mild 305 buildup, dyno test - Chevy Impala SS Forum
 
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post #1 of 7 (permalink) Old 03-20-2001, 11:44 AM
kdrolt
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I'm putting this one here instead of the engine section. It will be especially useful for 305 Caprice owners. It's a mild 305 sbc v8 buildup that is intended to be CARB- and emissions-legal.

A few notes: the engine was tested on an engine dyno, so it probably had no air cleaner, and absent the usual parasitic losses of an alternator and a cooling fan. So in a car, you might see 10-20 hp less at the flywheel (fw)... but that's ok because you'd still be getting 260-270 fwhp, and over 300 fwftlbs. Those numbers would exactly equal a 94-96 LT1 in a Caprice or Impala btw.

Also note that the cam used in the test has specs are very close to a F/Y 92-97 LT1 or LT4 (stock, non-HOT) cam, and a Quadrajet 4v carb was used.

Also note the comparo of

a) open headers (no cat, no muffler),

b) header + dual cat + low_restrict_mufflers,
and

c) stock LG4 type exhaust with iron manifold, single cat, single muffler.

The dyno test shows that the stock exh system chokes the hell out of the car, but only above around 2500 rpms. So for ordinary mundane driving (under 2500 rpm), the stock system was engineered correctly... and it was probably MUCH quieter (not that any of you would care <g> ).

What I would have also liked to see was the poor man's exh alternative: iron manifold, dual low-restriction cat, dual low-restrict-mufflers. I think that combo would have well.... but it would also show what can be done on a budget, and that might upset the aftermarket header vendors.

Here's the URL:
http://www.hioutput.com/tech/290hp/290hp.html

There are also 340 and 400 hp examples if you probe further:
http://www.hioutput.com/tech

but the 290 hp article is the most relevant for a street-driven 4000+ car like a Caprice, IMO.

Final note: they talked about that there are "no good" 305 heads, but also that the AFR 350 heads *can* be used. They NEVER said what heads they DID use in the test, so I have to assume that they used the AFRs and milled them to get the compression ratio they wanted. The cr appears to have been around 10.1:1, and that's fine for a 92-octane engine w/aluminum heads.

I further suspect the Vortecs would have done as well; the port volume is smaller on the Vortec than on the AFRs so that would have favored the mild cam, mild rpms, and been better suited for a 305... nevermind that the cast iron Vortecs would have retained the combustion heat better.

The AFRs on the other hand, would flow a bit better at high rpms but this is less of a factor on a 305 vs a 350. They also didn't use as high a comp ratio as they could have w/alum heads, so a AFR to Vortec head comparo (for the test data given) would be close to a wash. IMO.

This is a useful item for 305 owners, and it shows what a mild cam (LT4, $140), cheap GM head (Vortec), and exhaust swap *could* do provided you use a Vortec-matable manifold, and that

a) the ECM were trimmed to provide the fuel needed, and provided that

b) the TBI unit could match the fuel delivery of the Quadrajet used in the test described here. HTH. FYI. - Ken


[This message has been edited by kdrolt (edited March 20, 2001).]
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post #2 of 7 (permalink) Old 03-20-2001, 11:52 AM
kdrolt
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Re LO3 305 mods, read this one too, and note the links at the end of the page:
http://www.goingfaster.com/spo/modthel03.html

FYI. - Ken

post #3 of 7 (permalink) Old 03-28-2001, 12:04 AM
UltraZone
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that site is really helpful man, thanks for digging it up and posting. However, I did notice that the power for each engine configuration gives roughly the same power output at a given rpm. the only thing that changes is the final rpm, which obviously will yield more horsepower, but each different configuration give roughly the same torque curve.
 
post #4 of 7 (permalink) Old 03-28-2001, 11:02 AM
kdrolt
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<BLOCKQUOTE>quote:</font><HR>Originally posted by UltraZone:
that site is really helpful ... However, I did notice that the power for each engine configuration gives roughly the same power output at a given rpm. the only thing that changes is the final rpm, which obviously will yield more horsepower, but each different configuration give roughly the same torque curve.<HR></BLOCKQUOTE>

Yep. The engine and heads are either unchanged, or the changes are so slight, that the cam is the only thing altering the results. AT LEAST FOR THE WOT DYNO DATA LISTED.

Here's the stuff in-between-the-lines that EVERYONE (but the manufacturer) manages to forget, including ALL the auto magazines: all the dyno data are taken at wide open throttle (WOT). But what would the dyno curve look like a part throttle? And what would the dyno data look like at part throttle, and at WOT, from idle to 2500 rpm? (people hgeavily into drag racing should just move on to the next Forum and skip the following because it doesn't apply to you).

The answer in both cases is that engines with hotter cams will lose bigtime at part throttle, and at low engines speeds, or both.
If you've ever seen dyno data on a 350 with the GM LT4 HOT cam (218/228 d i/e), you see really great engine torque data from around 1700 to 6000 rpm... so both the torque and hp are awesome. It really is a great cam. But you DON'T see what the data look like at part throttle for that same engine & cam, i.e. for ordinary street driving. Said differently, the LT4 HOT cam WILL lose torque at lower engine speeds, and at part throttle.... and you will notice this more in a Bcar than you would in an Fcar.

What I just said above means even more for a 305 Bcar owner --- don't overcam the engine... or the part throttle performance will be lousy.

