HP Rating

[Greg D.]

I was told that the fuel injected 327 although rated at 375hp really put out 400hp+ and the 396 rated at 425hp was really only 330hp. And that the 427 435hp was really 500hp which I've read and heard.

Anyone have the facts.

[Steve Antonucci]

Re: HP Rating

Greg,

This topic was covered ( in part ) a couple of weeks ago. I believe Clem
mentioned that the '65 396-425hp and the '66 427-425hp were fairly close to
each other in performance. I do know that the 396 was nowhere down near 330hp.
That would have been a joke back then.

Steve

[Greg D.]

Re: HP Rating

Thanks. I've had a hard time with accepting 330hp for the 396 425hp. Do you know about the 375hp 327? I'll also check out the previous discussion you mentioned.

[Verle R.]

Re: HP Rating

Greg,

A friend recently had his engine on an engine dyno and produced about 400 HP with stock cam and other components set up at optimum specifications (blue printed).

Verle

[Mark #28455]

installed vs dyno HP

Back then, the factory rated horsepower with the engine on a dyno, optimal air temp, no exhaust system, probably with dyno headers, etc. Starting in the early 1970's the new rating system more closely approximated the power you really got in an "as installed" engine.

Check the rear wheel HP on a truly stock 1960's Corvette and I guarantee it'll be nowhere near what the rated HP was!

Mark

[Duke W.]

Re: HP Rating

Most pre-1972 SBs were significantly overrated and the SAE gross standard used back in the sixties was not representative of the "as installed" configuration with the fan, acceccories, and vehicle exhaust system. The key to achieving good top end power is the heads.

HEADS! HEADS! HEADS! That's what it's all about. If you don't pocket port/port match the heads and do a multiangle valve job, you're leaving a lot of top end power on the table, even on a 300 HP engine.

FI 327s produced only marginally more power than the same long block with a carb/manifold because the OE machined heads are choked, but once head flow is improved FI has a decided advantage because it has a very efficient "tunnel ram" manifold.

Pocket porting/port matching the heads will allow the configuration to achieve the SAE gross ratings or better. For example, a L-79 with LT-1 cam and a true CR of about 10.5 will make about 360 HP gross @ 6500. With FI the power climbs to about 400, but low end torque is less. Once head flow efficiency is improved the FI's single plane tunnel ram manifold has a decided advantage in top end power. Without head work the FI manifold still looses low end torque while returning only a marginal improvement in top end power.

As built by Flint, a typical L-79 will make about 300 @ 5500, which is some distance from the advertised rating of 350 @ 5800, and the 300 HP engine is closer to an honest 250 @ 4500 gross, but it can achieve close to the SAE gross rating with head work and four degrees of cam retard.

A "blueprint stock" 327/375FI with a true 11.25:1 CR, massaged heads as above, 30-30 cam retarded four degrees, and good race headers/open sidepipes will make over 450 gross HP @ 7000. A racing exhaust system can exploit the 30-30 cam's high overlap, but with mufflers, even with headers, the 30-30 cam is a looser compared to the LT-1 cam, which has less overlap. Big overlap and exhaust backpressure are mortal enemies and just fight each other at the expense torque bandwidth.

Most of the above data are from Engine Analyzer models using actual head flow data, and the models were validated with actual SAE gross dyno data from the LT-1 cammed L-79. My estimate for the SAE net rating is about 320, and though it has not been tested on a chassis dyno, I estimate it will make 270 RWHP corrected to SAE air density.

Big blocks were more realistically rated, but they still need some head work to achieve the SAE gross ratings. A L-72 with head prep as above should make about 450 gross, but net as a percent of gross is less in the car because the exhaust system is restrictive - 6 psi backpressure versus about 3 psi for a prepped SHP SB. The Corvette OE exhaust system is pretty good for a SB, but is restrictive with the higher flow of a BB.

Whenever "horsepower" is discussed the full context of the measurment should be understood, but it rarely is, so like cam data, it's usually just a bunch of out-of-context numbers, which makes them essentially meaningless.

It's also apparent to me that a lot of guys don't understand head work. In the case of a street engine there is no need to make the port "bigger" or only marginally so. Pocket porting/port matching/multiangle valve seats do not significantly increase port volume. What it does is increase the FLOW COEFFICINET, which allows the heads to flow more at the same depression - about ten percent more on the inlet side and a whopping 40 percent more on the exhaust side. This enables more top end power with virtually no effect on low end torque.

Duke

[Steve Antonucci]

Re: HP Rating

Duke,

I'm going to ask a question of you regarding the HEADS! HEADS! HEADS!

