How to Spec an Engine....the process..

[Jsup]

So...this subject came up in another thread I thought it would be an interesting philosophical discussion about the process about how to get the engine you want. I have seen too many people go to forums, get advice on how to build an engine, and wind up totally disappointed in the result.

In one instance, this "forum recommended motor" was put in the car. My friend spent a lot of money buying the most shiny and cool parts, in the end, it just didn't run. He was getting his ass kicked by "junkyard builds" as he called it. People who spent a lot less money but got it right.

So....I'd like to submit my thoughts for consideration and have your comments. Perhaps we can stimulate some good discussion for future reference....

Anyway, I would think that the first thing to do is consider how you're going to use the car. What the operational range is. A street car will rarely see over 5000 RPMs. You probably want the power down low, etc... A strip car will probably want to see power in the 3500RPM range and up...etc....

This, in my opinion, is the most important part of any build. The more time spent here, the happier the end result will be.

Short block: Not the most critical point, but can determine operational range, for example, a 377 is going to spin up and is more at home at higher RPMs than a 383. That's about all you can say for a bottom end. There's the debate about dished vs. flat vs. domed pistons, today the trend is dished, but past that...nothing much more.

The first place I'd go after displacement is determined is the cam. The cam determines the characteristics of the motor. How it will behave, operational RPM range, torque curve, HP potential, etc....the entire motor's personality is determined by cam selection. Probably the most critical decision in putting it all together. The cam ramp, an often ignored metric, will have an effect on performance.

Next, I'd have to say rocker ratio. not much to say there, but that will determine how the cam reacts.

Third, the intake. Why? The intake needs to match the cam in terms of application. Is it a low RPM cam and a high RPM intake? Won't work. Also, the intake has an effect on how the car behaves almost as much as the cam.

Fourth, heads. Given the cam operational specs, the bottom end and piston selection, the intake characteristics, and the desired compression ratio based on piston selection heads come into play. Since all you're really doing with heads, in a simplistic description, is passing air from the intake, into the cyl, then out to the exhaust, they are simply a means of moving fuel and air. Given the cam selection and intake selection head selection will stem from the operational characteristics of the cam and intake. Lift, flow, etc...to match the other two. Chrarcteristics include getting the right compression ratio you want by means of chamber size. Two schools of thought there, some people go larger chambers and more piston. Others go smaller chambers and more piston. Then you have flow within the correct range of the the rest. If your cam isn't pushing .600 lift, flow numbers at .600 lift are meaningless. Then there's the whole debate of chamber design and so forth, which is a entire thread by itself.

Fuel delivery. Simply a means of matching injector size or carb size to the rest of the engine. My only comment here is that I see many people on the FI side pick injectors that are too small for the application. There are many injector size tools in the internet, go up a step. If it's 39 go to 42.

Anyway, that's my .02 and it's worth about 1/2 that. Thought it would be interesting to hear your thoughts on the matter.

 

[BBShark]

I think your selection sequence is good. I started with the short block on my engine, next the cam. I had to go back a couple of times to "adjust" the cam based on head selection (compression ratio, quench etc.).

Everything is so inter-dependant that, by the time you get the whole package together, you have probably re-thought a lot of the components.

Kind of a pet peeve of mine is the information provided by cam manufacturers. Some show you specs for 2500 rpm and above. Like you said, a street car doesn't spend a lot of time above 2500 rpm.

 

[mrvette]

I spent most of my driving life with the Pontiac school of engines under the hood, my first car was a 283 '60 vette, only lasted a years, so went into Pontiacs after that because I could get INSURANCE on the Poncho but not any vette.....:suicide:

so only owned maybe 2 SBC and many Dodge trucks by time I got back into Vettes again with my '87, in 1992, that was the car that got me into DPFI and electronic engine controls, and got my curiosity up enough to put a Caddy DPFI system on a Pontiac 455 I had in a tow car.... 550 instant ft lbs at idle....freeking car would jump over trees when asked.....HUGE improvement in performance over the Qjet .....1000 cfm t-body.....

so with that in mind, I stuck a TPI on my stock L48 '72 here, engine was fresh from previous owner....then dumped out on it for more power and overdrive here in Florida, 12 years ago....