The 400 hp 305 article, for example, could be used on a Bcar, and the car would run some very nice track numbers if the car had a very loose converter (3500 rpm stall) and 4.56 in the rear end. But that same car would not be much fun for ordinary street driving. The difficulty is that nobody can know that, or see that, in the dyno data. Because almost ALL published dyno data are taken at WOT.

Said differently, you don't get to see the loss in low speed engine torque by looking at dyno charts taken at WOT. BUT GM does -- and they engineer the engine-cam-car together as a system to meet fuel economy, emissions, and overall performance.

You almost have to make the expensive mistake and learn from it, or benefit from someone else's experience making that mistake... by overcamming an engine in a heavy car.

The mild (and cheap) engine builds listed in the above links are probably quite good for a 305 in a Bcar, especially the 290 hp version. And I always advocate spending money only on parts for the 305 that could later be used on a 350. The best thing you can do for a 305 is replace it with a 350. But if that's not in the short term cards, then the next best thing is to use parts that COULD be used both on the 305 now, and later on a 350.

Food for thought. &lt;g&gt; - Ken
post #5 of 7 (permalink) Old 03-28-2001, 12:37 PM
SSmasher
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Ken,
For a guy like me with only a working knowledge of engine modifications, your 'article' was very enlightening. Thanks!
post #6 of 7 (permalink) Old 09-26-2001, 11:56 PM
UltraZone
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I just wanna bring this up again, because it's such great stuff.

I got a q for Ken tho:

If the 400HP configuration would lead to too little torque at less than WOT, lets say steady speed at highway driving, wouldn't the lower torque mean less horsepower, and therefore more fuel economy? In other words, from an engineering standpoint, wouldn't the engine be able to generate the amount of HP needed to keep the car moving at a steady pace in a more accurate way than an engine which generates more HP at part throttle? Inquiring minds wanna know.....
post #7 of 7 (permalink) Old 09-29-2001, 06:35 PM
kdrolt
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<BLOCKQUOTE>quote:</font><HR>Originally posted by UltraZone:
... If the 400HP configuration would lead to too little torque at less than WOT, lets say steady speed at highway driving, wouldn't the lower torque mean less horsepower, and therefore more fuel economy?<HR></BLOCKQUOTE>

No, it wouldn't give you more fuel economy... and sorry for the delay in responding to this.

What would happen is that at low engine rpms, and part throttle (i.e. steady cruise), the engine would have trouble maintaining speed (especially when loaded at part throttle, i.e. with lots of passengers and/or going uphill), BECAUSE the engine cylinders wouldn't fill very well. The engine would stumble all over itself.

Why?

Because part of the intake charge would get dumped right into the exhaust port (because the exhaust valve is still open when the intake is open) so you'd lose a lot of air:fuel right into the exhaust before it ever had a chance to get into the cylinder and get burned (and turned into torque and power). SO you'd lose fuel economy right there, AND you'd need to add more throttle to overcome it --- which would bite you in the backside fuel economy-wise again.

Go buy the latest issue of Hot Rod Magazine and read the article on the F*rd 302 buildup where they compared various bolt ons including a mild cam, and a very hot cam. The fact they they used the 302 as a guinea pig is irrelevant; what's important is that they were comparing cams and intakes on a single 5.0 platform, and then testing it on a dyno. This gives you a direct comparison of the effect of changing the cam, or the intake, or both -- in a comparison that's as fair as can be.

They then put the dyno numbers into a piece of software to estimate the track data. This is just physics, so it's a very sensible thing to do... and it's a lot fairer because there is no driver involved, nor changing weather conditions, nor effects of breakage.

The mild cam they used was a lot like the LT4 cam (not the LT4 HOT cam). It didn't put out the torque or power that the hotter cam engine did, but for a given set of street gears and torque converter, it was actually quicker in the quarter by a small amount, and it ran great under all conditions on the dyno. The hotter cammed test (on the same engine) ran great at WOT on the dyno but also ran LOUSY at part throttle and low speeds. Read the article and you'll see what I mean.

<BLOCKQUOTE>quote:</font><HR> In other words, from an engineering standpoint, wouldn't the engine be able to generate the amount of HP needed to keep the car moving at a steady pace in a more accurate way than an engine which generates more HP at part throttle? <HR></BLOCKQUOTE>

Unfortunately no. The large duration & overlap will make the engine make big dyno numbers, but at any part throttle case, and low speed (say under 2000 rpm) the engine is essentially on the verge of stalling because it has trouble enough running on it's own nevermind handling a load. That's the reason why hot-camed engines have such high idle speeds, because you can't get a motor like that to idle at low rpms and part throttle unless you use a massive flywheel (which no one ever does).

You can get away with a hot cammed big block because even at low speeds and part throttle, the engine still makes decent torque but not enough to prevent a good launch, and the big block has a lot more rotary mass so the energy stored in it is still enough to help it run even at low speeds. That's not true on a smaller engine, ESPECIALLY on tiny engines such as those in many imports. That's why variable valve timing (and variable induction) is so essential in making those small engines perform well at ALL speeds.

BTW, hot rodding a 305 isn't a bad idea. Sure, there are a number of reasons why it's better to do a 350 instead of a 305, but the fact is that many cars HAVE 305s and many of those 305 owners want more from them. Vortec heads will work quite well on a 305 (yes the smaller bore shrouds them a little, but you've also got heads designed to make a 350 breathe great and you are using them on a 305, so there isn't really any penalty), as will Fcar LT1 and LT4 cams, to the tune of approx 290-300 fwhp and 310+ ftlbs. And it will get better fuel economy than a 350 will, assuming a light foot.

FYI, HTH. - Ken
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