BACK
ENGINE DESIGN THEORY
From HO Racing's Pontiac High Performance Engine Design and Blueprint Assembly
If you want to build a Pontiac race engine intelligently - then reading this section is a must. However, H -0 Racing Specialties Inc. realizes that everyone who reads this book is not an engineer nor even inclined toward graphs,equations or numbers. So in addition to the following in-depth discussion, you will notice that all significant conclusions are CAPITALlZED. If you read nothing else in this section. read the capitalized statements.
Engine Parameters
What is an engine parameter, you ask? An engine parameter is some number which describes a physical characteristic of the engine. Examples of engine parameters are cubic inch displacement (CID), bore, stroke, valve size,rod length, rod/stroke ratio, bore/stroke ratio, etc. These parameters then are engine dimensions and relationships between these dimensions and they determine how fast or quick your car moves when you put your foot in it !
Now, there are probably as many engine parameters as there are engineers to think them up. But in this section we will show you the few that are really important, so that when you lay down your hard-earned cash. you will know what you are getting .
Performance Factors
Engine parameters have to be evaluated in some way in order to determine which are the "best" parameters. This evaluation is done using what we will call Performance Factors. Some are probably familiar to you, some are not. The ones we will use are torque, horsepower, and volumetric efficiency. They are defined thusly:
- Torque - twisting force. More strictly. it is the resultant moment of a force applied at a distance. In our analysis,we consider it the prime factor for street performance .
- Horsepower - motivating power. It is actually a unit of the time rate of work (torque times rpm in this case). We consider it the prime factor for competition perform ance .
- Volumetric Efficiency - How good an air pump an engine is. It is defined as the ratio of actual air inspired divided by the cylinder swept volume. In this section we use it to evaluate engine parameters and compute characteristic torque and horsepower figures.
Parameters That Matter
There was a wise old hot-rodder who once said. "When in doubt. bore it out." What he meant was that there is no substitute for cubic inches in street engines. Detroit generally, and Pontiac in particular, has followed this premise since cars have been built. Nearly every time the public demanded more performance, the response from Motor City was a larger engine.
It would seem that if you have one engine which has twice the displacement of another, the larger will put out twice the power of the smaller, but this is not true. Let us examine Pontiac engines and find out why.
Volumetric efficiency (VE), as defined previously, will be the common factor which will allow us to directly compare all Pontiac engines from 389 to 455 CID. It has been learned by experimentation (Detroit engineers, not us) that the VE curve has the same shape as the engine torque curve. VE is influenced by many engine parameters, such as cam timing, valve-area-to-bore-area ratio, intake port length and diameter, etc. But, as you will find out when you read the parts swapping section, all Pontiac production 4-bbl heads (1967-up) are essentially the same as far as valve size, port length, and manifold design are concerned and they will all interchange on any displacement block. The same can be said for exhaust systems, cam timing specs, and practically any other engine parameter which affects VE except for stroke. Even bore size changes only 2% (from 4.0625" to 4.150", 389 to 455 CID).
To make our power comparison, let us use exactly the same head design (corrected for constant compression ratio), intake and exhaust manifolds and cam on 389 thru 455 CID engines. This will be a very realistic comparison because you would probably pick the best high-performance top-end components regardless of the CID of your engine .
Figure 1 is the VE curve of the typically equipped Pontiac engines we are examining. Although this particular curve is unitized, it is also typical since we are assuming that all factors which influence VE are constant except for stroke. In other words, the only engine parameter which influences VE is mean piston speed (MPS). Mean piston speed is merely the distance the piston travels per crankshaft revolution times the crankshaft (engine) rpm. In formula form:
MPS = STROKE x RPM/6. For 389 thru 455 CID. the strokes 3.75",4.00" and 4.21" are applicable.
Notice that the peak VE occurs at 2000 feet per minute (fpm). This is the MPS at which the peak torque occurs also. From Figure 2 it is found that 2000 fpm MPS corresponds to 3200 rpm for 389's and 400's; 3000 rpm for 421's and 428's; and 2850 rpm for 455's. In other words. the more stroke, the lower rpm at which the peak torque occurs. This is not the whole story, however, because actual engine torque is also directly proportional to swept volume (CID). In formula form: TORQUE = VE x CID . Therefore. even though the maximum torque on a 455 occurs at a lower rpm, the 455's torque is 13.7% greater (455-400 x 100)/400 than the 400's torque.
