A simple porting question (for once)

[McDog]

I've got two 701 motors I'm tearing down to check out and put RIVA heads on. I was looking at the exhaust outlets and noticing how they are imperfect and don't match up with the sleeves exactly right.

If I use my dremel to clean this up, what are my chances of screwing the porting up? I've never done anything like that before, and I don't want to ruin the cylinders. But they are sitting on my workbench just begging for attention. Would it be worthwhile? Is there any performance gain from doing this?

 

[Matt_E]

How do they not match up right? Is it the roof of the port?

 

[McDog]

Well, there is a squared off piece that sticks out on the roof of the port on both engines. The bottom of the port doesn't match up perfectly on a couple of the cylinders too. I'm talking about very small imperfections, though. Like less than 1/16 of an inch. I'll post some pics tonight.

 

[keefer]

If you match the roof where the sleeve overhangs into the cast you will be raising the port timing which will hurt your bottom end. Port shape & width are mostly changed to get the desired power changes you are going for. I am not an expert in porting but I know that you need to do your homework before you start cutting & grinding.

 

[Matt_E]

Well, there is a squared off piece that sticks out on the roof of the port on both engines. The bottom of the port doesn't match up perfectly on a couple of the cylinders too. I'm talking about very small imperfections, though. Like less than 1/16 of an inch. I'll post some pics tonight.

Don't mess with it. If anything, you'll move your power peak up a little in the RPM band, that's it.

 

[Crab]

I read a post from Supertune a while back and he said leave the roof alone even if mismatched.

 

[McDog]

Thanks.

 

[Mouthfulloflake]

can you post pics Mcdog?

Ive never seen a mismatch on the TOP of a 61x cylinder, I have seen it on 62T 701 cylinders though.

I agree with not smoothing it in, I am just curious to see it.

 

[Matt_E]

What makes you think they're X cylinders?

 

[Mouthfulloflake]

I did not say that I thought they were X cylinders.

I was simply telling him what I have seen.

What makes you think they're X cylinders?

 

[McDog]

They are 61x. Sorry the pics are hard to see. My camera wouldn't focus right, I guess because of the reflective surface. You can sort of see the squared off extrusion on the roof of the port in two pictures. The other picture is of the lip of the sleeve on the bottom of the port. Both stick out about the thickness of your fingernail. All the cylinders are about the same in both engines. One is 1996 62t/61x and the other is 1994 61x/61x.

 

[Matt_E]

You can smooth it out if it makes you feel better, but I don't think you'll gain anything.

 

[McDog]

SBT has a video link off pwctoday on porting in their tech faq that says to clean up and match the sleeve to the manifold. The guy in the video says the ski will run better.
All I care about is low end power and a RELIABLE, smooth running ski.
Opinions?

 

[Big Kahuna]

That is an old trick with limited class racing, clean them up, not really legal, but I did it the last time I had my cylinders off before I had them ported, race season was over, gave a slight gain.