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Towing Engine Build

April 18 2010 at 1:01 AM
Nick  (Login moonley)

After nearly a decade of dreaming, I think I've put together some of the pieces and most of the plan for my next engine build. The engine will replace the tired 390 in my 1970 F-250 4x4, which is used for daily driving and frequent towing at the max GVW. The truck has an eaton FS4025B transmission (newer version of Clark 285V, which I swapped in to replace the NP435) which has a 4th gear of 1.28 and 5th gear of 1 (no overdrive). I will also be putting 3.54 gears in the differentials, which will drop my RPM at 60 mph from 2,734 to around 2,360, which isn't a concern for towing because the 1.28 4th gear would increase my RPM to 3,022 if I need it to go up a grade, etc.

So...based on the info above I would like to build an engine where the torque curve peaks around 2,500 RPM (best for driveability and mileage). I have also decided on sequential fuel injection, and I have a Vortech s-trim supercharger on hand that I would like to use as well. PLEASE NOTE that I already know a centrifugal supercharger isn't the best for towing, and that turbos are more efficient, blah, blah, blah... I have the supercharger already (got a sweet deal) so I would think that using it is better than not using it.

I'm thinking that I would like to build a relatively high compression (9:1) low boost (5 psi?) engine. My thoughts are that with a 9:1 compression ratio I could still have good power and torque at low RPM before the boost comes on. Would like to hit 5 or so psi of boost around 2,500-3,000 RPM, and using the EEC for the fuel injection I could retard timing at higher RPM and/or redline the truck around 4,500 or 5,000 RPM. Please let me know your thoughts on this, details are as follows:

390 most likely .060 over (depends on outcome at machine shop)
stock crank
stock rods
forged pistons, aiming for 9:1 compression with good quench (around .041)
aluminum Edelbrock heads
Crane 343901 cam? (better suggestions??)

Vortech s-trim supercharger with intercooler
A9L EEC-IV tuned with Quarterhorse and Binary Editor
42# Lightning injectors
90mm Lightning MAF
75mm throttle body (match supercharger outlet size)
Edelbrock Victor intake for MPFI

P.S. Do you think I could get by with 89 octane with this setup? I could also use a wastegate to recirculate boost and limit it like a turbo. Sort of make a "variable compression engine" where with a few keystrokes of the computer to adjust timing and fuel, adjust wastegate settings, and I could optimize for if I wanted to run, say, E85...

Sorry for the long post, thanks for your help!

This message has been edited by moonley on Apr 18, 2010 2:24 AM

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(Login RichFerguson)

Re: Towing Engine Build

April 18 2010, 8:41 AM 

Wow. I don't know, that sounds like it might be expensive. I wonder if a stroker kit, carbed and naturally aspirated, would get the job done as well?
Nice dream though happy.gif

[linked image]

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(Login bbr60)

Biggest concern is detonation

April 18 2010, 9:21 AM 

Living near a world class towing test track,Interstate 70 west out of Denver to the 11,000 foot Eisenhower tunnel,every summer the roadside is littered with theories run amuck.Without a world class knock sensor low grade destruction can be in progress before symptoms are evident.Only you know the terrain,temperature and gas quality you will encounter.Good luck and keep us posted,sounds interesting.

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(Login rpr546)


April 18 2010, 9:40 AM 

Diesel or a naturally aspirated 8.5 compression 460, or the same compression in an FE stroker kit and no blower either.

What you are trying to do at max gvw just does not compute for long life. You are asking for diesel performance, and that is what you ought to get.

This message has been edited by rpr546 on Apr 19, 2010 8:50 AM

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(Login CaptCobrajet)

Towing engine..........

April 18 2010, 9:44 AM 

I agree with the stroker idea. I don't think you want a .060 overbore. I think a clean-up hone on a 391 block and a 4.375 crank from Doug Garifo would be the ticket. Stroke is your friend at 2500 rpm. I like the low boost, but I think you still should be 7.5 or 8 to 1. A small cam will make alot of dynamic compression, especially with the blower. Heavy loaded at 2500 with forced induction will rattle the windows at 9 to 1. I would be about 210/210@ .050 and about .550 lift. You can still buy new sodium valves for the 391 heads, and they work. Also a really wide seat, at least .080 for the seat width on the exhaust. Both will transfer alot of heat to the aluminium head, which is a good thing. I also think the older trucks need louvers in the hood like an old street rod. That will really cut the heat down. I have some towing schemes also. Just a few ideas....

Blair Patrick

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(Login cammerfe)

I believe your outlined plan....

