I've been following along in many of the threads but something just isn't clicking for me. Compression(torque) vs. HP (speed). I understand that you can't have both. You either have a stump puller or a 1/4 mile motor, right? So if I raise compression (thinner head gaskets, flat head pistons, etc), does that increase the size of the stump I can pull or better my time to the finish line?

 

Increasing the compression, all other things being equal, does both. What it does, is increase the efficiency at which the fuel can be burned. Thus, you get more power per power stroke, making more hp and torque across the board.

However, when you increase compression too much you get preignition. To alleviate this, you go with a bigger cam. It bleeds off the compression down to a usable level. However, due to the increased overlap, you increase torque at higher rpms and decrease it at lower rpms. The HP goes up, low end torque goes down. Usually the peak torque goes down too. This is why we have race gas, so people can run high compression with smaller (relatively, they're still big) cams so the torque doesn't come on at some obsenely high rpm.

 

Okay, I get it. I'm guessing that I have average compression. If I go with thinner head gaskets and a cam between .530 and .560 lift, will that still keep me in the happy zone? What is the compression that I want to stay away from? I won't be changing pistons or anything like that.

 

I am not sure that the cam has anything to do with compression ratio – it is the volume difference in your combustion chamber/cylinder between bottom dead center (BDC) and top dead center (TDC). If you will post more details on your build someone may be able to tell you what compression ratio you can expect. Overlap helps draw the fuel/air mixture into the cylinder at higher RPM – I do not know but don’t think it changes the compression Ratio. The two cams I am looking at – the Crain 343941 and 343901 – both have valve lift of over .5, and no overlap. Which cam were you looking at?

Where I am struggling is with compression ratio – one piston option yields a CR of 9.8, another 10.2. I will be pulling the engine hard at low RPM and worry about pinging – anyone have any experience running low grade pump gas with these sorts of CRs?

Thanks -

 

Yes, the cam does indeed have a huge effect on compression ratio, but not because of overlap. The function of the cam that affects compression ratio is the intake valve closing event. As we know, the intake valve does not close until after the piston as gone past bottom dead center. This means that the as the piston begins to travel up the bore on the compression stroke the intake valve is still open, which bleeds off compression at lower rpms. If you add duration, which pushes the intake valve closing event farther into the compression stroke, it bleeds off even more. This effect is eventually overcome by the inertia of the incoming intake charge at higher and higher rpm until it can create a "ram" effect. If you intend to run a large duration cam, it is imperative that you compensate for it with increase compression. Have you ever noticed that the cam manufacturers always recommend higher and higher compression as their listed cams get longer in duration? Conversely, if you have higher compression, then install a shorter duration (lower rpm) cam, you will almost assuredly run into ping. The terms we use to discuss these things are "static compression", which is the most talked about type, and "dynamic compression", which is the real net compression after the cam has "bled off" it's share.

-Scouder

 

I have had two 390's now with a 10.25 to 1 comp ratio that were happy as s*&% running 87 octane gas. The first was a GT 390 from a 67 Stang, with TRW forged flat tops, can't recall the head casting #'s, but the cam in it was a Cam Dynamics grind with an advertised duration in the 290's range ( been 20 years so I can't recall the exact spec here) the lift was .516 with 1.73 rockers. I had this motor in both the Stang and later in a 70 F100. Never had pinging issues in either vehicle. Now I have another in a 68 Merc Monterey. This motor has the same comp ratio-- 10.25 to 1 as the GT motor. The heads are C8AE-H's, cam is a Crane Energizer with a 272 degree duration and .523 lift ( 1.73 rockers) This motor also runs fine on 87-89 octane gas. Now this motor DID ping like crazy with the stock cam it came with, it didn't even like 93 premium. Nothing else has changed, just the cam and .041 thick head gaskets in place of the stock .020 steel shims. This lowered the comp ratio 1/4 point, not enough to count as the reason it likes low grade gas now. I also have now a 10.4 to 1 331 stroker motor in my 89 Ranger with aluminum heads, a Z303 Ford roller cam ( 290 degrees w/.587 lift) this motor also does not ping with 89 octane. 87 it doesn't like, but it's fine with 89 .

 

Thanks, Scouder, again you educate me and likely save me from making an expensive mistake. I forgot or missed that the intakes stayed open at the start of the compression stroke. With the 941, they are open for the first 35 degrees; with the 901 they are open for the first 29 degrees. That is assuming I am reading the charts correctly.


Crain 343941
Cam Timing: TAPPET @.050
Lift: Opens Closes Max Lift Duration
Intake 1 BTDC 35 ABDC 107 216 °
Exhaust 51 BBDC (3) BTDC 117 228 °

Crain 343901
Cam Timing: TAPPET @.050
Lift: Opens Closes Max Lift Duration
Intake (5) ATDC 29 ABDC 107 204 °
Exhaust 45 BBDC (9) BTDC 117 216 °

I wasn’t able to find a Crain Energizer cam in their catalog, but that is most likely why baddad’s 68 Merc stopped pinging after he went with a long duration cam – he had lowered his “dynamic compression.”

I will add yours and Baddad’s post to my notes – I have about got all I need to build my perfect engine.

 

De nada.

-Scouder

 

You can access the Energizer's specs on Crane's website. Summit list's it in thier catalog, cam & lifters for $110. It's got good bottom end and pulls hard in the Merc up to 5500 rpms, enough to push the 4200 lb Merc to mid 14's, with the std bore stock pistons, a 428PI intake, a 3310 750 Holley carb, FPA headers the stock C-6 trans and 3.70's out back. Idles like a stocker, only on the coldest days can you hear it lope a little. Ought to work good in a pickup too.

 

Close you eyes and imagine if you will-hypothetically speaking...10:1 compression, Crane 941 cam, iron C6 heads (stock) port matched to Edel Performer intake. Dual headers with 3" collectors to 2.5" exhaust. Edel 4bbl carb.
I think I may be putting all this information together in my mind and coming out with an end product finally. Does this setup sound desireable? I know it's not ProStock or anything but work with me.

 

Sounds exactly like my build, with the exception of the intake. I'm using the RPM intake with head ports matched to that.

Should work good.

 

That is what I am working towards - except that I still dream of finding a 428 crank and rods.

I am also uncertain about the headers - all I have been able to find are long tube, and the tri-y style is supposed to be better for low RPM work. It is looking like I will have to build them.

 

Long tube headers wont hurt you at all at low rpm. I've got a set on my 360; they work great at hauling gravel around.

Unless you're looking for that last 1 ft-lb to lose 10 hp up higher, I wouldn't worry about it. Either option is so much better than the stock logs it's unbelieveable.

 

Rusty, do you have a good takeoff from the stop and still able to haul gravel?

 

You bet! While I am still running the stock internal 360, it has enough power to set you back in the seat and it will haul gravel like it's not there. Last year I hauled a ton of gravel in the back of it, worked great. Didn't slow it down much, hardly noticable.