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Finally got my '71 coupe on the dyno. Turned out my butt dyno is very close. I told my son yesterday, I figured 360rwhp @5200 rpm and 430 rwtq @ 3400rpm.
well, it made 387rwhp at 5200+ and 442 rwtq at 3000rpm.
Combo is as follows: 9.25:1 CR, 468 ci BBC, Pro Comp rect port heads, smallish solid roller (240 dur, .570" lift), "pancake" LS6 intake, Hooker headers with 3" exhaust all the way back, T56 transmission.
makes over 400rwtq from 2500-5000. It is obvious that the intake is a significant limiting factor, but that is the best option with rect port heads and retaining the stock BB hood. But an L88 hood and RPM AirGap would give me another 20-25rwhp from 4000 on, and probanly increase the efficient rev range another 300-400rpm. HP is flat from 5000-5700, where we ended the pull. Tempting to try and get more top end pull out of it... however, this is a 2-bolt block. It has decent internals, but nothing lightweight/high-$$. Not sure I trust shifting it at 6300+ frequently, even if it made power up there.
Honestly, im very happy with it. there is more in it with a better intake manifold, but that means a hood and corresponding paint work. the paint is fantastic on this car. What I may very well do is add some gear. it has 3.36 gears and with the 6-speed, I could easily get away with 3.73s or 4.11s. right now I can do 52-55mph in 1st. no need for that. 4.11 is probably overkill, but I am running 27.50" tires, so 5500rpm in 4th would be 109mph. 3.73s @ 5500rpm in 4th would be 121mph.
You are under estimating what a 2 bolt bbc block can handle.
Since I have very little long-term experience with them (owned a '69 for a few years, back 15+ yrs ago... it had a Gen VI 4-bolt 454), I just don't know.
This one has a prepped stock crank, forged rods and pistons. I didn't build it, but the PO seemed pretty meticulous with the entire build, and has lots of documentation on the prep work that was done before and as it was built.
Think so?! I was thinking low-12s. But maybe with the flat torque curve, it would ET better than that.
When I had a 383, it rear dynoed at 373 rwhp, and about 450 rear wheel torque. It's a '69, auto trans, and had 4.11 gears. It ran 11.62 seconds in the quarter. So, yes, you have the potential.
Since I have very little long-term experience with them (owned a '69 for a few years, back 15+ yrs ago... it had a Gen VI 4-bolt 454), I just don't know.
This one has a prepped stock crank, forged rods and pistons. I didn't build it, but the PO seemed pretty meticulous with the entire build, and has lots of documentation on the prep work that was done before and as it was built.
its been a long time ago but i had both a 396 and a 454 both with two bolt blocks and turned the engines 7600 rpm, both with factory rectangle port heads. Street cars never turn high rpms for long time periods like a roundy round car or even road racing. Ya the rotating assembly is heavier then sbc but the blocks are thick everywhere in comparison and they dont have paper thin cylinder walls. So if you want to go past 5200 rpm the block not going to give you problems.
Last edited by Little Mouse; 05-01-2020 at 08:50 PM.
its been a long time ago but i had both a 396 and a 454 both with two bolt blocks and turned the engines 7600 rpm, both with factory rectangle port heads. Street cars never turn high rpms for long time periods like a roundy round car or even road racing. Ya the rotating assembly is heavier then sbc but the blocks are thick everywhere in comparison and they dont have paper thin cylinder walls. So if you want to go past 5200 rpm the block not going to give you problems.
After seeing the dyno sheet, it makes sense to shift it upwards of 5700rpm right now, so that's where I will.
Moving forward, I figure with nothing but a high-rise intake, I will pick up power above 4500rpm and it would probably push peak HP up in the 5700-5800rpm range, so shifting at 6100 or so would be the trick.
Glad to have some understanding of these. Most of my experience is with SBF and GM LS-engines.
My builder routinely runs 2 bolt blocks to 600hp. Drag racing.
But after 500 he does put studs in them.
Under 500 no problem.
