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Very high RPM on pump gas #2706715
10/14/19 01:48 PM
10/14/19 01:48 PM
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hemienvy Offline OP
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I saw an ad for a 3.250-stroke Hemi crank.
So I started wondering about how one would build a 9-10,000 rpm engine to run on pump gas.
I thought there were some motorcycle engines that could rev "pretty high", but I don't have a number to put on that.
You couldn't have 15:1 compression, but you probably would need a cam with lot of duration, or.....????
How would you build such a motor ?
Big heads, low compression ?

Re: Very high RPM on pump gas [Re: hemienvy] #2706717
10/14/19 01:52 PM
10/14/19 01:52 PM
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I don't know the compression ratio but a guy I know from Drag Week just finished up what I think is a cleveland headed 351 ford. 427 cubes 762 horsepower to the tires at 8300RPM on 93 octane. I don't know the RPM limits but an uneducated guess of mine would be in the 8500 range on pump gas.

video

https://youtu.be/s3oxTGDiGSc

Last edited by Bad340fish; 10/14/19 01:54 PM.

68 Barracuda Formula S 340
87 "Chrysler" Conquest
Re: Very high RPM on pump gas [Re: Bad340fish] #2706718
10/14/19 02:08 PM
10/14/19 02:08 PM
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It depends on flame speed, with a much shorter stroke the pistons are actually moving slower MPH up and down thus you can run a higher RPM and not outrun the flame front. Like was mentioned motorcycle engines routinely turn 12-15,000 RPM on pump gas butt they are very small displacement but the flame speed and piston speed are probably similar to a stroker engine. A piston going 12,000 RPM with a 2 inch stroke is not traveling any faster on average than a piston in a 4 inch stroke engine running 6000 RPM.

I think what your considering is actually going to be tough because you need a big dome for a hemi to have much compression already, to get a small cube engine to have a lot of compression with those ginormous chambers you are going to need a massive dome that will make a heavy piston and take much longer for the flame front to work it's way around and that is not RPM friendly. Maybe you could weld up the sides of the chamber some like a GEN III hemi so you don't need as big of a dome. You will also still have things like pushrod weight limiting what you can do with the valve train.

Sounds like a fun project, I just don't think the pump gas is going to be a limiting factor.


I am not causing global warming, I am just trying to hold off a impending Ice Age!



Re: Very high RPM on pump gas [Re: HotRodDave] #2706721
10/14/19 02:21 PM
10/14/19 02:21 PM
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I should add that I am thinking in generic terms, I am not necessarily talking here
about a 10.725 deck 426 Hemi. What characteristics would a 10,000 RPM pump gas engine
of any sort ideally have/need ?

Re: Very high RPM on pump gas [Re: hemienvy] #2706724
10/14/19 02:32 PM
10/14/19 02:32 PM
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polyspheric Offline
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Basis for "9-10,000 RPM" with 3.25" stroke?


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Re: Very high RPM on pump gas [Re: hemienvy] #2706727
10/14/19 02:55 PM
10/14/19 02:55 PM
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Light pistons, strong light rods, well balanced crank (center counter weights on a V8) light weight, big bore short stroke, really good flowing heads, light weight valve train (narrow valve stems, titanium retainers, beehive springs), on a pushrod engine you tend to want very high rocker ratios so your lifter and PR can have shorter travel, intake and exhaust carefully tuned to an exact RPM range instead of off the shelf stuff, a shorter deck and/or higher cam position for shorter pushrods, meticulous oil control considerations, smaller bearing journals for lower bearing surface speeds, flat top pistons for best flame travel.


I am not causing global warming, I am just trying to hold off a impending Ice Age!



Re: Very high RPM on pump gas [Re: HotRodDave] #2706730
10/14/19 03:08 PM
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Thanks Dave, what about CR ?

Poly, Some "short stroke", 3.250 is kinda short.