On a BB, exactly what head work would you recommend to acheive optimal
power? Your 10% intake & 40% exhaust flow numbers got my attention.

Does this have, by chance, anything to do with the '074 (L-88 & ZL-1 ) head
exhaut port design ( round ) or the "D" exhaust ports of the later '177 heads?
Everybody talks about pocket porting, but I am not sure everybody does it well.
Is there a more definitive guid to pocket porting that can be followed? Just
calling a head shop and asking for pocket porting scares me....

BTW, did you ever think about writing a book?

Steve

[Mark #28455]

pocket porting

Pocket porting is just knocking off the sharp angles where the factory valve seat cutters meet the cast shape of the port. If you're not sure what you're doing it may be better to let the machine shop do it for you. You are not trying to alter the stock shape of the port or make it bigger.

The 077 head was a great improvement over the 074, but if you're running a stock exhaust with mufflers, you probably wouldn't notice all that much improvement as they were both designed for high RPM use and are quite "soggy" below 3000 RPM. The oval port heads run better in the lower RPM range.

Mark

[Duke W.]

Re: HP Rating

As stated "pocket porting" primarily involves cutting down and blending the annular ridge where the conical cutter that rough shaped the valve pocket meets the as cast section of the port. There's a little more too it than that, but the rest of the details are covered in the books.

"Port matching" involves opening up the port at the manifold interface to equal the manifold section dimensions.

In both cases grinding is only required for the first one-half to one inch above the valve seat and downstream of the manifold interface. For a street engine there is no need to increase the port section area in the interior of the port and doing so may actually be detrimental.

Multiangle valve seats are also beneficial because the valve spends most of its time at relative low lift, so the approach and departure from the annular valve opening area (the "curtain area") is critical.

All the above techniques have been documented for decades in books like "How to Hot Rod your Small Block (or Big Block) Chevy", David Vizard's books, and the old Chevrolet Power Manuals. Google on these names to find copies either new or used, and I'm sure the old Power Manuals show up on e-bay. With these references, anyone can do their own head massaging other than the final valve job.

I don't have any hands on experience with big blocks, and I've had a tough time gathering meaningful data to aid my simulation efforts, but BB heads share much in common with SB heads, so most probably will respond similarly to the same modifications.

The vintage BB world is actually more complicated than the vintage SB world since there were two different head architectures (rectangular and oval ports) with variations among each type. There are only minor differences between most of the small chamber SB big port heads including 461, 462, 186, 492 and others. The later large chamber heads have essentially the same port layout and may flow a little better due to less valve shrouding, but they don't allow the higher compression ratios that are desireable with premium unleaded fuels.

Because the SB exhaust port is relatively short and geometrically simple, pocket porting results in much better relative flow improvement than on the inlet side, and this has serious implications on the valve timing requirement to achieve optimum torque bandwidth.

Since the OE machined I/E flow ratio is about 0.65, a rather early opening exhaust valve is desireable, which means more total exhaust duration relative to the inlet side at constant overlap, and this also yields a rather early point of maximum lift, and the LT-1 cam is dead on these requirements.

But when the I/E flow ratio ends up at about 0.85 after pocket porting an early opening inlet valve is not needed and delaying it will improve low end torque somewhat without hurting top end power though the difference is not really signficant.

I'm doing all my simulation research now using what I think is an accurate model of the Corvette exhaust system that tells me how much backpressure is generated and have come to conclude that the OE SHP cams have too much overlap. I've always stated that aftermarket cams of similar duration to OE have too much overlap, and nearly all have more than OE, and I've now concluded that even the OE SHP cams have way too much overlap for a street engine with mufflers, even if it has headers.

So I'm designing a new SB cam. The inlet side is the L-72/LT-1 inlet lobe retarded several degrees. (The LT-1 cam inlet side uses the same lobe as the L-72 cam, which is the same, both sides.) The exhaust side is a "cut down" L-72/LT-1 inlet lobe, but the details to achieve the combination of suitable duration with acceptable dynamics has proved more challenging that I originally thought.

To get an idea of what might work, consider the new LS7, which has very broad torque bandwidth and very high specific output at heretofore unheard of mean piston speed for a pushrod street engine. The CNC machined ports yield an I/E flow ratio of about 0.65. The exhaust was intentionally sacrificed to get as much port and valve area on the inlet side as possible. To compensate, the exhaust valve is opened very early. Also noteworthy is a very retarded inlet event relative to "tradional" valve timing. The durations at .050" lifter rise are 211/230 and the points of maximum lift are about 123/117, which yields a LSA of 120 degrees. This cam has barely more effective overlap than the 300 HP SB cam and a lot less than even the relatively mild L-36 cam.

