Mild 390 build advice
#31
12-07-2018, 09:11 AM
Logistics Pro
#32
12-07-2018, 09:47 AM
Logistics Pro
I'll run it this weekend and post results , I'll run with the 256/262 & 262/268 as i already have them In my Cam folder for DD5
I have the HEI dizzy it is big and ugly , only good is replacement parts are available at every local Auto parts . so If something fails you are not stranded with parts on special order .
I have the HEI dizzy it is big and ugly , only good is replacement parts are available at every local Auto parts . so If something fails you are not stranded with parts on special order .
#33
12-07-2018, 12:50 PM
#34
12-07-2018, 01:08 PM
. As someone else pointed out, the forged piston is also overkill in this power range.
Ha..."mild cam = 290* duration"...
The stock c8/d2 heads vary widely in combustion chamber size. I've seen 68-72 being most common ranges. If you got some that were 72 cc or maybe even some 74 cc outliers, that makes a world of difference. I quoted compression ratios above with a .050" gasket and a 68 cc chamber.
68-72 is the typical casting tolerance in any Ford head. 4 ccs makes about a tenth of a point difference in the comp ratio.
Just avoid louvered glass packs and get perforated. A turbo-muff isn't going to give you more power over a perforated glass pack on this planet. Just as importantly, stay reasonable with exhaust pipe size (2-1/4" rather than 2-1/2" +) for your rpm range.
Unless you let the muffler shop mount them backwards. That happens a lot, then mounted backwards, there's a whole lotta difference in power..
Ha..."mild cam = 290* duration"...
The stock c8/d2 heads vary widely in combustion chamber size. I've seen 68-72 being most common ranges. If you got some that were 72 cc or maybe even some 74 cc outliers, that makes a world of difference. I quoted compression ratios above with a .050" gasket and a 68 cc chamber.
68-72 is the typical casting tolerance in any Ford head. 4 ccs makes about a tenth of a point difference in the comp ratio.
Just avoid louvered glass packs and get perforated. A turbo-muff isn't going to give you more power over a perforated glass pack on this planet. Just as importantly, stay reasonable with exhaust pipe size (2-1/4" rather than 2-1/2" +) for your rpm range.
Unless you let the muffler shop mount them backwards. That happens a lot, then mounted backwards, there's a whole lotta difference in power..
#35
12-07-2018, 03:38 PM
#36
12-08-2018, 02:58 AM
And how does that relate to a tenth of a point in the ratio ? You're nit picking here. I ran the stock 10 to 1's with 87-89 octane with two different cams and no detonation
#37
12-08-2018, 07:29 AM
How do you get 1/10? Have you actually done the math? With a 10 cc piston, the difference from a 68 to 72 cc head goes from 10.1 ish to 9.7 ish. That's hardly nitpicking. Change that to a 19 or 20 cc piston and now we are in the 9 to 9.4 range: something much more appropriate for a cam that tight. This is *hardly* nitpicking. I just threw in a .050" gasket and there will obviously be some error in parts and some other variables to adjust slightly up or down from here. To truly know you would have to have everything measured.
It is not wise to push yourself to the edge on compresstion, especially on a mild build. There is no point.
I once did squats after binge eating Mexican. It worked out for me, but I don't recommend it to others because I know I got lucky and that the outcome would likely be disastrous.
Last edited by the_hetz; 12-08-2018 at 12:07 PM. Reason: fixed misspeak
#38
12-08-2018, 11:12 AM
First, a 290 duration cam is not a mild cam, something we keep going back to. This allows you to run higher compression. Even the 272 cam you mention is slightly bigger than what he is planning to go with. That's great that it worked out for you, but if you didn't measure everything then you really don't know where your comp landed, and you may have gotten lucky.
How do you get 1/10? Have you actually done the math? With a 10 cc piston, the difference from a 68 to 72 cc head goes from 10.1 ish to 9.7 ish. That's hardly nitpicking. Change that to a 19 or 20 cc piston and now we are in the 9 to 9.4 range: something much more appropriate for a cam that tight. This is *hardly* nitpicking. I just threw in a .050" gasket and there will obviously be some error in parts and some other variables to adjust slightly up or down from here. To truly know you would have to have everything measured.