Re: Very high RPM on pump gas [Re: hemienvy] #2706745
10/14/19 03:38 PM
10/14/19 03:38 PM
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I don't know the full spec or CR (wasn't pump gas) of my old mates 351 Pinto that raced in the 80's, but I do know he ran a stock 351C crank and a [img]https://ibb.co/LnrQrkF[/img]stick, launched it around 7800, shifted it near 10000 rpm, ran a best of 8.16@164, survived for a while!!

Last edited by rb446; 10/14/19 03:57 PM.

1969 'Cuda 446ci, best 9.96@133.9 in 1990
1971 340 'Cuda, best 11.01@122.8 in 1987
Re: Very high RPM on pump gas [Re: hemienvy] #2706761
10/14/19 04:43 PM
10/14/19 04:43 PM
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Originally Posted by hemienvy
I saw an ad for a 3.250-stroke Hemi crank.
So I started wondering about how one would build a 9-10,000 rpm engine to run on pump gas.
I thought there were some motorcycle engines that could rev "pretty high", but I don't have a number to put on that.
You couldn't have 15:1 compression, but you probably would need a cam with lot of duration, or.....????
How would you build such a motor ?
Big heads, low compression ?

Let me ask you why would you want to do this, are you crazy work
426 gen 2 type Hemiroid motors have inherently bad valve train geometry down
To answer your question with pump gas a good set of individual shaft rocker and the proper valve train and rocker gear may get you there but with that little tiny short stroke like that it won't have any torque to speak of N/A work twocents A stock stroke 426 Street Hemi with that type valve gear would be a lot better choice in my opinion for a high RPM street pump gas motor work twocents
Torque is what moves the mass, NOT high RPM HP runaway

Last edited by Cab_Burge; 10/14/19 04:44 PM.

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Very high RPM on pump gas [Re: hemienvy] #2706787
10/14/19 06:45 PM
10/14/19 06:45 PM
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polyspheric Offline
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I was asking how you arrived at 9-10,000 RPM as a target? The rod-breaking stress level @ 10,000 RPM is double that @ 7,000 RPM.

Double the budget, much less power than stock stroke.
The whole "the heads will work much better with smaller displacement" has been tried by everyone in the last 80 years.
The heads will work very differently, but not necessarily better.

The rod ratio goes to hell. Unless you want a boat anchor for a piston, the rod gets longer by half the stroke change (more if low piston weight is desired):
6.86" ÷ 3.75" = 1.83:1 ratio
7.11" (+ .25" longer rod) ÷ 3.25" = 2.19:1 ratio
Do you know what that does to vacuum? To air motion around overlap?

You're throwing 50 years of hemi cam development over the cliff, and starting with a blank sheet of paper.

Ask Barton for the price on a complete set of rockers (take a stiff drink 1st).


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Re: Very high RPM on pump gas [Re: polyspheric] #2706847
10/14/19 10:13 PM
10/14/19 10:13 PM
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No need to spin them that hard. I have shifted at 7500 with a 4.5 stroke and it was still pullin!!


HEMI-ITIS has no cure.
My condition is fully BLOWN!!
Re: Very high RPM on pump gas [Re: hemi-itis] #2706894
10/15/19 01:07 AM
10/15/19 01:07 AM
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hemienvy Offline OP
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OK.
Thank you gents, good points made here.

What I'm getting is that there's no reason on God's green earth to ever build a 10,000 RPM engine,
let alone a 10,000 RPM Hemi.

This isn't quite the direction of inquiry I thought I was posting.

Re: Very high RPM on pump gas [Re: hemienvy] #2706935
10/15/19 09:56 AM
10/15/19 09:56 AM
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there's no reason on God's green earth to ever build a 10,000 RPM engine
To win races


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Re: Very high RPM on pump gas [Re: polyspheric] #2706939
10/15/19 10:14 AM
10/15/19 10:14 AM
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Originally Posted by polyspheric
there's no reason on God's green earth to ever build a 10,000 RPM engine
To win races


Unless you are running NHRA Pro Stock then 10,500 is your limit ......

