Motor Choice for Superformance GT40 MK1

[Russ Noble]

Just to give you guys something to think about, The peak piston speed at 7000 RPM on a 4" stroke is about 83mph, on a 3" stroke it is around 62mph. Now at that RPM the piston is accelerating from 0 to those respective mph and back to 0 (( 233 times per second )).

Do you not think that the rings/pistons will have an easier job at the shorter stroke!

Jac Mac

Yeah, OK I'm thinking about it! But you should only have to rev the 4'' to about 5500 to make the same power as the 3'' @ 7000 with the same heads, bore, etc. Now do the maths and work out the loads in everything remembering that the loads go up in direct proportion to the stroke and by the square of the revs! Not to mention valve train loads also. Valve train is often the weakness with big revs. IMHO

 

[David Briggs]

Jac Mac has the right idea, I took a very valuable 289 Hipo out of my Tiger years ago and replaced it with a built up 5.0 w/j302 heads. The new engine does not need to rev over 6500rpm(T-5 w.80 5th). I missed the 7000 Rpm plus for about a second and 1/2. The difference in torque took a lot of the drama out of the car and it is a lot faster, plus the engine is still running great 15 years later. I race FF also and those engines typically need a rebuild every 1000-1200 miles, I would not want to do that on my GT40 and with the correct choice of engine it is completely unnecessary, you can go very fast for years.
I have always built my own engines but I will put a Roush 342 in my R model clone so that I have a demonstrator.

 

[Russ Noble]

David,

I think Jac Mac was trying to say the smaller motor is better. You appear to have just blown that theory into the weeds.

Cheers

 

[Jac Mac]

David,

I think Jac Mac was trying to say the smaller motor is better. You appear to have just blown that theory into the weeds.

Cheers

Not really Russ, In Davids case where he used a 302 & intends doing a (342ci - 3.25/3.40 stroke depending on which combo he used ) the piston speed would have been reasonable @ 6500. In the other previous 402/427+ cu in cases where strokes are in the 4.00" area piston speed is much more of a factor as per my above calculations. However if you are content to live/drive within a safe rev limit there should not be any problems with the long stroke versions.
Russ,you and I probably tend to think in race car type operation whereas most of these Superformance types will be content with blowing off the odd Ferrrari/Porsche in a quick burst on the freeway or a couple of hot laps at a track day. To each his own!

Jac Mac

 

[Howard Jones]

wooooooweeeeeeeeeeee! 500..600....700Hp!!!! Holly crap. This whole thing reminds me of COBRA owners. The thing about a double wishbone mid-engine prototype is handling!!!! If you want to be the fastest guy at a track day then get the weight down to 2000Lbs, put your money into brakes and chassis setup. A set of top of the line shocks will probably take more time off a lap than another 100hp. Then put in a nice easy motor to drive. 350Hp or so. Bolt on some slicks and have at it. I promise, you will be in the top 10% at any track-day based on lap times. IF you can drive the car.

I swear to god! These cars don't need 600Hp. Most people can go faster with 300 HP than 600 HP. Then again if it's chest thumpin were talkin then what the hell go for 800Hp. Thats about where the big hairy Cobra guys are.

How about a alum 302 at about 400Hp. Take all the bling, carpets, AC, stereo? out of the car and really get it light.

I sorry if I sound like a wet blanket. But really... its the grip that makes it fast. It will take a hell of a street legal GT40 to run lap times as fast a pro FF.

 

[Russ Noble]

Not a wet blanket Howard,

Everything you say is dead true. But bench racing is so cheap!!

But for the serious racer 500, 600, 700 hp in a well sorted, light GT40 is going to be much more manageable than in a Cobra or a Mustang. Ask Ross Nicol. Ultimately it's not a matter of being in the top 10%, but crossing the finish line ahead of the guy that gets second! There's more and more GT40's around and they are going to be the ones to beat. In the end it's going to be the lightest best sorted car that wins unless you're getting blown off down the straights! It's bloody hard work pegging it back and trying to get in front just to be blasted into the pavement down the straight again! IMHO

Cheers

 

[flatchat(Chris)]

Got to agree with Howard and Russ, I think this topic has been thrashed around enough to write a "paper" on it:dead: -- therefore I think that these guys need help! --I mean help to spend their money:coolgleam: -- so, what ever you spend on a 600 hp blunderbus you'll need to spend on transmissions,brakes,suspensions,tyres etc without adding any more weight -- I bid thee all good luck

 

[Johan]

You need an 8000 rpm 302.

 

[Keith]

But for the serious racer 500, 600, 700 hp in a well sorted, light GT40 is going to be much more manageable than in a Cobra or a Mustang

'kin 'ell!

Where's this all leading to?

Howard is right - this is all silly Cobra "mine's bigger than yours" pissing contest.

 

[JimmyMac]

Keith,
I believe that you can't put a plough on a race horse, nor a Clydesdale in the Derby.

 

[Keith]

I do believe you're right Jimmy.......rockonsmile

 

[Ron Earp]

Howard is right - this is all silly Cobra "mine's bigger than yours" pissing contest.

If we get to that point around here threads will be shut down for sure. We aim to keep the signal to noise ratio high on GT40s.com and those sorts of discussions don't contribute to that goal.

Ron

 

[Russ Noble]

Sorry if I upset someone.

