small displacement V8

[Guest shortyz]

Originally posted by pparaska:

quote:

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Originally posted by shortyz:

im trying to think what i can do to improve the perfomance of my motor. im buying a MSD ignition next. and im stumped on what to do. ive been seeing threads about aluminum heads? how many pound do you loose and how much hp is gained with a resonably priced alum head. (i got camel hump heads)

Depending on your build, what's been done to the camel humps, displacement, you could gain from 30-70 (or more?) HP with good AL heads. AFR, Pro Top Line, Canfield, TFS, etc. It's not just for weight savings! And you can usually add a 0.5:1 or even 1.0:1 to your compression ratio as well if you use AL heads.

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ok cool. i will look them up. i dont need any higher compression im running 94 octane already.

ppraska will my camel humpies kick the can after 5000RPM. cause im not looking for that

thanks again!

[pparaska]

One thing I've found while playing with Desktop Dyno 2000 (and no, I don't believe everything the computer tells me) is that better flowing heads that flow better will raise the hp and the rpm at which it peaks. This is true if everything else in the engine is the same, including the cam.

[Doug71zt]

Just to follow up on an earlier post - How much is an original LT-1 worth these days? I just took one out of my parents's neigbours '72 3500 GMC. It was taken out of a Camaro in the late 70's and installed in that truck by my uncle. I've always kept an eye on it and it was retired last year. The engine was a freebee for getting rid of the truck. Do I have a goldmine or what?

 

Thanks - Doug

[Mike C]

There was one on eBay last night ~$400 and reserve not met. I would consider a good rebuildable one worth about $750 with original big valve heads and the steel crank.

It is my opinion that unless you just miss the boat with the port volume of your heads (putting a set of 230cc heads on a 305ci motor), the power curve should remain almost identical to what it was with a poorer flowing set of heads but output should be higher everywhere with more overrev on top. In all of my experiences, low and mid-range torque were increased with larger valves and better flowing heads. Now remember, this is with a moderate performance cam, meaning over 210 degrees at .050 and not a truck cam where peak power is 4000 rpm. This was confirmed in the CHP cylinder head tests when the 215 Dart Pro Actions had the second highest torque production below 4000rpm, only about 2-3 ft lbs off of the 170cc Darts. The myth that square port BBC heads had poor low end throttle response was squashed by a mag that swapped to a little cam. There conclusion was that it wasn't the heads that were responsible for the lousy low speed performance but the fact that all of the square port head motors came with MONSTER camshafts. Duh, I say.

[pparaska]

Duh is right. You'd think these "performance" mag writers would know more.

 

As a knowledgeable engine builder said to me lately: "heads for power, tune with the cam".

 

This guy is saying to me what a few others are saying, and it's a bit the opposite of the "standard" belief is:

"Go with the best flowing heads you can, and don't worry too much about the port volume. Tune the power band with the cam. The port volume just adds to the intake runner volume."

 

I'd love to hear more on this - maybe a separate topic?

[Guest Anonymous]

Spot on Pete... the port (everything before the intake valve) is effectively a part of the intake runner, and should be considered as such for the sake of calculations. So an extra 30cc in port volume isn't going to make THAT much of a difference, considering it's a small fraction of the overall intake runner volume (around 900ccs for an EFI intake, and 600cc for a a carb) What you DON'T want is the air velocity dropping too much too quickly (think rate of change, as well as magnitude),causing port wetting and all sorts of problems. Thing is, heads that come with bigger port volumes usually come with bigger valves, which necessarily reduce port velocity and low end torque... that's something to consider...

[DavyZ]

Originally posted by Dan Baldwin:

Misconception #1 Shorter stroke engines rev up quicker, but longer stroke engines have more torque.

 

Given engines of similar displacement, cam, CR, etc, they will rev up about the same speed under a given load, and will have similar peak torque (largely a function of displacement and CR). The benefits of having a shorter stroke FOR A FIXED DISPLACEMENT is that you can rev to a higher rpm limit for the same peak piston acceleration, AND you would have the opportunity to have bigger valves, which would improve high-rpm breathing. This does NOT mean you can get more power by destroking an engine. You will lose more than you gain, since you've only increased your redline by the ratio of the square roots of the two strokes, while you've lost the straight ratio (i.e., redline may go up 5%, but you've lost 10% displacement, so potential power is reduced by about 5%). Unless you're in a displacement-limited class, you want as much displacement as your engine configuration will allow. Assuming you want to be as fast as possible. And, believe it or not, a longer-throw crank WILL rev up quicker under a given load than a shorter-throw crank. Greater leverage. Displacement increase through longer stroke or greater bore both have the same net result: more torque to spin the crank more quickly.