If the rules of the class dictate a cubic inch limit then RPM is the alternative to make horse power - there was a car running a small HEMI on the salt flats as I recall - I would guess that crank came from a class racer at some point

But that would be a real difficult and expensive way to make HP in a HEMI

Re: Very high RPM on pump gas [Re: DoubleD] #2706954
10/15/19 10:57 AM
10/15/19 10:57 AM
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Originally Posted by DoubleD
Originally Posted by polyspheric
there's no reason on God's green earth to ever build a 10,000 RPM engine
To win races


Unless you are running NHRA Pro Stock then 10,500 is your limit ......

If the rules of the class dictate a cubic inch limit then RPM is the alternative to make horse power - there was a car running a small HEMI on the salt flats as I recall - I would guess that crank came from a class racer at some point

But that would be a real difficult and expensive way to make HP in a HEMI






Not to many pro stock guys on this site since AJ retired and our other member switched teams


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Re: Very high RPM on pump gas [Re: polyspheric] #2706985
10/15/19 12:36 PM
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Originally Posted by polyspheric
there's no reason on God's green earth to ever build a 10,000 RPM engine
To win races

There is a member on here who has a BAE #8 race hemi that has been above 10,000 RPM a bunch of times shruggy
I believe they where told by BAE that the blown alcohol racers are spinning their motors up to 11,500 RPM shock confused
I would never advise any one to go their (above 8500 RPM) unless they had a unlimited budget and really like to do a lot of preventive maintenance while racing it work wrench

Last edited by Cab_Burge; 10/15/19 12:39 PM.

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Very high RPM on pump gas [Re: hemienvy] #2706992
10/15/19 12:46 PM
10/15/19 12:46 PM
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Originally Posted by hemienvy
I saw an ad for a 3.250-stroke Hemi crank.
So I started wondering about how one would build a 9-10,000 rpm engine to run on pump gas.
I thought there were some motorcycle engines that could rev "pretty high", but I don't have a number to put on that.
You couldn't have 15:1 compression, but you probably would need a cam with lot of duration, or.....????
How would you build such a motor ?
Big heads, low compression ?



You not going to do it with factory lifter placement and factory cam tunnel. Would need a staggered lifter pattern in block, preferably a keyway lifter. also would do it with a 55-60 cam with roller bearings.

Re: Very high RPM on pump gas [Re: cuda499] #2707020
10/15/19 02:13 PM
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Lots of variables to consider biggest being the differences in pump gas that's available out there, there WILL be a power difference in different brands of fuel. Next I would think being able to keep the water and oil temps under control would be another problem. Extra heat is going to make it more detonation prone, so I would imagine that would be a big concern. Next the RPM level you are looking at will create more heat inherently. Im not sure anyone has ever built a pump gas large non boosted automotive V8 type engine for that type of rpm. The parts needed to simply achieve that RPM are not generally whats seen in pump gas type mills. 10,000rpm is a LOT of RPM to be sure. We spin the heads up motor a little harder than that. We do it on a 100 octane fuel as well. However that fuel is HIGHLY oxygenated and of course leaded. FWIW that engine used to be an unleaded pump gas SB engine back when the car went 8.80's


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Re: Very high RPM on pump gas [Re: Al_Alguire] #2707152
10/16/19 02:30 AM
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Thanks again for the additional posts.

It's agreed you would need strong parts.

I can't estimate if you could take a P/S engine and just lower the compression, you probably would lose
all the bottom end torque if you kept the same cam duration. Is it accurate to say that cam duration/timing
is what primarily determines RPM range ?
Also, it could be the ignition advance curve for such an engine would look quite unusual, but maybe not.
If they flame front burn rate increased with increasing RPM, maybe due to the violent mixture motion,
the advance curve wouldn't be too weird.
So if you had breathing and combustion, it could work.

Re: Very high RPM on pump gas [Re: polyspheric] #2707165
10/16/19 04:27 AM
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Originally Posted by polyspheric
there's no reason on God's green earth to ever build a 10,000 RPM engine
To win races


Not since modified production died.


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