Push Rod vs. DOHC engines

[ZDan]

Originally Posted by Dimman

Going to quote myself from the 'Vette thread:
I've thrown a bit of a challenge @ZDan a couple times, with regards to what may be a practical displacement limit to a V8 engine, but got nothing from him.

?

I guess i missed this. Practical limits? Depends on application. For sure you can go bigger than the LS7's 7.0 liters.

In 1911, FIAT built a car with a 28.3 liter inline-4.


So surely if someone wanted to they could make a 56.6 liter V8. I don't know what they'd put it in, though...

I don't think that the OHV V8 is up against some "practical displacement limit" at ~7.0 liters. In any case, for a given car design, with a given limited volume allotted for the engine, you'll be up against a significantly lower practical displacement limit if you go with DOHC instead of OHV cam-in-block.

[Dimman]

Originally Posted by ZDan

I guess i missed this. Practical limits? Depends on application. For sure you can go bigger than the LS7's 7.0 liters.

In 1911, FIAT built a car with a 28.3 liter inline-4.


So surely if someone wanted to they could make a 56.6 liter V8. I don't know what they'd put it in, though...

I don't think that the OHV V8 is up against some "practical displacement limit" at ~7.0 liters. In any case, for a given car design, with a given limited volume allotted for the engine, you'll be up against a significantly lower practical displacement limit if go with DOHC instead of OHV cam-in-block.

What I'm getting at is GM able to keep expanding displacement to stay in the game vs DOHC? Stroke increases will force higher mounting as well as taller deck heights, bore increases will lengthen and widen the block as well as requiring more valve lift to push open the bigger valves necessary to take advantage of the new displacement. Then we get into reciprocating and rotating mass increases, piston speed/load increases...

At 7.0L, it still compares favourably to my hypothetical 5.3L (99mm bore x 86mm stroke). But I picked reasonably conservative power to airflow numbers (not spun like crazy yet).

But going for higher rpm (say with an iVTEC/VVTL-i setup, which is not a massive amount of weight by any stretch), geared appropriately, then what?

[ZDan]

Originally Posted by Dimman

What I'm getting at is GM able to keep expanding displacement to stay in the game vs DOHC?

You may as well ask if DOHC engines will be able to keep expanding displacement to stay in the game vs. the Corvette.

Worth noting that the base Corvette engine has only 10% more displacement vs. the 80s and the LS7 only 23% bigger, whereas Ferrari's V8 has gone from 3.0 to 4.5, a 50% increase, over the same time.

[Dimman]

Originally Posted by ZDan

You may as well ask if DOHC engines will be able to keep expanding displacement to stay in the game vs. the Corvette.

Worth noting that the base Corvette engine has only 10% more displacement vs. the 80s and the LS7 only 23% bigger, whereas Ferrari's V8 has gone from 3.0 to 4.5, a 50% increase, over the same time.

Hmmm... This is an excellent point.

I used 1985 figures and they do a pretty fantastic job of showing how well GM is developing the Corvette motor.

L98 5.7L 230bhp
LS7 7.0L 505bhp

Displacement gain, ~22%
Bhp gain, about 2.2x

Ferrari 328 3.2L 270bhp (shady factory claim)
Ferrari 458 4.5L 562bhp (when not on fire)
Displacement gain, ~40%
Bhp gain, about 2.1x

1985 Ferrari bhp over Corvette: ~17% (but Ferrari had some major truth issues back then...)

2012 Ferrai bhp over Corvette: ~11%.

Dimman is forced to admit that ZDan is on to something... Dammit!


Before I go sulk, I did some block space estimates with some simple trig to compare my hypothetical 99 x 86 5.3L vs GM's 4.125 x 4.000 427cid

Using the same 1.6 rod:stroke and a completely out of my ass 0.6 bore:piston top from pin center, the 427 block would be ~35mm taller (from journal center at bdc to highest corner of piston), ~23mm longer (based on 4 bores) and ~55mm wider (widest corner of piston to the opposite).

Now obviously the DOHC heads are what make the size a major factor, but at least it could start from a smaller point.

RPM for 4200 ft/min piston speed
5.3L: ~7500rpm 701cfm gross airflow using a bit worse than the FA20's 1.23:1 maybe 1.30, and using
the same 7000rpm power peak 655 cfm 1.30cfm:bhp = 503 bhp.

427cid: ~6300rpm 778 cfm gross airflow 1.54cfm:bhp = 505 bhp

Well double goddamn. It seems my 550 bhp estimate was complete horseshit... I have to go check my notes.

You win this round ZDan, but I will return.

Fistshake and crazy mumbling...

[OrbitalEllipses]

I too thought he made an excellent point, but thanks for doing the math.

[Dimman]

If I assume the 5.3 is tuned like the 427 to make peak power at the same piston speed, and a little better efficiency 1.27, still not as good as the FA20, I get the 550 bhp number...

[Spartarus]

I found this recently, and I think it's relevant to the discussion, now that the dust from the battle has settled somewhat.

http://www.superchevy.com/how-to/vem...oke-of-genius/

I'm fascinated that they took a cam-in-block 2v pushrod engine, reduced the displacement, and rapped it up to 8000 rpm and made just about spitting distance from 100 hp/liter, NA.. Those are 4v DOHC numbers, for significantly less weight and cost. hrm.

[Bobblehead]

Its been two years, I came in here to say that DOHC is more complicated, but better than pushrod. I found math and necro.

I'll take my leave now.

[Chad11491]

Originally Posted by Spartarus

I found this recently, and I think it's relevant to the discussion, now that the dust from the battle has settled somewhat.

http://www.superchevy.com/how-to/vem...oke-of-genius/

I'm fascinated that they took a cam-in-block 2v pushrod engine, reduced the displacement, and rapped it up to 8000 rpm and made just about spitting distance from 100 hp/liter, NA.. Those are 4v DOHC numbers, for significantly less weight and cost. hrm.

the old L series straight six from nissan was a SOHC and OS Giken made/make a DOHC head replacement, and the hp difference from just that was well under 10%.