Inliners International Forums General Discussion Hi Performance Chevy 250, 350-375 HP NA Possible?

Hey, I'm new here and have a few questions about the 250 inline 6 for my '66 Chevy C10. I'm building the truck as a daily driver that will also be capable going out to open track days or autocross events. I'm not looking for it to be competitive, just something that handles pretty well and be more fun to drive hard. I'd like to stay with the 250 six since it's what the truck came with from the factory, and also would be unique in the sea of V8's at shows and track events. V8's just don't do it for me.

With that said, what is possible with the 250? I'm more familiar with hi-po VW engines (current one should make 1.5-1.6 hp/ci) but I've not found alot of info on the inline 6 engines. I would like to be able to run in the mid 13's in the 1/4 mile, and I've got a T56 six speed to back the engine.

From what I've found so far I've come up with this as a rough idea of what might work:


1966 block, probably go with 40 over.
Ported original head with bigger valves, lump port kit.
I'd like to run a roller cam and lifters.
Triple Dellorto side drafts (very familiar with these- I run Dellorto downdraft's on my VW)
I can make the intake manifolds from readily available intake/carb flanges.
Forged rods/pistons.
13:1 ish compression on E85- readily available, ~50 cents cheaper than 93, runs much cooler.
Aluminum/steel faced adapter flywheel for the T56, ~20 pounds.


My questions are; what valve sizes should I shoot for, who sells forged pistons (pretty sure I'll need custom pistons though), who's good at porting heads for this level of performance, and who sells roller cams? I guess the big question is; is 350-375 hp even doable for a streetable engine? That would be roughly 1.4 hp/ci, which sounds reasonably achieveable to me, but I'm no expert on Inline 6 engines. I think that the high compression, roller cam, and separate runner intakes (reversion isn't a problem) would help tremendously with low end torque (atleast, it does with VW engines) and would help keep the engine driveable. I'd like it to make power to about 7000 rpm; is the stock crank capable of handling that, or are there other cranks I need to look into? Thanks in advance for any help!

Here's a link to the truck's build thread on 67-72chevytrucks.com

http://67-72chevytrucks.com/vboard/showthread.php?t=474456

It isn't too hard to get that much HP out of these engines. Most all the top name piston companies offer pistons for these engines, so you wont have any problems with that. A roller cam wont be needed to reach your goal unless you just have an extra $600+ to spend just for the cam. There are a bunch of guys in Brazil making 450 HP on the street with these engines using DCOE Webers, so you should be able to do it with no trouble.

250 non supercharged cubic inches is not the best way to move a 4000 pound truck no matter how much HP.

It's been pushed around by a 250 all it's life, so anything will be an upgrade! The six speed with 4.56 or 4.88 gears will also greatly help get the truck up and going. My daily driver has 4.37's, and I really like the short gearing in it.



Thanks for those links! I've been on both sites before drooling over what those guys are able to do!!

I'd really rather stay N/A. Won't individual runner intakes like used with Webers/Dellorto's reduce the low rpm reversion that causes lower rpm drivability problems in plenum manifolds? Higher compression should help lower rpm torque production, and as I mentioned I'll be running it on E85 which is 105 octane so I'll be able to run pretty high compression on the street. It's 85% alcohol so it should stay cool as well. It's readily available here and is around 50 cents cheaper per gallon than 93 octane.

The Sissell blue engine is awesome, and the intake is very much like what I run on my daily driver VW engine; individual runner intakes with twin two barrel Dellorto's. The low end torque and throttle response is unreal!

It sounds like the cylinder head will be the greatest stumbling block, but I can't afford a 12 port head. What kind of CFM numbers will it take to make around 350 hp?

Here's a pic of one of the carbs and intake from my daily driver VW Bug (small one on the right), and the other is for my stroker build. I really think that IR intakes are the way to go if you want driveability with a large cam. On my VW stroker motor, I'll be running a cam with 301 advertised duration, 268 duration @50, and 12.5 compressioin ration, and it will idle smoothly at 800 rpm. I've heard of V8 guys with similar experiences with IR intakes- huge cams that pull to 7k plus, yet idle and drive as smooth as stock. Can't this work on an 250, or do the siamesed intake ports throw a stick in the spokes?

