What's the bottleneck in my engine?

[MidOHasp]

Bevan, do you have a flat bottom on your SPF? I start losing balls around 140 for fear of takeoff and abrupt landing.

[BevanWright]

I dont have an SPF, it is a NZ made replica by Almac Cars. It does have a flat bottom now which I made out aluminium, but I was running at the 185 - 190 mark without it, and it just started getting light at that speed. That is when I decided to make the flat bottom, and intend on making an air-damn as well. The intention was to go over 200, but I was young, single and without kids at that stage - maybe I would think twice now?

I dont know why my car is so stable at speed, it is low, it has an aluminium tonneau cover and a small aeroscreen, so i guess that allows significantly faster air flow over the top of the car. Also the bonnet has lots of louvres to let air out, and the flow of air out the side louvres is pretty open from the engine bay.

I have been surprised by the comments of the 140-150 mph being the tops for most Cobras, but then I havnt driven a Cobra with a full windscreen etc over 100mph.

[ItBites]

I know, I know, I said I'd evaporate from this thread, but I guess I lied... Durn Chevy folks... I couldn't resist talkin to another Chevy Cobra guy... I been so lonely...


Bevan,

I think an LS-7 should have had rectangular port heads(?). If they are ovals, you are past their usefulness at 7000 RPM, but apparently it runs pretty good. I've had my Cobra past 170 mph (Radared at Brainerd Int Speedway) and it still had quite a lot in it, but 190 - wtf dude?

I use Chevy W-port aluminum heads that are flowed BIG time. My cam is something like .690 lift and 270 deg at .050. I never had it on an engine Dyno, but RWHP was 505 at 6000 RPM when the tires smoked on the roller. I shift at 7400 and the slope of the HP curve was still pretty steep. I also have injection and that is good for a few HP since manifold density can be raised to near atmospheric density without losing proper mixture.

I'd guess about 625 for your combo with oval iron heads, but at 190, unless its really light, its better than that, so either I'm conservative or your heads are better than I could imagine for oval ports. With rectangular heads flowed etc, I'd guess 700 at 6800 RPM.

BTW, you're right about much of the HP claiming going on. My car is so loud to get the HP I have that I've been kicked off several tracks across the country. I've never heard one louder or with as much pop/crackle as I get. 12.5:1 does that.

Anyhow, keep the dirty side down (at 190 - wow!!!)

[BevanWright]

Sorry - you are right - they are rectangular ones - I was thinking that the options were oval or round.

[BevanWright]

and ps - the car is reaonably light at around 2300pounds. (is this light?)

It is a Kevlar body, ladder chassis, but heavy all iron engine. A lot of the weight is Engine, box, diff.

The weight wouldnt make a huge difference to top speed tho from my knowledge.

[CJ428CJ]

Blykins,

Yet another example of how accurate Desktop Dyno seems to be. I'm impressed with its accuracy every time I hear some comparison numbers. Let us know what you come up with after you play around with different combos a bit.

Chris

[MidOHasp]

Bevan,

Not sure why I thought you had an SPF...nothing written up there to indicate that.

In any event...damn.

Yes, I agree about inflated HP claims. Have to be some out there. I know that if I swapped intakes, I might be able to pull 530-540, but to get north of 600 would require some major work. Or a big block

You Chevy guys.......well, I won't argue with BBC guys. I know that the BBCs work from some experience

Not sure if I will ever wander much north of 150. Maybe some day.

You guys are nucking futs.

[BevanWright]

haha - yes, I think that helps!

[Excaliber]

The "book" that came with my ERA says:
665 horse power (was listed 675 and X'd out 665 hand written in) without side pipes. WITH side pipes 96 horse power lost.

Crane roller cam but I have no specs. Roller rockers. 12.5 to 1 compression. Twin 660 cfm Holleys, 75 jets all around.

Radical idle, don't run "good" till about 3000, then "explodes" with power to around 7000. I drive it to work, but it takes a certain "technique", it aint always easy. I'm in trouble if I get stuck in traffic!

I backed off the timing, changed the jets to 68's and set the rev limit at 6,000. I lost some horse power (no problem there). It's "OK" on the street now, but 500 plus horse is a handful. Once in 3rd gear traction is no problem with the BIG letter bias Goodyears.

Close Ratio top loader, 3:31 rear gear.
At 6000 rpm:
1st 62mph, 2nd 85, 3rd 111, 4th 143

,,,just thought I'd throw out some numbers, comments/critiques welcome. I'm still learning the car.

[petek]

Poor Ernie. $35k ERA witha 665hp motor and it can be difficult to drive in traffic ... and he lives in Hawaii where there is ALWAYS traffic!