Inliners International Forums General Discussion Hi Performance Chevrolet 261 build and head questions

I have a 261 chev motor bored .010 over and stock on the bottom end. The head is an 848 head decked .025 over and has stock valves in it. The cam is a 264 cam with Int 19/53 and exh 65/10 and a lift of .436. It has a clifford water heated intake with a holley 390/4 on it and long tube headers. It also has a mallory distributor on it which to date hasn't been a problem.

If I were to want to make more power out of this engine, what would be the next step or should I leave it alone? I've had various responses and would like to know what you think. Larry at Clifford seems to think that his magic head would be worth it which I believe has larger valves and it heavily decked. Would it be worth it and a noticeable improvement? (I would just have my local shop do it instead)

Any thing else I can do to squeeze some power or is it not worth the gain without going to a different motor?

Thanks

What's your deck clearance now?

man, are you in luck, there is a (no, THE) authoratative ( I spell like hank sometimes)article on the 261s. On this site go to "Engines",under "261 head swap", find the post#66090 by stocker 49. He links you to that article from a 1955 HRM MAG.

There are about 8 or 9 dyno tests showing what can be done and what they're worth. These various tests even include boring one out to 278 inches, as well as several cam swaps and 2, and 3 single carb setups (in 1955 I dont think anyone had 4 bbl intakes for the sixes)

Me, I'm old and have a distrust for the former Hudson man's Chevy stuff. And remembeer you're talking to a salesman over the telephone, not a far resemblence from a bleeding man in a shark cage.

I think many of the old products will be hard to find but Substitutes can be made with sim results. Try to sick with his dual pattern cams-they work-thats his foremost signature product. There are still some machine shops that regrind cams that may still have abilities to reproduce his stuff- There was one or two in Ft. Worth a few years ago-thats what the internet is for!

Frank would steer you to towards more streetable products than most folks--He also promoted 3.55 rear end gears to that end!

Thats a real good article, hope it helps you!

And thank you Stocker 49 for offering it to us!

Edski

The 1955 (Racer Brown) article has no information at all about installing a 235 head on a 261 except this: "A head from any 1953 or later 235 cubic inch engine is completely interchangeable with the 261 head". The only reason they used a 235 head is that the chamber volume is smaller, which requires less milling to achieve a specific compression ratio increase.
No mention of steam holes, bolt tightening sequence, or gasket difference.

right, On those sixes,due to where the Intake valve is positioned in the combustion chamber you must sink the valve the same amout as the mill job to provide valve/piston clearance during TDC begining of intake stroke. So McGurk's recommendation is to save a little effort and use the Pass head which will give you some compression increase without the need of milling and the sinking of the intake valve. I'm not certain, but it seems that both truck and std shift pass heads have 1-7/8" dia valves, but the Pass PG head uses 1-15/16" dia. intake valve. Its been some 35 years or so since I was messin' with that motor- getting a little fuzzy. That and wether you use Palmolive for dishes or Tide, both are good but Tide is sure rough on your guitar calluses!

Indeed panic . . .

I read the lack of mention of the so called steam holes in that article to mean that they simply didn't bother with them. A 235 head with a 235 gasket will seal these additional water jacket holes at the block; with a 261 gasket it will seal them at the head. But neither leaves the space above the combustion chamber high and dry. The primary water jacket hole is in the exact same spot on both heads - with the pressure from the water pump filling the entire space with coolant.

If we are arguing that the old school hat rod was under engineered when compared to modern standards - you will get no arguments here.

If we are arguing that GM had solid reasons for providing additional flow of coolant around the cylinder walls and up into the jacket above the combustion chamber on the 261 versus the 235 - again you'll get no arguments here.

In my mind the question is: Does a 261 tasked with making a '53 Vet go fast share the same cooling requirements as a 261 tasked with lugging timber up a hill on a hot summer day?

Racer's article suggests no (by omission).

If you disagree then the experiences of others indicate that one must drill with caution if one chooses to drill . . . But drilling is apparently a choice not a must . . .

regards,
stock49

Hi PFP . . .

It would seem that your post has gone a bit sideways . . .

The urge to continue to tweak is hard to resist. I guess it is question of how much more you want/can get out of the setup.