We also know that horsepower is torque times rpm. Peak horsepower for this VE curve occurs at 2800 fpm MPS where VE = 0.85. From Figure 1
2800 fpm corresponds to 4500 rpm for 389's and 400's.
4200 rpm for 421's and 428's
and 4000 rpm for 455's.
The corresponding characteristic horsepower is computed from HP = VE x CID x rpm/ 5252. The chart below summarizes the rear wheel torque and horsepower for the engines of interest (street cam).
| | MPS = 2000 fpm | MPS = 2800 fpm |
| CID | Max Torque @ rpm | Torque @ Peak HP | Max HP @ rpm |
| 389 | 389 ft-lb @ 3200 | 321 ft-lb | 283 @ 4500 |
| 400 | 400 ft-lb @ 3200 | 340 ft-lb | 291 @ 4500 |
| 421 | 421 ft-lb @ 3000 | 358 ft-lb | 286 @ 4200 |
| 428 | 428 ft-lb @ 3000 | 364 ft-lb | 291 @ 4200 |
| 455 | 455 ft-lb @ 2850 | 387 ft-lb | 295 @ 4000 |
The figures above show us that the more cubic inches, the more torque results, everything else being equal, but resultant horsepower is nearly independent of displacement. In fact, if we had done a more strict analysis and not assumed everything but stroke as constant, we would see that horsepower is completely independent of displacement as far as VE is concerned.
So what does all this prove? It shows two things:
o FOR THE STREET: RUN THE LARGEST ENGINE YOU CAN. You want maximum torque and Pontiac's 455 fills the bill admirably. It has the largest torque output and at an rpm you can use on the street.
o FOR COMPETITION: HORSEPOWER IS NOT NECESSARILY DEPENDENT ON CUBIC INCHES. If you are into competition racing, you know that there are a lot more factors which affect horsepower than VE. The rest of this section is devoted to analyzing some of these factors, so you can pick your engine accordingly.
Design for Competition
If we forget for the moment that we will have to run our race car under some organized body's class restrictions, then we can do a simple analysis of possible Pontiac engine parametric combinations to find out which engine is potentially the best race engine.
If engine cubic inch displacement, per se, is not a concern, then what ratio of bore-to-stroke (the parameters which produce displacement) is the best? Almost all successful race engines are "over- square" meaning that bore is larger than stroke. A short stroke relative to bore size is beneficial because of less piston drag. A large bore relative to stroke allows larger valves to be used with less cylinder shrouding. Let us look at the bore/stroke ratios (B/S) of some currently successful production-based race engines.
| Make | Displacement | Bore | Stroke | B/S |
| Chevrolet | 302 cu. in. | 4.000 in. | 3.000 in. | 1.33 |
| Chevrolet | 331 cu. in. | 4.024 in. | 3.250 in. | 1.24 |
| Chrysler Hemi | 426 cu. in. | 4.250 in. | 3.750 in. | 1.13 |
| Chevrolet | 454 cu. in. | 4.250 in. | 4.000 in. | 1.06 |
By comparison. listed below are the Pontiac engines with significant over-square B/S ratios.
| YEAR | Displacement | Bore + .060" | Stroke | B/S |
| 1956 | 317 cu. in. | 4.000 in. | 3.250 in. | 1.23 |
| 1958 | 370 cu. in. | 4.123 in. | 3.5625 in. | 1.16 |
| 1968-76 | 400 cu. in. | 4.180 in. | 3.750 in. | 1.11 |
| 1968-69 | 428 cu. in. | 4.180 in. | 4.000 in. | 1.05 |
| 1969 | 303 cu. in. | 4.125* in. | 2.840 in. | 1.45 |
| 1970 | 366 cu. in. | 4.153* in. | 3.375 in. | 1.23 |
| 1970-76 | 455 cu. in. | 4.210 in. | 4.210 in. | 1.06 |
* Stock bore
Another parameter which is important to race engines is the ratio of connecting rod length to stroke length (R/S) . This ratio determines the maximum piston inertial loading and the optimum crankshaft angle (the number of degrees after top dead center (TDC) at which the crank throw and connecting rod are at right angles). From Figure 3
the greater the R/S, the less the maximum inertial piston loading and the more reliable the engine or the greater the red line. From Figure 3 the greater the R/S, the further past TDC the optimum crank angle occurs and the higher rpm at which the same maximum torque will occur with resultant higher horsepower (remember HP = torque x rpm). Let us look at the rod/stroke ratios of the engines of interest.
