Inliners International Forums General Discussion Hi Performance 261 Build Up

Hi all, I am new here to the forum and I have some questions for you about my 261 buildup and what your thoughts are as to performance and there's no better place to take this question to you folks right here.

I am building up a 49 Suburban and an hoping to build a fairly stout 261 motor while still keeping it on pump gas and making it driveable for the family. Here are some details:

261 rebuild .030 over factory on the bottom end.
235 #848 head rebuild as stock with hardened seats installed
Patrick's mild cam (220@50, .433 lift) I think this is Deltacams Howard M4f, but don't quote me.
Headed milled .030
Clifford Intake / exhaust (not sure what exhaust size I need yet)
Holley 390

I know I am not going to win any races, but I was curious to know if I were going to do any headwork on it, what would it be? I know Clifford will sell you a head with larger valves and deck the head for higher compression, but I'd rather just buy and communicate to my local shop what I want done on the head. If I do any head work beyond what I have, what is the pros / cons (if any) and how much extra Torque / HP should I expect above and beyond what I have now as a buildup?

I know you guys know you're inine stuff here, so any suggestions would be greatly appreciated. Also, how shoould I match exhaust for peak performance and backpressure? X pipe, etc.

Thanks in Advance.
Patrick

You could do some mild porting and polishing especially on the exhaust ports.Just match the ports to your gaskets. Other than that it seems like a nice reliable street performance build.

and the full flow oil conversion! You're set, and if you can't win races you're racein' the wrong guys!

Thanks Guys. So what are your thoughts on the exhaust? I am thinking one muffler crossed over going one or two out potentially. What are your thoughts on exhaust, or will it make much of a difference in terms of performance with this engine? Should I just choose what I want in 2 inch and be done with it?

I don't think performance will up that much to be noticeable, but duel pipes have a nice sound. Mufflers are personal preference for sound too I guess. Anything from Cherry Bombs (really loud) to Flowmasters(mild) have been discussed in these forums. Sometimes it just has to be what your wallet can handle.

I've been running the same engine, a 270 GMC, in the same pickup since 1978. it has had different intakes, exhaust, transmissions. and rear ends. In an old book my brother had in the '50s, Souping the Stock Engine, the stressed breathing. The old engines were very restricted in both intake and exhaust. The largest and cheapest power gains were made with multiple carbs and duel exhaust. A good cam came next. I have had three different exhaust systems on my pickup and there is one more to come. First was Clifford Viper tube headers, 2 1/2 collectors, duel glass packs, full length pipes with an "H" pipe, one on each side. This produced the highest RPM. Next was Fenton headers into one three inch flowmaster exiting in front of the driver side rear wheel. This sometimes hurt my ears not with noise but some strange pressure thing. It literally sucked but it sounded great. It lost both bottom end power and top end. Last was Fentons and 2" full length straight duels,both exiting at the read bumper on the driver side. Way too loud! Two trips to Reno through Virginia City and I added two 18" Cherry bombs. This is a great sounding set up and pulls well through the full RPM range. It still does not wind as high as the first system. I will put the Vipers back on with these pipes. Exhaust is important to these engines and I think duel exhaust works best. Just my thoughts. Tom

Patrick,

I assume that you have seen the Racer Brown-Frank McGurk article in the May 1955 HRM? Note that the addition of a Corvette six exhaust manifold and dual stock truck mufflers increased the torque (and HP) by four to better than five percent from 2000 rpm and up. I've been told that Fentons flow better than the Corvette six manifold,but I was happy with the Corvette six manifold that I had on my 261 back in 1958.

As Tom says, these engines need help in breathing. Note that even with three carbs and a pretty good cam the speed for peak torque was still only 2000 rpm.

There is only one way to definitively answer those questions - Cut one up. Available material will no longer be a mystery and with creative toolpath and cutter geometry there is plenty that can be done using modern machine tools. I see McGurk’s work as a case of “ When your only tool is a hammer,,,,everything looks like a nail” and if he had access to a flow bench I think he would have done things a bit differently.

I took the head I cut up to a couple of race engine builders for show and tell and they were quite amused by my tales of how plunging a shell reamer in from both ends were performance improvements worthy of dyno testing and documenting in Hot Rod Magazine. Allowing me to study the features in modern performance heads and how to apply them here has been a great learning experience. I can say it is possible to change the short side turn from 1/8” radius to 3/4” and still maintain .100” minimum wall thickness consistently and repeatedly. How well this change as well as others improve flow I should have documented by spring.

Great info guys. Thanks. I think I am going to try a 2 1/4 system, and maybe a 2 into 2 muffler, that way, I can get the x pipe at the same time. I think that I'll just start with the setup I have, and then maybe look at some polishing and maybe change the valves at a later time, but I think from all of your advice, I am on the right track. I did see that article from the 50's. I actually found the Hot Rod magazine on ebay and bought it, so I have a hard cover version. Pretty cool.

Curt,

You have me very curious now. Let me know when you find these results out this spring. I will definitely be curious what happens.

Curt Me too. This is imfo. I'm looking for now. Thanks

I ran 2" duals, Fentons, out of my '56 235 with 30" cherry bombs and sounded good with a little rap. After a few years I put on an x pipe in front of the mufflers and it quieted everything down and I seemed to have gained power as I go over the Afton Mountain pass on my way west. This application is for a daily driver only vehicle so I am not interested in weekend or seasonal "hot rod noise" driving as I frequently take 1 K trips in my '49 1/2 ton.

I would like to try just the x pipe or an X-Pipe Muffler that I saw in a recent issue of Jegs Speed equipment. I have a '58 261 that I have yet to get started on so I will be watching this thread and am curious as to the route you will take with your build.

IIRC Dyno Max makes a muffler that's 2 in, 2 out and the flow path Xs between the two.

I don't recall at the moment what pipe sizes it's available in though.