MG Motorsports Forum

Hey, guys. What should we set our Rev Limit to? Our new to us engine has a balanced bottom end but stock rod bolts, stock main bolts and a center main strap. 20 over flat tops, calculated 11.6 to 1 comp. APT SPVP4 cam, Mini Spares high lift valve springs. Currently the rev limiter is set to 7200. Is this safe with our stock rod bolts? Should I lower it for longevity. Next winter we will most likely pull it down and at least add ARP rod bolts and main bolts. Any opinions appreciated.

Chris W

I am not sure that I would go over 6k with the stock bottom end bolts. ARP. Are a must for everything both bottom and top. Think ARP rod bolts for 1275 rods are actually big block Chevy rod bolts, go figure. Also the center main must be reinforced either with the cap top straps or better is a 4 bolt main.
With this you should be able to go to 7k with no worries even with stock crank and rods, if the rods are shot peened and polished.

John

Center Main is Strapped. I know you can't add the ARP Rod Bolts without resizing the rods so that will have to wait till next winter.

I agree with John on the rod bolts, once a stock rod is sized, even listened and use apr rod, it limited factor other than weight. I tell folks people don't get aftermarket MG rods, 1275, or MGB because they break, they get them because aftermarket is lighter. Your weak point is your rod bolts. We lightened, polished , re-sized and balanced stock rood with ARP rod bolts in SCCA limited prep and tun 7200 rpms all days long and never saw rod failures, they would easily worked at above 7200, but that is about where a LP SCCA 1275 quits making power.

Here's what I did to the 1275 rod for racing (Stock vs Raced Prepped) . SCCA required stock cranks and rods for the H production liite prep 1275, but we could lighten, and polish them. Mike Miller and Klaus Bock them CNC programed to even lighten them more than this, but now SCCA allows aftermarket rods on limited prep engines.

Of course this was something many didn't do, because most used aftermarket rods in full prep engines. So when LP HP 1275 got popular back in the early 00s,I bet I did 50 sets of these lightened stock 1275 rods.

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Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
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Edited 3 time(s). Last edit at 2024-04-22 01:29 PM by Speedracer.

Are you using the 526 or 527 springs? That's what is going to be the rev limiting component. Stock rod bolts good to 8000+ rpm. Many many years experience building and racing these engines.

526 springs have a 200 lb per inch rate (advertised)
527 springs have a 280 lb per inch rate (adv)

For comparison, Isky VWE005/SP116 have 240 lb per inch rate (adv).

Use closed seat pressures of 85 to 90. 95 if using 526. Look for 200-230 over the nose.

With 11.6 and the SPVP4, peak power will likely occur 7400-7600 depending on timing into engine. I'd set my rev limiter to 7900 to catch a missed or forgotten shift. The 1275 LP engines built in-house made peak power when 11-1 at 7200 and the 12-1 at 76-7800 on Stuska dyno. Porting allowed only 1" in from manifold. Stock HS2 carbs and manifold.

BB Chevy rod bolts share 3/8"-24 thread size with Spridgets, but different diameters for press to bolt bore. Also head shape different. Special modifications necessary to correctly use BB bolts in Spridgets. ARP bolt specific to 1.625 rod A-Series and is shaped very much like a stock bolt.

Just re-read the OP. 20+ flat tops. If the AE 21251 type piston I would back the rev limiter down to 7000 rpm because of the crappy oil control groove type. These are "W" oil type grooves, not small diameter holes or narrow slots. The piston tops have a tendency to separate from the bottoms when rev'd past 7000 towards 7500. If using the AE 21253 or the County equivalent or the MS Mega type, this is not a problem for that type oil control groove.



Edited 1 time(s). Last edit at 2024-04-22 01:57 PM by Spridget64SC.

We are using the 526 Springs i think, but I'll check the paperwork that came with the car to be certain. I plan on checking the install height of the springs and bought a tool to check the spring rate so I'll know that here soon as well. I'll check to see what pistons we have. The one pic in my phone shows C-AJJ3382-20 GLD on the tops of the pistons. I'll see if I can do a quick internet search since I'm away from the build book that came with the car. I have the cam timed to 101.5 ATDC just as the APT paperwork stated. The engine builder had it installed with no offset keys or Vernier so it was not optimal at all. The head we have is a "big" intake valve street ported head for now until we get the rest of the running gear up to snuff, then we'll pony up to a real race head.

Thank you all for the replies so far!

Chris W

Looks like we have the "Mega" Ae pistons.

Yes, those pistons should be quite adequate for compression ratios in the +13-1 range and rpm well past 8000.