Thanks for the information, Dan. This point is what I was trying to make in my previous posts, but I just could not verbalize it worth a darn. This totally makes sense to me and that is what I believe. Now throw reciprocating weight differences into the mix and let's party!

 

Davy

[Guest Anonymous]

Dan B.. No one is disputing your short stroke/long stroke theory. Some people like the color blue while others like red..and some like both or all in between.... A choice between a 302 or a 383, the 302 would win with me becuz it has piszaz! But I would not go out of my way to obtain a 302 SBC. LOL! A lot of L6 thread questions going un-answered.

[Dan Baldwin]

Damn, I KNEW I shouldn't have clicked here. I see the same misconceptions here that have run rampant in L6 land, and have been around for as long as I've been alive.

 

Misconception #1

Shorter stroke engines rev up quicker, but longer stroke engines have more torque.

 

Given engines of similar displacement, cam, CR, etc, they will rev up about the same speed under a given load, and will have similar peak torque (largely a function of displacement and CR). The benefits of having a shorter stroke FOR A FIXED DISPLACEMENT is that you can rev to a higher rpm limit for the same peak piston acceleration, AND you would have the opportunity to have bigger valves, which would improve high-rpm breathing. This does NOT mean you can get more power by destroking an engine. You will lose more than you gain, since you've only increased your redline by the ratio of the square roots of the two strokes, while you've lost the straight ratio (i.e., redline may go up 5%, but you've lost 10% displacement, so potential power is reduced by about 5%). Unless you're in a displacement-limited class, you want as much displacement as your engine configuration will allow. Assuming you want to be as fast as possible. And, believe it or not, a longer-throw crank WILL rev up quicker under a given load than a shorter-throw crank. Greater leverage. Displacement increase through longer stroke or greater bore both have the same net result: more torque to spin the crank more quickly.

 

Misconception #2

V8s are torquey low rev engines, 6s are revvier, and 4s are even revvier and have no torque.

This one must be due to the fact that most 4s are less than half the size of V8s, and to make acceptable power must be tuned for higher rpm operation.

The fact is that for a given displacement, more cylinders = more rev potential, allowing for more peak power. Of course tuning for high rpm torque (maximizing power) typically reduces low-rpm torque. So for a given displacement, and assuming tuning for peak power, 4s would be the low-rpm torque monsters, 6s would be less low-end torquey and more high-end revvy, and V8s would be even more revvy and less low-end torquey. The F1 guys have found that the optimum setup for their application is a V10. Lots of revs for peak power. Adding cylinders beyond 10 apparently introduces enough additional friction from greater total cylinder circumference (less additional power, more fuel consumption) to offset the additional rev potential.

 

Stuff to think about, anyway.

[Mudge]

...and with all this in mind what would I most want to have? A stroker with a nice fat cam, I'd get some torque back and I'd still have my beautiful top end!

 

Leave the short stroke stuff for real race cars in limited size classes!

[Michael]

At this point we can observe that the remaining justification for suggesting that long-stroke engines have inferior rev performance, is that subject to the ubiquitous "all else being equal", the long-stroke setup will have a higher moment of inertia, implying that it would take longer to spool up. However, there is a straightforward (but expensive) remedy: use lightened components; a lighter flywheel, lightened crank, compact harmonic damper. Considering the costs of getting back the horsepower lost when losing displacement to destroking, this is a reasonable option.

 

When I first wrote this, I guessed that in a light car with a large engine, it is not inconceivable that in first gear the torque needed to spool up the engine is a large fraction of the torque to accelerate the mass of the car (with a given tire radius). A short calculation showed that this is completely false! Example (in S.I. units): 1000 kg car, 0.3m tire radius, accelerating in first gear at 8 m/s^2. Torque at the driveaxle is 2400 Nm. Assume that we begin at 1000 rpm and end at 5000 rpm, and that the ratio of crank rpm to axle rpm is 10:1, and assume constant acceleration (I know, that's a crude assumption, but it makes the math simple). Then the engine has to spool up from 1000 rpm to 5000 rpm in 1.57 seconds, giving an angular acceleration of 267 rad/s^2. Model the crank by two coaxial cylinders: one with 1.25" radius (the "main journals") weighing 40 lbs, and the other with 3" radius (the counterweights), also weighing 40 lbs. Converting to S.I. units and using the formula: moment of inertia = 0.5*mass*r^2 for a cylinder, the crank's moment of inertia is 0.062 kg*m^2. Also, since torque = moment of inertia times angular acceleration, the torque expended at the crank to spool up the crank itself is about 16 Nm, or 160 Nm at the driveaxle. This is a big crank in a light car accelerating rapidly. Even so, the crank spool-up absorbs <7% of the torque! Including the flywheel, the clutch and the harmonic damper of course raises this ratio, say by a factor of 2. Which perhaps explains why race cars tend to feature small-diameter multiple-plate clutches - another option to consider for a "rev-challenged" long-stroke engine.