April 18 2010, 9:53 AM 

is eminently do-able. The idea of extra torque from a blower is excellent for your purpose. A possible alternative, hardware-wise, might be to sell your Vortech and use the proceeds to buy a couple of usable Garrett TBO 344 turbos---the kind used on the 2.3 Mustangs and T-birds. I say this because of the experience I've had with both centrifugal superchargers and turbos. Financially, you should be able to make it a 'wash'. But operating characteristics would seriously favor the turbos.

In order to get boost at low RPM with the Vortech, you'd need to monkey with pulley sizes. It's certainly do-able, bur far from ideal. But because the referenced Garretts are sized for 144 CID, and using one on each bank of your 390 will actually have 195 CID feeding each of them, you should have little trouble getting boost in your operating range. And turbos , being 'demand-type' mechanisms, will have good effects on economy of operation as well as offering the performance enhancement.

By all means, make use of higher-than-usually-considered compression. It's a torque-builder and no matter which blower scheme you go with, you'll have the intercooler to help your efficiency. I have also found that a simple, home-made, methanol injection system, triggered by a Hobbs switch, will be of great help.

I'm thoroughly in favor of your projected use of EFI. And the fact that you can also control your ignition with the same system is just a 'plus'. Please continue to post regarding this project. I'm very interested to see how things work out.


This message has been edited by cammerfe on Apr 18, 2010 10:05 AM

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(Login Torino390)

Re: Towing Engine Build

April 19 2010, 1:35 AM 

I don't see too much of a problem with 89 octane gas. My S code 390 in my '68 Torino GT is .30, Ross forged custom pistons, with fully ported C8AE-H heads, 2.09 & 1.66 stainless valves, stock stroke with 10.75:1 compression & was running 91-93 octane & NO detonation.

I do have thermal barrier coating on the piston domes, head chamber, valve faces & out the exhaust port. Which also helps reduce detonation & improve performance.

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(Login moonley)

Thanks for the input...

April 19 2010, 9:40 AM 

Ok...as much as I hate to admit it you've got me thinking about the twin turbos...

Still not sure on the 9:1 compression. If I max boost at 5 psi and calculate "effective compression ratio" at sea level (14.7 psi) then I get:

(((5 / 14.7) + 1) * 9) = 12.06:1 compression ratio at my maximum boost of 5 psi...

Even with aluminum heads and good quench I should keep my compression down to 8:1, but I'm worried about losing the low end torque.

As far as the knock sensor, I have a "plan" for that as well. Bosch sells a generic vibration (i.e. knock) sensor (BS65008). If I can map the output of that sensor when bolted to my engine during normal operations with premium gas (i.e. no knock) relative to RPM, I could then devise a way for it to trigger a signal to the ECU when it senses noise above the pre-determined range. Something similar to this:


Obviously have to work out the details on this one yet, but it's got me thinking.

In the meantime, anyone have a cam grind that they would recommend for low RPM torque with good LSA for boost? Maybe a custom grind is in order...

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Barry R
(Login Barry_R)

Kinda with Blair on this one

April 19 2010, 3:31 PM 

Boost and towing can go together. But in order to get that boost "happening" at 2500 RPM you're gonna have to really spin that sucker up as far as pulley speeds. Means you'll be seeing some boost at pretty low speeds - hence lower compression than you'd spec out in a hot rod. Which in turn mandates a pretty small cam so it ain't real soggy off the line. I'd bet on it really rocking until 4000 or so and then going flat. Knock sensor might be darn important - a piece of copper tubing and a syrofoam cup is surprisingly good in a pinch....

Barry Rabotnick

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Blair Patrick
(Login CaptCobrajet)

Lower C/R............

April 19 2010, 10:12 PM 

Yeah, I kept getting a mental picture of going up the mountain over by my house, loaded heavy, gas pedal on the floor for about 10 minutes, at 2500 rpm and full boost. I'd make it about 7.5 if I thought that might ever be the situation. I hate a diesel, but I've pulled that grade at 26000# in my Powersmoke in 100 degrees, on the floor the whole way, thinking I'd really like to have a gas burner that would do that. I think it is doable but I'd have to really cover all of the bases. That's when a manual load valve on a dyno could really help simulate the hard pull, like up and down the hills, and even part-throttle loads. I wonder if I will live long enough to play with all of the ideas I have floating around in my head to try. If I get to finish half of my personal list of junk before I check out I'll be pleased! Seems like customer projects always trump mine and my stuff ends up on the back burner. It could be worse, at least I enjoy it and it is a constant learning experience.

Blair Patrick

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