302 cid Small Blocks

[Joe L.]

A recent thread had me bring up, peripherally, the subject of 302 cid small blocks. There is something that's always had me wondering about this engine configuration. As most know, the 302 cid engine used in Camaro Z-28s from 1967 through 1969 was an "artificial" engine. It was produced by GM to fit within the then-required cid limitations of the Trans-Am racing series and sold to the public in order to qualify the engine for Trans-Am racing under other rules of the sanctioning body. This engine used a 327 bore (4.00") and a 283 stroke (3.00"). After the engine was dropped at the end of 1969, this engine configuration was never used again by Chevrolet or GM.

Chevrolet did make several smaller cubic inch small blocks in the 67+ period, including the 262, 267, 305, and 307. The 262, used for 1975-76, had a 3.67" bore and a 3.10" stroke. The 267, used for 1979-82, had a 3.50" bore and a 3.48" stroke (same as 350). These 2 engines were "abortions" and never really were used widely. However, the other 2 small cubic inch small blocks were used very widely.

The 307, used from 1968-73 had a 3.875" bore (same as 283)and a 3.25" stroke (same as 327). The 305, first used in 1976 and still in production to this very day for SERVICE engine and marine use, used a 3.736" bore and a 3.48" stroke (same as 350).

The only explanation that I've ever read for all these different configurations was fuel economy and emissions control. From a performance standpoint, NONE of the above-referenced small block configurations ever achieved any positive notoriety. Also, NONE were ever popular for aftermarket "hop-up".

Now, here's the part that's always had me wondering: in 1968, Ford also built an engine for Trans-Am racing purposes. It was a follow-on to their 239/260/289 small block engine series and it was a 302 cid configuration. In fact, it's bore and stroke were identical to the Chevrolet 302----4.00" bore and 3.00" stroke. Ford didn't drop this configuration in 1968, though. They continued to manufacture it as, essentially, their sole "small cubic inch" V-8 engine for years until about the year 2000. During that period the engine enjoyed much success in every manner of use. It's wildly popular among the Mustang set and is widely popular in aftermarket "hop-up" circles.

So, what makes me wonder is if Ford could use the 302 "4 X 3" configuration so successfully for so many years, how come Chevrolet dropped this configuration after 1969? Ford had to meet the same fuel economy and emissions standards as Chevrolet.

[Bryan L.]

Re: 302 cid Small Blocks

I've wondered too why Chev went through so many variations in the small displacement SBC while Ford kept right on making millions and millions of 302s. Some of those SBC were absolute turds.

I love the 302 SBC. I've been lucky enough to own 3 69 Z/28s. There is nothing like the sound of a 302 at 7000 rpm. They weren't big on torque engine so you had to drive it like you hated it.

BL

[Reba Whittington]

Re: 302 cid Small Blocks

Interesting questions, Joe. You would be suprised how many people (at least in my neck of the woods) don't realize Chevy ever made a 302. My sister and I both had 1969 Z-28s, and we had people look at the hood emblem and say what kind of engine is that?

[Duke W.]

Re: 302 cid Small Blocks

There's no pat answer to this question, but the following might explain, at least from a logical standpoint, why GM never pursued the 4"x3" bore and stroke configuration beyond the 302 Z-28, which was a "homologation special".

As a rule, for a given displacement a bore/stroke ratio that is nearer to "square" or even undersquare (longer stroke than bore) is best because the combustion chamber will have a lower surface area to volume ratio for a given chamber architecture and compression ratio, and a longer stroke will enhance low end torque relative to a same displacement engine that is more undersquare.

The Ford 221/260/289/302 small block has a shorter deck height than the Chevy small block, so a 3" stroke is about its limit without creating unfavorable crank/rod geometry. The aftermarket strokes them to 3.25", but the aftermarket will always stretch limits beyond where and OEM feels comfortable.

For more stroke Ford produced the Windsor block for the 351 and larger displacement, which has greater deck height to accomodate longer strokes.

Many contemporary engines, especially inline fours used in transverse applications are now "undersquare" with strokes longer than their bore diameter. Since this makes for a shorter engine for a given displacement it aids in packaging. The long strokes provide good low end torque, and the four-valve heads allow high specific peak power output by maintaining volumetric efficiency to high mean piston speeds with conservative valve timing, which is necessary for good emissions and idle quality.

