"Cruisable" with hot cam?

[J.Jensen]

Note: I don´t have a V8 car - and the only ones I´ve driven have been big heavy auto cars, so bear with me if its a dumb question.

Whats the problem with a "hot" cammed for high revs engine that makes it unsuitable for street and cruising use?

Is it stalling if the throttle is opened too fast or low torque in the bottom compared to a bottom-stronger engine that makes it harder to start/stop?

"over here" (Europe) we´re sort of stuck with tiny I4s - so I´m struggling a bit with the concept of a 5 litre not being capable to cruise along at idle in gear when a 1.2L 4 cylinder can in even 1:1 gears.

I´ve driven a few cars, porsches and the likes with on/off racing type clutches - and they all behaved as long as they were rolling, albeit a bit difficult to start on low rpms and uphills...

[mdross1]

Not all engines are created equal but I4's to mega inch bb will all run the same in the wrong setup . The cam certainly does effect low rpm reponse,big cams in to high a gear or with rear-end geared too high, cruising through town will cause bucking. Go down one gear get the rpm's up where the engine has no problem all is well.
Bottom line is a well designed thought out drivetrain these problems do not exist.

[scottj]

The biggest issue is part-throttle cruise. Running 264°/272° @ .050... would get bucking at part-throttle cruise if the revs dropped to 2200 rpm. As long as I was accelerating it would pull smoothly.

[blykins]

The bigger the cam, the more overlap it has. The more overlap, the less efficient the engine is at lower rpms. As Scott says, you can get "the buck" if they are too inefficient and you also get a "dirty" idle quality, along with softer power on the bottom end and mid range.

[Wbulk]

As has been said one issue is the wrong cam for street use, which is very subjective. However, you will find that in the US a very common mistake is picking the wrong cam with the wrong transmission, and wrong differential gears. It's all in the math!!!

Guys will spend a small fortune on pimping the car out but will miss this critical element of it's design. It has been a consistent issue in the US since the 60s. Usually by guys that have more money than experience of mechanical training, but not always. HP sells but most guys never ask what rpm range does an engine produce that HP. It's often in a range you are not in very often or maybe not the skills to drive in. I swear there is a formula somewhere that figures the cost of high HP vs. longevity, and the cost per mile of use goes really high the higher HP goes. Mostly, the higher the engine spins the sooner it breaks. For the street the trick is to find the balance.

[Tommy]

J.
Here's a non-technical answer. If you have a very modern engine with computer controlled direct injection fuel system, variable valve timing, a seven speed transmission, etc., you can have the best of all worlds. Look at the newest Corvette for an example. It has astonishing acceleration, near 300 KPH top speed, and gets reasonably good gas mileage running on only four cylinders when cruising. ... But, if you want to spend a lot less and get a good daily driver that gets good gas mileage but not much in the way of high performance, you can do that by optimizing a small engine to do that one thing very well.

The "hot cam" issues I suspect you are asking about regards the 1960-70 vintage American V8 engines commonly used by many of us with Cobra replicas. Most of these are not sophisticated engines in the modern sense. They can be optimized however you want, but most of us get some variation of the big horsepower version. That means they run very well at high RPM and near wide-open-throttle settings, but not so much at low RPM cruise settings. We could make them more friendly to stop-and-go traffic and low speed cruising if we wanted, but it would be at the cost of the some top end performance. And you seldom see anyone proudly putting their MPG after their signature on this forum.

The last thing I'll offer is that by choosing an older engine design with a cam that is optimized for high RPM, full throttle operation, you get an engine that runs a little uneven at idle. For Americans of my age, that sound is synonymous with the highest performance engines of my youth. I understand that there are smoother running engines today that greatly outperform my engine, but they just don't sound as good to me. It's a personal taste thing, and the sound of my car is just about my favorite thing about it.

[lippy]

I have a pretty hot solid roller cam in an FE. It idles fine at 1000 RPM and above and cruises just fine with 0.82 OD and a 3.54 rear, which turns the engine at about 2400 RPM at 70 MPH. The engine will cruise just fine down to about 2000 RPM but below that it's not as happy. To me, the tradeoff is less torque and lower RPMs and a higher RPM range and more power up top. But it's definitely driveable.

[DanEC]

One aspect is that the hotter the cam and rougher the idle, the worse is low-speed bucking or surging. I saw where someone referred to it as "trailer hitching". I believe that the Cobra being such a light car also accentuates it somewhat. If you intend to drive the car at all in traffic situations this can be an issue. It can be reduced somewhat by running more initial timing or maybe even vacumm advance. If low speed traffic isn't going to be a concern then a hotter cam shouldn't be too much of an issue.

[lippy]

Dan is right but I live in NoCal, where traffic is unavoidable, with a hot cam and it's no big deal in heavy stop and go traffic.

[J.Jensen]

I would never put a modern V8 in a Cobra. At least not one build to replicate the real thing closely.

