm will have a higher lower BSFC than a large engine soft footed at 4000 rpm making the same power. 

FIFY.

 

Lower BSFC means it's using less fuel per hp/hr so more efficient. I know that's what you meant.

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4 hours ago, wvumtnbkr said:

This was my power goal as well.

 

I went gm 60 degree v6.  There is a manual trans that bolts right up for fairly cheap (wc t5 for around 250$).  The engines themselves are cheap (200 to 500$) .  

 

Never heard of an issue with one that was maintained (replace intake gaskets on older ones).

 

You can get speed parts for them (cams, port and polish, etc).

 

Easy to service (pushrod engine).

 

Available from around 130 hp up to 260 hp.

 

We burn just about 8 gph with a 201 hp engine.

 

We used to run sbc back in the day.  Did all of the mods.

 

It seemed they would assplode in new and different ways almost every race.

 

Pushrod through the hood....  spun bearing....  massive oil leak...  part of the cam broke off...

 

We had fresh rebuilds, a gm crate engine, you name it.  We just couldn't keep them together for some reason.

 

Kept to 5500 rpm max.

 

 

 Curious,what engine did you pick? Researched a swap a while ago but went another route.

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On 10/11/2020 at 10:20 PM, karman1970 said:

Sold our old car the other day.  Life unfortunately dictates that our next build will probably be postponed a while, though no too long I hope.  Never stops one from bench-racing though.  So that said, I've been thinking about engine options.  Brought up two separate but related questions I haven't been able to figure out.  Hoping some of you more experienced guys might have some input.

 

1) I have read both here and the Lemons forum that American pushrod V8s (especially the SBC) don't do well in endurance racing unless you keep the revs low.  I've seen 5000 max mentioned quite a bit for both Fords and Chevys.  One Lemons team said they had to move down to 4000 max to keep their SBC alive.  Why is this?  From what I've read, the circle track guys don't have a problem keeping their stuff alive, and I've sure they rev way higher.  Other than a windage tray, good oil pan and pump, and making sure the oil drains back to the pan without restriction, I haven't seen any special stuff they do or run to keep them alive.  Definitely nothing that couldn't be done for little to no points in Champ or Lemons.   What is it about the pushrod V8s, and especially the Chevys, that cause them to be so fragile in endurance racing?  Do GM's 4.3, 3800, and 60-degree V6 suffer the same issues?

 

2) Been struggling to find a way to word this clearly and succinctly, and I'm pretty sure this fails both.  Here goes anyway... Platform in question is limited to around 190-200hp for swapping.  If you had a bunch of engines of different displacements and design but the same max horsepower, which one would theoretically have the lowest fuel consumption across the usable powerband?  Opposite ends of the spectrum are: multi-valve 4 or 6 cylinder making 200hp vs smog-era 400ci+ big-block from a truck also making 200hp.  All other things equal, which one theoretically moves a 2700lb tunaslapper around a track faster?  Is one option enough more fuel efficient to make a difference?  My gut says for a given max power, the larger engine will probably turn a faster lap, but it will also burn more fuel.  I'm also assuming the fuel burn is somewhat proportional to displacement and peak torque output?  Thus, it's kind of a toss-up in an endurance situation - go faster and pit more often, or slower and fewer stops?  Or is there a happy medium, and things go bad on the extremes (very small or very large displacements but all the same max power)?

??? we ran the original L69 305 in our 84 T/A (149k when we got it) for at least 4 weekends plus a few track days. It was pretty much stock except cam, headers. After a hard day at NJMP on Saturday we went to fire it on Sunday and heard that noise. the 2nd 305 (88 WS6 car), we actually pulled the pan, replaced the bearings, rings, ported the heads, added a larger roller cam, ported the intake and headers. That engine is still running strong with no issues. It has a 3 or 4 weekends 2 practice days and some street pounding on it.  We run a larger oil filter but no cooler and always run full syntho oil.  I'm sure we are north of 300hp at this point. This is from NJMP Thunderbolt. www.youtube.com/watch?v=GpZHzZYXhJQ 

 

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19 hours ago, enginerd said:

This part is incorrect. Typical car engines make peak (or nearly peak) BSFC when at full throttle. It has a lot to do with “pumping losses” and “friction losses. Basically, floor it always. A small engine pinned at 4000 rpm will have a lower (better) BSFC than a large engine soft footed at 4000 rpm making the same power. 

 

The bold part is the invalid assumption.

 

I was thinking within practical limits, say 20% torque reduction on the large engine that is 20% bigger displacement than the smaller engine. Agreed that if you are talking about a 454 chevy at idle position vs a go kart engine at wide open the bsfc is going to be very far apart. 

 

Look at the example bsfc chart below, and imagine where the rpm on track is going to be Say 4000 and above. The map is pretty constant at the high rpm in the upper 1/4 of the torque range. Add to that convo that the 20% bigger engine will most likely need 20% less engine speed to make the same power (lower operational rpms), potentially have a less aggessive cam and may be able to run a leaner mixture (lower cylinder pressure). Very likely you could operate the bigger engine at 20% less torque at the min rpm and taper the throttle to match the power curve of the small engine (from 20% to 0 torque reduction as you go 4000 rpm to redline) and get very similar fuel burn. 