So with a pocket ported SB head having an E/I ratio of 0.85, swap the durations and points of maximum lift and you get the idea of what I am up to. Of course, total lift is less than the LS7's prodigious .591" at the valve to keep dynamics within acceptable limits with the vintage SB OE valvetrain at up to 7000 revs.

Current indications are that this valve timing closes about half the low end torque gap between the LT-1 cam and 300 HP cam and eliminates the typical SHP "hole in the torque curve" between 2500 and 3500 while making the same top end power as the LT-1 cam. SHP aficianados will probably not care for the fact that it will idle exactly like the 300 HP engine, so they may not be interested, but it will turn a 300 HP engine into a helluva sleeper! The current design has clearance ramps for mechanical lifters, but they can easily be converted (famous last works)to a suitable hydraulic lifter ramp design, and the dynamics should also allow hydraulics to rev to 7000 especially if they are "zero lashed" - or maybe no more than a quarter turn down from zero clearance.

I may write a book if I ever finish my research and get enough dyno data to validate the results, but I don't see the end on the horizon.

I'm interested in extending this effort to vintage BBs, but without good flow data for similarly reworked heads of the various types, it's impossible to optimize valve timing.

BTW, this morning I witnessed some chassis dyno tests on a 2.8L Fiero that the owner runs at Bonneville. About a year ago we system engineered some mods to improve top end power without sacrificing low end torque or driveability within a tight parts budget. We had baseline chassis dyno data on the OE engine with open exhaust from about 18 months ago.

The owner is an excellent machinist/fabricator and most of the work involved the same. He ported and matched the heads and ground three angle valve seats and the OE cam was retarded four degrees. There was no budget for flow testing the heads, so we had to make best estimates based on what little published information there is on this engine. We ran some simulations on a couple of available aftermarket cams that have a little more duration, but there was no improvement relative to just retarding the OE cam. The biggerst change was turning the upper portion of the two-piece manifold into a plenum, which cut total inlet tract length from 13.5" to a more desireable 10" and also eliminated the sharp bend in the runner at the interface of the two manifold sections. (The long manifold runners make excellent and broad torque in the midrange, but limit the top end.) Finally, he fabricated a set of 18" primary length headers with downpipes that merge into a single 2.5" collector and tail pipe. They should effectively function as "tri-wye" headers, but it was impossible to get reasonably equal length downpipes due to space constraints. The final product is beautifully fabricated showing great attention to detail.

The Engine Analyser simulations predicted about a 20 percent increase in power in the desired 5000-5500 range. Actuals fell short to only 15 percent, but the A/F ratio started out at about 9.5 at 1500 and only leaned out to about 10.9 at the top end, and this overrich condition is costing about four percent relative to a more ideal 12.5-13.0 A/F ratio. He went the wrong way on injector size and pulse width. Once corrected, the actuals should correlate well with the simulations.

There was a slight loss of torque below 2000, but any way you cut it, a 15-20 percent improvement on regular unleaded gasoline without raising the CR and installing a "big cam" that would screw up driveability and low end torque is not too shabby.

Duke

[Mark #28455]

BB flow numbers

Duke,
The BB flow numbers you're looking for are at:

http://www.strokerengine.com/page/page/1268049.htm

In addition, I have been messing with the Vortec BB heads, they have a fantastic chamber that gives max power with 28 to 30 degrees advance and measures approx 100cc, so good compression with a flat top piston. They flow as follows:

Lift Exh Int
.050 32.3 40.7
.100 63.7 74.0
.150 92.5 106.8
.200 117.5 135.7
.250 136.3 162.2
.300 147.3 184.8
.350 162.9 207.0
.400 175.3 227.0
.450 186.0 245.7
.500 195.5 258.6
.550 198.9 271.7
.600 202.0 274.8

Hope this helps,
Mark

[Duke W.]

Re: BB flow numbers

Thanks for the link, but it's not much help. What is needed are flow data for a set of vintage heads that are modified as I've discussed. The data listed for the current casting that is a replacement for early heads is of no use because it's for an OE machined head and pocket porting will considerably change the flow numbers and, and in particular, the E/I flow ratio, which is what is needed to optimize valve timing.

Duke

[Patrick H.]

Re: HP Rating

Duke,

Very interesting. I even think I understood more than half of it.

If you ever get around to setting one of those cams up in hydraulic configuration, I'd love to see it.

Patrick

[Steve Antonucci]

Re: BB flow numbers

Duke,

I'm going to do my level best to help you with this research on the next
BB I complete.

Steve