I have no experience with this Clifford magic head so I cannot comment about how it compares with the 848 head you are currently running.

regards,
stock49

The ONLY desirable feature of the 848 head is the smaller chamber volume (same reason as 235 on 261) - a small but cheap compression increase.
It's nowhere enough to get actual high compression (above perhaps 9.0:1), which requires decking the block, milling head or using a domed piston.

Clifford's halcyon days are long past.
They have an unfortunate tendency to, shall we say, use poetic license to describe the benefits of their products...

do I hear an "Amen"?.....Amen

I'M building a 261 we speak.There's info stating the 848 head combustion chamber cc's is 79.5 on an unilled head.Assuming it's correct.....
Plugging in the numbers and using the many available compression ratio calculators online..... if your piston sits about .010 below deck and you run a flat top piston,compression is just about 9-1.
And .010 below deck gives around .060 quench with the typical .050 compressed head gasket.

If possible, you want to tighten that up to .040" or so.

Considering the available headgaskets are .050-.055 compressed,that means a piston coming out of the block .010 at TDC.A lot of pro engine builders shy away from this on a street engine.
Detroit Gasket used to have a .038 gasket for the 261. Don't know if you can still get one.
Tighter quench is usually better,but almost all the info you see on it is for common wedge shaped V-8 heads,the 235-261 head is not one of these.To know for sure would be to build and test two engines identical except for the quench and see in real life if there is any actual performance difference.

Hey Guys,

Sorry it's taken so long to reply. I thought I subscribed to this, but I guess I didn't! Let me see if I can answer all the questions.

The head has been decked only 25 thou
The gasket thickness, I have no idea. I guess I could measure it and see?
I had the cam reground by Delta Cams. Nice guys

I do have a saginaw 4 speed and 3.55 rear end gears.

When I got the core, it had the 848 head on it originally, but no steam holes drilled. I ended up doing it. The block and head was maged and check out ok.

I have a copy of that 1955 HRM with that article and have read it front to back. Some of the info is sketchy, but I do get most in principle. I'm not sure I want to start playing with Cams at this point, but if it's the only way to make some extra power without touching the head, then maybe that's the route i'll take

I am glad somebody has 'confirmed' that while Cliffords magic head is long past, I do wonder what gain I would get by installing larger valves and if it's worth it?

Hard to about modifying the head since there's a lack of actual before and after dyno or road test results.
Patrick Dykes told me he's seen a few 235-261 engines on dynos over the years and no street engine makes more than about .7 hp per cubic inch even with extensive porting on a stock iron head.

Yesterday I cc'd the 848 head going on my 261.This head had a clean up mill of .008 and appeared to be never milled before.New exhaust valve seats and new stock style 1.5 inch Chevy V-8 valves and extended core spark plug,like a Champion Y type
The combustion chambers were 79 CC on average.So if this engine is zero decked,using available .050 compressed head gasket,the static compression ratio is 9.15.

You didn't say, but did you change to GMC rods and have new pistons made?...Just thought I'd ask

No.The .190 longer and stronger GMC rod might be a good idea for the usual reasons. But for this 4200 rpm engine I don't think the swap is all that important for a street engine.

Tony - Thanks for the info that helps out too.

A few years back, Doc on the webrodder site began a 235 stroker project, bring it out to 255", with some modern tech put into the build. They did some flow testing on a stock intake port, reviled that terberlence in the port began as low as .250" of valve lift, got really bad past .450". They thought porting the entire port would be cost-prohibitive, so went with pocket-porting above the valve seat. I don't recall the details, but believe they picked up flow by 10-15 CFM. Think they stayed with the 1.88 valves, didn't test the 1.94 PG valves.

Sadly, Doc has had some health issues and the build wasn't completed and dyno tested. All 8 parts of the build have been deleted.:(

I do know that Billy at Comp Cams was working on a dual-pattern cam with 256 intake duration, 260 or 262 exhaust. Tom Langdon has a dual pattern cam available, but don't have the specs. I think the idea behind both cams is to close the intakes earlier to build DYNAMIC COMPRESSION and help the crappy exhaust port with more duration, like Mcguirk did.

A local performance engine shop did my pocket porting for $150 4 years ago. From what I've read, the last 1/2" of the port is the most critical, should not be any larger than 80% of valve size. If I was to start over, I would go with a 15-deg. back-cut on 1.94" valves, pocket-porting and port clean-up. Maybe try a cam change, Edelbrock 500 carb too! Can't say it would produce a huge seat-of-the-pants difference, but every lil' bit helps on these ol' turds WE LOVE!!!