Yes, individual runners will help tremendously.

Running DCOES as I have can really tame the wild cams. Idles pretty good.

Bottom end torque & mid range w/DCOES are great.

The siamesed head is what lacks to make real good HP N/A I.M.O.

All I can say is try it out, 44 MM Dellortos, 13:1 compression, cam of around 246- 258 range degrees duration @ .050 should be a real torque motor.
Need to use a nice aftermarket harmonic damper/balancer.

At one time I ran 3 - 48 MM DCOEs (too big) 12:1 compression 236 in 246 ex duration @ .050 it had 220- 230 psi cranking compression.

Never raced it at a track but raced friends cars that did to compare & I would say it would have run mid 13's @ a 3000 ft altitude track, but my Camaro was about 3000 LBS

I had also Paxton supercharged my 250 blowing through the 3 DCOEs, but the best overall performance/driveability my car ever ran was w/the 3 DCOEs.


MBHD

with your current rear gears you can probably run the big cams suggested,(torque motors?- thats a term usually denoting good torque at a lower rpm) but without those ratios a good mod would be to remove your doors so you can help push it away from the stop lights. Inches have a more beneficial effect on torque than anything on a natural breathing motor, and 250 aint a lot.

I've run 2.54 low B/W T-10s with stock cam 194 and 230s and feel that they kept the motor too strung out, but allowed fast shifting due to closer ratio spacing, whereas the std 2.94 three speed allowed the motors to come back down to a better rpm level at every (though slow) shift. A radical cam will work better closer ratio'd tranny and vice-versa.

Hank, a siamese head can easily make 425-450 HP with a 250 N/A with high compression and a flat tappet cam. I've prepped many heads for guys in Brazil at that HP level. They do not use nitro and you don't have to to get 450 HP out of these heads or engines, they have E85 or better pump gas and many street racers down their have much more radical engines than guys run here. So, 350 HP is just as easy. I think you are under-estimating how much HP you were making with yours. Tlowes 250 dyno engine made 300 HP with just flat tops and a 70cc head(9.8-1 compression), so another 50 horses isn't too hard to get by upping the compression to 13-1 or higher and getting more radical with the cam and more head work. Again, you just didn't realize how much HP you were making with what you had.

A lot of variables,but mid 13's in a 3000 pound car is about 300 hp.
The OP doesn't appear to mind driving a radical built small engine with deep gearing.But for many of us that gets old real fast if a daily driver...............

How do you use a 13:1 motor as a daily driver without using gas around 112 octane?

You can't unless it's a late model fast burn combustion chamber with full electronic engine management...E85 is about 110 octane??? Then tuning the carbs to flow about 50 percent more fuel...

French My 250 was between 13.9-1 to 15-1 when I first had the 194 head on it sometime before I joined Inliners and start running the drags More with the car. BUT anyway I ran it on Lcheap 87 for the better part of 6yrs It would start with a Bump of the key But had to Load the clutch to shut it down.(But if i ran 112 it would shut down fine other wise. And Those Same Pistons are still being used. I have Photos of when I would blow the head gasket out and NO Pitting in Head Or pistons.
And I pulled 16mpgs with it like that with a 3.08 rears

Your engine has an actual 14-1 static compression and ran fine on 87 octane? I got a very hard time believing this even with a brutal duration cam.It just slams all that is known about octane and compression ratios from every pro engine builder.You have discovered the Holy Grail

My main reference on what guys in Brazil run is from Douglas.

IIRC,going off memory, the top 250 CI guy that runs in a street radial class (no drag radials) is 11.70 ET & his engine along w/Douglas' 250 were bored to 4.0" running 13-15:1 compression & up 10% nitromethane & the top guy makes 490ish HP, again going from memory.

He is planning on making 700 HP w/a 292 N/A currently.


MBHD

My L6 acted 180 that of my BB 396 which had a 12-1 and you couldn't get it fire off On Pump gas Not even by pushing it @ 45mph in 4th gear. But drop a little 110 down the carb and it would fire up and Run But Once the motor was stone cold again NO Way it fire.

Here was how the motor went. a 307 12-1 pop-up Fly cut .225 Still had a good size dome and a 194 head milled.Back In 84 I had asked Glen Self and a few others what they thought the copression may have been and they all had said with Out really crunching the numbers that motor was in the range of 13.9-1 to 15-1. Mind you if I took the car to the track It would Maybe??? make 3 passes before blowing that gasket. But it would run pretty much for ever on the street as Long as I did not get on it Hard to offen.another words Not RPMing it over the 6000 mark If i pulled it past that The gasket would blow. with out a dout.

It had to be detonation blowing out that head gasket, right? Not due to the compression, per-se (except as compression effects burn rate). You were running forged pistons, right?

I'm not questioning you, just thinking out loud, trying to wrap my brain around it.

I do not think it was ever published here on the outcome of the 250 dyno results.

A.F.A.I.K., I do not think the 250 ever made over 280 HP N/A?? & w/the turbo, which helps a lot made how much HP? 350 or so?

Tlowe?


MBHD

Your right Hank, it was never published here what the final results were for many of the tests. It did make 280 HP with the balancer coming apart, but made close enough to it once fixed to call it that. With the turbo, it was pretty good over 300.

That's my thoughts too. And a high compression engine with starting issues wouldn't be fuel octane.

Very interesting discussion, I'm learning alot. It sounds like an engine with enough power to push the truck into the 13's will be fairly tempermental on the street. I guess I have to make the decision to settle for less power but better drivability and reliability, but a slower truck, or build the other engine I had in mind for the truck.

The other engine would be (plug your ears!) an aluminum block 5.3 LSx destroked to 4.8 using a 4.8 crank and longer forged rods, forged pistons, ported heads and a cam/valvetrain to handle 8500 rpm. It would also get an IR intake and four Dellorto's. Even being a V8, it would be a pretty unique build and definitely not "cookie cutter" under the hood.

If you couldn't tell already, I like high revving engines- while my daily driver VW has a ton of low end torque and will climb any hill in fourth gear just barely touching the throttle, it only pulls to a little over 5000 rpm. With short gearing it just runs out of steam too quickly, and I don't want the truck to be the same way. The VW actually has too much low rpm throttle response- it can be hard to drive smoothly in first and second gear if you're not very smooth with the gas pedal, and again the truck doesn't need to be this way. I think a bigger cam that would shift the powerband up the rpm scale would actually help cure some of the low end snap or "touchiness" and be a bit easier to drive smoothly actually. A 3800 pound truck probably won't have that problem though.

Just throwing some thoughts out there so you guys can see where I'm at with an engine build for the truck. My sister has a Trailblazer SS that runs 14 flat, and I'd like it to run a bit quicker than it does. A friend has a WS6 Trans Am that runs high 12's, and honestly I don't need it to be that quick on the street. I don't need a ton of power on an autocross track, mainly would need a wide powerband since any shifting kills time. I would defintely rather stick with the 250 if it can be made to work, but the extra power an less weight of an aluminum block LS has it's appeal too, as much as I don't want a V8.

Another option might be the Atlas 4200 Vortech I6 engine from the later model Trailblazers and stuff. They have almost 300 HP in stock form and are all aluminum and capable of 500 HP+ in turbo trim. They are high RPM capable also, just a thought....

I had originally considered an Atlas 6 before I bought the 250, but there seemed to be a bunch of support issues (oil pan, bellhousing pattern, flywheel, etc.) when I first looked into those. I've already got a T56 that wouldn't bolt up to an Atlas. There would be alot of money tied up in parts just to get it in the truck and mated to a manual transmission that I wouldn't have to spend if I stuck with the 250, or go with an LSx engine.

You might want to price what it costs to build a 350-375 HP 250, the 4200 is still cheaper even having to buy an oilpan, bellhousing adapter, etc....Just sayin'

This motor had no issue with detonation ,If it had the pistons would have been all pitted up and so would have been the valves Or even the head maybe. And I have Photos of the pistons.And Like i said I am still useing those same pistons in another motor.

just thinking... as you rev, you have less and less time to try to fill that cylinder so thats less volumetric efficiency and so less pressure as the revs increase. Only way hp can increase is by adding more events per time. So any blown gaskets could well be caused by slight detonation rather than actual firing pressures blowing out the gasket because max cylinder pressure (and volumetric efficiency) occurs at torque peak.

HORSEPOWER FORMULA
P= pressure caused by the firing
L-length of stroke
A=Area of bore
N=number of firing times per minute.

Change anything in the above horsepower formula and you change hp.

Maximum torque occurs at maximum cylinder filling which is also point of maximum volumetric efficiency.

According to Blair it's a bit more complicated.

who is Blair, and why come?

The formula is more of a logic explaination- but seems to be faultless.

It explains why a wild cammed & carbed 283 can produce say 390 hp ,and a stocker 454 the same.

Should have been more specific.
The P L A N is accurate, slightly better if you insist on BMEP instead of pressure (because it includes loss on compression and pumping).
I meant that Prof. Gordon Blair uses VE to identify how much the intake system is capable of getting into the cylinder, but that's not always the same as what's actually trapped when the valve closes. The math is pretty scary and I don't have his book handy.
The max torque and max VE almost cross, but with a slight wiggle due to parasitic drag etc.

agree with BMEP, VE and max torque relationship was my assumption, it seemed right at the time- still seems generally right. Too much much math takes the fun out of it.
I was trying to understand how too many rpms would blow a head gasket when I thought that the percent of cylinder filling was pretty much dimishing as rpm increased past the torque peak, and hp would increase only as the number of events were happening per time--I think.

Thanks

Yup, pressure and number of events will cross at 5,252.
However, just as an example?
Let's make the torque curve really flat (we wish!) around 4,000, then dip slightly at 4,100. Depending on the parasitic load (valve springs, ignition, pump, bearings), 4,100 may actually be the highest pressure but the indicated torque drops a bit because the p. load has gone up by 2.5% (directly proportional to the RPM change) - but the torque didn't.

I think I'll sit down now, I have a headache. This torque formula (and oh yes, the pumping losses) with the 5252 rpm factor in it sure messes up a good story, wish I had attended Duke University in 1968 like my friend did when he took the Internal Combustion Engine classes & labs -it sure would be easier to understand when your mind hasnt had 40 years to jell in its ways.

And Gordon Blair, you sure he wasnt the Brit who went along with Bush on the weapons of mass destruction thing?

An',an' how come the Inliners Directory got you living in Oklahoma? Were you exiled? Are you a leper? Oh Lord, here come the Okies!

The 5,252 isn't magic, or in some way controlling the output of every motor.
It's a coincidence, and only occurs when using foot pounds for torque and horsepower as units of measurement. If you use newtons, watts, BTU, joules etc. it crosses at a different spot.

Oh I wasnt meaning that at all, I've played around with the torque formula and knew of the 5252 (but didnt know about the other measurements making that kind of difference) and have taken a horsepower graph that showed no torque and ran the hp thru the formula and then plottied the torque on top of the hp graph in a diff color. It really gets interesting when you plot a blown or turbo'd engine for torque--thats impressive. For a street engine a turbo car needs no rpm to be bad news-good news for our sixes.

Expert: Gordon Blair
Lying sack of shite: Tony Blair

I found this gem in "Engine Design", by a J.G. Giles in a 1968 automotive technology series book:

"For a fuel of a given octane number the onset of detonation is influenced by the compression pressure and therefore compression ratio at fully open throttle, by the temperature of the incoming fuel/air charge, by the mixture strength, and by the characteristics of the combustion chamber. It is therefore not possible to say what octane number fuel may be used with a given compression ratio or the other way round, and the actual requirement of the engine in terms of octane number must be determined experimentally."

So the correct answer is: There is no correct answer regarding simple compression ratio vs octane rating. I like it!