Thank you! It did turn out a lot like yours... if you ever get yours on a chassis dyno, we will really have a good idea on driveline losses in these cars. The TKO and T56 are more efficient than the Muncies, I think... but the IRS in these things isn't nearly as efficient as a solid axle car, IMHO.
Figuring somewhere between 15-20% driveline losses, I'm ~460-480hp at the crank now, TQ ~520.
I think I will do gears before considering a new L88 hood, prep and paint, and an AirGap intake. It's a fun car, just fast enough to enjoy it without getting myself hurt. If I want to do 200mph or go around a road course, the newer car can handle that.
Your heads being a new form of rectangle port im sure have a lot of volume and big valves, your at to low of rpm to take advantage of the heads.
I agree... need more intake, more cam. The guy who built it said he wanted to model it after the LS6. I'd say he pretty much hit the mark as far as power and torque goes (its a 460hp/520tq motor at the flywheel), but nowhere near optimized.
If I ever pull it apart, I will re-do all of it. AFR heads, more compression, hi-rise intake, probably a stroker crank in a 4-bolt block and rev it to 6500+.
I went for almost an exact duplicate of an LS6, even down to the heads and carb. Yours and mine both should be at my number, 485HP. I've got a point more compression but you have better heads. Cams are real close.
I personally like the BB hood way more than the L88 hood, so I am really limited on intake.
If I was going to build it over again, for a different car, I would go with AFR heads. The exhaust flows soo much better than these 50 year old heads they could easily be worth 60 HP. Those and a Victor Jr and an 850 or so would gain lots of top end, and with a little bigger cam you're pushing near 600!
And I love the wheel / tire look you have! You are making me rethink my Ralley wheel plan.
I went for almost an exact duplicate of an LS6, even down to the heads and carb. Yours and mine both should be at my number, 485HP. I've got a point more compression but you have better heads. Cams are real close.
I personally like the BB hood way more than the L88 hood, so I am really limited on intake.
If I was going to build it over again, for a different car, I would go with AFR heads. The exhaust flows soo much better than these 50 year old heads they could easily be worth 60 HP. Those and a Victor Jr and an 850 or so would gain lots of top end, and with a little bigger cam you're pushing near 600!
And I love the wheel / tire look you have! You are making me rethink my Ralley wheel plan.
Thank you. I get a ton of compliments on the new wheel/old car thing I have going with this car.
I know on the '69 I had, it was 9.5:1 CR with AFR heads, hyd roller a little smaller than my current solid roller, Air Gap intake and Hooker sidepipes with STS baffles opened up, 950 carb. Made 434 rwhp / 450rwtq. Here is the old thread on that build. https://www.corvetteforum.com/forums...bbc-combo.html
The biggest difference is that even though peak TQ is not much higher than my current combo... it came on a little higher in the rev range (peak TQ was at 4400rpm, but it was still over 400 at 2500) and stayed there longer. HP peaked at 5600rpm and nosed off, but it still easily pulled to 6300... I think it was still at 410rwhp at 6000rpm.
The engine in this car is strong and tuned well. I recorded an A/F ratio of 12.85:1 across the entire rev-range. Maybe could run a touch more ignition timing, but that might get me 5hp... and then when it gets really hot here in TX, I may have some detonation issues. Easy for an EFI car, but I am a set it up, be done, and go drive it person. I didn't fool with it on the dyno. Made two pulls and they were within 1hp/1tq of each other. I think just adding an Air Gap and maybe a little more carb (if it really needs it) gives me another 20-25rwhp. Not sure peak TQ goes up at all, but it would carry it out farther up the rev-range.
Congrats, that is a nice engine and beautiful car. I am sure it is a lot of fun driving. Now you know how nice another 25 and an L88 hood would look on your car lol! 😀😀😀
My 467 with a 2 bolt block in my drag car made 702 hp at 6600 rpm and I shift at 6800 rpm, no issues. Has the stock main bolts. (Engine dyno numbers) I think you are safe to push it a little harder.
05-01-2020, 01:48 PM
C3 chassis dyno report
4.11 is probably overkill, but I am running 27.50" tires, so 5500rpm in 4th would be 109mph. 3.73s @ 5500rpm in 4th would be 121mph.
