H. P. Versus Fuel Injection

[Duke W.]

Re: H. P. Versus Fuel Injection

You've been suckered in by all the marketing hype. Faster opening/closing flanks increase the valve spring force requirement, which reduces valvetrain reliability. The internet is full of "wiped lobe" stories, and the most common is the popular Comp Cams XE series. There's no free lunch. More aggressive lobe dynamics requires more spring force, which increases valve train loading, and this leads to a less reliable valvetrain.

The GM engineers of the day had a very good understanding of valve train dynamics and their "soft action" lobes and well thought out lobe phasing make better road engine torque bandwidth than aftermarket cams with worry free valvetrain reliability.

Most aftermarket designs have more than OE overlap, which kills low end torque and degrades idle quality, and their earlier closing inlet event limits top end power. At best you end up with "more power" in a very narrow range in the mid-rev band, but not at the top end compared to the best OE SHP cams and certainly not at the bottom end. OE cams provide amazingly flat torque curves, which is ideal for a road engine.

All OE cams going back to the mid-fifties (except the Duntov) have asymmetric lobes that soften the closing event to improve valvetrain durability. Most of the aftermaket manufacturers haven't discovered this yet, and those who have act like they invented it, yesterday.

"Beehive" valvesprings don't significantly reduce valvetrain reciprocatiing mass. Most of this mass is in the lifter and valve. The "beehive" design is essentially a variable rate, which can help stave off the onset of valve spring surge, and I would recommend them IF there was one that duplicated the OE spring characteristics, but none that I know of meet this requirement. Most are signficantly stiffer than OE springs, and the OE damper has proved to be a very effective way to limit spring surge to acceptable levels.

Regarding "custom grinds" the only way to determine a custom cam specification is to (1) define a realistic set of target engine performance characteristics for the application, which is most cases is a road engine, so the broad range of engine performance from off idle to peak revs must be considered (2) understand the total flow characteristics of the engine - induction system, heads, and exhaust system and (3) use time tested system engineering techniques including simulations to predict "as installed" (including front end accessories and vehicle exhaust system losses) engine performance to achieve your specified performance requirements. Anything less is nothing more than a guess, at best!

Comp Cams is one vendor that I specifically recommend Corvette engine restorers avoid like the plague - bad news all around.

Duke

[Tim S.]

Re: H. P. Versus Fuel Injection

I would say we agree to disagree.........................

[Todd A.]

Re: H. P. Versus Fuel Injection

Duke,
I had been following your work on developing a "special" SHP cam, and was disappointed when you found that current flat tappet cam blanks couldn't support the designs you intended to try. I was curious if your lobe designs could be translated to a solid roller format that could actually be realized. I understand it would essentially require starting from scratch - finding proven roller lobes that create similar valve lift curves to the vintage cams, but I was just curious if you had already looked into the idea.

[Duke W.]

Re: H. P. Versus Fuel Injection

There was only one design that could not be ground on the one available flat tappet cam blank that is used by all vendors to grind the hundreds of available designs. This particular design combined the 30-30 lobe on the inlet side and Duntov exhaust lobe on the exhaust side with a very wide LSA and late phased inlet event.

The problem is that the blank is cast for the relatively narrow LSAs typical of aftermarket cams, but such narrow LSAs are not well suited to road engines, especially when you get to relatively high .050" lifter rise durations - say 220 degrees or more.

Also that particular design didn't provide a huge improvement. It did show slightly better low end torque due to the later opening exhaust valve relative to the LT-1 cam. Since typical head work increases the E/I flow ratio the long exhaust durations typical of many OE and aftermarket cams are not necessary, but they don't do too much harm.

A properly designed roller cam does appear to provide some improvement in performance - at least in part of the rev range, but IMO the gains are not worth the added cost to convert a vintage engine to a roller design. As is the case with most flat tappet aftermarket designs, most currently available roller designs for vintage engines have too much overlap.

The heavier roller lifters and more aggressive dynamics limit valvetrain speed to about 6000 RPM - at least for hydraulic roller designs and require much stiffer springs than OE. Mechanical roller designs are really "racing only", and I know of none that are even remotely suitable to a road engine.

Any flat tappet hydraulic design that works well for a particular application can be "converted" to a hydraulic roller design by chosing a lobe from the manufacturer's library that is a few degrees less duration than the flat tappet lobe and have them ground at the same POMLs. Such a design should provide similar idle quality, a bit more peak torque, and about the same top end power, but IMO the cost/benefit ratio is not very good.

For best broad range performance, soft action flat tappet mechanical lifter designs like the LT-1 cam are the best choice. With OE valvetrain reliability you get a very flat torqure curve from 2000 to peak revs and a very flat and broad peak power bandwidth from 5000-7000 on a 3.25" stroke configuration.

Duke