| Displacement | Rod Length | Stroke | R/S | Inertial Loading Factor | Opt. Crank Angle |
| Chev. 302 | 5.700 in. | 3.000 in. | 1.90 | 1.263 | 75.3 |
| Chev 331 | 5.820 in. | 3.250 in. | 1.79 | 1.280 | 74.4 |
| Chrysler 426 | 6.861 in. | 3.750 in. | 1.83 | 1.273 | 74.7 |
| Chev 454 | 6.125 in. | 4.000 in. | 1.53 | 1.327 | 71.9 |
| Pontiac 317 | 6.625 in. | 3.250 in. | 2.04 | 1.245 | 76.2 |
| Pontiac 370 | 6.625 in. | 3.35625 in. | 1.86 | 1.269 | 75.0 |
| Pontiac 400 | 6.625 in. | 3.750 in. | 1.77 | 1.283 | 74.2 |
| Pontiac 428 | 6.625 in. | 4.000 in. | 1.66 | 1.301 | 73.2 |
| Pontiac 303 | 7.080 in. | 2.840 in. | 2.49 | 1.200 | 78.3 |
| Pontiac 366 | 7.080 in. | 3.375 in. | 2.10 | 1.283 | 76.6 |
| Pontiac 455 | 6.625 in. | 4.210 in. | 1.58 | 1.316 | 72.4 |
Compression ratio (swept volume plus dead volume, both divided by dead volume) is another important parameter to consider for a race engine. We must be practical though because we cannot simply pick a compression ratio (C.R.) and let the piston manufacturer worry about getting enough dome volume on the piston. As a matter of fact, for superior flame propagation (best horsepower), it is best to use a flat top piston. We will also have to use a production cylinder head, and there are physical limits on the possible chamber volumes .
What C.R. would we like to have?
Figure 4 shows how much power we lose off optimum power if we have something less than an infinite C.R. For instance, we would be about 2% off optimum at C.R.=13:1. Listed below are the C.R.'s we can achieve using the Pontiac engines of interest and a 1970 Ram Air IV head.
| Displacement | 1970 RA IV | Mill Head .063" | Use Thin Head | Reduce Deck by |
| | Stock (71cc) | (61 cc) | Gasket (58 cc) | .023" (53 cc) |
| 317 (+.060") | 8.3 cr | 9.1 cr | 9.5 cr | 10.0 cr |
| 303 | 7.7 cr | 8.6 cr | 8.9 cr | 9.4 cr |
| 366 | 9.2 cr | 10.1 cr | 10.5 cr | 11.1 cr |
| 370 (+.060") | 9.5 cr | 10.5 cr | 10.9 cr | 11.5 cr |
| 400 (+.060") | 10.2 cr | 11.3 cr | 11.7 cr | 12.4 cr |
| 428 (+.060") | 10.8 cr | 12.0 cr | 12.4 cr | 13.2 cr |
| 455 (+.060") | 11.1 cr | 12.4 cr | 12.8 cr | 13.6 cr |
Theoretical Evaluation
We now have sufficient information to reasonably answer our original question:
which Pontiac engine is potentially the best race engine? WE WOULD RANK THE ENGINES IN ORDER OF PREFERENCE as follows :
366- This engine has a B/S (1.23) about the same as the Chevrolet 331 (1.24).
It can be argued that an R/S greater than 1.9 (Chevrolet 302) starts to entail valve overlap breathing problems, so maybe there is no actual advantage with the 366 R/S of 2.10. In any case, with the 366 it is possible to achieve a C.R. of 11.1:1 with flat top pistons, which may be slightly too low only for drag racing .