Maybe you should try doing math here. What cc the piston top volume here is irrelevant to a 4 cc difference in the head, the difference doesn't change. And funny you should chastise me on math, what with your spelling errors.
It is not wise to push yourself to the edge on compresstion, especially on a mild build. There is no point.
Forged rods are cheap insurance against a broken rod. Studded heads and mains are cheap insurance. The list goes on and on. Once you add up all the cheap insurances, it is no longer cheap. For a build this mild, quit at the beginning and buy the correct parts.
I once did squats after binge eating Mexican. It worked out for me, but I don't recommend it to others because I know I got lucky and that the outcome would likely be disastrous.
#41
12-08-2018, 12:40 PM
Well you've never run these cams I see. If you had, you'd know they are quite mild in a 390. The 272 in that Merc's 390 idled smooth as silk except when cold, even then you had to listen to hear the little lope it had. Ditto for that Cam Dynamics 290 grind. Don't know why you'd segue off into forged rods here. ALL Ford pushrod V8's had forged rods. You don't have to go all out in any build.
My math is solid and not made up; I'm not sure what you have against it. Let me explain: I showed rough ranges for both a 10 cc piston with a 68 cc to 72 cc head (approx 9.7 to 10.1) and similar ranges for a 19 cc piston with a 68 cc to 72 cc head (approx 9 to 9.4). The 4 cc of head difference is neither insignificant, nor is it "only 1/10 of a point of compression" in either case. It could be enough to make or break the build. You may have had chambers at the high end or even above the 'spec' while his may be at the low end. Without measuring, play safe. In a mild build, play safe.
By the way, a 4 cc difference in head volume actually WILL affect the compression more significantly with a 10 cc piston than a 20 cc piston, but not much. This is not a point that I was trying to make before as it doesn't really become significant within reasonable bounds, but you seem to have brought it up. Change in cc is not a linear relationship with compression ratio.
Please be specific about my spelling errors and I can correct them. I do what I can to proofread, but there is a diminishing return on time spent trying to catch every single thing, especially on a cellphone. I did not "chastise" you; I questioned whether you did the math because the "1/10 of a point" claim was solid evidence that you had not.
Regarding the cams: NO, I have not run either, nor have I claimed to. I did not say the 272 was a big cam, I only pointed out that it was slightly wider than what he has chosen and is therefore a step in the right direction. This, combined with other variables (such as actual head volume) may be enough to make or break things. That is the point I'm trying to make. A 290 cam will allow a LOT more pressure to bleed off on the compression stroke which again could make the build work. Dropping this down to a 256 or even 262 is HUGE difference and makes it an apples/oranges comparison.
What did you use for intakes? How aggressive were the ramps on that 290 cam? A 290 cam with similar .050" specs to the ones at hand would be a pretty sluggish ramp/old design/not efficient. If it had big .050" specs then it is no longer mild (I noticed that you didn't claim the 290 to be 'smooth as silk'). All of these things affect cylinder filling and therefore pressure and therefore detonation characteristics.
What did your timing end up being through the RPM range and was it optimized for an old school, slow burn head?
#42
12-08-2018, 01:18 PM
Logistics Pro
#45
12-08-2018, 06:40 PM
The advice given was that a small cam and the flat tops (very high compression) together is a *terrible* combination. That high of compression will require a big enough cam that stock heads are out of breathing room where it starts to shine, and the bottom end will start to die...the worst of both worlds.
You NEED pistons with a dish and a modestly larger cam (up 1-2 more sizes). Even the 268/276 cam recommends compression down in the 9.1-9.5 range.
Good luck. If it works, let us know. If your machinist is an SBC guy and that's his evidence for what works, you're in deep.
You NEED pistons with a dish and a modestly larger cam (up 1-2 more sizes). Even the 268/276 cam recommends compression down in the 9.1-9.5 range.
Good luck. If it works, let us know. If your machinist is an SBC guy and that's his evidence for what works, you're in deep.