I want to build a USRRC 289FIA replica, with a Weber fed 289 that happily does 8000. Like Tommy said - its a sound thing

Ofcourse - these things are built for speed and fun, but it must also do some street miles. The occasional drive through town, but mostly highway/backcountry roads and a track event or two.

[blykins]

It takes a lot of highly specialized and expensive parts to happily do 8000 rpm. Also, turning 8000 rpm and making power at 8000 is a whole other story. Making peak horsepower at 8000 rpm with only 289 cubic inches will pretty much insure that you have zero bottom end whatsoever, a lot less than the "hot cam" scenarios that we've been discussing so far.

[J.Jensen]

No not aiming at peak power @8000, but being able to go that far every once in a while would be nice.

[J.Jensen]

And if what I'm thinking isn't possible to make, I'd happily take the technical lesson on how the mechanical side of it puts a stop to it.

I'm used to I4s with turbos and EFI - where the air is pressed not sucked in...

[blykins]

Most engines that make peak hp in the mid 7000 range will require a fat bank account. Doing it happily and reliably add more to the bill.

Four cylinders have very small main and rod journals, light rods, light pistons, etc. V8's have heavier components, larger bearing sizes, etc. Larger bearing sizes mean higher bearing speed.

A 289 is a very small V8. They don't have much hp and torque to begin with and when you cam them up to make hp up high, hp may go up, but torque goes way down. Horsepower and torque peak rpms go way up. This makes for a very finicky engine on the street. Combining the large overlap with webers will also prove to be a tuning challenge.

The valvetrain parts alone (springs, etc) necessary to support very high rpm use will be very expensive.

Anything can be done, but this sort of engine would not be as fun as what you are thinking.

[kevins2]

Originally Posted by J.Jensen

And if what I'm thinking isn't possible to make, I'd happily take the technical lesson on how the mechanical side of it puts a stop to it.

I'm used to I4s with turbos and EFI - where the air is pressed not sucked in...

I think you're on the right track with this post. May I suggest that you focus on telling Brent the specifics of how you want the car to perform and how you intend to use it (you've already done some of that in this thread) and let him design the engine specifications for you. I believe you'll be much happier that way.

Kevin

[scottj]

When I ran my “old-school” motors they were 434 cid SB. Peak power was at 7200 rpm and the valvetrain was built to go 8200. We ran a 264/272 @ .050 solidroller, titanium valves, beryllium copper seats, ti retainers, Isky Toolroom springs, and shaft rockers. We watched the lash and checked valvespring pressure religiously. We also ran 2.00” or Honda rods and ultra-light cranks with a drysump.

I never had issues with solid rollers on the street, but that could be because the only street driving I did was go to straight to the back roads, rip it up, and go straight back to the garage.

Would I build them this way if I was planning on doing any real street driving with only occasionally ripping up the back roads? Absolutely not… I built them this way because I only drove them hard.

[DanEC]

Just from what I've read - if you are planning on Webers, there is somewhat of a science to making them run right - cam timing, ignition timing, etc. I guess they make good power - but I think you may have to let the Webers dictate the engine parameters that the builder builds it to.

[xb-60]

Originally Posted by J.Jensen

No not aiming at peak power @8000, but being able to go that far every once in a while would be nice.

J.Jensen, I like the way you're thinking.

This is from Road & Track magazine, written about the Cobra 260ci....

"...it would idle with only a trace of lumpiness, pull strongly at almost any speed, and buzz past the 5800rpm power peak to 7200 rpm before it began to sound distressed. And surprisingly, the power did not appear to fall off much even at 7000rpm - 1200 rpm over the point of maximum power."

....and this was 1962!

Cheers,
Glen

[Rick Parker]

J Jensen: Without getting on a soapbox let me reiterate Brents comments. I am one of the few here who run a relatively highstrung 289 with 48 IDA Webers in a street environment. With Webers to get a 289 to RPM to 8000 let alone make power at that RPM the chokes will need to be about 44 to 45 mm. it will require 12-13: 1 Compression, the required cam will be unusable at anything below 3500 RPM with NO low end torque, at the very least at which point it relegates the car to a track only application unless you are running 4:56 gears at which point you are spinning about 4000 RPM at 60+ MPH . Requires aftermarket block, Expensive rods ($1300) Steel crank $1700, Heads with Shaft mounted rockers $4000, Webers $3500 and on and on. Huge Money. When done it will be a maintenance nightmare.

You may dream of an 8000 RPM SBF push rod engine, but unless you are ready to drop close to $20K + for a fully developed engine you will be well advised to rethink your project ore retitle it : Bench Racing 101.

[J.Jensen]

I really like being able to post an idea, or thought, and have you guys teach about why it can´t be done
I reckoned it would take $20k to make power at the top end though.

Blykins; to flip it the other way then - how should I spec a 289 around webers, for mostly backcountry roads cruising and the occasional track day - and where would such a build produce peak power? How far up could that be pushed and not make stupid compromises to the streetable part of it?