370 - With an overbore of. 060" (B/S = 1.16). valve shrouding is no problem.
The R/S of 1.86 is also quite attractive. although this engine also does not have quite enough C.R. at 11.5:1 to make the best drag engine.
400 - An overbore of .060" provides a quite satisfactory B/S of 1.11. which is comparable to the 426 Hemi's 1.13. The R/S of 1.77 is a little small, however,
and this limits the high rpm potential somewhat. C.R. is no problem in any application with 12.4: 1 available.
317 - This early engine overbored .060" also has about the same B/S as the Chevrolet 331. The R/S ratio of 2.04 is excellent also, but there is simply not enough C.R. (10.0:1) available to make this an outstanding race engine.
303 - Parametrically. this engine is really far out! Maybe too far out. A B/S of l.45 and R/S of 2.49 are theoretically fantastic, but since you can only get 9.4:1 C.R.. there is no good way to use it.
428 - At .060" overbore, the B/S of 1.05 is good, the R/S of 1.66 is OK and essentially any C.R. is available. If cubic inches are your preference. this engine is a good way to go.
455 - This engine has to be at .060" overbore before it is even "dead square" (B/S = 1.00). The R/S is fairly small at 1.58, but nearly any C.R. is achievable up to 13.6:1! This is not a super-high rpm engine, but that does not mean it will not make horsepower .
Practical Evaluation
Now that we know something about engine theory. which engine do we pick on a practical basis?
Let us use these ground rules:
1. The engine must use a 1965 (some 1964) or later block.This assures that we can put it into any late chassis with no hassle .
2. The engine must use factory production parts whenever possible.This will make the price reasonable and the parts available .
Rule 1 eliminates the 1956 - 317 and the 1958 - 370. This is just as well because you would have trouble finding one and converting it to late model.
Rule 2 eliminates the 1969-303 and the 1970- 366. Although both these engines were developed by the factory, they were never put into production .
Complete 303 or 366 assemblies are available from aftermarket sources (not H-O Racing Specialties, Inc., however) if the price does not bother you.
This leaves us with the .060" 400 AS THE BEST CHOICE. There are certainly a lot of these engines around, so why are they not blowing everbody off the track? Well, as a matter of fact, some of them are! But let us look at the practicality of campaigning one of these cars in, say, NHRA Super Stock. The 1969-70 Ram Air IV 400 (your best configuration) is factored at 380 HP. There are simply other makes in the same class which are factored by NHRA with lower lb/cu. in. ratios (although the lb/adv. hp is the same, of course). NHRA modified production is probably worse than Super Stock because head modifications are allowed. There just are not many companies around who know how to beneficially port a Pontiac head. But after you read this book, you will know how!
You can play with the NHRA rules as well as being done in by them. As described above, the 455 is not that good a high-rpm engine, but it is a great medium rpm engine. Also, it just so happens that the 1971-72 455 HO engine is factored by NHRA at 335 HP and the 455 HO uses essentially the same heads as the Ram Air IV.
You can see the potential of this engine, particularly in one of the automatic classes in Super Stock.
Here is a suggested configuration for a 400 that is really attractive. Use a late 400 HO or Ram Air Block with 4- bolt mains. The stock crank is fine if prepared properly, although a 389 Super Duty or Ram Air V 400 crank would be better. Bore the block. 060" and use +.030" forged 455 pistons. Make up the piston pin height difference by using 6.855" rods ( aftermarket steel rods or specially made aluminum rods) . You will have to pay a fair price for these long rods, but good Pontiac rods are going to cost you some $ anyway, so why not reap the additional benefits of an R/S = 1.83. This combo should really scream and while not 100% NHRA Super Stock legal, it should pass almost any cursory technical inspection.
Valve Sizing
If you have a 1967-up engine. this is one parameter that you cannot do too much about since all 1967-up 4-bbl heads (except Ram Air V) use the same size valves. But it is of interest to determine what the effect on performance is when big valve heads are swapped for small valve ones on the same engine .
