Hmmm... Pushrod VS DOHC V6

donk_316 · Post by **donk_316** » Sat Jan 01, 2005 12:39 pm

==DISCLAIMER==
All the following info is based on theory and numbers. Not actual dyno results.

FACT - DOHC head does and will flow more than a gen3 head.

FACT - PUSHROD block design is superior with cross bolted main caps. Both blocks share a structural oilpan design and re enforced 'timing' covers.

FACT- DOHC motor wieghs more than a PUSHROD. Enough of a wieght difference to make a difference to someone who is making every pound count. I dont have exact numbers but anywhere from 50 to 120 pounds has been qouted.

FACT - The DOHC has 5 "camshafts" VS the PUSHROD having 1. Less rotating mass would (and should) mean less frictional losses and parasitic drag in the PUSHROD. The "wieght" of lifters, pushrods and rockers isnt even close to enough to say they "even out" so dont go there.

FACT - Near limitless camshaft options for the PUSHROD block. Solid or Hydraulic. Roller or non-roller. Mild to Wild.

FACT - The DOHC camshafts are considered the bottle neck and limiting factor of the entire motor. Only regrinds are available and they are... questionable.

I was considering building a DOHC motor instead of this PUSHROD motor due to the "way better" head design especially under boost conditions.

TEST 1
With a DOHC stuck with its factory cams / stock heads and a boosted VS the PUSHROD with custom cams / gen3 heads and the same turbo...

The PUSHROD makes 10 peak hp MORE at the SAME RPM.
The PUSHROD makes 20 peak ft# MORE at the SAME RPM.

Near identical curves. No noticable advantage for either motor.

TEST 2
With a DOHC customs cams / stock heads boosted VS a PUSHROD custom cams / stock gen3 heads same turbo...

The DOHC makes 120 peak hp MORE at 2000 RPM higher. (!!!)
The DOHC makes 40 peak ft# MORE at the SAME RPM.

Should also be noted that the DOHC torque curve is near table flat ( and over 300ft#) from 3000 to 8000rpm now.

Tell me what you think. Which direction would you go if you were building an all out V6?

crzyone · Post by **crzyone** » Sat Jan 01, 2005 1:08 pm

DOHC cams can be built up by a company that welds underwater and then reground with a higher lift and agresive profile. With moderate mods to the heads, intake and exhaust with these cams you can easily see 300-350fwhp.

For NA hp there is no substitute for 24 valves.

Boost is the great equalizer. Pound for pound of boost, the dohc should make more power.

donk_316 · Post by **donk_316** » Sat Jan 01, 2005 2:37 pm

crzyone wrote:DOHC cams can be built up by a company that welds underwater and then reground with a higher lift and agresive profile. With moderate mods to the heads, intake and exhaust with these cams you can easily see 300-350 hp.

With moderate head work, cams and compression a 3400 can do the close to the same. 275-325 hp

crzyone wrote:For NA hp there is no substitute for 24 valves.

I would agree with you if the flow numbers didnt say the exact opposite when comparing stock VS stock.

crzyone wrote:Boost is the great equalizer. Pound for pound of boost, the dohc should make more power.

Thats kinda my point. The DOHC obviously COULD make more power but even 350 hp is relatively 'low'....you need the cams to make decent power out of the DOHC.

My biggest point is....for the increase in wieght (especially at the top of the motor) and relatively weak block i still dont see the 'DOHC advantage' not to mention having to cut the deck hinges and so on.

If someone can seriously prove to me its worth it to build the DOHC over the pushrod, ill go with a DOHC instead. My reciprocating assmebly will work in either block.

crzyone · Post by **crzyone** » Sat Jan 01, 2005 3:34 pm

GM put mild cams in the dohc from factory.

Stock for stock, the 3.4tdc, even with mild cams, still kills the 3400 for hp.

I've heard the heads on the tdc add no more than 50 lbs over cast iron 60* heads. Aluminum gen 3 heads are probibly good for a 70lb weight advantage.

deh cams is where it be at YO!!

donk_316 · Post by **donk_316** » Sat Jan 01, 2005 3:44 pm

crzyone wrote:GM put mild cams in the dohc from factory.

yeah ive heard the original cam specs is a 270hp motor.

crzyone wrote:Stock for stock, the 3.4tdc, even with mild cams, still kills the 3400 for hp.

I agree.

crzyone wrote:I've heard the heads on the tdc add no more than 50 lbs over cast iron 60* heads. Aluminum gen 3 heads are probibly good for a 70lb weight advantage.

So 70 extra pounds at the top of the motor is a good thing? No! Bad Levi! Sit in the corner! 70 pounds!! thats a crack whore sitting on your decklid man!!

Shaun41178(2) · Post by **Shaun41178(2)** » Sun Jan 02, 2005 4:24 pm

Well there are ways to take 70 pounds out of the rear of the Fiero if you are that concerned with it Donk.

