Cylinder pressure bleed down...
 

I have heard/read that: cam selection is critical to bleed down cylinder pressures when running over 9.5:1 comp ratio on 93 octane.

Can anyone clarify this statement please?

Nope.:D

Did you get around to checkin your cyl. pressures? Generally a shorter cam will raise cyl cranking pressures, and the opposite for a longer cam. If a cam is advanced it raises cranking pressures, and the opposite if it is retarded. Lobe separations like 114 to 118 will lower pressures when compared to 112 or 110. If cranking pressures are to low, the motor will be a soggy performer. This happens when to large of cam is installed without enough cubic inch or compression. If the cranking pressures are extremely high the motor will be powerful down low, but may be prone to detonation at the higher end of the torque curve where the dynamic compression is at max. A rule of thumb I have read is that motors running on pump gas with 9 to 10 to 1 static compression should try to keep the cranking pressure between 140 and 180 psi. Hopefully your 540's crank 160 to 170. If they are 200 or higher you may need more cam or more exhaust duration or the cams may simply need to be retarded a few degrees. Let us know what you find out when you run a compression test.

Mark,

Wette Vette has the story straight! If you have more questions go to Lane Automotive and pick up the book titled How to build Max Perfromance Chevy Rat Motors by Ed Staffel.. Some very good reading with formulas.. that will answer your questions!!!

I added this one to my recommended reading list some time back..

Dick

Thanks again you guys! :)

My cams are 236*/244* @ .050 on a 114* LSA. They have been dialed in at right about 108* or so. My comp ratio is about 9.7:1

I have yet to do a comp test

With solid lifters cylinder pressure can be a tuning aid along with setting the valve lash. Tighten the exhaust a little and watch the pressure go down. Tighten the intake and the pressure goes up. You are limited on how far you can adjust the lash in both directions, but it is fun to see the effects of slightly more or less cam lobe area can effect the cranking pressures.

Hey KAAMA, I spoke to some more people about which has more potential between solid flat tappets and hydraulic rollers. So far the overwhelming decision is that solid flat tappets spank hydraulic rollers. This comes from some pretty good engine builders and I tend to agree with them. Talk at ya later!

Craig/Wette Vette,

Thanks for your responses. I have the hydraulic roller lifters/cams. Some guys have told me that a "roller" cam, (even a hydraulic roller), that because of its nature of having a roller profile is always going to make more power than a flat tappet cam----even over a solid flat tappet. It just keeps the valve open longer because of the nature of its roller profile. Whether my engine has a hyd roller or a solid flat tappet cam---I'm only going to run it to about 5400-5600rpm tops---so which one is best up to and including 5400-5600rpm tops?. Will the flat tappet beat the hyd roller BEFORE 5600? I like the flat tappet solid lifter because there are no moving parts and that they're lighter than a roller lifter, BUT I like the power of a hyd roller----that is IF they really do make more power over a solid flat tappet.

I love talkin' about this stuff. :)

Straight from my Erson Catalog I would say with your compression a mechanical flat tappet cam with 246/254 at .050" with .585 lift would be real close to matching your cams output throughout the RPM range, and probably more at the higher revs, with good reliability. Only my opinion. I'll see what the desk top dyno says.

The flat tappet solid made 719 HP at 6000 RPM and 675 Ft. lbs at 5000 RPM. With a ROLLER cam of 236/244 @ .050 on a 114 installed on a 108 made 708 HP at 6000 and 710 Ft. lbs at 4500 RPM. Good numbers by both cams. The motor chosen was a 540 at 9.5:1 compression with fully ported heads and large tube headers with open exhaust. 1000 CFM carb on a single plane manifold. The only thing that may be misleading is they don't specify whether the cam is solid or hydraulic roller. It just says roller cam. For the flat tappet stuff thay have a choice between hydraulic or solid lifters. Desktop Dyno 2000 is the one I have.
What do you guys think??

Craig, did you use the "Seat to Seat" duration vs the "@ .050" numbers? I have that program too! It shows that if you go by the "@ .050" duration numbers that the results are much poorer. I like it though--good ref material. I noticed that too about the roller cams not specifying hyd vs solid---what's up with that?

Kamma;

I would talk with some of the cam companies on that issue, not some flunkie on the desk either someone who really knows...like a Gordon @Comp or Steve Tanzy @Erson. I'm interested in what they say on the solid flat vs hyd. roller @5600ish as well. Solid flats will obviously kick a hyd rollers ass over 6000. My hunch is the solid flat will make just as much in the 5500-6000 range as well. :cool: The best thing is they are unbelievably cheap.;)

jdnca1, the Desk Top Dyno is suprisingly close (SOMETIMES) to some real dyno sheets I have using the same data i.e. cam specs, carb size, header types, comp ratios, various heads, etc. It's interesting and can be a lot of fun to play with. I will check into Erson & Comp. Thanks :)

When using Desk Top Dyno, the cam type you choose is defining how aggressive a lobe design you're running more than just the type of lifter. For instance a good hydraulic roller really has a cam lobe more similar to a solid flat tappet than a solid roller. A stock GM hydraulic roller probably more closely resembles a flat tappet hydraulic lifter cam. I generally specify a solid lifter design for engines with aftermarket hydraulic rollers and it works out pretty close.