Cam Recommendation for Vacuum Rule Class...

[Travis Gibson]

A good family friend has been involved in "mud racing" up North here (the dakotas) for several years now and every year has been a struggle against the vacuum rule to compete in his racing class. The trucks race through 150' mud pits and ET's are not measured.

The rule states that the truck must pull 15 inches (of Hg) of vacuum at an 800 rpm idle. Other rules include a 468 max cube limit, no aluminum heads, no transbrakes, no fuel injection, single carb, gasoline only (high octane race fuel allowed), 37" tall tires max, and new for this year, roller valvetrains allowed.

So, with the addition of the new rule the cam must be changed, and this is actually a good thing because the old 274 Xtreme Energy Comp cam (hydraulic flat tappet) did not pull enough vacuum.

The engine is a 468 big block Chevy with heavily ported #781 iron heads with 2.19/1.88" valves, combustion chambers were large since unfortunately the intake valves had to be sunk heavily and measured on average 122cc. This put the static compression at approximately 12.4:1. The induction consists of a Holley Strip Dominator with a 1000 CFM 4150 Holley mechanical secondary carb with annular boosters. The ignition is MSD, from the billet distributor to the CD-box. The engine has 47cc hollow-dome Probe pistons, a GM iron crank, and stock length rods. The engine stalls at around 3000 rpm or slightly lower and has only 4.10 gears but this is deceiving because of the transfer case multiplication, it does use a 37" tall "Super Swamper" tire. The vehicle is a '86 S-10 with a big block somehow stuffed in with 14 bolt rear-ends, it weighs in at around 3200 lbs. Transmission is a TH-400.

The plan is to use a small solid roller as the engine revs to about 6500-7000 rpm depending on the condition of the "track." Also, the valves are fairly heavy Manley street flow pieces with 3/8" stems, so the control will be difficult and the valve springs of course will need to be replaced. Beehives will probably, and very unfortunately so, be out of the budget. The only piece of the valvetrain that will stay will be the Scorpion 1.7 aluminum rockers.

We are out of ideas for camming this engine. The first cam used was a fairly wide-lobe separation (112°) hydraulic flat tappet, I'm unsure of the duration but this was when the engine had very little compression, a dual plane intake, and a 800 CFM carb and it pulled 17.5" of vacuum. The second cam was a Comp 270XE4x4 (hydraulic flat tappet) that also pulled about 16" of vacuum and it was on a 111° LSA. The next cam was a Comp 274XE on a 110° and vacuum dropped to 14.5" and it wasn't usable. The trend with all the other racers is to use big duration on a wide lobe separation (for example 280-290 degree profiles on 114 or 115° LSA's). Everybody but us have been using solid tappets for years, and maybe that's why we were behind at times. I'm hoping that the roller will help pull vacuum and that it will work miracles for power production, as the "competition" (mostly 385-series Fords) are dyno'ing at about 620 horsepower (if that means anything). It seems all of the engines rev way past their peaks (reason for single plane intake and "big" carb) so maybe a wide lobe separation is beneficial, what do you think?

Thank you very much for taking the time to read this huge wall of text. Anything you can say about how to "deal" with these engines or style of racing will be beneficial.

[rookie]

Where is the cam installed at, and what is your timing at idle?

[Travis Gibson]

The current cam is 4° advanced and the initial timing is 24°.

The elimination of the vacuum advance on the MSD billet distributor hurt the timing curve a little, as it has to be restricted heavily since it starts so high. Total is around 42°.

[Travis Gibson]

In case anyone was wondering, the exhaust system is a set of Schoenfeld "out the hood" headers with 2" primaries into 4" collectors. I don't choose all of the parts for this engine, and this proves it because the secondary length is INCREDIBLY short and would look a little funky with extensions sticking a foot into the air

Anyone have any ideas how to make the engine think it has a longer collector/secondary length?

[rookie]

Going wider on lsa is going to kill power on 468ci. I would advance the 274 cam to 101 or 102 center line and dial in a little more inital advace, if I understand correctly you only need a half inch of vac. from where you are at, you could probably get that with timing alone.

[Travis Gibson]

We don't intend to keep the flat tappet cam in a class full of guys using rollers, I would think it's too much of a disadvantage, or is it not with these small cams?

Wouldn't having the cam 8-9° advanced really hurt power as well? I can't imagine that the engine would even want to rev to 7000 with that much advance, or am I completely wrong?

[rookie]

Here is an article on a 468 , Big Block Chevy Motor - Popular Hot Rodding Magazine it ia calling for a lsa. of 108, if you put that in the engine 4 degrees adv. then you are at 104, it only looks 8 degrees advanced if you start with to wide of a lsa. ie: 110-112.
The intake closing point is what builds cylinder preasure and prevents reversion and creats vac. if you close the intake to late you loose vac.

[rookie]

The whole idea of a vac. rule is to keep guys from running big dollar, high rpm engines, so if you can build the most usable power in the given range, then you don,t need the rpm and most top end power.

[rookie]

Here is another a few more cubes than you are running but as you see it has a 107 lsa with a 103 center line.

Assembling The Bottom End Of An Engine Block

Note power graph Part 2

http://www.popularhotrodding.com/eng.../photo_19.html

[BlackCat13]

plenum divider will probably add 1.5 vacuum and 20 lb/ft torque with that same CC 274XE cam.

No matter what cam, try the divider.