14 replies to this topic

[86CR-X]

i just had a quick question, i refered to the faq and couldn't find it.
what is the main differance between a dx head and an si head?
i want to get more power out of my ew2,
i was thinking of putting a larger than stock single weber carb on , but i hear u need an si head for it?

[LudeBehavior]

I think one of the big differences are the cams, ones made for carbs the other is not.

my two cents

[oversteer]

I don't se any reason why you can't use the DX head with a single Webber carb, but the Si head will make much more power than the DX. May have something to do with the auxiliary valve?
I know the Si head has a more agresive cam, I think the ports have better angles and are larger, the valves may be larger too and no auxiliary valve.

[Ghost-One]

umm the difference in the dx head is that it has CVCC and sucks ever wonder why ther are 16 v on the dx

[Surestick]

You should find the answers to your questions in this thread motorcycle carb/CVCC head
Hope this helps!

[Greg Gauper]

The US DX w/CVCC head was designed to get good gas mileage w/low emissions. It wasn't designed to make lots of horsepower. The US Si head does not have the prechamber, plus it is FI.

I'm sure our Canadian friends will correct me if I'm wrong, but I was told that the Canadian DX & Si's are the same head and just the intake system (carb vs FI) is different? The Si might have a slightly hotter cam as well?

From a tuning standpoint, the CVCC heads are a bit tricky to get working properly with a single Weber downdraft carb, but it can be done. A lot of IT racers run that set up.

Other problem with CVCC head is there is a practical limit on how much you can advance your ignition timing before you start to literally eat sparkplugs. With the Si, the limit is fuel octane/compression limited i.e. crank it up till it pings and then back it off.

The CVCC heads DO respond very well to a properly designed header and this is IMHO the single most cost effective way to boost power.

[gtpilot]

I'm sure our Canadian friends will correct me if I'm wrong, but I was told that the Canadian DX & Si's are the same head and just the intake system (carb vs FI) is different?

Im not Greg's Canadian friend (I am an American and I backed into his tow rig...not sure if that makes us friends or enemies!?!) but the suble difference between the USDM Si PE7 head and the CDM DX PE3 head is the combustion chamber design - I can't find anything else different in the castings. The PE3 head has very nice quench pads built into the combustion chanber and therefore makes an easier head to build into a race head.

Kirk

Edited by gtpilot, 10 February 2004 - 06:23 PM.

[eldo]

Ed's head(Can.DX - PE3)



Rampage's head(Can.Si)





Ed

Edited by eldo, 11 February 2004 - 02:11 AM.

[gtpilot]

Eldo,

What is the casting number on that CDM Si head and what models/years did it come from?

Kirk

[Greg Gauper]

I was going to comment on that too! That doesn't look at all like my US Si head.

[balladeSports]

Now there is one other diffrence between the si head and canadian Dx head. The exhaust valve on the si head is bigger. The vlave springs on an si head also has the duel valve spring setup
Putting an si head on a US DX bottom end will also lower the comprssion to low. you also need the si pistions to make it work.
What you realy want is the Canadian head, intake and carb. bolt this on a US 1.5 bottom end and your laughing.

[86CR-X]

ok how about a cdn si head bolted on a cdn dx bottom?
what do i look for to identify if its cdn ,us , si or dx, if i get one at an auto wreckers?

[Surestick]

I've got a Canadian DX & I'm planning to adjust the valves in a couple of weeks.
I'll be able to tell you for sure if it has a CVCC head then.
Just gotta get a new valve cover gasket & arrange some heat for the garage.

[eldo]

Eldo,

What is the casting number on that CDM Si head and what models/years did it come from?

Kirk

You'll have to ask Mark(Rampage) because he has that head.I believe that it came off a 86 Crx Si and the 87's are the same(according to the Haynes manual).Where are the casting numbers on the head?(i have 2 Si motors in the garage and could give you the numbers if i knew where to look.At the exhaust side it reads PE7 - 5 3 2 6 9 24 1
The heads are still on the block so i can't see any other numbers.

Ed

[Surestick]

Question for Greg Gauper
You said "Other problem with CVCC head is there is a practical limit on how much you can advance your ignition timing before you start to literally eat sparkplugs."

Why is this? Why do they eat plugs with advanced timing? -Just curious.