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Low compression… ignore it and build as if it were already a high compression engine, ignore all power down low.
Add supercharger later…..
compression or flow?
- Thread starter mopfried
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Low compression… ignore it and build as if it were already a high compression engine, ignore all power down low.
Add supercharger later…..
Phreakish
Well-Known Member
I'm no expert, but you can increase airflow the engine a couple of ways. ports can flow more or if the ports can't be opened up then more duration will flow more.
More duration tends to perform better with more compression to make up for dynamic losses from later closing intake.
So if you go with better/bigger heads/intake/headers you can (theortically) make the same/more power with less duration and not be quite as crippled by the lower compression (since the IVC is sooner).
Higher compression might support more duration but then the peak power will come in later - and a streeter with 3.55 gears and a 4spd isn't going to get to tap into that as often.
It's this dynamic that would personally lead me to favor flow vs compression.
More duration tends to perform better with more compression to make up for dynamic losses from later closing intake.
So if you go with better/bigger heads/intake/headers you can (theortically) make the same/more power with less duration and not be quite as crippled by the lower compression (since the IVC is sooner).
Higher compression might support more duration but then the peak power will come in later - and a streeter with 3.55 gears and a 4spd isn't going to get to tap into that as often.
It's this dynamic that would personally lead me to favor flow vs compression.
BINGO! The easier it is to breath in and out the more effective the package is. I say it a lot, get the best cylinder head you can on top of your engine. This does not mean put a pro stock head on your 7.8-1 - 318!
I feel the same way for exhaust. I’m done with the 1-5/8 headers! The only worth while one is Summits w/the thick flange but it has 3 tubes under the steering linkages and IIRC, Doug’s has a 1-5/8 that’s worthy.
Otherwise, 1-3/4 @ a min!
Belvedere2
Active Member
What's still out there for W2 headers? I have TTIs. I remember Hooker used to have a Super Comp 1 7/8 I think. It's been awhile.
Newbomb Turk
Well-Known Member
Ideally you want compression and flow. Due to decades of myths, lies and downright chicanery guys don’t use enough compression for the cam they want to run.
If you can’t get compression get some flow and crutch it with cam timing.
Newbomb Turk
Well-Known Member
The Hooker W2 headers were 2 inch and have a tube on each side going through the fender.
Another option is Tubular Automotive. They make W2 headers. I’ve never used them but I’ve used one of their Pontiac headers and they are very nice.
Hedman, Tubular automotive in Ct.
Dang! Got tree’d!!!!
Oh! You can also get flanges through summit and Schoenfeld.
Schoenfeild? Spelling?
Oh! You can also get flanges through summit and Schoenfeld.
Schoenfeild? Spelling?
Newbomb Turk
Well-Known Member
Lol I saw that but I was going to let it slide.
BTW, what did Milodon say about your gear drive being undersized like that?
Never spoke them about it. Currently I have a bunch of stuff going on in between posts walking around inspecting the garage going up, contractors for the pool, garage floor finishing, honey do list and my sons request to donate his old cloths to St. Vincent DePauls cloth donations for the poor.
I took a minute to run a tap in the 352 crank snout…. I just sat down …. Gotta guy due in 15 minutes…. Mo-Money spent….
IMO, calling Milodon will probably result in “It’s my fault” conversation…. I’ll get around to a machine shop later…. When IDK… I’ll get there. Otherwise it’s just timing the cam and assembling everything. It’s been mocked up a few times for fitment issues/checks… I’m minus a distributor drive. I have 3 somewhere in my mess and I can’t find them worth a crud!
Anyone who questions why a low compression cannot make power needs to look at some of Jim Laroy's low compression builds posted on here. They're not hard to find. So yes, if you think a low compression engine cannot make power, you are indeed wrong.
If you built a 400 hp 8.5:1 cr for every point you go up in cr you'll gain about 3-4% or in this case about 12/16 hp, 9.5:1 = 412/416 hp, 10.5:1 = 424/432 hp etc... Given the fact most street engines are gonna be between 8.5-10.5:1 cr were talking 0-8% hp difference.
It matters but not the end of the world. But unlike other mods it's loss/gain is across the entire rpm range.
It matters but not the end of the world. But unlike other mods it's loss/gain is across the entire rpm range.
33IMP
Well-Known Member
If .I remember right.....
O.P is talking about losing a half point, down to 10 from 10.5. Hardly a "low compression" engine.