We previously did a simple analysis that "determined VE as a function of stroke only. When stroke got larger, maximum VE occurred at a lower rpm.
This was because mean piston speed was constant. Also held constant (although not discussed) was the corresponding gas speed through the intake port and valve.
which was actually the determining parameter. Gas speed is proportional to swept volume times rpm divided by valve lift area. Therefore, the intake valve size and (cam) lift determines the gas speed if swept volume per unit time (mean piston speed) is held constant. If we determine the increased rpm required to compensate for the increased valve area (gas speed again constant), we will have a good idea of what the new peak torque (VE) and horsepower rpm will be when we swap heads.
The most probable head swap is a 1967-up 4 bbl head for a pre-1967 or late 2- bbl one. In either case, the change in valve diameter is 2. 11 " for 1.92" or 1.96 " . The corresponding increase in valve lift area for the former is 10% if the cam and rocker arms are unchanged. The rpm must be increased by a like amount to maintain the same gas speed. Since peak torque is the same as before (VE and CID have not changed) it is apparent that the horsepower (torque times rpm) has been increased by 10% also. This is the simple reason why a swap from small valve to big valve heads will produce a performance gain.
It is advisable to also change rear end gear ratios by a like amount (10% in this case, i.e., 3.42:1 should be changed to 3.73:1) to compensate.
Engine RPM Limits
There are two engine rpm limits that are important to consider. These are the rpm levels for continuous safe cruising and the maximum rpm that the short block can safely handle. The specific maximum rpm is determined by:
where:
R = Rod strength
M = mass of reciprocating parts
S = Stroke
ILF = Inertial loading factor
F = Factor of safety
For a reciprocating mass consisting of 830 gm. for piston, pin, and rings plus the rod. small end weight as noted ( i.e.. 1252 gm. for 541000 rod), the following chart lists the maximum rpm limits for a factor of safety of 2.0 : 1.
|
|
Stock 541000
|
Early forged 532294
|
RA V 545855
|
63 SD 529238
|
455 SD 485225
|
|
Rod Small End Wt.
|
422 gm.
|
422 gm.
|
411 gm.
|
422 gm.
|
355 gm.
|
|
Stroke
|
|
|
|
|
|
|
2.840 in.
|
7267 rpm
|
7438 rpm
|
8574 rpm
|
8886 rpm
|
9624 rpm
|
|
3.250
|
6670
|
6828
|
7869
|
8156
|
8834
|
|
3.375
|
6572
|
6727
|
7754
|
8036
|
8704
|
|
3.563
|
6310
|
6458
|
7444
|
7716
|
7357
|
|
3.750
|
6116
|
6260
|
7216
|
7479
|
8103
|
|
4.000
|
5881
|
6019
|
6939
|
7191
|
7789
|
|
4.210
|
5700
|
5835
|
6727
|
6970
|
7549
|
If your piston, pin and rings weigh a different amount than the reference mass, the rpm limit is changed by the square root of the reference mass divided by the actual mass. Example: piston, pin and ring assembly weight is 730 gm.
830 + 422
730 + 422 = 1.042 (New rpm limit is 1.042 times higher than old limit)
If you run your engine at a higher rpm than that listed in the table, then the factor of safety will be reduced by tht square of the ratio of the listed rpm divided by the actual rpm. Example: 3.75" stroke with stock rods run at 6400 rpm.
(6116/6400)2 = 0.913 (New factor of safety is 2.0 x 0.913 = 1.826)
If the ratio of the rpm's is less than 0.7071. the resultant factor of safety will be less than 1.0, which indicates that the stress level exceeds the rod strength and the rod will probably break immediately. At factors of safety between 1.0 and 2.0. the expected life is correspondingly less than maximum.
For cruising. the exact rpm is not as critical. If you observe a cruise limit of 2500 fpm mean piston speed you will have no problems.
Main bearing diameter also has an effect on maximum engine rpm limits. A large bearing diameter allows a large crankshaft cross-section which means greater strength, but it also means increased oil pump pressure to overcome the centrifugal force of the oil in the main journal. Figure 6 below shows the minimum oil pressure for any rpm .