in this latest go round, it's probably the highest tech V8 I have ever owned....roller motor, later L98 heads D port,

I build for torque.....455 ci got me 550 ft lbs....this engine spec is 355 and 425 ft lbs....ok by me.....52mm dual blade t-body....LT1 intake, runs like a raped ape, once I put GAS into the fuel lines....:sos::crutches:

I tend to like torque way in hell over HP, as too many friends over the years have built cars that won't idle below 2000 rpm, and get 2mpg, bag that crap,

I'll take 400+ ft lbs at idle over some 600 hp thing at 7000 rpm's any damn day....

Kinda the way I like my stereo systems.....I have 4 12" speakers PER CHANNEL in my house, McIntosh all the way.....I like engines with a nice LONG TUBE HEADER, like a Lo note on a Cathedral Pipe Organ.....

I know HP will get you a better ET, but I still like enough torque to snap axels....:lol:

 

[Jsup]

I think your selection sequence is good. I started with the short block on my engine, next the cam. I had to go back a couple of times to "adjust" the cam based on head selection (compression ratio, quench etc.).

Everything is so inter-dependant that, by the time you get the whole package together, you have probably re-thought a lot of the components.

Kind of a pet peeve of mine is the information provided by cam manufacturers. Some show you specs for 2500 rpm and above. Like you said, a street car doesn't spend a lot of time above 2500 rpm.

This is my third engine in this car. Stock, I modded the stock 350, dropped a screw into the cyl, didn't know it, well, that was the end of that.

This motor I went to a well known, reputable builder, Pat Musi. Internationally known and just let him do it. I did little thinking on this build, just told him specifically what I wanted and let him figure it out.

My first run I built the motor three or four times on paper before pulling the trigger. I'm being fairly general about this, as you said, there's a lot of finessing that goes on as the build progresses.

 

[guru]

I think there is something to picking the intake before the heads. Especially when you consider the lack of good options in a corvette. Hood height is a bitch.

I think once you pick your desired RPM range, the heads, intake and camshaft sort of follow.

I think displacement and RPM range dictate your head choice, more so than your camshaft does. Your camshaft should be for the advertised RPM range you want, as properly sized heads will accomodate the increased displacement. It is my feeling, and I may be wrong, that the concept of displacement mellowing a cam is an indication that your heads (and maybe intake) are too small for the task at hand.

Head selection is a choice of preference and budget in todays market. Call the manufacturer, and ask them what they think about heads given your displacement, RPM range and your racing goals. For me HP and racing times is secondary to RPM range. I firgured the RPM range and displacement would be the ultimate decider on the power so why worry about it. Its all going to be faster than where I was.

I choose 2400-6800rpm.

Brodix I think has the best selection guide, if you like charts

So here is my process if I were to do it again:

1.) Pick an RPM range
2.) Pick a valve train type (solid, hyd, roller, etc)
3.) Call a few head makers and find out what they think
4.) Compare the various head selections based upon their attributes, cost, port size, exhaust port location, casting quality, company reputation, and materials selection
5.) Pick one
6.) Call a few cammakers and right down their recommendations, or just call a company adept at cam spec'ing and take their word for it. If you do call a couple. Average em all up and pick the one closest to the average.
7.) Choose an intake based upon your head selection (port size varies greatly between various brands), and RPM range.
8.) If you have a big single plane you want to drive on the street, make sure you get a carb with enough booster gain. It will do alot for the no low RPM torque problems that are out there. Annular venturis can be your friend. If you have a carb with stick boosters in 2008 slap yourself and go get something with downlegs. No excuse for it. Bad dog.
9.) your cam and head selection will dictate your compression ratio and piston choice
10.) rod length can be considered if you want, its typically not that big of a deal, but cammotion definitely cares when they spec a cam.
11.) Your crankshaft material has to be suitable for your intended application

And so forth.

 

[mrvette]

Rpm range is easy, I never seen a engine blow under 5000 rpm....

so 550 ft lbs at idle.....

solved THAT one in a hurry, did't I????

:lol::stirpot:

 

[guru]

LOL my truck has a higher RPM range than that :lol: