Q Heads and 1.8 heads

[ornjkr8]

Hello everyone, Rooting through my stuff looking for some heads for my 1800 AN facsimile. I have some small valve 1800 heads and a few sets of 1700 q heads. I have no other reference, but are 1800 heads the same except for the carb/EFI valve differences, or since my heads are the small valve ones, am I better off boring the q heads to fit the 1800 jugs and take advantage of the chamber designed for the dome?

[raygreenwood]

I think reliability wise....they are close. The Q heads are very good all around if they have not been abused. If your build is fine with the smaller valves...bore the Q-heads.

Do you have domed 1800 pistons? if so....use the Q-heads.

Ray

[ornjkr8]

Yes, I have a set of 1.8 domed pistons

[Piledriver]

The 1700 heads have a 100mm cylinder register, so will need opened up to 105mm to fit the 93/94 or 96mm jugs.

I'm seriously considering using a set of 1700 heads with some 100mm Mahle birals that need use and only opening the first ~3mm to 108mm for a thicker deck.(and a free deck height extension)

[Clatter]

I always thought the 1.8 heads had bigger valves than the 1.7..

[Piledriver]

They do, the 1.8S heads are 41x34, but the 39x33 1.7 heads only flow ~5cfm less in stock form.
I'm after the ~6mm thicker deck surface for turbo use, T4s need all the help they can get for head sealing.
Intake 160cfm@28mm at .500" is plenty, much easier to add a psi or 3 of boost vs huge port$ and valves.
As a side benefit, the "top of the bore" in the head can get insulating coating along with the chamber...
and possibly a c-ring or oring.

I wanted to do it with the AMC 1.7 heads but I have a good set of 1.7l vw heads, 1K for a pair of stock valve amc heads just to check if it helps the head seal is a bit hard to swallow. If it works great I'd consider having a high flow set built with the std 100mm register,100mm bore step.

[Bruce.m]

Side note…
The latest breed of modern turbo engines sacrifice peak flow for better swirl (& use a dense charge pocket). The principle is, they can always add a few psi of boost to compensate but the swirl is critical to the power goals. For what it’s worth, dome pistons are now a big no, as they spread the charge. They want the charge to be shaped into a ball or as close to that as possible.

[Piledriver]

Modern GDI motors also tend to run near diesel compression ratios and boosted to hell.

The Mazda Skyactiv-X motors run around 17:1 CR and only run right on regular gas, premium burns too slow...
Its essentially a diesel running on gasoline with spark plug assistance running it its ideal modes.
They are supposed to come to the USA at some point.

That and 30% better fuel efficiency than their already very efficient gasoline motors...and lower emissions.

Even relatively pedestrian ecoboost or vw tfsi motors are running 14:1 compression ratios, same as TDis.
...preignition or detonation very unlikely with fuel getting injected a few crank degrees before ignition.