For now, I'd try a 500 4-bbl, Langdon's HEI dizzy with VACUUM ADVANCE. To get more than that on the street, your looking at work and $$$ for a turbo or blower set-up to fry the tires - anyplace, anytime!!! https://www.inliners.org/ubbthreads/images/icons/default/grin.gif

I'm a newbie here, but have enjoyed sixes since I bought my '53 Chevy PU back in 1974. Also have a '57 Chevy 150 2-dr. with a 3/4 cammed 235, Vega 4-sp, 3.08 gears, with a 261 in the planning stages. A very lo-buck, VERY FUN ride!!! Hope my $.02 offers food for thought...

Best Wishes, Tim

What specs does a "3/4" cam have?

Is there a 1/2 or 7/8 cam?

3/4 is an old term for a hotter than street cam, ie "3/4 race" grind.

From what I read, it would have been roughly halfway between a hot street (fast road as the brits like to call it) and a full race grind.

Perfect for hooligans on a budget (can only afford a nasty sounding cam) and strokers.

Or were you poking fun?

How old are ya 'Flyer???

The cam in my truck is a Sig hyd 264/.480" lift, installed back in '79 or '80 when good hydro lifters could be had. Came through Clifford.

The '57 has a Crower solid cam, 260/.450" lift. It came in a 3/4 built 235 that was a friggin JNIX. Broke a valve spring (Crower) causing a dropped intake valve. Fully ported head flunked mag with a crack above a water jacket. A year ago it spun a rod bearing. Despite Corwer being a hometown co. (San Diego), I will NEVER buy anything from them, as I also broke a rod of theirs in a NEW race motor that lasted 3 laps. I would rather say "its a Schneider..."

'Guess by today's standards, both would be closer to RV grinds... but 3/4 sounds better - don'tcha think???

I'm still looking for a solid referral to a shop in SoCal that can weld up the ported head. Any help from the troops here???

Thanks 'Flyer for keepin' me honest - I think!!!

- Tim

Do you know if Comp Cams is regrinding or do they have new cores? Delta Cams regrinds a dual pattern,I believe they were or are doing Langdon's cams.

Single pattern ("normal") cams presume that the flow efficiency of the intake port at .75 psi of vacuum (= 1.5" Hg @ WOT throttle) is roughly equal to the flow efficiency of the exhaust port a 50 psi or more (exhaust pressure when the valve cracks open, dropping to almost nothing by TDC).
If it's not (one port works better) adding duration and/or lift to the weak side balances this out.
If measured at the same constant 28" H20 pressure (which never happens in an engine) the exhaust CFM flow is about 70-80% of the intake flow. E.g., intake is 150, exhaust should be 105 to 120 CFM, etc. If the exhaust is 140 (93%), the intake duration should be extended, or the test re-done for higher lift, or both.
For a bad intake port, both lift and duration generally help (which one is more depends on the head design).
For a bad exhaust port, only duration has much effect: open the valve earlier; if still more is needed close it later.
Flow work posted here shows the exhaust port to flow substantially less CFM than the expected 70-80% of intake flow, so extending the exhaust duration works.
How much to extend it depends on how much work you've done to either port.
If only the intake has been modified, you need even more help on the exhaust side. If only the exhaust, less bias.

Your head porter's bench should be your last stop before shopping for a cam.

From what I've read, Isky ended up with alot of Mcguirk's patterns and stuff after Frank passed away and "Mcguirk Engineering" became just a small shop under Isky's roof. I wonder if Ed kept any of Frank's cam patterns? Wouldn't it be interesting to tune up those grinds with what we know today about airflow???

It sure would be cool to hear from someone who has done "FULL RACE" porting and air flow work on these heads to see if there is much of anything us shadetree boys could do to help out the exhaust ports - time vs. benefit.

Thanks panic for putting in the theory behind single pattern cams and port balance. Good food for thought!!!

- Tim

Again,the only problem with any of this is it's all theory. No hard numbers on the engines in discussion to back up the info.
From my basic measurements on several stock GM 235-261 cams I see the exhaust lift is about .010-.020 higher than the intake.Looking at the repair manuals,stock cam exhaust timing is about 12-20 degrees longer than intake at advertised duration.No secret that inlines have restrictive exhaust and intake systems .
Now,once the engine is modified with a better exhaust and intake who knows what really goes on.McGurk famous dyno test of the 261 was using a 54 Vette dual exhaust manifold.I would think a typical set of Fentons may flow better.Or of course what effect does a tuned steel tube header have?
Flow bench work needs to include the intake system to get a true idea of actual flow.