 

To the original poster, the reality is you would be carrying the weight of a larger engine and asking your guys to not use all of it's potential. Your better bet would most likely be to short shift the bigger motor to get the same wide open average power across rpm (again assuming same bsfc) and then have the ability to open her up at odd hour tracks. 

cOgSj.jpg

Edited by Black Magic
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14 minutes ago, Black Magic said:

 

I was thinking within practical limits, say 20% torque reduction on the large engine that is 20% bigger displacement than the smaller engine. Agreed that if you are talking about a 454 chevy at idle position vs a go kart engine at wide open the bsfc is going to be very far apart. 

 

Look at the example bsfc chart below, and imagine where the rpm on track is going to be Say 4000 and above. The map is pretty constant at the high rpm in the upper 1/4 of the torque range. Add to that convo that the 20% bigger engine will most likely need 20% less engine speed to make the same power (lower operational rpms), potentially have a less aggessive cam and may be able to run a leaner mixture (lower cylinder pressure). Very likely you could operate the bigger engine at 20% less torque at the min rpm and taper the throttle to match the power curve of the small engine (from 20% to 0 torque reduction as you go 4000 rpm to redline) and get very similar fuel burn. 

 

To the original poster, the reality is you would be carrying the weight of a larger engine and asking your guys to not use all of it's potential. Your better bet would most likely be to short shift the bigger motor to get the same wide open average power across rpm (again assuming same bsfc) and then have the ability to open her up at odd hour tracks. 

cOgSj.jpg

Gotcha, that makes sense. And thanks for providing a graph.

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10 minutes ago, enginerd said:

Gotcha, that makes sense. And thanks for providing a graph.

 

I forgot to add, your racer mind is spot on with lower power setting bsfc....the Nascar guys under caution save fuel by coasting engine off, bump starting the engine and running it at high load near torque peak for a few secs, then turning it off.  This saves a reasonable amount more than part throttle operation. 

 

 

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2 hours ago, Black Magic said:

 

I forgot to add, your racer mind is spot on with lower power setting bsfc....the Nascar guys under caution save fuel by coasting engine off, bump starting the engine and running it at high load near torque peak for a few secs, then turning it off.  This saves a reasonable amount more than part throttle operation. 

 

 

So a decent fuel saving mode would be:

 

- Coast a little before braking

- Short shift (to be inside peak torque, 5000-6000 instead of 6000-7000 rpm on my car)
 

If duty cycle and PW was monitored, how could you tell that it was running more efficient? Or can you?

 

edit: it's fuel saving, not power saving (we are not building ASICs here...)

Edited by turbogrill
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If you think about where you are full throttle around a lap and compare that to where you aren't gaining much lap time, coast at those spots.

 

In other words, the end of the straight is where you gain the least amount of time.  You also are using a lot of fuel.  Lift a couple seconds early and coast into the corner.

 

This will save a good amount of fuel if you are diligent any chance you get to do this.

 

Also, short shifting does the same thing with potentially more fuel savings but at the cost of greater lap times.

 

I have found that my pace doesn't really fall off when I coast at the end of the straight, because I give myself more time to brake (easier on brakes) and more time to set the balance of the car up for corner entry.  Therefore, I typically see a few tenths lost on the straight, but also make that up in the first corner.

 

I am not a pro, obviously.  A pro would already be maximizing the braking zone and corner entry no matter what.

 

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53 minutes ago, turbogrill said:

 

 

Hmm...why would this be the case? You are going fastest so you cover more distance?

 

You are accelerating much more slowly.  Depending on the vehicle and straight in question, lifting a few seconds before the braking point may only cost you a couple MPH.

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2 hours ago, turbogrill said:

So a decent fuel saving mode would be:

 

- Coast a little before braking

- Short shift (to be inside peak torque, 5000-6000 instead of 6000-7000 rpm on my car)
 

If duty cycle and PW was monitored, how could you tell that it was running more efficient? Or can you?

 

edit: it's fuel saving, not power saving (we are not building ASICs here...)

saving fuel is over-rated. Build it and pass people at Daytona by 20+mph. 

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45 minutes ago, karman1970 said:

 

You are accelerating much more slowly.  Depending on the vehicle and straight in question, lifting a few seconds before the braking point may only cost you a couple MPH.

 

aha, ofcourse.

 

24 minutes ago, TiredBirds said:

saving fuel is over-rated. Build it and pass people at Daytona by 20+mph. 

 

Good point. DID I HEAR TURBO???????????????????????????????????

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15 hours ago, turbogrill said:

 

aha, ofcourse.

 

 

Good point. DID I HEAR TURBO???????????????????????????????????

we have about 100 points to burn.... humm. We get about 1.5 hours out of a tank of fuel, but these F-body's have a tiny 15.9 gallon tank. With a turbo it'd be about an hour... a fun hour thou. 

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On 10/13/2020 at 7:37 AM, TiredBirds said:

??? we ran the original L69 305 in our 84 T/A (149k when we got it) for at least 4 weekends plus a few track days. It was pretty much stock except cam, headers. After a hard day at NJMP on Saturday we went to fire it on Sunday and heard that noise. the 2nd 305 (88 WS6 car), we actually pulled the pan, replaced the bearings, rings, ported the heads, added a larger roller cam, ported the intake and headers. That engine is still running strong with no issues. It has a 3 or 4 weekends 2 practice days and some street pounding on it.  We run a larger oil filter but no cooler and always run full syntho oil.  I'm sure we are north of 300hp at this point. This is from NJMP Thunderbolt. www.youtube.com/watch?v=GpZHzZYXhJQ 

 

What was the failure on the first engine?  One of the many engines I have considered is the 4.3 V6, but I'm worried all the bad things I've heard about SBCs are going to afflict it as well. 

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