At the opposite end are current F1 engines with bores of about 3.5" and strokes well under 2". Such highly undersqaure engines with high compression ratios yields a combustion chamber that is a thin disk, which has very high surface area to volume ratio. Such a chamber has very poor thermal efficiency, but the shorter the combustion period the less heat they lose. Thus these engines can make very high specific output at extremely elevated revs, but they only make competitive output in about the top ten percent of the rev range, which is why the gear spacing in FI gearboxes is only about 10 percent. This keeps the engine working in the 16,000 to 18,000 range if not a little higher. Below this the torque curve falls off a cliff.

Everyone knows that 302s are very "edgey" engines - very different than long stroke SBs. Consider the case of "similar" engines - everything the same except stroke. If such engines were tested one would note that peak torque and power will occur at about the same mean piston speed for both long and short stroke versions. Say a Z-28 engine with the 30-30 cam makes peak torque and power at 4500 and 6500. If you increased the stroke from 3.00" to 3.75" with no other changes, you would observe that the torque and power peaks would drop to about 3600 and 5200, respectively. Peak torque would be increased, but peak power would be about the same. The 377 would be a better street engine because if would produce more average torque and power over the useable rev range. It would idle better and feel much less edgy and would be a vastly improved street engine because of the greater torque and more usable torque bandwidth.

This is why building the biggest displacment possible within the physical limits of the package will always make a best street engine.

Duke

[Bryan L.]

Re: 302 cid hemis

I read an article a few years ago about the hemi heads that were developed for the Trans-Am engines. It seems that Chevrolet engineering was worried about the big port Cleveland heads that were being developed by Ford for the upcoming Trans-Am season and decided to one-up Ford with the hemi. They couldn't get them to perform like they expected and sent them to Smokey Yunick for some fine tuning. Smokey sorted them out and they didn't really make any more horsepower than the Cross-Ram 302s from Traco were making. They were abandoned but not before making a few sets. Pretty neat looking setup with magnesium valve covers.

I wonder if Joe has a set in his collection?

BL

[Jack layton]

Re: 302 cid hemis

A picture of said hemi heads with a 2x4 cross ram manifod installed in a 1969 Daytona yellow Z-28 RS owned by Mick Price of Atwood, Illinois ( at that time) is located on page 105 of the Super Chevy special edition "30 Years of Camaro" published for the summer of 1997. Vol 1, No. 1. I thought some folks might like to see a picture of this "rare" Chevy set up.

[Duke W.]

Re: 302 cid hemis

Were those heads the same basic pieces that were in work for the Grand Sport Progam before is got shut down. At least one set was built. I've seen a picture - I think it's in Ludvisgen's book.

Duke

[Mike Cobine]

Re: 302 cid Small Blocks

That was great Duke, but I was surprised to see two things left out: Emissions and The 283.

Part and maybe most of the reason for the 302 going away is that emissions in 1968 began to dictate a lot of what Chevy would do. The undersquare engine (longer stroke/smaller bore) was much cleaner and provided more torque like you said, even as cam profiles dropped and horsepower dropped in the emission years. So the 307 and later the 305 could be cleaner and yet maintain torque to a respectable level for driving around town.

Really, if you think of a 302 as a bored 283, there is no reason to be edgy. however, what made it edgy was the cam and the heads. For the heads and cam to flow at 7000 and 8000 rpm, there could be no bottom end. The 283s never ran the big valve big port heads like the 302 did.

Even with Chevy's conservative ratings, the 302 was 290 hp at 5800 rpm. That is FI 283 land, except that the 302 didn't stop at 6000 rpm. Also noticable is that the torque was 290 at 4000 rpm, about 1800 rpm higher than where other engines made that amount.

With the 1.94 heads and a milder cam, it would have worked like a larger 283. But there really wasn't a need since the 350 was being used in almost everything. And there was no economical advantage such as the 307 and the 305 had, so there was no reason to keep it.

With the emissions aspect, there was every reason to get rid of it.

[Joe L.]