Just a guess?
The exhaust restriction where the valve head comes too close to the back wall has been explored courtesy of Curt B. This is fixable in part without spending lots of $$, and IMHO required for any performance use (along with bowl enlargement, multi-angle seat, small radius where margin meets the valve's seat, back-cut valve at 30°).

The intake/exhaust flow bias is also affected by rocker ratio, especially the intake - but there's nothing available for the 235 except the odd museum piece.

How does overlap come into the equation?
I always noticed that Delta cam grinds have significantly less overlap, but more valve lift compared to stock Chevy and McGurk cam grinds.

Delta's 254 adv. duration single-pattern grind has IO 4.4 BTC and EC 4.4 ATC timing events.
The popular 'Bulldog' 254/264 dual-pattern grind has even less overlap at IO 2.7 BTC and EC 2.7 ATC.
I don't know what overlap they use on their longer duration race grinds, but it may not differ much.

I wonder why Delta uses less overlap and single-pattern cams compared to the supposedly tried-and-true McGurk dual-pattern approach. They apparently ignore this theory.
Did Delta learn from the decades of grinding cams for these engines and get better performance?
Or is Delta's approach simply more tailored for street engines without lopey idle, which may be their largest customer base for early Chevy 6?

Yes,without back to back actual testing there is no way to know for sure what really goes on.On something like a SBC the thousands of comparison tests of various setup shows what's the best bang for the buck.And many time extensive porting increased top end power a bit and destroyed power elsewhere in the powerband.Other than McGurk's 50 year old testing there's little to no reliable real world info.
You can flip over the 235 head and easily see the short comings.You can grind away and increase flow on the bench. But only actual testing on a dyno ,dragstrip or road will tell if it's worthwhile.

Single pattern cams are cheaper to make.

On the overlap,generally lower speed engines have less. And perhaps more likely is too avoid piston to intake valve interference on the 235 Chevy.
Do you think Delta ties out cams in running engine themselves or just copies older designs and customer feeback?.The old Howards M4F was a popular short overlap cam single pattern cam.
If you run down the cam specs from let's say Comp Cams for V-8's you'll see what appears to be the same specs for the various entry level performance cams be it a Chevy,Ford or Mopar. I gotta think the different head designs,stroke etc might demand engine specific cams for each type engine.But I guess it works good enough basic the street engine.
You'll also see dual and single pattern grinds offered with nearly the same specs for a particular engine. I suppose this fills in the blanks for particular tuning situations.

Probably need to do some modern day dyno testing on these engines like Tlowe did with the 250 and 292. Im sure some if not most of the old tried and true cam and intake swaps of days past will have been found to be outdated and replaced with more modern cam grinds, but it will still be good to know and see the results from it.

Arias pistons in a '54 261 block bored out to 280 ci.

Another angle.

GMC rods.

another shot....

Very nice . . . what compression ratio will you get with this setup?

What are those main bearing caps? Are they customs or what?

Yeah, there billet main caps!

Those bumps look very big, like shaped for a head with a large exhaust pocket.

What kind of lifters are you going to use? Solid or hydraulic? What brand?

Solid lifters, the whole Clifford mech. cam kit. Bearing caps machined by a retired Flint, MI General Motors machinist.

FWIW, I talked with Larry at Clifford today and he said my 261 bored out to 280 was "...impossible, we've bored 10's of 261's @ 80 over and came out some with paper thin walls and one with a hole in a cylinder."

I'm just trying to educate myself, here; could you use hydraulic lifters? If not, why not?

Re: "we've bored 10's of 261's @ 80 over and came out some with paper thin walls and one with a hole in a cylinder"

I can understand that as a matter of principle a shop doesn't want to encourage people to request work that may not come out safe, or buy a block that's already big and waste money.
However, it appears that boring 1/8" was pretty common in those days and most of the failures I read about were after a severe beating.
Perhaps they bored it based on a poorly located centerline from the previous rebuild, so they took too much out of one side?
Didn't notice severe corrosion?

I wouldn't suggest it as anything but the last stage for an otherwise known-to-be-good block (rather than start all over again), but I certainly don't think it never works.