His W2s are assumed to flow better than his RPMs, and drop his compression into a safer zone for pump gas. The extra 50 lbs or so might matter to a 2000 lb racecar, but won't be felt in a 3500 lb streetcar.
WIN-WIN for the W2s imo.
O.P is talking about losing a half point, down to 10 from 10.5. Hardly a "low compression" engine.
His W2s are assumed to flow better than his RPMs, and drop his compression into a safer zone for pump gas. The extra 50 lbs or so might matter to a 2000 lb racecar, but won't be felt in a 3500 lb streetcar.
WIN-WIN for the W2s imo.
Newbomb Turk
Well-Known Member
TT5.9mag
Two atmospheres are better than one
Generally the ones I see not upping cr are just adding parts to a running short block and are probably in the 200-399 hp range. I agree if building from scratch cr that works with your cam and gas and tuning abilities should be apart of your plan.
Don't forget loose ignition timing too. Between those two, you can make a pretty dang good "crutch".
Use the Wallace (or other) compression ratio calculator
If your pistons are the domed kind, the chambers open or closed, AKA, how many cc’s are your W2 heads?
If your pistons are the domed kind, the chambers open or closed, AKA, how many cc’s are your W2 heads?
Newbomb Turk
Well-Known Member
Per the Chrysler book your pistons should be out of the deck about .040 or maybe a bit more if your head gaskets are thicker than .028 or whatever the thickness the book says. It also depends on how much have been milled off the heads.
They should be out of the deck gasket thickness plus the depth of the cut out on the head minus .040 for gasket thickness. That’s assuming you have an .040 gasket. You can use whatever number of gasket thickness you have.
If you have an .080 cut out and a .040 gasket you need to be .080 out of the hole.
Correctly made pistons already have the deck of the piston higher than the rest of the piston.
If you have a .028 gasket and a .060 cutout you’d need to be .048 out of the deck.
I'd already have um on there.
Well I might just go with the 340 and W-2 heads then. I need to mock it up and do some measuring to see exactly how far the stock pistons are out of the block. The block is apart and I dont have main or rod bearings to do that yet.
I also need to CC the heads to see exactly what CC's they are. They are open chambers and from memory they were 70. I really need to know for sure. I did check and the W-2 heads have long valves. I have nearly an inch from closed valve to retainer clearance. I have both iron and roller rockers with the offset shafts and stands. Dont know anything about the springs.
Not opposed to a big cam jerking around at low RPM on the street isnt a deal killer for me.
And im not opposed to using my 391. I just thought the 355 would be a little more highway friendly.
BTW you guys are awesome and a lot of help.
Thanks. I have been out of this game for a while.
I also need to CC the heads to see exactly what CC's they are. They are open chambers and from memory they were 70. I really need to know for sure. I did check and the W-2 heads have long valves. I have nearly an inch from closed valve to retainer clearance. I have both iron and roller rockers with the offset shafts and stands. Dont know anything about the springs.
Not opposed to a big cam jerking around at low RPM on the street isnt a deal killer for me.
And im not opposed to using my 391. I just thought the 355 would be a little more highway friendly.
BTW you guys are awesome and a lot of help.
Thanks. I have been out of this game for a while.
I’m kinda running into a compression vs flow deal right now. My buddy wanted 2.08 valves in his Edelbrock heads but his also going to run a 4.040 Cometic head gasket. Well like I said before the Speedmaster heads have a nicer chamber than the Edelbrock heads. With that small gasket I’m not even sure I can work the chamber enough to make his 2.08 valves pay off. I try to size everything close to the 4.180 felpro gasket size.
Belvedere2
Active Member
I'm curious about this question myself. Personally want to run a solid with those specs; 240ish duration and 550 to 600 lift. I've been told that my 10.1 comp is going to make my car a dog. Thought about pulling the heads and shaving them to get ratio up but if it's not totally necessary....
If the world and planets lined up…..
I wouldn’t worry about it. While another 1/2 point is what I’d shoot for, it’s not the end all. You could…. Just sayin… move the cam around and close the intake valve earlier.
If you don’t have the cam yet, I’d contact Bullet or Schneider cams and discuss it with them. FWIW, it’ll help to have your cylinder head flow numbers handy when you talk about this.
If you have the cam, just stab it and rock it, it’ll be fine @ 10-1.
Just what exactly is 240-ish?
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