I'm a bit new to building American V8's and I find there is often new information or a different way of looking at things that come out of these discussions. In particular, things like rod ratios, long/short stroke, piston speed. In particular Jac Mac said a short stroke motor was better than a long stroke. Then David said he went to a 302 from 289 and it was better and was going to go with a 347 for his next one. Seemed to be a contadiction here! I can undrstand that to you guys that have been building these things since high school it's all old hat. Guess I should just use the search button.

Howard brought up the high HP, I merely illustrated there was a place for it. Don't forget GT40's were race cars first with very few original road cars. Replicas generaly got dumbed down as road cars. Cobras and Mustangs were originally road cars some of which became modified for racing. There are lots of guys running big HP in these front engined things, I pointed out that a GT40 would be a much better platform.

Didn't mean to tread on aybody's toes.

Sorry.

 

[Howard Jones]

Oh oh..... I didn't want to suggest that anyone was wrong. Just to add my opinion. Of course all things being equal more power is better. My point is for most people building one of these cars. Money IS a object. The chassis dollars spent will equate to lower lap times quicker and cheaper than drive train dollars. I will make one exception to that a very high quality limited slip in the diff.

For a project at lets say $85K. You have the choice of a 20K motor and its 15K+ power train including gearbox or a 7K motor and 8K in the rest of the power-train leaving you with 20K worth of chassis upgrades including a second set of wheels and slicks. I submit that the second car would be faster on the lap AND run more of them without breaking.

Of course if you have the money to turn a 85k project into a 105K project then way cool! Go for it!

The last thing to consider is more and more track day groups are frowning on 600Hp plus racecars in the hands of weekend warriors. I have heard rumors of a few, very few, 600hp+ cobras asked not to participate because of insurance reasons. Maybe that was only the excuse who knows for sure. It seam to me to be the beginning of a trend though.

 

[Ron Earp]

Hey, don't let my comments worry anyone. This thread has not degenerated to a low level yet so don't be concerned. As usual, all you guys are civil to one another and we've never had any trouble except from one or two personalities on the forum in over five years.

I'd vote short stroke and high RPM for my track car. What is a NASCAR rod ratio? 358 inches is about all I know, and that might not even be right. Those turn up to 9500 RPM (and higher) for long periods of time, 7500 RPM would be no trouble for these motors.

Ron

 

[Alan in Nevada]

The chassis dollars spent will equate to lower lap times quicker and cheaper than drive train dollars. . . . . . leaving you with 20K worth of chassis upgrades . . . . .

At the risk of getting briefly off-topic here . . . .

Howard:

What upgrades would you suggest for a SPF GT for 20K?

 

[Hi-Tech Auto]

Alan,
As the chassis and suspension of the SPF GT40 is a direct clone from the original gt's,except for the more modern shocks, springs and tyres that are fitted, there is not much that you need to do for general use. When it comes to racing, the first thing we need to know is what type of racing and what are the rules that are applicable. Only then can one start tuning the shocks, springs and rollbars around the choice of tyre allowed. For road use and the odd track day the current car, as supplied, is more than adequate.

 

[Howard Jones]

Alan, Shocks/ springs will be the single biggest expense assuming that the car comes with adjustable rod ends all around. Konis, maybe Penskis(sp?).Then some custom made adjustable anti-roll bars. If I was spending your money I would do a cockpit adjustable rear bar. Have the car corner weighted and have the same guy get the bump steer as close as possible. Maybe the SPF's don't need this but I would check it.

Brakes, put on at least 13X1.25 on the front with 6 pots and 12X1.25 rotors on the back with 4 pots. If you are not fugal here you can really eat up some bucks. The good Wilwood stuff is OK. If you REALLY want to spend money Alcons will do it for you! Ask SPF about brakes and see if you can get this done right the first time. Duel masters and a cockpit adjustable brake bias bar for sure.

I would add a roll cage to any GT40 I build as well as a onboard fire system.
Again ONBOARD FIRE SYSTEM!!!!!! Best 500 bucks you ever spent! Also if the gearbox you use isn't equipped with a limited slip do this as your first upgrade. This is the most important upgrade. I really like my Quaife LDS. I would try to order my gearbox with one in it. Works like magic. You don't need a 6 speed. 2.3 1st, thouugh .8 5th with a 3.50 ish final is about right.

Now you need to spend some of that money on track time to get the car set up. This takes time. Take notes and one thing at a time! You will need a tire temp gauge, toe plates, caster/camber gauge and someone to help.

Lets say you spend a thou on some open track time. Gas, hotdogs, tires, brake pads and 3 or 4 days of track time ough to pretty much finish of the thou.

I would think you are fairly close to 20 thou. This is for parts and some labor. I assume you are going to put most of the parts on yourself or order it with then on the car from the beginning. If you have any money left have a custom alum race seat made. Add a 6 point harness if it has a 4 point and install it correctly.

By the way. If this is a track only car leave off the side windows. Or put a nice big hole in the drivers side one that you can get your hand out of.

Put 400Hp in this and I garrr--onnn-teee it will be FAST.

 

[Alan in Nevada]

Hi-Tech Auto: Thanks. We know that this is "straight from the horse's mouth."

Howard: Thanks for the ideas. When I get mine, I'll get it on the track (or a really big parking lot) and start sorting out.


For me, it will be mostly the street and a half-dozen local SCCA time trials a year, and maybe the Silver State Classic (formerly known as the Eastern Nevada Road Race. (If they keep those damn steers off the highway)) Formal racing? We'll see.

Again, thanks to all for your input.