Pontiac Head Swap

[hydroli]

In this thread I will post developments on a head swap that I don't have any I idea about the feasibility. The heads that can be considered for such a swap are from from pontiac and other GM brands established in brasil and europe. They have the same bolt pattern, cam and valve train configuration as the chevette head has.

You can find informations about the heads in these web sites.

GM LT3 Web site
lt3engine.i8.com/info.html

Wikipedia : Sunbird
en.wikipedia.org/wiki/Pontiac_Sunbird


The heads doesn't have the same intake and exhaust bolt pattern as the chevette and it will change too from the 1.8 to the 2.0. The exhaust side is different as well from one to each other models.

2.0 heads have better runners and better flow pattern than the 1.8 one. Valves are bigger too. Combustion chamber has a better shape in the 2.0 late model while the early model has the same chamber shape as the 1.8 engine.

Earlier heads were known to be weak. From an engine machinist, problems with this head can be avoid with a colder thermostat and a good warm up before being raced. I think that a 1/8" hole in the thermostat can insure a constant flow and avoid thermal shock.

I will take a try with the 1.8 head.

[hydroli]

I found an Intake prepped on ebay and that's why I choose to go with the 1.8 head. It's a rare model and it can be hard to find. The one I got has been modified to receive a spacer on which any carburetor may take place, spacer will have to be custom made. Since the flange that bolts to the head has a 5 degrees angle, reported to the carburetor flange, the spacer should have this angle or the float consequently adjusted to still have a good level in the carb.

Here are pics of the intake I received a few days ago.







As you can see with the middle one, runners are bigger the the chevette ones. Quarter piece and one cent are placed for scale comparison.

The next link shows a 1.8 head compared with the chevette one.

chevetteracing.proboards23.com/index.cgi?board=engines&action=display&thread=299&page=2#1456

I'll post news of this project here as soon as I have developments. Cam carrier fitment and measurement related, water and oil passages as well as any related modifications will be explained the best I can.

[DOCTOR FIELDS]

Good luck with your head swap. One thing i would recommend is boring the block to .110" which i believe is the stock bore for a Pontiac Sunbird. As you can see in the pictures i posted of the head bolted to a Chevette block, the cylinders did not match the combustion chambers. In other words, you would need to bore the block larger to giver proper clearance around the valves for proper flow. Hope that makes sense.

[hydroli]

Boring the block .110... how many races with such pistons? Is it something to look at prior to hone .110 oversize??

[DOCTOR FIELDS]

I'm not sure what you are asking? Are you asking how long a .110" block will last?

[hydroli]

I have to admit it's not clear if I read it. I asked how many racers runs .110 oversize and if it's something special to verify before honing the cylinders that big?

[DOCTOR FIELDS]

It used to be common to run a .110 Sunbird piston. I never ran them, but maybe someone who has can give you more info. You might look into sleeving the block before going that big. .080" is about as far as you want to take a stock block, due to core shift and getting the cylinder walls too thin. Put a larger sleeve in and then bore to .110.

[rad15E]

i've built several .110 motors for people over the years using the sunbird piston,the block will take the bore just fine but use a torque plate to hone with for sure,unless you can find a lighter brand than what i could find i always thought these pistons were to heavy to be a race piston,for a stock stroke motor you have to cut a lot off the piston to flattop and then a lot off the block,its also hard to get these small heads to flow enought to keep up with this much cyclinder area,i;m not putting these motors down at all they do run great but you can't stop at just the big bore---just me 2 cents worth lol

[hydroli]

Yes, I agree and it's why I'm looking to swap the head . As for the pistons, I may buy some from CP Pistons. I called today and gave all informations I could. What Mike from CP said is that he could make a set of custom pistons that would have a 84mm diameter, .079" oversize from stock. They are forged and will be able to handle 15 PSI of boost. I can order any ring configuration wrist pin size and compression height I want by the way. I will have to pass by a revendor though cause they won't sell directly to customers.

From .079" to .110, there's only .015" on each side remaining. It's almost nothing from my point of view. I'll radius the top edge of the cylinders with a sand paper, just to break it, and everything should go right.

That's for the cylinder size to combustion chamber issue.

Other developments may follow in order that it appears to me.

Thanks for reading

[hydroli]

I have a question to who ever tried flat pistons in a chevette block. Do you need valve reliefs? Have any one ever measured the clearance with " of lift and " shaved between valves and pistons?

[rad15E]

we all run flat top pistons mostly,most motors the piston is left .003 in the hole and you do not need reliefs even with the high lift cams----[make shure you cam timing is correct]

[hydroli]

Good to know. Thanks Radford.

[DOCTOR FIELDS]

Oliver / Doug,

The reason i suggested sleeving the engine before boring to .110, is minimum cylinder wall thickness. I've cut apart these blocks before and there's quiet a bit of core shift during the casting process. On average the stock cylinder wall is .235" thick, some have been more, some less. That is measured about half way down in the bore. The upper 1/4 of the cylinder is somewhat thicker, never measured it.

The rule of thumb is .150" MINIMUM cylinder wall thickness. If you go less than that, you risk cylinder wall flex. When this occurs, your rings un-seal and you will loose compression. If you do the math, boring a stock block to .110" will leave you with only .125" cylinder wall thickness, which is definitely well below the .150" mark. I've seen cylinder walls crack that were down to .103" thick (.132" overbore Chevy Luv pistons)

So i would highly recommend sleeving any engine that you intend to bore over .080".

____________________________________________________

I would stick to a piston size that you can get a good ring package for. I know that .040" and .080" over you can get Honda ring packages for (1.2mm 1.2mm 2.8mm) which reduces friction greatly over the stock (1.5mm 1.5mm 4.0mm) ring configuration. I would also recommend Total Seal rings. You can go to their website www.totalseal.com and browse through their bore size pages, it will list each bore size and what ring packages are available.

Hope this helps, keep us updated!

[sobeit]

i bought a 2.0 sunbird head from a wrecking yard, guy just put a new head on and wrecked car . almost no carbon build up or anything still looks all fresh. i also got the cam which looks very similar to a chevette, along with the lifters, cam followers, cam carrier and valve cover, oh and the water outlet. i ordered a chevette head gasket today to get a better look of things. from what i can see right now the area where the oil goes back down on back side shows about 1/8 or so. every thing else looks good so far.

[hydroli]

Hello sobeit,

Is there a 1/8" OFFSET from head to block on the oil passage or they are different size? Let us know if you notice something else.

I bought one from ebay yesterday. It was cheap at $95 since it's been serviced and all checked up.

I should get it soon and check what don't fit. I'll have more time until August 4 and I will be able to work on the chevettes.