 

Regarding port sizing - Jim McFarland had some articles a few years ago, discussing "optimum" velocity of the intake charge. According to him, the mean speed of the flow before reaching the valve curtain area should be around 240 ft/s. Given an intake runner volume and geometry, we get a notional cross-sectional area of the intake port. McFarland pointed out the importance of comparing port cross-sectional area instead of volume. Big-block intake ports have much large volume than small-block intake ports, in part because of large area, but also because the runners are longer.

 

From the cylinder displacement and a guess for volumetric efficiency – at a given rpm, we get a figure for the volume of requisite intake charge per intake stroke. From the cam profile one can back out a time interval during which a fictitious mean intake velocity would fill the aforementioned volume - again, at a given rpm - it's not the same as camshaft duration or a simple integral. It does not appear to be a simple calculation. Anyway, ideally, the port cross-sectional area times 240 ft/s, should equal cylinder volume times volumetric efficiency divided by the time interval. Deciding upon the rpm at which we want the best match would then size the port cross sectional area. All that remains is to slog through the calculations.

[Moridin]

Man, you guys know quite a bit. Some amazing stuff is be put on this discussion board. Anyways, please let me now if I'm wrong, but didn't the F1 guys used to use turbo 4 cylinder engines making huge amounts of power. I thought they reved up quick and made mucho power. Could it be that with the introduction of higher strength metals that they went to more and more cylinders? Easier to make more power? Thanks for all the information you guys are pouring out onto the pages here. Thanks again.

[KiD-ViD]

they barred forced induction if I remember correctly. without fi more cylinders was a logical next step I think.

[Moridin]

I remember reading some news threads for F1. They were saying something about a new trick or material was being banned every year. Anyways, thanks for the info.

Oh, that's why I love CART. One of the few left using turbos. Anyways, thanks for the info.

[Drax240z]

The change in F1 happened when forced induction was outlawed. The fuel they were running was quite the witches brew, and I believe that those additives used (toluene, xylene, sp?) to produce the big power numbers were outlawed around the same time.

[KiD-ViD]

too bad CART is phasing out turbos too... they already lowered the amount of boost they are allowed to run. oh well we will see what happens.

[Dan Baldwin]

Originally posted by Michael:

the long-stroke setup will have a higher moment of inertia, implying that it would take longer to spool up. However, there is a straightforward (but expensive) remedy: use lightened components; a lighter flywheel, lightened crank, compact harmonic damper.

The longer stroke crank (allelsebeingequal) WILL have a higher polar moment of inertia. BUT, this is WAY MORE than offset by the greater LEVERAGE offered by the increased stroke. So under a given load, the longer-stroke engine WILL rev up more quickly. The rotational inertia of the crank is all but negligible, anyway, as you pointed out. The ONLY rev issue regarding stroker engines is the slightly reduced redline (assuming equivalent-strength pistons). But this is also MORE than offset by the greater displacement.

 

Anyway, surely by now we can put to bed the myth that shorter stroke engines rev up more quickly. I hardly consider my street L31 to be "rev-challenged".

[Moridin]

I thought CART had put some sort of freeze on the engines for a year or something of the sort. Originally they were going to go to NA V8s that were compatible with IRL, so they could easily/cheaply interchange between the two. I think its a shame that CART has been so poorly run in the last few years that it has to resort to dealing with the IRL. I'm not a big fan of oval racing. Anyways, sorry for ranting, but turbos rule.

[Guest Anonymous]

It all boils down to rotating mass. if bores and strokes are bigger and longer respectively, that means more material is needed for bigger pistons and bigger rods, therefore increasing weight, friction, and stress. this is why superbikes can rev to 15 grand.the engines are tiny. i know guys with 289's that have forged internals, who can rev to 10,000 rpm. if you can supercharge a 289, you get the best of both worlds, high hp and high redline. thats my 2 cents

[Guest Anonymous]

i know i just said what everybody said, but i like things in lehmans terms. speaking scientifically gives me headaches.