Don Terrill’s Speed-Talk

Anyone with experience on sb chevy circle track STEELHEAD racing.
It seems that out of 8-10 engines that I have built that when I shorten the stroke & enlarge the bore that I lose some power.
These engines are all in the 360ci range with RHS RAISED RUNNER steel heads,titanium valves,shaft rocker with ratios varying from 1.90-1.85 to 1.70-1.70,durations @ .050 from 258* to 262* on intake,262* @ .050 to 268* @ .050 on exhaust,all with 6" 530 gram Pankl rods..Used various intake from GM 1103 to Edelbrock 2926 ported by Wilson to Holley 300-41 ported by HVH.Cranks have all been Winberg or Bryant's,283 mains & Honda rods,some with center counterweights & some not.Strokes were all in the 3.335 to 3.290 Power has been from 690-705 & torque is 530-540.
Here is the problem.
Last engine was 4.185 bore and 3.270 stroke.Same RHS RAISED RUNNER head ported by Ron's.Wilson ported 2926 intake,1.80-1.70 T&D rockers.Since the stroke was so short I used a 5.85 rod that weighed 570 grams and a 262* @ .050 on intake & 266* @ .050 on exhaust camshaft.This engine lost 10-15hp & 10-15# torque.
Question for the engine builders is what happened?

I don't think I've ever seen an engine with steel heads...

That's a lot of help.

Maybe with the larger bore block the cylinder doesn't seal up quite as good and you lose some power.

NASCAR engines run at 4.250 bore so why wouldn't a 4.185 bore work?

perfconn wrote:NASCAR engines run at 4.250 bore so why wouldn't a 4.185 bore work?

Are you using the same block that they use or a production block? Was the cam shaft that you used ground on the same base circle in both engines or was the base circle reduced on the long stroke engine for cam clearance?

How much of a difference did you see in the timing requirements of each engine?

Using a Bowtie block.Always have used a 50mm cam & did in this one also.All the engines and I'm talking about 8 or 10 of them always like 33*-34* You don't need a reduced based circle for a 3.335 stroke.

perfconn wrote:Using a Bowtie block.Always have used a 50mm cam & did in this one also.All the engines and I'm talking about 8 or 10 of them always like 33*-34* You don't need a reduced based circle for a 3.335 stroke.

So you're saying that you shortened the stroke a max of only .065 or so and it dropped that much power? I was thinking that you had made a much larger change from say a 4.030 bore 3.625 stroke to the short stroke stuff that's why I mentioned the cam base circle which still shouldn't be an issue.

What about just difference in the heads themselves? Maybe the ones you used before were from a previous/slightly better batch. I had been thinking that maybe the chamber was laid out to fit a lot larger bore and it messed up the pressure recovery vs what it did on the smaller bore.

This particular set of heads was done by Ron's with a CNC program as were all the others.Everything checks pretty much the same and I think the engine will race good just doesn't make good Dyno numbers,I'll try to post the Dyno sheet and flow sheet if possible.

I lowered the inertia factor to make those numbers.They were actually about 19hp off what the sheet shows.

When you tried different spark advance did you notice any effect on opposite ends of the power band? Did more or less timing increase peak torque and decrease peak HP or vice versa? Does it appear one timing setting favors torque peak and another favor the power peak?

Timing on these engine are normally 34* & I've tried 33* with no other changes and saw no difference anywhere.This is with Renegade 112 gas and 1.400 venturi carburetors.In the 4 or 5 years of building these engines I have used customers carburetors.One of them was an 1.560 that I thought was so big it wouldn't run but actually ran OK.Have tried 1.450 venturies and not saw much difference.
I'm just curious if the shorter rod affected the cam timing or something that I don't know a lot about.I knows the cam experts talk about cam events at certain degrees but that's over my head.I normally use 106*-107* LS and put them in on 101*-102*

Did you try less than 33 degrees or retard it until it definitely lost power to find the minimum timing that makes the best power? How do you know 33 isn't too much? Some cylinder heads with recent improvements in port and combustion chamber shape are "fast burn" and run best with less than 30 degrees advance.

No I didn't go below 33* The RHS RAISED RUNNER STEELHEAD is not a fastburn head.This is a 21* SB Chevy head not a Vortek.

You know where this is headed ....

If you didn't go below 33, how do you know 32 or 30 isn't better?