You can do a performance rebuild of a 3400 and get about 240 hp with headwork, intake work, headers, and the standard bolt ons.

A dohc will do about 240 with a few mods, like headers, exh, intake, PS delete and prob the cam retard. Thats not much really with big gains coming from the exh and the timing. Some head and intake work should help out on that motor too.

I think a dohc in the right hands can be a good fiero motor. So can a 3.4 PR

It all comes down to what you want to do.

Don't worry about the main caps and all that crap about them being weak in the dohc motor cause its not crossbolted or even 4 bolt mains. People like to cry about everything. You have to push 500 hp before I myself would start to worry about them. There are some 60 degree guys running 400 on the stock crank and bearing support system with just ARP studs. No probs at all. The block is strong. Its not going to break.

fierod · Post by **fierod** » Thu Jan 06, 2005 11:32 pm

I have a 3.4 pushrod in my Fiero now. I've considered the 3.4 TDC but I don't see alot of aftermarket mods for that motor. First thing I see is the need for 4 good cams, but that gets pricey. Another thing to consider without money in mind, just how far can you go with a 3.4 pushrod n/a? It looks like you can get better punch from the 3.4 TDC if you have the cash.
Correct me if I'm wrong.

crzyone · Post by **crzyone** » Sat Jan 08, 2005 1:17 am

You are right. The 3.4 pushrod has a bit of aftermarket but any mod you do to the dohc will make big gains. Cams are the biggest bottleneck, and for $400-600 there are companies that will regrind them to add lift and duration. They do this by adding metal to the lobes and regrinding.

That mod plus a good port and polish, headers and a free flowing intake should put the engine in the high 200s low 300s for hp. Upping the compression and shimming the valve springs for valve float prevention should also add a few more hp up top.

Aaron from the other forum figures he has a formula for over 400 crank hp NA. Wether it has that potential or not remains to be seen.

donk_316 · Post by **donk_316** » Sat Jan 08, 2005 1:59 am

I wouldnt put too much wieght into what that "Aaron" retard says. Over on 660 he just admited the magical "400hp NA DOHC" is based on "make believe cams"

Just read the thread in the off topic on that site with the same name as this thread.

Shaun41178(2) · Post by **Shaun41178(2)** » Sun Jan 16, 2005 12:46 am

$400-600 for reground cams are worth it I think.

I mean think about it. thats around $100 per cam. Thast not bad. TIs just thaat you have 4 of them. If you pick up 25 hp I say its worth it. If I had that engine in my car I would so do it. I wouldnt' wait. I would find the company to do it and pay to have it done. throw the bottle on it and run 12's Simple as that.

crzyone · Post by **crzyone** » Sun Jan 16, 2005 7:43 pm

Cams, headers and a late style intake should get this motor in the high 200s crank hp. Bumping compression is also easy.

If I was running nitrous there would definitly be a reason to go with sbc rods and forged pistons.

When my other motor is done, I should see how big a shot a stock 3.4 will take. :thumbleft:

fierod · Post by **fierod** » Mon Jan 17, 2005 1:45 pm

crzyone wrote:
That mod plus a good port and polish, headers and a free flowing intake should put the engine in the high 200s low 300s for hp. Upping the compression and shimming the valve springs for valve float prevention should also add a few more hp up top.

.

Shit, so what does the stock 3.4 DOHC make for HP/torque? This is sounding a lot better than my 3.4 pushrod.

My 3.4 pushrod is built up about as much as possible for a N/A. Ported heads matched to intake and headers, Crane H272 cam, roller timing set, roller rockers (1.5 ratio), MSD ignition system (6AL) with coil. Motor is carbuerated with a holley 4bbl.

About the only thing I have left thanks to some advice from others: underdrive pulley, good free flow exhaust, higher rocker ratio assuming the springs can take it, and maybe shaving of the heads to up compression a bit. Even with all that, I think I would be lucky to be much above 200hp at the wheels.

My 3.4 is quick, but I'm getting to the point where I need something faster. I'm going to put the 3.4TDC on my list.

crzyone · Post by **crzyone** » Mon Jan 17, 2005 6:42 pm

Stock a manual 3.4dohc is 210hp 215tq at the flywheel. With a 13* exhaust cam retard, better flowing exhaust and no a/c or ps it should be around 230-240 at the fly. This is as much power as your highly modified pushrod 3.4 is making.

Then the more expensive mods need to be done but they will give big gains. Deck the heads, new cams with bigger lift and duration, headers, short runner intake, shim valve springs = 300hp easy.

The bebefit of the 3.4dohc is the ability to use the stock front mount as a 2.8 or 3.4 pushrod. Only a small amount of grinding needs to be done. And you have to fab up a dogbone mount.

I have this engine and enjoy it alot. I wouldn't consider a non turbo pushrod engine in a fiero. The 3.4 is all about smoooth power. Until 4000rpm its going to feel alot like your pushrod, from 4000-7200rpm it pulls hard.