Restricter Race Motors -make more power than your opposition

[Travis Gibson]

A comment made by someone else really interested me...

In an engine built entirely for fuel economy would the intake runner need to be as small a restriction as possible? I would believe that a fuel economy engine would benefit from the reduction of pumping losses but at what point would reducing pumping losses be counteracted by adding in excess charge weight?

Great article by the way, it's wonderful that we have someone writing here that can whip up something that quickly that can really spur the minds of dedicated engine builders and novices as well.

[DavidVizard-GFN]

Travis,
Don't confuse small ports for small carb venturi for economy. Having said that an engine designed for economy does need to have a smaller port as it should by sized more for lower rpm use and strong torque to accelerate the vehicle without necessarily having to down shift.
DV

[rookie]

Also check out this build.
Engine Build - Competitive Power Late Model Stocker

[80' 427]

Its been a bit but we have been racing and had to build a new car because of a bad early season wreck. I did add a pan evac thinking any aid in reducing internal pressures would help 2 fold. One in would lower the internal pressures to help in ring seal and second it would help prevent oil leaks which can get us a black flag. One question I have come up with as of late came from working on a hobby stock whick is even more restricted with stock exhaust and heads. When building a cheap motor to replace a claimed engine the owner used a "pro am" cam shaft that he got for free. They were a much more open class motor so the engine is now down 40 hp compared to the previous engine. In an attempt to crutch the cam shaft 1.6 rockers were added to the intake. This caused a instant loss of 15 more horsepower. Switching the 1.6 rocker to the exhaust returned the power but had no gain over the 1.5. I guess I am stumped about this. Of course the cam is unknown so the knowledge gained is limited. But thinking why this happened is it the reduction in intake port velocity? That the exhaust is so restricted that a increase in intake charge actually is casing more exhaust gas than the system could handle causing a contaminated air/fuel charge? Why did the exhaust gain nothing? It confuses me. Should we have tried a 1.3 brake in rocker on each side?

[DavidVizard-GFN]

80'427

Sounds like too much cam here for an intake and exhaust that cannot effectivly flow in/out or make use of pressure wave tuning. The drop when a 1.6 rocker was used is a sure sign of too much cam in general and too much overlap in particular.
DV

[80' 427]

the intresting thing is since my last post the guys tried a smaller isky that nearly everyone here runs in this class. It is up 20 hp but there is no gain or loss with a 1.6 on intake or exhaust. I am wondering if they should try a break in rocker to see if there is a loss or gain.

Also talking to a local hobby stock racer/machinest that has just quit racing. By going from a rochester to a autolite 1.33 2bbl he typically saw a gain of 10-15 lbs of torque. The cfm is the same but the autolite uses annualar boosters.

[80' 427]

Still more intresting info. First the hobby cam specs I was talking about before. The cam that 170 rear wheel hp, duration @ .050 was 257 intake, 261.5 exhaust, .544 in .552 ex lift, 105 lc, 101 In cl, and 49 degrees of overlap. The 191 rwhp cam was Dur @ .050 248 in 254 ex, .534 in .525 ex lift, 106 cl, 73 degrees of overlap. I guess I would have guessed the cam had too much overlap but it is backwards. Remember the 1.6 rocker on the intake of first cam lost 15 hp more.

Now just some more fun facts. This weekend is cheater night (anything goes) so the we used this as chance to see what needs improved. Just a swap from lt1 manifolds with 2" pipes to 1 5/8" header with 3" collectors (an old dented and rusty set I had) jumped 40 hp. The jetting was still perfect. The 2" pipes are required per rules. Time to get out the grinder and do some manifold porting.