Re: 302 cid Small Blocks

Mike----

The 302 Ford engine with its 4" bore and 3" stroke produced over the years, generally, just about the same peak torque at just about the same RPM as the Chevy 307 and 305. So, I don't see where the longer stroke of the small inch Chevy engines gained them anything in the torque department. As far as emissions go, like I mentioned, the Ford 302 had to meet exactly the same emissions standards as the Chevy engines for any given year. Ford kept the 302 around for 32 years. The 307 lasted only 6 model years, although the 305 has lasted 26. Nevertheless, the 305 never established the "performance credentials" and anywhere near the popularity of the Ford 302.

I think that if GM had chosen to continue to use the 67-69 302 cid configuration for its long-term small cube V-8 in greatly "de-tuned" form, it could have achieved everything that it did with the 307 and 305. Plus, the 302 would have achieved much more impressive "performance credentials" and would have been a very popular "hot rod" engine.

[Clem Z.]

Re: 302 cid Small Blocks

just a guess on my part but with the short stroke and the 9.025 deck the 302 chevy needed a tall piston unless the went to longer con rods and the tall pistons are very heavy. 302 chevys with 6.0 and 6.125 rods really reved fast because the weight reduction in the piston. it is a 2 to 1 ratio,for every gram added to the rod weight you would loose 2 grams from the piston. maybe GM did not want to tool up for longer rods.

[John H.]

Re: 302 cid Small Blocks

There was much hand-wringing at the time over emissions strategies and what would meet requirements with the least piece cost (A.I.R. systems were very expensive); and reduced surface-to-volume ratio was a key element in reducing engine-out emissions. The 302 had the worst S/V ratio, so the other avenues were taken.

[Duke W.]

Re: 302 cid Small Blocks

The short deck Ford SB was stretched to the limit at 4x3 inches. It was the only way to achieve 300 CID capacity. The Chevy SB with it's taller deck offers more flexibility in picking various bore and stroke dimensions to achieve displacements between 300 and 350 CID.

The 262 to 307 (except the 302) are not good bases for performance engines because the blocks start out with less than a 4" bore. Any performance engine is best based on the largest displacement possible, so a 4" bore block is the logical starting point, and, unless you are building a limited displacment class engine, the longest stroke possible for maximum displacement will always yield the highest average power across the rev range from off idle to peak revs. That makes a 377/383 the best choice, but it will be much more expensive because custom components will have to be purchased, whereas virtually any junkyard 350 can be a basis for a budget high performance engine using most of the major components that come with it like the bottom end assembly and heads.

Duke

[Clem Z.]

Re: 302 cid Small Blocks

for max HP in a SBC you need at least a 4.125 bore to unshroud the valves. the 358 CI NASCAR engines all have a 4.155 bore to take advantage of the better head flow. the dodge engines at first had a larger bore but when it turned out to be a advantage NASCAR limited all engines to a max of 4.155 bore.

[Dino L.]

Re: 302 cid Small Blocks

Does anyone ever remember seeing a green 1972 Corvette coupe with a COPO 307 engine in it? Detroit area...

[Mark Ring]

Re: 302 cid Small Blocks

Just a little note to add. The SCCA Trans-Am rules were changed in 1970 to allow de-stroking of factory engines to get them under the 5.0 liter maximum. The 302 was not a great daily use road engine (think L-88) but it was a great under 5.0 Liter race engine and dominated the track at the time. The longer stroke 350 engine is a much better street engine for many reasons already stated. There was no need for the factory to continue to produce the 302 for the street so the LT-1 was made available in the Z/28 for 1970. A 67-69 302 is almost identical to a 1970 LT-1 except for the stroke, and the cam. Internaly the engines are basicaly identical. So you could still have Homologation, de-stroke the 350 to a class-legal 302 race engine for Sunday (actualy a hair over that in race trim) and still have a streetable car to sell on Monday.

-Mark.

P.S. - I run a basicaly stock 302 and it is definately a lot of fun but they are realy pretty inefficient at low RPM. If you can cope with that, they are a blast. The production single 4 barrel carb setup works fine for me but some of the guys like to run the service package dual 4 barrel GM crossram intake setup with the GM "-140" first design off-road cam. That is even closer to full race trim. WOW!